Match complimentary pair in HK-730?

[Kirk E]

I'm replacing a complimentary pair of faulty driver transistors on a Harman Kardon 730 power amp board. Q408/Q412, at least one of which is faulty, and Q407/Q411 for symmetry. You can see the power amp schematic on page 18 of the service manual.

Would there be any advantage to matching hfe or other specs between this complimentary pair? I understand the differential pairs Q401/Q403 and Q402/Q404 should be matched, but I am less sure about the pair above. What do you think?

Thanks,

Kirk

 

[xXBrunoXx]

I would say , if you can match every pair of transistors by Hfe that you want to replace , than do it , is there a shop where you can get them pairs so you dont have to buy a bunch and pair them ?

 

[Kirk E]

Since I found "matched pairs" of the transistors I wanted on ebay right of the bat, I didn't think I'd need to think about this so hard, but unfortunately the seller was not using the term matched in this context :-| After searching these forums on the subject I find a lot of people saying it's easy to match pnp or npn with itself, but very hard to match pnp with npn. If the hfe of the ones I got from ebay are any indication, I think I'd try and find an already matched substitution rather than buy a large quantity to sort through. The ones I got are relatively consistent with each other, but all wildly different than the complimentary part. On top of that, I gather from reading these forums that matching itself is not as simple as matching hfe on a multimeter. The difficulty matching is exactly why I wonder how critical it is to match this particular pair in the circuit.

 

[xXBrunoXx]

doesnt match the Hfe is enough ? or transistors must be matched by other critaries not only hfe .. let's say for example bd140 = hfe 100 and bd139 = hfe 101 so they are matched ?
and using bd140=140hfe and bd139=80hfe would be a huge difference and so the wave form will be " damaged" the negative cycle will be slightly lower than the positive one , creating distortion , because of different hfe , correct me if im wrong

Im a newbie in audio

 

[Kirk E]

I'm not the best one to answer this since I'm still studying the subject, but it's my understanding a multimeter hfe is not very accurate because it only measures at one current, which might not be the current the device is used at. Better to measure with a curve tracer.

As I study this I think it less and less likely Q408/Q412 need to be matched, but if anyone has input on this I'd appreciate the help.

 

[Kirk E]

Perhaps this answers my question?:

http://www.diyaudio.com/forums/soli...tors-vas-stage-need-matched-pnp-npn-pair.html

If Q408/Q412 is indeed a single ended VAS + current source, they might not even need to be a complementary pair let alone hfe matched.