Pepping up a Silicon Chip SC480 audio amplifier

[Terry Given]

Hi All,

this is for the audio gurus:

I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a
couple of amps.

I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.

It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!

For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.

I havent got much time right now, and specifically dont want to break
it, cos then I'd have to rebuild it. and I dont have the time to measure
the open-loop transfer function repeatedly as I mess with the various
compensation caps.


So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled
in here) then a 2k2 into the base of Q2 with a 330pF shunt.

it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which,
of course, I cant, and which will lower the corner further.

there is a 10pF miller cap across the feedback input transistor Q3, and
a 68pF miller cap across the B-C junction of Q5. Im guessing the 68pF
does the bulk of the work here.

Any suggestions as to what I can get away with?

Cheers
Terry

[Bruce Varley]

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a couple
of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it,
cos then I'd have to rebuild it. and I dont have the time to measure the
open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled in
here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of
course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a
68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does
the bulk of the work here.
Any suggestions as to what I can get away with?
Cheers
Terry

This isn't a direct response, more something related that you may want to be
aware of. I've cooked a few hifi amps in the past by trying to use them as
low region ultrasonic drivers, what appears to happen is that applying high
power at high frequencies, phase shifts cause overloading around the
internal feedback loops, predriver and driver devices can get overstressed.
The amp may be rated up to 20+ KHz, but the assumption seems to be that the
spectral distribution of what it's handling is 'audio', and not bat-speak.

[Eeyore]

Post by Bruce Varley

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a couple
of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it,
cos then I'd have to rebuild it. and I dont have the time to measure the
open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled in
here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of
course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a
68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does
the bulk of the work here.
Any suggestions as to what I can get away with?

This isn't a direct response, more something related that you may want to be
aware of. I've cooked a few hifi amps in the past by trying to use them as
low region ultrasonic drivers, what appears to happen is that applying high
power at high frequencies, phase shifts cause overloading around the
internal feedback loops, predriver and driver devices can get overstressed.
The amp may be rated up to 20+ KHz, but the assumption seems to be that the
spectral distribution of what it's handling is 'audio', and not bat-speak.

Yup.

You need to check the full power bandwidth figures. I like to see 40-50 kHz
myself in my audio designs using MJ150xxs.

Graham

[Terry Given]

Post by Eeyore

Post by Bruce Varley

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a couple
of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it,
cos then I'd have to rebuild it. and I dont have the time to measure the
open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled in
here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of
course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a
68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does
the bulk of the work here.
Any suggestions as to what I can get away with?

This isn't a direct response, more something related that you may want to be
aware of. I've cooked a few hifi amps in the past by trying to use them as
low region ultrasonic drivers, what appears to happen is that applying high
power at high frequencies, phase shifts cause overloading around the
internal feedback loops, predriver and driver devices can get overstressed.
The amp may be rated up to 20+ KHz, but the assumption seems to be that the
spectral distribution of what it's handling is 'audio', and not bat-speak.

Yup.
You need to check the full power bandwidth figures. I like to see 40-50 kHz
myself in my audio designs using MJ150xxs.
Graham

all valid points.

in my case, full power is either DC or 50/60Hz; the network anlayser is
injecting a small signal (OK, for higher power stuff I use a coupling
transformer to inject several amps, but I have a 15:1 turns ratio) over
a range of frequencies (5Hz - 200MHz), so I'm not worried too much about
this. And if I break it, I'll go get a bigger amp :)

Cheers
Terry

[Phil Allison]

"Terry Given"

Post by Terry Given
in my case, full power is either DC or 50/60Hz; the network anlayser is
injecting a small signal (OK, for higher power stuff I use a coupling
transformer to inject several amps, but I have a 15:1 turns ratio) over a
range of frequencies (5Hz - 200MHz), so I'm not worried too much about
this. And if I break it, I'll go get a bigger amp :)

** By removing the input RC filter and output LC filter, the SC480 would
then have a small signal bandwidth of about 1 MHz.

The full power bandwidth is limited by the drive stage slew rate and output
device "cross conduction" heating - 50 kHz would be about it before there
is gross waveform distortion and or output stage over temp failure.

So, at 1 MHz, expect it to deliver about 1 volt rms.

BTW

Dunno why you think adding reverse diodes to the TIPs makes any difference
to inductive load driving - as the SC480 has no VI limiting to generate
kick back spikes.


