Amplifier transistor matching?

I am trying to fix an old Marantz 2245 stereo. one of the amplifier blocks had severe distortion problems. On inspection, two transistors were suspect. One I can find and fix. The other transistor is part of two pairs of transistors on the amplifier block. Apparently these two pairs of transistors come in matched pairs. One is a 2SC960/LA43 the other one is a 2SA607/LA43E. Replacement transistors have been reported not to work satisfactorily on these Marantz circuits. I was finally able to find 2SC960 transistors but not with the same LA43 subscript. My question is, what kind of transistors parameters do you guys know should be matched among pairs of transistors to see if I got a suitable matching pair before replacing them.

Reply to
cor
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Without knowing their biasing circuitry, I don't think a complete answer can be given. Some biasing circuits are fairly forgiving, others seem like balancing a pencil on it's end...

But, in my opinion, Vbe and hfe are primary parameters.

I would be interested in learning a better answer myself, tho.

You might consider sci.electronics.design also, if you haven't already.

There are some competent IC designers there, who deal with problems like this when they design their miracle circuits.....

Andy

Reply to
Andy

For most gigs, the price of a battery is negligible compared to the show going 'down.' That said, the new Shures seem to do that long on a 9v. The really nice ones have battery meters on the actual receiver, so that you can monitor the battery condition remotely. I've seen a bunch of these lately. They've performed flawlessly IME, but the included mic is a little large. There's an ultra small mic option which is less noticable, but the (big) stock mic sounds better than most lav's I've worked with....

jak

Reply to
jakdedert

I've probably repaired many hundred such units, so here goes.....

We stocked generic parts for all of these repairs with minimal hFE=75 and VCE=100. Then parts were matched to the following characteristics:

1) Case style: TO-3, TO220, or small signal 2) NPN, or PNP

3) Regular transistor, or darlingtons.

4) Replace all transistor in the complete channels output circuit, usually 5 to 7.

5) Turn the bias current adjustment to minimum resistance.

6) Plug the AC power cord thru a 40 watt lamp circuit in series with the power.

7) Turn on the unit with no signal, and watch for a dimming light bulb. If it goes full brightness, kill all power and go back to step 4 looking for more fried parts.

8) If the light goes dim, connect directly to AC power.

9) Feed 1kz sine wave in errant channel while driving a 100w 8ohm resistive load. Run a very low power level to accentuate the 'notch'. Watch the output on a scope and tweek the bias pot until this 'crossover notch' dissappears.

10) Additional testing using a harmonic distortion analyzer may find a more optimum setting for the bias at full power level.

You may or may not get original factory specs using generic parts, if you can get 'factory' parts it may work better, or not.

Be aware, if you apply full AC power with even one of the transistors in a failed mode, it will 'take down the whole show' all over again.

Good luck,

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Luhan Monat: luhanis(at)yahoo(dot)com
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Reply to
Luhan Monat

An open device in a totem pole configuration makes the amp able to drive in only one direction, so you get only the positive or negative half of the waveform, i.e. 50% distortion.

Reply to
Ol' Duffer

In the output stage, it's usually Beta (DC current gain) so that they share the load.

For an input diff amp pair VBE may be more important.

Reply to
Ol' Duffer

You should check a batch of power transistors on a curve tracer sometime. The results may scare you. I routinely see a factor of three variation in Beta within batches. Of course you can use big, wasteful swamper resistors, or a bank of 5 unmatched devices where 2 matched would be sufficient. Or you can let the amps blow up and they I buy more transistors than I need and select a good grouping from the middle of the range and fix them so they don't blow up anymore...

Reply to
Ol' Duffer

Hello Graham,

I'll second that, it's what I saw most in matched pairs or quads. Except for FETs where the match is usually Vgs versus resistance.

Yes, it is best to avoid matching. But when you can't avoid it and then specify a transistor array it can be acceptable. That shouldn't be some boutique part though. I have done a few matched designs (where there was no other choice) based on SD5400 arrays. All RF stuff though, not audio.

