TIP2955/3055 matched pair - where to find?

[nigelwright7557]

Originally posted by AndrewT
I don't think so.
The emitter resistor is intended to provide feedback and balance of higher currents when the output is delivering current.
The quiescent emitter resistor voltage is so low and the difference between Vre as a proportion of the Vbe+Vre is so small that unmatched transistors gain little benefit in balancing quiescent current.
However, before one puts such faith in the measured Vre one must first match Re to far better than the usual 5% tolerance of these low power resistors.
I select for <0.5% as a matter of course and try for 0.2%.

But who cares what happens with no ac signal applied ?

I jus twant it to sound good with music applied to the input and be reliable. And that is what has happened with my power amps.
I have certainly had no complaints from customers.

[AndrewT]

at low AC signals the transistors will be operating at different temperatures if the quiescent currents are unmatched.

Some of the consequences are that the parallel transistors will have different handover times in the cycle, have different gains, have different fT, have different SOAR.

Once you have these different temperatures between the individuals of the unmatched parallel sets, this temp. diff. will exist even when larger AC signals are passing.

[djk]

Over 35 years ago I repaired a quasi-comp amplifier with a random junkbox sourced NPN output.

It measured quite poorly (0.8% THD vs less than 0.1% on the good channel). I suspect the interstage driver transformer may have been damaged, but I really didn't know.

No one could HEAR the difference between the repaired channel and the original channel, and I have heard many amplifiers that measured better actually sound worse.

[infinia]

I think the problem with random use of 2N3055s for audio is that there was no contol of the spec's. everybody and his brother where making them with different processes and even die sizes over all these years. RCA rebranded theirs just for audio use providing at least some control in audio apps versus the normal ubiquitous uses as Vreg pass devices. Probably just use a good name brand device and measure/select for higher gains beta > 50 would be what I'd do. Not matching really.

[bobdole369]

RPL all the output 2955/3055's, checked around the channel where it had shorted and did not find any issues. After replacing, the amp works fine. It's quasi-burning in now - playing all 4 chans at a low volume, just don't want to install it and have to take it all apart again.