Pepping up a Silicon Chip SC480 audio amplifier

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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

Re: Pepping up a Silicon Chip SC480 audio amplifier

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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.
 



Re: Pepping up a Silicon Chip SC480 audio amplifier



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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


Re: Pepping up a Silicon Chip SC480 audio amplifier
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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

Re: Pepping up a Silicon Chip SC480 audio amplifier

"Terry Given"

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** 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





Re: Pepping up a Silicon Chip SC480 audio amplifier
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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.

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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

Re: Pepping up a Silicon Chip SC480 audio amplifier

"Terry Given"

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** 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





Re: Pepping up a Silicon Chip SC480 audio amplifier
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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

Re: Pepping up a Silicon Chip SC480 audio amplifier

"Terry Given"
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** Not much praise in that remark.


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




.....   Phil






Re: Pepping up a Silicon Chip SC480 audio amplifier
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I love my digital video recorder. I shall never watch another ad.

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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

Re: Pepping up a Silicon Chip SC480 audio amplifier

SNIP
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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?



Re: Pepping up a Silicon Chip SC480 audio amplifier
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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

Re: Pepping up a Silicon Chip SC480 audio amplifier



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**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.


--
Trevor Wilson
www.rageaudio.com.au



Re: Pepping up a Silicon Chip SC480 audio amplifier
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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

Re: Pepping up a Silicon Chip SC480 audio amplifier
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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

Re: Pepping up a Silicon Chip SC480 audio amplifier
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Ah, how does it know when the logo is about to re-appear?

Dave.

Re: Pepping up a Silicon Chip SC480 audio amplifier
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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

Re: Pepping up a Silicon Chip SC480 audio amplifier

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100%
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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 .

Re: Pepping up a Silicon Chip SC480 audio amplifier
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<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

Re: Pepping up a Silicon Chip SC480 audio amplifier

"Terry Given"
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**  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






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