..... Phil

[Terry Given]

Post by Phil Allison
"Terry Given"

Post by Terry Given
in my case, full power is either DC or 50/60Hz; the network anlayser is
injecting a small signal (OK, for higher power stuff I use a coupling
transformer to inject several amps, but I have a 15:1 turns ratio) over a
range of frequencies (5Hz - 200MHz), so I'm not worried too much about
this. And if I break it, I'll go get a bigger amp :)

** By removing the input RC filter and output LC filter, the SC480 would
then have a small signal bandwidth of about 1 MHz.
The full power bandwidth is limited by the drive stage slew rate and output
device "cross conduction" heating - 50 kHz would be about it before there
is gross waveform distortion and or output stage over temp failure.
So, at 1 MHz, expect it to deliver about 1 volt rms.

excellent, thanks Phil.

I could probably change the output devices, but improving the slew rate
involves messing with the previous stage.

Post by Phil Allison
BTW
Dunno why you think adding reverse diodes to the TIPs makes any difference
to inductive load driving - as the SC480 has no VI limiting to generate
kick back spikes.
..... Phil

no it doesnt, but I have a screwdriver, and sooner or later will
disconnect/reconnect the output transformer while its running. I figured
they couldnt hurt, and I had some diodes.

Cheers
Terry

[Phil Allison]

"Terry Given"

Post by Terry Given

Post by Phil Allison
** By removing the input RC filter and output LC filter, the SC480 would
then have a small signal bandwidth of about 1 MHz.
The full power bandwidth is limited by the drive stage slew rate and
output device "cross conduction" heating - 50 kHz would be about it
before there is gross waveform distortion and or output stage over temp
failure.
So, at 1 MHz, expect it to deliver about 1 volt rms.

excellent, thanks Phil.
I could probably change the output devices, but improving the slew rate
involves messing with the previous stage.

Post by Phil Allison
BTW
Dunno why you think adding reverse diodes to the TIPs makes any
difference to inductive load driving - as the SC480 has no VI limiting
to generate kick back spikes.

no it doesnt, but I have a screwdriver, and sooner or later will
disconnect/reconnect the output transformer while its running. I figured
they couldnt hurt, and I had some diodes.

** If you use the SC480 to drive a 50/60 Hz iron lam transformer up anywhere
near the saturation limit - then inductive kicks on load connect /
disconnect are the LEAST of your worries.

Think "inrush surge" and accidental low frequency drive ( like 5Hz instead
of 50Hz).

Think no VI limiting.

Think smoke.......



..... Phil

[Terry Given]

Post by Phil Allison
"Terry Given"

Post by Terry Given

Post by Phil Allison
** By removing the input RC filter and output LC filter, the SC480 would
then have a small signal bandwidth of about 1 MHz.
The full power bandwidth is limited by the drive stage slew rate and
output device "cross conduction" heating - 50 kHz would be about it
before there is gross waveform distortion and or output stage over temp
failure.
So, at 1 MHz, expect it to deliver about 1 volt rms.

excellent, thanks Phil.
I could probably change the output devices, but improving the slew rate
involves messing with the previous stage.

Post by Phil Allison
BTW
Dunno why you think adding reverse diodes to the TIPs makes any
difference to inductive load driving - as the SC480 has no VI limiting
to generate kick back spikes.

no it doesnt, but I have a screwdriver, and sooner or later will
disconnect/reconnect the output transformer while its running. I figured
they couldnt hurt, and I had some diodes.

** If you use the SC480 to drive a 50/60 Hz iron lam transformer up anywhere
near the saturation limit - then inductive kicks on load connect /
disconnect are the LEAST of your worries.
Think "inrush surge" and accidental low frequency drive ( like 5Hz instead
of 50Hz).
Think no VI limiting.
Think smoke.......
..... Phil

good point. but Im using a 50VA toroid from jaycar, and driving the 230V
winding, so Im pretty sure I'll stay a long way below Bsat (about 10%)

but I *do* drive it at 5Hz. but seeing as its no greater than 10% rated
volts, I wont overflux the transformer. I may, at full voltage, get
close to the peak design flux, which for some cheap shit transformers is
often > Bsat

Cheers
Terry

[Phil Allison]

"Terry Given"

Post by Terry Given

Post by Phil Allison
** If you use the SC480 to drive a 50/60 Hz iron lam transformer up
anywhere near the saturation limit - then inductive kicks on load
connect / disconnect are the LEAST of your worries.
Think "inrush surge" and accidental low frequency drive ( like 5Hz
instead of 50Hz).
Think no VI limiting.
Think smoke.......

good point. but Im using a 50VA toroid from jaycar, and driving the 230V
winding, so Im pretty sure I'll stay a long way below Bsat (about 10%)

** A 50VA, 230 volt tranny will have a primary R of about 50 ohms.

The SC480 is safe.

