A Tiny Pye Amplifier

[AlanBeckett]

If that input cap really was 5n6, then the LF -3dB point would be 130Hz at best. In reality, the input impedance of the IC will increase that. They say >750k, so taking that as a lowest worst-case value, we're at 170Hz. Perhaps 56n?

Perhaps so. That's why it's got a '?' against it. I didn't measure it.

If you preferred to be more conventional, I see no reason why you shouldn't return the LS to the -ve rail once the polarity of C6 has been reversed.

But that would mean that the orientation didn't match the Silk Screen which would be confusing in the future, and I'm easily confused

I'm surprised at the lack of PSU decoupling. In fairness, the datasheet doesn't show it in any of the suggested applications, but there is one on the diagram on page 2. Often, they are omitted for clarity on the application examples. While it certainly seems to be behaving, I'd consider adding something. Perhaps 100n on the rear of the PCB, directly under the IC might be a good plan. Of course, there might have been some bulk decoupling on the PCB it plugs in to.

That' w hat I assumed. It seems quite happy as it is although of course the PSU is very well Regulated in its own right.

Having said that, there isn't much on the positive rail, assuming the internal schematic is accurate. The VAS (T9) is returned to ground. Apart from the upper output pair, there are just current sources. Perhaps it is quite tolerant in itself, but returning the LS to +ve with no decoupling seems odd - all the signals are returned to ground, so the LS should be as well. Any variation on the +ve caused by a lack of decoupling will superimpose themselves on to the output, causing measurable distortion.

I've no easy means of measuring Distortion short of getting a Spectrum Analyser out which it hardly seams to warrant. It looks pretty good.

Regarding heat sinking, a sine wave is a tough test for an amplifier. When playing music, unless it's modern stuff that's been heavily compressed, then there will be a Peak to Mean Ratio of 20dB at least. Of course, in power terms, 20dB is a factor of 100, so if the peaks are on the cusp of clipping, the average level will be 40mW. Try it - I suspect that your heat sink will be absolutely fine for audio use.

Indeed so, although I really didn't want to spend too much time on it. Perhaps I'll hook up some Music and a real Speaker and see what it looks like.

This could be the core of a really useful bench amp or signal tracer or something similar. Just sorry that I can't add any more info about the origins, but I could imagine it being used in a TV or mono record player or something similar - 4W peak is really quite generous for this when used with a typically efficient loudspeaker (a Hacker portable set goes very loud with its ~1W amp). Zin is probably high enough to work with a ceramic pickup and a 1M volume pot. Of course, there might have been a really quite elaborate pre-amp on the main PCB.

Depending of course on what it was meant for. I was rather hoping someone would come out of the Woodwork with the answer. No responses rather push me towards the PMR Theory because there's lots of TV and Radio people on here.

Any date codes that might give a clue?

Not obviously. The Type Number is stamped into the Aluminium. The Datasheet is dated 1970 which fits in with the QIL Pack. There still seems to be plenty about.

Alan