BTW, since you seem to be an audio guru: Is there any truly digital wireless mike system with a reasonable battery life (like >5hrs for the lapel mikes)? Preferably with AA and not with 9V batteries.

Regards, Joerg

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Reply to
Joerg

Usually it's current gain that's matched. I've never specified matched pairs in any design ( pro-audio btw ) in my entire life despite being responsible for some 10s of thousands of amplifiers out there. It seems like poor design to need matched pairs to me.

Some circuits almost don't care. It depends a lot on the driver stage.

Incidentally I can't really see how a failed output device can be responsible for severe distortion. Normally it's a works or not situation with output devices.

Graham

Reply to
Pooh Bear

Indeed.

Hmmm.. I'm not specifically into wireless mikes and I think both the older VHF ( certainly ) and newer UHF ones are good old analogue.

A good place to ask would be alt.audio.pro.live-sound. The issue of battery life with wireless mikes has come up a good many times. That's where you'll find ppl who use this stuff all the time. I don't think you'll avoid 9V batteries though from what I understand. You might make your 5hrs with rechargeable NiMH but the pros seem to prefer alkalines - just in case of a bad charge perhaps. The battery ( alkaline ) gets chucked at the end of the gig.

Graham.

Reply to
Pooh Bear

If one of the pair fails, the result is severe distortion - a rectified waveform, low-pass filtered by the speaker. Only about 30% THD... some people don't even notice!

Reply to
Walter Harley

Dunno about digital. The Shure analog UHF packs, with lav mics, last more than 6 hours with a pair of alkaline AAs. One of my gigs uses a dozen or more channels of them; we put fresh batteries in at 4:30pm, and at 10:30pm when the show ends they're usually still showing three or four out of five bars on the battery life indicator. We replace them every night anyway - if we went for two nights, by the end of the second night we'd be too nervous. As jak said, the price of batteries is small compared to the price of the show going down.

Reply to
Walter Harley

Hello Graham,

The analog ones are ok, except for a suboptimal squelch and a nasty pop when muting it. In church you have to do that a lot.

I had asked in rec.audio.pro but I'll try your suggestion tomorrow. Not today, the barbie is almost ready. Marinated ribs and potatoes tonight.

We use Ansmann 9V NiMH which seem to be the only ones with 250mAh, plus nifty uC charge stations. But even with top notch Alkalines our Sennheiser EW system doesn't reach 5hrs. Actually the Ansmanns hold out a bit better. Thing is, two AA cells pack a whole lot more energy than a

9V battery. I wonder why they didn't design for 3V or even better 2.4V.

Regards, Joerg

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Reply to
Joerg

These devices which I currently use for example are pre-graded by the manufacturer. Worst case match is 2:1 in either gain grade.

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A simple low value emitter ballast resistor overcomes the bulk of beta mismatch anyway and I would never fail to use them. You can't depend on paralled device temps being identical - in fact quite the reverse - never mind thermal runaway !

Graham

Reply to
Pooh Bear

Good god ! I'd forgotten about non complementary outputs ( thankfully ).

It's been that long. ;-)

Graham

Reply to
Pooh Bear

But your system sems to be outdated. Here is a 2AA 6-8hrs transmitter

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ciao Ban
Bordighera, Italy
Reply to
Ban

rec.audio.pro is good but mainly recording oriented. It's the live guys who use wireless mics most.

Sounds very nice. Enjoy !

Having the volts available is often handy, although I admit I don't know the exact answer.

250mAh is good though. What's the terminal voltage when charged ? If it's only 8.4V that seems to be problematic. I gather some NiMHs have an extra cell.

Graham

Reply to
Pooh Bear

Sorry, there was still another page in my clipboard, I meant this one (in German for Joerg)

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--
ciao Ban
Bordighera, Italy
Reply to
Ban

You mean ?

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Which is an in ear monitoring ( IEM ) *receiver* !

Not the same thing at all !

Graham

Reply to
Pooh Bear

Ah - ok - tech spec here.

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Unfortunately not a lapel mic. I wonder if Sennheiser do a simple transmitter pack ?

Graham

Reply to
Pooh Bear

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