I’m currently working on a Luxman L-31 amplifier but doubting my choice on a transistor substitution. Lots of people replace the 2SA836 and 2SC1345 transistors with BC560 and BC550 transistors. But for Q201 and Q202 the VCE exceeds the maximum VCE the BC560 can safely handle.
Without checking the further specs I picked two KSA992 transistors out of my stock and used those. The amplifier works fine but I suddenly realized the KSA992 has a maximum current of 50mA while the 2SA836 has a maximum current of 100mA.
My transistor calculation skills are practically gone after 30 years so I tried simulating the circuit in Falstad circuit simulator. That resulted in a maximum emitter current of ~2.9mA in Q201 and ~1.9mA in Q202. Those values feel a bit low to my idea.
Can someone explain, or advise me a website with a decent explanation, how I can calculate the current in a differential pair like this?
Without checking the further specs I picked two KSA992 transistors out of my stock and used those. The amplifier works fine but I suddenly realized the KSA992 has a maximum current of 50mA while the 2SA836 has a maximum current of 100mA.
My transistor calculation skills are practically gone after 30 years so I tried simulating the circuit in Falstad circuit simulator. That resulted in a maximum emitter current of ~2.9mA in Q201 and ~1.9mA in Q202. Those values feel a bit low to my idea.
Can someone explain, or advise me a website with a decent explanation, how I can calculate the current in a differential pair like this?
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Current to be split between Q201 and Q202 is apprx (Vzener-Vbe201)/(VR201+R205), depends on setting of VR201
You get vbe across R206 (I think that’s a 6) which is 1.2K. That’s around half a milliamp. Adjust VR201 to give a total current of twice that (somewhere around half rotation) to balance the diff pair. It will WORK if it’s out of balance but there are limits to how well. When properly balanced it will tend to have low offset voltage, but the best setting for most amplifiers is when the two emitter currents are EQUAL.
I recognized the differential pair so I matched the transistors and the DC offset was easy to adjust between +/-5mV. It’s the current flow calculations around transistors that get me. Had some in school back in the mid 90’s but electronics was a minor and mostly focused on digital logic.
But this helps me understand things for the next time.
In the meanwhile another L-31 arrived so I can do the trick once more.
But this helps me understand things for the next time.
In the meanwhile another L-31 arrived so I can do the trick once more.
Black deposit on silver plated wires does not do any harm.
You will not throw away your silver cutlery when it is turning black.
Look what the reason is for popping noise.
If you’re getting a greenish deposit, they need to GO. I’ve seen components corrode themselves right off the board. An open circuit in a bad place can take other things with it.
I know silver oxidizes and that it can be cleaned but in this case the transistors really are bad. After replacing them the popping is fixed.