Component Matching

[Pete K]

Hi

I'd be grateful if somebody could tell me if the components that went into AP1 & AP2 power amps were selected and matched before use. What sort of tolerance say on hfe would there be between transistors if they were matched?

Thanks

Pete

[Pete Wilson]

I don’t know.

From the circuit diagram the output stages look to be triples, so there should be gain to spare.

I suspect that getting closely-matched devices these days wouldn’t be very expensive.

Unless you’re trying to be “authentic”, you might want to try out more modern devices, specially for the actual output transistors. These days you can get devices which don’t beta droop as current goes up. Might be fun…

The chap who can answer the question is Stan Curtis.

— P

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[Stan Curtis]

Hi Pete

 

The input stage transistors in the AP1 & AP2 were matched although without reviewing the schematics I couldn’t tell you the parameters. It was to balance the positive & negative signal gain. The resistor tolerances were 5%.  Today it is just as cheap to fit 1%

 

To answer the other question there are much better transistors available today that would really improve the performance of these amplifiers.  Unfortunately you can’t just substitute & expect improvements. The open loop response would change & the compensation capacitors would need to be recalculated

 

Best wishes

Stan

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[Pete Wilson]

Yes.

In making the suggestion, I’d assumed that anyone trying new devices would be sensitive to the compensation issue….

But I shouldn’t have made that assumption.

And thanks for info!

— P

On Dec 2, 2019, at 3:14 AM, Stan Curtis <ma...@stancurtis.com> wrote:

To answer the other question there are much better transistors available today that would really improve the performance of these amplifiers.  Unfortunately you can’t just substitute & expect improvements. The open loop response would change & the compensation capacitors would need to be recalculated

 

[Pete K]

My original replies appear to have gone astray....

Many thanks for the information and suggestions. I've picked up a heat-damaged AP2 and done an assessment of the damage, and had a good look at the components used.

Some of the 'MPSA' & 'BC' transistors are marked with paint, which I assume is part of the matching/selection process. 

One channel apparently works but the other has quite a bit of heat damage to the PCB, and to a few of the components downstream from the 'epicentre' of the damage, which is in the BC144L/BC812L area. The components could be tested and replaced and I might patch that board up, but the FRP material itself is carbonised which must mean it's quite leaky to microamps/milliamps of current in a critical area. 

I'm taking my time and collecting components as close to the originals as possible, and I'll draft a new board to be manufactured in China by somebody like JLCPCB. I believe the board layout is part of the 'magic' of these amplifiers so the traces will match the originals as close as possible, so no auto-routing, all curved routes and essentially only a single layer.

The amplifier itself is very '1970s' in its wiring so I feel compelled to modify the electrical safety side of the design. Also I'll probably install a secondary line of protection against overheating - one that doesn't self-reset.

Regarding replacement components the only stumbling block appears to be the D42C7 and D43C7 devices. Looking on line there are suggestions for replacements that aren't silly, but they appear to miss the need to maintain the amplifier's 'balance', so I'll take some time rather than leaping in. Manufacturers quite understandably have moved on from the 1970s with devices now having increased bandwidth and large hFE values, so 'remainder bin' selections from ebay etc appear to be the best route. I will need to select these with care because there are quite a few fakes out there that are generic 'farthing bead' devices dressed up as luxury 'million dollar diamonds'. However, they may be as good as the originals given 45 years of improvements in fabrication techniques! They are mostly housed in TO126 packages though, so the board drafting can proceed.

If you can hear a creaking sound it's around 35-years' worth of rust falling off my analogue electronics knowledge. 

PK

[Pete Wilson]

:-)

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