Post by Terry Given
but I *do* drive it at 5Hz. but seeing as its no greater than 10% rated
volts, I wont overflux the transformer. I may, at full voltage, get close
to the peak design flux, which for some cheap shit transformers is often >
Bsat

** There is a lot of thin, copper wire preventing any large current flow
with that baby size tranny.

Operate it in reverse and you will have a whole nuther story though.



..... Phil

[Terry Given]

Post by Phil Allison
"Terry Given"

Post by Terry Given

Post by Phil Allison
** If you use the SC480 to drive a 50/60 Hz iron lam transformer up
anywhere near the saturation limit - then inductive kicks on load
connect / disconnect are the LEAST of your worries.
Think "inrush surge" and accidental low frequency drive ( like 5Hz
instead of 50Hz).
Think no VI limiting.
Think smoke.......

good point. but Im using a 50VA toroid from jaycar, and driving the 230V
winding, so Im pretty sure I'll stay a long way below Bsat (about 10%)

** A 50VA, 230 volt tranny will have a primary R of about 50 ohms.
The SC480 is safe.

Post by Terry Given
but I *do* drive it at 5Hz. but seeing as its no greater than 10% rated
volts, I wont overflux the transformer. I may, at full voltage, get close
to the peak design flux, which for some cheap shit transformers is often >
Bsat

** There is a lot of thin, copper wire preventing any large current flow
with that baby size tranny.
Operate it in reverse and you will have a whole nuther story though.
..... Phil

Gidday Phil,

just spent a couple of hours messing around with it - better than
watching telly.

no load (measurements relative to passband gain) and small-signal (input
= -20dBm):

HF -3dB = 192kHz
HF -10dB = 285kHz


removing the input 330pF LPF cap:

HF -3dB = 248kHz
HF -10dB = 750kHz

*and* rolloff decreases by 20dB/decade


removing the zobel network:

HF -3dB = 1.38MHz
HF -10dB = 3.3MHz *and* 10.4MHz

there is also a nasty resonant peak at 9.5MHz, going back up to -3dB


A little bit of twiddling - to whit, can I bang an RC network across the
output, or the Q5 68pF miller cap, or the Q3 10pF miller cap - lead me
to look closer at the 10pF cap.

changing the 10pF cap to 5.6pF removes the resonant peak, and I get:

HF -3dB = 2.6MHz
HF -10dB = 11.7MHz

the resonant peak is nicely attenuated.

this'll do me just great.


for my next trick (later, Im gonna stop messing with this and go hang
out with my lovely wife) I shall try a variety of loads and see what
sort of response I get as a function of load power. a quick whiz with a
100R load showed no issues at 0dBm..

thanks again :)

Cheers
Terry

[Phil Allison]

"Terry Given"

just spent a couple of hours messing around with it - better than watching
telly.

** Not much praise in that remark.

HF -3dB = 248kHz
HF -3dB = 1.38MHz

** Close enough to my figure of " ... about 1MHz " - eh ?




..... Phil

[Terry Given]

Post by Phil Allison
"Terry Given"

just spent a couple of hours messing around with it - better than watching
telly.

** Not much praise in that remark.

I love my digital video recorder. I shall never watch another ad.

Post by Phil Allison

HF -3dB = 248kHz
HF -3dB = 1.38MHz

** Close enough to my figure of " ... about 1MHz " - eh ?
..... Phil

pretty darned good indeed. I cant sleep, so Im gonna do the load tests
now. And test the input-voltage-to-secondary-current transfer function
with my toroid driving a short.

Cheers
Terry

[Moi]

"Terry Given" <***@ieee.org> wrote in message news:***@ftpsrv1...
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work 100%
(in the face of all the efforts of the advertisers?) How does it work?

