This post is about a gorgeous Accuphase E-202 integrated amplifier I’m repairing, and the need for careful transistor matching when repairing audio gear like this.
Choosing and matching transistors correctly is critically important when repairing or restoring audio electronics. I’ve learned this over the years, through experience and from people who’ve been doing this for much longer than I have. In this article, I look briefly at matching small signal transistors.
Matching transistors helps an amplifier run at its best. That’s why Accuphase took the care to do it when they built these amplifiers, but sadly, many repairers seem to take less care. Most just whack in whatever transistors they have lying around and hope for the best.
Get transistor matching wrong though, and your unit will exhibit high levels noise and distortion, DC offsets, or worse. The other consideration is that many faults are mistakenly attributed to capacitor problems, when they are in fact caused by drifty, noisy or mismatched transistors.
This beautiful Accuphase E-202 had some gremlins. Apart from needing a major service, the unit had a protection fault that was especially irritating, for her owner and me, causing the unit to randomly go in and out of protection.
In this E-202, there were several pairs of mis-matched transistors from repair goofs, and several original pairs and single transistors that had drifted over time. These caused DC asymmetry in the amplifier modules and instability in the protection circuit. The DC asymmetry in the amplifier blocks can be trimmed out, but noise and distortion are also compromised. There’s no trimming possible in the protection circuit though.
Whoever worked on this before me apparently liked replacing only one device out of a matched pair. Worse, they used a different device type for these replacements, creating a problematic mis-match. Worse still, these different parts didn’t match the specs for the original parts and this is commonplace.
From a technical standpoint, this isn’t acceptable even in a simple amp or preamp. It’s a definite no-no in a high-end hi-fi amplifier, the distortion and noise specs for which hinge on those carefully matched pairs.
Transistor Matching Basics
Let’s say one transistor in a differential pair becomes noisy, a common occurrence in preamplifiers and amplifiers. From worst to best practice, here are the common repair options:
- Replace only the noisy transistor, with a different, incorrect type. Worst idea, very common.
- Replace only the noisy transistor, with an identical, correct part. Better idea, less common, good practice.
- Replace both transistors with an identical gain-matched pair from one batch. Least common, best practice.
If we are dealing with complementary pairs, ie two different but well-matched transistors designed to work as a pair, one should always use the correct complementary pair, or another complementary pair with very similar specs.
When transistor matching, it’s critically important to:
- Select the right transistor for the role
- Have enough of those transistors to enable you to
- Hand-match parts where possible
Achieving the Match
The trick is to be set up so that you can match parts when needed. A few things help, like learning about common transistor types, the important specs when matching, substituting parts and so on.
Having enough of the parts one is trying to match is critical. You want at least 50, preferably 100+ transistors of a given type, from the same batch. Spread this over some commonly found transistors and you can see the need to carry some stock.
You also need equipment. A curve tracer is a great first choice, something like the Tektronix 576 is ideal. These are big, heavy, complex and expensive however. Instead, I use a Peak Atlas DCA Pro DCA75, an excellent, compact and accurate semiconductor analyser and curve tracer.
Then, you just get testing, recording results as you go. The idea for differential pairs is to match current gain or hFE as closely as possible, within 1 or 2 is best. It’s also best not to hold the transistors when testing, as this changes their hFE.
I replaced some complementary pairs in this E-202, plus some differential pairs. Not all of them needed replacing, but whilst I was inside, I went ahead and did it anyway. Two pairs of transistors on the protection board were especially important in this repair, and one device was leaky. In the end, I tested all eight, and replaced five transistors in the protection circuit
With the correct approach, it’s possible to fix most semiconductor madness one might find in even very old gear like this E-202. Installing several matched pairs of devices greatly improved the overall performance of the amp and the protection circuit now runs reliably. The unit will run and sound so much better with correctly matched parts.
If you’d like to discuss matched transistors for your gear, just get in touch!