A Tiny Pye Amplifier

[AlanBeckett]

I blame this deviation from what I firmly intended to do entirely on Young Joe, late of this Parish.

A couple of Months ago I bought this from an eBay Seller:-

   

I was stoically ignoring it apart from looking up the Amplifier Datasheet and reverse Engineering the Circuit.


  TAA621_DATASHEET.pdf (Size: 77.95 KB / Downloads: 44)    

Rashly, a Week or so ago, I mentioned it to Joe. He redrew the Circuit very nicely:-

   

So I thought 'What the hell, it can't take much doing'. My Trusty Advance 81A and Farnell PSU soon got it going with a 12V Supply and a 5R Load.

       

Clearly it wasn't going to survive for long without a Heatsink so I bent up a bit of Ally and bolted it on.

       

To be continued ......

Alan

[Refugee]

You now need a pocket media player to go with it.

[AlanBeckett]

The Datasheet implies that the optimum Operating Conditions are a 24v Supply and a 16R Load. My TF won't do 16R so I had to go with 15R.
It really will go to 4W although I didn't risk it for long - 2W seemed a more reasonable Continuous Sin Output.
Taking 1kHz as a Reference the measured Gain was as follows:-

24v Supply, 15R Load, 400mV p-p Input.

20Hz    1.7v p-p    -18dB
50          7                  -6
100         11               -2
200         14                 0
500         14                 0
1,000     14                 0
2,000     14                 0
5,000     12               -1.3
10,000    6.4             -6.8
20,000    3.6             -11.8
50,000    2.0            -16.9      

So, it's Fi 'aint very Hi, but probably acceptable.

After a bit it started to get unstable running at 2W so clearly my Heatsink isn't adequate for that, although it runs fine for ever at 1W even with the Garage at over 30 deg. C.

There was however a sting in the Tail. I left it running for a few Hours and when I went back to it the Output Electrolytic had died horribly although it was still working.

   

I changed it to the nearest I had handy but then realised that I should have returned the Load to the Positive not the Negative - Doh. However, it still works fine.

What does it belong to? I haven't a clue. The Pins on the 'Bottom Edge' imply that it was plugged into a Socket.I don't think the Output is adequate for a TV although it might suffice for a small one. My guess is one of their PMR Sets.

I'm certain I could get a cheaper, better and more modern unit from China, but then it wouldn't have Pye written on it 

Alan

[AlanBeckett]

I should have said by the way that it really is tiny, around 2 3/8" Square in old Money.

Alan

[Refugee]

It just looks like it belongs to a rental TV set to me. One of the ones where the engineer just changes it and leaves the home after the job is done.

[AlanBeckett]

Maybe, although I can't see it lasting long in a TV environment.
I expect someone will recognise it.

Alan

[pwdrive]

It has the look of Philips about it.

Lawrence.

[AlanBeckett]

Indeed, although it does have a genuine Pye Logo on it, which leads me to believe it belongs to Pye Telecoms.

Alan

[Mark Hennessy]

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?

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.

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.

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.

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.

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.

Any date codes that might give a clue?

[pwdrive]

It could well be Alan (Pye Telecoms)

When I was in the trade Philips/Pye spares were ordered from C.E.S.

Lawrence.