A Tiny Pye Amplifier

[Old Sparky]

That IC reminded me of a similar type, also made by SGS-Ates, which was used in some Car Radios, although, IIRC, that one was a TBA641, which was made in two versions, the ony difference, AFAIK, being the shape of the mounting bracket fitted to them. In car radios 'back in the day' 4 to 5 watts o/p into 3 or 4 ohms was usual, and, of course the radios ran from a nominal 12v supply, sothe chip (and all other components!) had to be capable of operating from a supply of between 11 & 14.4 volts. I still have a couple of NOS TBA641s somewhere!

[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

[pwdrive]

Had a poke around on the web.....No, not found it.... But look at the pic in the link and the style of the vertical PCB, through the hole soldering plus the PYE logo plus an number with the prefix "AT" near the logo, Alan's photo shows what looks like the same "AT" prefix , the module Alan's got looks like a similar height as the ones in the link below:

http://www.qsl.net/gm8aob/images/PYE%20P...DE_IMG.jpg

Lawrence.

[AlanBeckett]

Is that it right at the top in the middle? It certainly looks promising although I can't see a Heatsink.
What's the Unit? It looks pretty fierce.

Alan

[pwdrive]

Alan, there is another shot from another angle but can't see any heats ink as wot your's has, I'll post the other shot shortly, yours could have come out of something of the same era.

Lawrence.

[pwdrive]

The other shot(s):

http://www.qsl.net/gm8aob/pages_2/pmr2.htm

Lawrence.

[AlanBeckett]

Thanks Lawrence, that's the sort of place I'd expect to find it with the little Plug-in Boards.
I had a look for a Manual but didn't find anything free.

Alan

[Mark Hennessy]

I wouldn't mind seeing a picture of C1 - it appears to be hidden by the mounting tab of the IC. Just curious - I know it's not really important. I do have decent stocks of Mullard/Philips components from that era here, so might recognise it.

<Digression>

Perhaps surprisingly, it's unusual to measure audio distortion with a spectrum analyser, and practically unheard of back in those days. Most measurements are THD+N, which means a notch filter is used to remove the fundamental, then an RMS measurement is made of the remainder, including noise. Good instruments (e.g. HP8903) will get nearly 100dB of fundamental rejection in the 1kHz region, meaning measurements down to about 0.001% are possible.

In more recent times, instruments that can measure THD (without the N) have come into existence, but obviously they need a high-end ADC and lots of computer processing to perform an FFT. I've got one here - a Tek AM700 - but it doesn't boot, sadly. One day I'll try to fix it. Audio Precision are the default choice if you have the budget. Prism Audio make the Dscope for rather less money if you can bear to use their software (I struggle!). Of course, freeware+soundcard is a reasonable, if uncalibrated route.

Of course, there are exceptions. The HP3580A is a spectrum analyser as RF folk would recognise it, but instruments like these are very rare. We have one at work - it's rather slow and clunky compared to the PC-based solutions, but it has a certain charm about it.

</Digression>

Anyway, all tangential to the original question, but hopefully of interest to some. I have no knowledge of PMR gear, though looking at the pictures provided by Lawrence, it's not much of a stretch of the imagination to make the connection. That said, it looks like a totally "generic" Philips/Mullard construction to me, using their standard parts of the time - really, it's only the glass fibre FR4 PCB material and gold-plated connectors that separate it from consumer-level gear. I've seen identical construction in Philips test gear - though it's clearly less likely to be from that segment - especially with the Pye branding.

Good luck with the search

[pwdrive]

I used to check THD using a Ferrograph RTS 2 (or was it RTS1?)

The left hand side of it can just be seen towards the bottom right of the picture.

Lawrence.

[AlanBeckett]

Yes Lawrence, that's the sort of Gear I used to use as well. I've spent many a long Hour wondering whether I believed the Results.

Anyway, back to the Amplifier, which was just meant to be a quick bit of fun

C1 is a tiny Ceramic.

       

I don't know how I arrived at 5n6 as I can't see any Value on it. I fished it out and it actually measures 7n5. By the way, I'm really glad that this is a Fibreglass PTH PCB because Single Sided SRBP would never have survived.

I've put it back together and it still works

Alan