Pepping up a Silicon Chip SC480 audio amplifier

T

Terry Given

Guest
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
 
"Terry Given" <my_name@ieee.org> wrote in message
news:1219228123.661821@ftpsrv1...
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
Click to expand...
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.
 
"Terry Given"
this is for the audio gurus:

I recently got an SC480 kit from Jaycar,
Click to expand...

** 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 wrote:

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?
Click to expand...
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
 
Bruce Varley wrote:

"Terry Given" <my_name@ieee.org> wrote in message

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?

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.
Click to expand...
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
 
Eeyore wrote:
Bruce Varley wrote:


"Terry Given" <my_name@ieee.org> wrote in message


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?

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
Click to expand...
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 wrote:
"Terry Given"

this is for the audio gurus:

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
Click to expand...
Hi Phil,

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

Cheers
Terry
 
Eeyore wrote:
Terry Given wrote:


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?


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 ?
Click to expand...
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.

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

Cheers
Terry


Graham
Click to expand...
 
On Thu, 21 Aug 2008 08:16:08 +1200, Terry Given <my_name@ieee.org> put
finger to keyboard and composed:

Phil Allison wrote:
"Terry Given"

this is for the audio gurus:

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
Click to expand...
http://us1.webpublications.com.au/static/images/articles/i302/30285_4mg.jpg

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
 
Franc Zabkar wrote:
On Thu, 21 Aug 2008 08:16:08 +1200, Terry Given <my_name@ieee.org> put
finger to keyboard and composed:


Phil Allison wrote:

"Terry Given"


this is for the audio gurus:

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
Click to expand...
Thanks, Franc!

Cheers
Terry
 
"Terry Given" <my_name@ieee.org> wrote in message
news:1219228123.661821@ftpsrv1...
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?
Click to expand...
**If you want high frequencies, then build a MOSFET amp.


--
Trevor Wilson
www.rageaudio.com.au
 
Trevor Wilson wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:1219228123.661821@ftpsrv1...

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?


**If you want high frequencies, then build a MOSFET amp.
Click to expand...
Jaycar didnt have one

Cheers
Terry
 
"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 :)
Click to expand...

** 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" <my_name@ieee.org> wrote in message
news:1219274335.153874@ftpsrv1...
Trevor Wilson wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:1219228123.661821@ftpsrv1...

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?


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

Jaycar didnt have one
Click to expand...
**You could be right. Altronics have them.


--
Trevor Wilson
www.rageaudio.com.au
 
"Trevor Wilson"
"Terry Given"

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

Jaycar didnt have one

**You could be right. Altronics have them.
Click to expand...

** 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
 
Phil Allison wrote:
"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.
Click to expand...
excellent, thanks Phil.

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

I'd be tickled pink with 1V @ 1MHz.

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
Click to expand...
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
 
"Terry Given"

** 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.

I'd be tickled pink with 1V @ 1MHz.


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.
Click to expand...
** 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
 
Phil Allison wrote:
"Trevor Wilson"
"Terry Given"


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

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

... Phil
Click to expand...
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
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:6h43q7FionnaU1@mid.individual.net...
"Trevor Wilson"
"Terry Given"


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


--
Trevor Wilson
www.rageaudio.com.au
 
Phil Allison wrote:
"Terry Given"


** 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.

I'd be tickled pink with 1V @ 1MHz.


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
Click to expand...
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
 
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