[Terry Given]

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work 100%
(in the face of all the efforts of the advertisers?) How does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).

it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

Cheers
Terry

[Trevor Wilson]

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work
100% (in the face of all the efforts of the advertisers?) How does it
work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt we'll
see commercial products that do that anytime soon.

**I looked at doing just that a dozen or so years back. Back then, TV
stations used to send a 200mS of black level, before and after ads. Trouble
was, the system was not infallible, since some programmes go to black.

[Terry Given]

Post by Trevor Wilson

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work
100% (in the face of all the efforts of the advertisers?) How does it
work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt we'll
see commercial products that do that anytime soon.

**I looked at doing just that a dozen or so years back. Back then, TV
stations used to send a 200mS of black level, before and after ads. Trouble
was, the system was not infallible, since some programmes go to black.

you could almost do it looking solely at volume levels. ads are *always*
louder, some channels more so than others. Still, 8x or 20x FF works
pretty well, and requires little or no effort on my part.

Cheers
Terry

[cth]

Post by Terry Given

Post by Trevor Wilson

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it
work 100% (in the face of all the efforts of the advertisers?) How
does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

**I looked at doing just that a dozen or so years back. Back then, TV
stations used to send a 200mS of black level, before and after ads.
Trouble was, the system was not infallible, since some programmes go
to black.

you could almost do it looking solely at volume levels. ads are *always*
louder, some channels more so than others. Still, 8x or 20x FF works
pretty well, and requires little or no effort on my part.
Cheers
Terry

Elektor published a design a few years ago that identified the
station/broadcaster watermark in the video stream and output start/pause
codes via IR to the recorder at the appropriate time. The recorder IR
codes were entered using a learning mode like the universal remotes.

http://www.elektor.com/magazines/2004/july/tv-commercials-killer.57016.lynkx

Never built it but it seemed a sound concept.

Chris.

[David L. Jones]

Post by cth

Post by Terry Given

Post by Trevor Wilson

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it
work 100% (in the face of all the efforts of the advertisers?) How
does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

**I looked at doing just that a dozen or so years back. Back then, TV
stations used to send a 200mS of black level, before and after ads.
Trouble was, the system was not infallible, since some programmes go
to black.

you could almost do it looking solely at volume levels. ads are *always*
louder, some channels more so than others. Still, 8x or 20x FF works
pretty well, and requires little or no effort on my part.
Cheers
Terry

Elektor published a design a few years ago that identified the
station/broadcaster watermark in the video stream and output start/pause
codes via IR to the recorder at the appropriate time. The recorder IR
codes were entered using a learning mode like the universal remotes.
http://www.elektor.com/magazines/2004/july/tv-commercials-killer.5701...
Never built it but it seemed a sound concept.

Until they change the logo position, which they tend to do on a
regular basis.

Often much easier to download the show you want, were some geek has
already removed the ads for you ;-)

Dave.

[Jasen Betts]

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work 100%
(in the face of all the efforts of the advertisers?) How does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

it's just a software problem as others have said there's a watermark
that can be used, all that needs to be done is detect it and stop
recording when it disappears and start a few seconds before it
reappears, as it typically turns up late.

I'd not be surprised if there isn't already a non-commercial product
that does that.

Bye.
Jasen

[David L. Jones]

Post by Jasen Betts

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work 100%
(in the face of all the efforts of the advertisers?) How does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

it's just a software problem as others have said there's a watermark
that can be used, all that needs to be done is detect it and stop
recording when it disappears and start a few seconds before it
reappears, as it typically turns up late.

Ah, how does it know when the logo is about to re-appear?

Dave.

[David Segall]

Post by David L. Jones

Post by Jasen Betts

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work 100%
(in the face of all the efforts of the advertisers?) How does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

it's just a software problem as others have said there's a watermark
that can be used, all that needs to be done is detect it and stop
recording when it disappears and start a few seconds before it
reappears, as it typically turns up late.

Ah, how does it know when the logo is about to re-appear?

MythTV <http://www.mythtv.org/> uses the logo and a couple of other
clues. It is conservative so it does miss some advertisements but mine
has never lost any program content. Like any other PVR it always
"records" a program so that it, and you, can skip backwards and
forwards. My computer is not powerful enough to remove advertisements
in real time so I have to wait about the same time as the length of
the program before I can watch the advertisement-free version.

