Bias matching replacement transistor - Parasound 2250

[Haaheim]

Hi guys!

I am currently working on a Parasound 2250 (class AB), which had one defective channel. Components in the signal paths have been replaced, including an output transistor (2SC3519).



According to the manufacturer, bias is adjusted by measuring 1-3mV across the output resistors. The original ones are completely fine around 1.5mV, but the replacement is not. Initially, I got a couple from eBay, assumed they were fake (they were) and was able to source a replacement locally. This exhibits similar behaviour, being around 0.6-0.8mV on its emitter resistor. The resistor is fine, as I have swapped the position of two transistors, and issue moved with it.

Since the transistors share base and collector, is this difference solely caused by unmatched transistors? (difference in beta/hFe (rank) etc.) Using a basic tester, the beta is different between them - when trying to research the topic, I find heated topics on whether matching is relevant and that the resistors should balance the mismatch.

Any input on this topic would be highly appreciated and educational to me

[petehall347]

Re: Bias matching replacement transistor - Parasound 2250

adjust with vr1

[Haaheim]

Re: Bias matching replacement transistor - Parasound 2250

Of course, that adjust everything - the replaced transistor is still roughly 1mV below the others regardless of VR1 position.

[petehall347]

Re: Bias matching replacement transistor - Parasound 2250

you really need a new matched set of outputs

[Haaheim]

Re: Bias matching replacement transistor - Parasound 2250

That is what I feared, as getting replacements will be hard..2SC3520, 2SC4140 are compatible, but seems they are out of production as well. I guess running the setup as it is will cause an extra strain on the two original transistors.

I assume complimentaries should be replaced as well? Will this change cause audible difference between the channels?

Thank you for your input!