Pepping up a Silicon Chip SC480 audio amplifier

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"

** 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

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

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

Hi Phil,

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

Cheers Terry

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.

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

Cheers Terry

On Thu, 21 Aug 2008 08:16:08 +1200, Terry Given put finger to keyboard and composed:

- Franc Zabkar

--
Please remove one \'i\' from my address when replying by email.

Thanks, Franc!

Cheers Terry

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

--
Trevor Wilson
www.rageaudio.com.au

Jaycar didnt have one

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.

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

Here it is:

Dave.

**You could be right. Altronics have them.

--
Trevor Wilson
www.rageaudio.com.au

"Trevor Wilson" "Terry Given"

** 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

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.

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"

** 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

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

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

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

--
Trevor Wilson
www.rageaudio.com.au