It is not a commercial program and it is totally free but not the
faint hearted. You can build a MythTV PVR for a little less than a
similar consumer product. Commercial programs do what the content
providers want them to which resulted in some Microsoft Vista users
missing not just one program but several days recording
<http://arstechnica.com/news.ars/post/20080514-nbc-vista-copy-protection-snafu-reminds-us-why-drm-stinks.html>.

[Terry Given]

Post by David Segall

Post by David L. Jones

Post by Jasen Betts

Post by Terry Given

Post by Moi
SNIP

Post by Terry Given
I love my digital video recorder. I shall never watch another ad.

Can you set that up so that it omits all adverts then? And does it work 100%
(in the face of all the efforts of the advertisers?) How does it work?

I press the FF button, and with practice can go *very* fast and not
overshoot (well at least not too often).
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

it's just a software problem as others have said there's a watermark
that can be used, all that needs to be done is detect it and stop
recording when it disappears and start a few seconds before it
reappears, as it typically turns up late.

Ah, how does it know when the logo is about to re-appear?

MythTV <http://www.mythtv.org/> uses the logo and a couple of other
clues. It is conservative so it does miss some advertisements but mine
has never lost any program content. Like any other PVR it always
"records" a program so that it, and you, can skip backwards and
forwards. My computer is not powerful enough to remove advertisements
in real time so I have to wait about the same time as the length of
the program before I can watch the advertisement-free version.
It is not a commercial program and it is totally free but not the
faint hearted. You can build a MythTV PVR for a little less than a
similar consumer product. Commercial programs do what the content
providers want them to which resulted in some Microsoft Vista users
missing not just one program but several days recording
<http://arstechnica.com/news.ars/post/20080514-nbc-vista-copy-protection-snafu-reminds-us-why-drm-stinks.html>.

yeah, my computer guys have suggested I try similar things, but always
with the "not for the faint-hearted" caveat. My preference, however, is
to buy a thing in a box, unpack it, plug it in and turn it on, then sit
back with a beer.

but VFF is a lot better than watching ads in real time, so Im happy
enough with that. I try hard to *only* watch time-delayed telly.

Cheers
Terry

[Jasen Betts]

Post by David L. Jones

Post by Jasen Betts

Post by Terry Given
it would be lovely to have it eat the ads automagically, but I doubt
we'll see commercial products that do that anytime soon.

it's just a software problem as others have said there's a watermark
that can be used, all that needs to be done is detect it and stop
recording when it disappears and start a few seconds before it
reappears, as it typically turns up late.

Ah, how does it know when the logo is about to re-appear?

one way would be to buffer a few seconds worth in RAM
when it sees the logo it saves the buffer

recording doesn't have to happen in real-time, only playback.

Bye.
Jasen

[Phil Allison]

"Terry Given"

Post by Terry Given
I recently got an SC480 kit from Jaycar,

** Scan the schem and post it somewhere - like ABSE.

Most of my ratty old SC, ETI and EA mags went in the bin last year.

I only kept the EA ones with project articles by me.



..... Phil

[Terry Given]

Post by Phil Allison
"Terry Given"

Post by Terry Given
I recently got an SC480 kit from Jaycar,

** Scan the schem and post it somewhere - like ABSE.
Most of my ratty old SC, ETI and EA mags went in the bin last year.
I only kept the EA ones with project articles by me.
..... Phil

Hi Phil,

sorry my lousy ISP wont support ABSE. email me, and I'll send you a scan.

Cheers
Terry

[Franc Zabkar]

Post by Terry Given

Post by Phil Allison
"Terry Given"

Post by Terry Given
I recently got an SC480 kit from Jaycar,

** Scan the schem and post it somewhere - like ABSE.
Most of my ratty old SC, ETI and EA mags went in the bin last year.
I only kept the EA ones with project articles by me.
..... Phil

Hi Phil,
sorry my lousy ISP wont support ABSE. email me, and I'll send you a scan.
Cheers
Terry

Loading Image...

- Franc Zabkar

[Terry Given]

Post by Franc Zabkar

Post by Terry Given

Post by Phil Allison
"Terry Given"

Post by Terry Given
I recently got an SC480 kit from Jaycar,

** Scan the schem and post it somewhere - like ABSE.
Most of my ratty old SC, ETI and EA mags went in the bin last year.
I only kept the EA ones with project articles by me.
..... Phil

Hi Phil,
sorry my lousy ISP wont support ABSE. email me, and I'll send you a scan.
Cheers
Terry

http://us1.webpublications.com.au/static/images/articles/i302/30285_4mg.jpg
- Franc Zabkar

Thanks, Franc!

Cheers
Terry

[David L. Jones]

Post by Phil Allison
"Terry Given"

Post by Terry Given
I recently got an SC480 kit from Jaycar,

** Scan the schem and post it somewhere - like ABSE.

Here it is:
http://us1.webpublications.com.au/static/images/articles/i302/30285_4mg.jpg

Dave.

[Eeyore]

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a
couple of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break
it, cos then I'd have to rebuild it. and I dont have the time to measure
the open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled
in here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which,
of course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and
a 68pF miller cap across the B-C junction of Q5. Im guessing the 68pF
does the bulk of the work here.
Any suggestions as to what I can get away with?

Well, you of all people should know about stability issues which will affect
how high it goes.

The typical limiting devices are the output transistors. What are they ?

Also you could reduce the open-loop gain and close the loop at the same
gain. That should extend the bandwidth if done right because of larger
margins.

Graham

[Terry Given]

Post by Eeyore

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a
couple of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break
it, cos then I'd have to rebuild it. and I dont have the time to measure
the open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled
in here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which,
of course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and
a 68pF miller cap across the B-C junction of Q5. Im guessing the 68pF
does the bulk of the work here.
Any suggestions as to what I can get away with?

Well, you of all people should know about stability issues which will affect
how high it goes.
The typical limiting devices are the output transistors. What are they ?

TIP3055 & TIP2955. I downloaded the data sheets, and the rat bastards at
bourns dont even spec Ft, but with storage times approaching 1us, Im
picking its only a few MHz. sounds like this is, indeed, whats gonna
stop me.

Post by Eeyore
Also you could reduce the open-loop gain and close the loop at the same
gain. That should extend the bandwidth if done right because of larger
margins.

didnt think of that. email me & I'll send you a copy of the schematic -
audio amps sure aint my speciality.

Cheers
Terry

Post by Eeyore
Graham

[Eeyore]

Post by Terry Given

Post by Eeyore
The typical limiting devices are the output transistors. What are they ?

TIP3055 & TIP2955. I downloaded the data sheets, and the rat bastards at
bourns dont even spec Ft, but with storage times approaching 1us, Im
picking its only a few MHz. sounds like this is, indeed, whats gonna
stop me.

You bet. They're as fast as slugs on mogadon.

Post by Terry Given

Post by Eeyore
Also you could reduce the open-loop gain and close the loop at the same
gain. That should extend the bandwidth if done right because of larger
margins.

didnt think of that. email me & I'll send you a copy of the schematic -
audio amps sure aint my speciality.

Oh the email in my headers works fine.

Graham

[Trevor Wilson]

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a couple
of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it,
cos then I'd have to rebuild it. and I dont have the time to measure the
open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled in
here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of
course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a
68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does
the bulk of the work here.
Any suggestions as to what I can get away with?

**If you want high frequencies, then build a MOSFET amp.

[Terry Given]

Post by Trevor Wilson

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a couple
of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it,
cos then I'd have to rebuild it. and I dont have the time to measure the
open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled in
here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of
course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a
68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does
the bulk of the work here.
Any suggestions as to what I can get away with?

**If you want high frequencies, then build a MOSFET amp.

Jaycar didnt have one

Cheers
Terry

[Trevor Wilson]

Post by Terry Given

Post by Trevor Wilson

Post by Terry Given
Hi All,
I recently got an SC480 kit from Jaycar, as I needed to attach a power
amplifier to the output stage of my network analyser. Its a nice little
kit, very easy to make, and works well, although I can hear the zobel
choke singing as I sweep thru the audio range whilst sticking out a
couple of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC
offset in addition to my ac-coupled input, and clamp diodes across both
sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC
line filter driving a SMPS *while it was running* with the amp supplying
all 50W. cool!
For my next trick I want to increase the upper corner frequency. at
present its just on 100kHz (I have a pretty plot of it), and I'd like to
push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it,
cos then I'd have to rebuild it. and I dont have the time to measure the
open-loop transfer function repeatedly as I mess with the various
compensation caps.
So what limits the upper corner frequency? the input network goes thru a
(well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled
in here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff,
with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of
course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a
68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does
the bulk of the work here.
Any suggestions as to what I can get away with?

**If you want high frequencies, then build a MOSFET amp.

Jaycar didnt have one

**You could be right. Altronics have them.

[Phil Allison]

"Trevor Wilson"
"Terry Given"

Post by Trevor Wilson

Post by Terry Given

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Jaycar didnt have one

**You could be right. Altronics have them.

** They have one - the K5173 200W amp module with psu for $300.

As Given is after a power amp with VHF bandwidth - I suggest he design his
own.



... Phil

[Terry Given]

Post by Phil Allison
"Trevor Wilson"
"Terry Given"

Post by Trevor Wilson

Post by Terry Given

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Jaycar didnt have one

**You could be right. Altronics have them.

** They have one - the K5173 200W amp module with psu for $300.

I thought about getting this one, but I wasn't certain I could make it
all work, so only spent $50 or so.

Post by Phil Allison
As Given is after a power amp with VHF bandwidth - I suggest he design his
own.
... Phil

perhaps, one day. doing it properly is a big job, one I am unlikely to
tackle without cash incentives.

in practice 100kHz is a great start (it allowed me to critically damp my
AC & DC filters), and 1MHz is excellent.

at present the SNR degrades significantly above 100kHz, as the output
amplitude rolls off. The analyser does have something like 130dB dynamic
range, and I got good results to 1MHz with a 2,000 second sample time
using 1Hz RBW. it just took half an hour. I can improve that further
still with n-tuple trace averaging, but now we are looking at the best
part of a day to take a single measurement.

So with Phils mods, I ought to get useable results to 10MHz (if I can be
bothered waiting that long), and excellent high-speed results to 1MHz,
which will probably suffice. When I next spark up the HP3577 I'll take
some before-and-after plots, and figure out a way to post them.

thanks all, especially Phil

[Trevor Wilson]

Post by Phil Allison
"Trevor Wilson"
"Terry Given"

Post by Trevor Wilson

Post by Terry Given

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Jaycar didnt have one

**You could be right. Altronics have them.

** They have one - the K5173 200W amp module with psu for $300.
As Given is after a power amp with VHF bandwidth - I suggest he design
his own.

**Indeed. 200MHz is a bit of a stretch for an audio product.

[Eeyore]

Post by Trevor Wilson

Post by Terry Given

Post by Trevor Wilson

Post by Terry Given
Any suggestions as to what I can get away with?

**If you want high frequencies, then build a MOSFET amp.

Jaycar didnt have one

**You could be right. Altronics have them.

May not actually be that fast. Remember gate capacitance. You need a beefy
driver stage to get them moving quickly as Terry will surely already know.

Graham

[Eeyore]

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

Graham

[Terry Given]

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.
Graham

any suitable part numbers?

and yeah, I knew the xx55's were slow as all hell.

I havent done my power-bandwidth tests yet, but my small signal
bandwidth is now 0-10MHz, which is good enough for a lot of what I have
to do.

If I can get ***@1MHz I'll be doing well indeed.

I sent you a pretty picture, too.

Cheers
Terry

[MoiInAust]

Wierd Terry!

Post by Terry Given

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.
Graham

any suitable part numbers?
and yeah, I knew the xx55's were slow as all hell.
I havent done my power-bandwidth tests yet, but my small signal bandwidth
is now 0-10MHz, which is good enough for a lot of what I have to do.
I sent you a pretty picture, too.
Cheers
Terry

[Terry Given]

Post by MoiInAust
Wierd Terry!

LOL

[Phil Allison]

"Terry Given"

Post by Terry Given

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

any suitable part numbers?

** If you want wide power bandwidth, then lateral mosfets are the clear
winners.

Eg, Hitachi 2SK176 / 2SJ56 or the similar Semelab BUZ 901/6 devices.

Main reason is lack of charge storage in the C-B junction means they turn
off so much faster ( circa 50nS ) you avoid the cross conduction disaster at
high slew rates.

Also, these mosfets self protect at high chip temperatures, " throttling
back " instead of simply blowing up.

http://www.magnatec-uk.co.uk/pdf/magnatec/BUZ900.pdf



..... Phil

[Eeyore]

Post by Terry Given

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

any suitable part numbers?

Straight off the top of my head no and I don't have a Toshiba Data book
here although I may have in pdf form. Their 'audio' power devices vary if
fT from 10-40 Mhz. Sanken do some fast ones too IIRC.

Hah - from a QSC schematic, 2SA1943 and 2SC5200 (widely used) are good for
30 MHz and plenty of volts and amps.

Can post in abse

Graham

[Phil Allison]

"Eeysore"

Post by Eeyore
Hah - from a QSC schematic, 2SA1943 and 2SC5200 (widely used) are good for
30 MHz and plenty of volts and amps.

** Shame how Ft is a strong function of Vce and Ic.

Ft drops like a stone below 0.5 amps and above 5 amps for those devices -
and nearly all other power BJTs are much worse.



..... Phil

[Trevor Wilson]

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

**As Phil has already stated, MOSFETs are MUCH better at HF than even the
Toshiba BJTs.

[Terry Given]

Post by Trevor Wilson

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

**As Phil has already stated, MOSFETs are MUCH better at HF than even the
Toshiba BJTs.

yeah, but I'd want to be damned sure its not going to burst into
oscillation at 200MHz. especially when I jam 200MHz up the input :)

mostly, this little amplifier was good enough to solve the immediate
problem (needed 100kHz), and now I've vandalised it further, it'll
probably do everything I want. it had the added advantage that I could
buy it in one bag from Jaycar, and spent 1hr putting it together. it
took longer to drill the heatsink than build the kit.

Cheers
Terry

[Eeyore]

Post by Trevor Wilson

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

**As Phil has already stated, MOSFETs are MUCH better at HF than even the
Toshiba BJTs.

Don't forget the nice beefy driver stage to charge and discharge that gate
capacitance.

Graham

[Terry Given]

Post by Eeyore

Post by Trevor Wilson

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

**As Phil has already stated, MOSFETs are MUCH better at HF than even the
Toshiba BJTs.

Don't forget the nice beefy driver stage to charge and discharge that gate
capacitance.
Graham

In the '90s I did a design for a low-power 2MHz diagonal half-bridge
SMPS. INitially I used FETs, but when I looked at the H-field emanating
from the gate traces (which were constrained to be on one layer, and
long), I changed the switching devices to bipolars - for the same speed
I required much less peak driver current. Sure, I had to cough up some
continuous driver current, but that was a lot less than the peak.

I have a SMPS Im finishing off now that sucks ***@12V, just to drive the
gates of 6 hefty FETs - so much that I actually dropped Fsw to keep it
under control. Once this design is in production I will design a
resonant gate driver, and see how low I can reduce the gate losses.

oh yeah Graham, I cant see ABSE, so if you could email me those Toshiba
datasheets it would be appreciated.

Cheers
Terry

[Franc Zabkar]

Post by Terry Given
oh yeah Graham, I cant see ABSE, so if you could email me those Toshiba
datasheets it would be appreciated.
Cheers
Terry

http://www.toshiba.com/taec/Catalog/pages/search.jsp
http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7673.pdf
http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7890.pdf

- Franc Zabkar

[Terry Given]

Post by Franc Zabkar

Post by Terry Given
oh yeah Graham, I cant see ABSE, so if you could email me those Toshiba
datasheets it would be appreciated.
Cheers
Terry

http://www.toshiba.com/taec/Catalog/pages/search.jsp
http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7673.pdf
http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7890.pdf
- Franc Zabkar

ta. If I wasnt so dim, I could have typed them into google. duh.

Cheers
Terry

[Phil Allison]

"Eeysore"

Post by Eeyore

Post by Trevor Wilson

Post by Eeyore

Post by Trevor Wilson
**If you want high frequencies, then build a MOSFET amp.

Not even necessary with Toshiba bipolars.

**As Phil has already stated, MOSFETs are MUCH better at HF than even the
Toshiba BJTs.

Don't forget the nice beefy driver stage to charge and discharge that gate
capacitance.

** Not at all necessary for either wide small signal bandwidth ( as wanted
by the OP) or where the load Z is fairly high - as is also the case with the
OP's stated needs.

Mosfet gate drive is requirements are VERY modest when the output power
level is lowish.

In any case, even with high power operation and parallel groups of lateral
mosfets, the standard drive circuit is nothing more than a pair of TO92
plastic pack transistors like the MPSA42 and MPSA92 and gives slew rates of
about +/- 50 volts per uS or 100kHz at full power.




..... Phil