Q205a acts as a current source, Q206a and Q207a as a current mirror, Q209a and Q210a as a common-emitter stage (I don't know why they used two paralleled transistors for that), I haven't a clue what Q211a is for. Chances are that the hFE of Q205a, Q206a and Q207a is not critical, and that the hFE of Q209a, Q210a and Q211a affects the loop gain of the amplifier.
Q205 is a constant current source which creates a constant voltage drop across R211, allowing the cascoding JFETs to establish a constant drain-to-source voltage across Q201 and Q202.
Q206 and Q207 are a current mirror.
Q209, Q210, and Q211 are the amplifier transistor half of the cascoded VAS. This revision of the schematic uses two of them in parallel, and disconnects the third one. Other revisions probably used all three transistors in parallel, and the easiest & cheapest way to perform the revision was just to unsolder one collector leg.
Q206 and Q207 are a current mirror.
Q209, Q210, and Q211 are the amplifier transistor half of the cascoded VAS. This revision of the schematic uses two of them in parallel, and disconnects the third one. Other revisions probably used all three transistors in parallel, and the easiest & cheapest way to perform the revision was just to unsolder one collector leg.
I think Q211a makes Q209a+Q210a a crude ~2x current mirror, which may be related to the "compensation" C204a+R213a. Reducing the OLG is a way to stabilize an amplifier, but the usual way is to degen the LTP. Today variations on the "Blameless" topology have become common knowledge and standard practice but in the past there were all kinds of things done to much the same ends.
Degenerating the long tailed pair would worsen the noise of the phono preamplifier (unless you put a small inductor across the degeneration resistors, but then you have to be extra careful with magnetic hum fields).
Interesting hypothesis. Still, if Q211a, Q210a and Q209a are a 1:2 current mirror, why didn't they just strap the collector of Q211a to its base, like they did with Q207a?
Interesting hypothesis. Still, if Q211a, Q210a and Q209a are a 1:2 current mirror, why didn't they just strap the collector of Q211a to its base, like they did with Q207a?
It's part of the preamp, don't think its part of the phonostage, could be wrong though. I've attached the schematic just scroll down to the last page.
Would any of these transistors have to be matched?
Educated guesses: the base-emitter voltages at a given collector current have to match within a few millivolt for Q209a, Q210a, Q211a. The base-emitter voltages of Q206a and Q207a also have to match, but they are presumably less critical because of the emitter degeneration resistors. 10 mV should be good enough.
If you have a cheap digital multimeter with diode test function, you can check the base-emitter voltage matching of PNP transistors by connecting the emitter to the positive terminal and the base and collector to the negative terminal. Do that for all transistors and compare the read-outs. Mind you, a temperature difference of 1 degree Celsius already causes a voltage difference of about 2 mV.
If you have a cheap digital multimeter with diode test function, you can check the base-emitter voltage matching of PNP transistors by connecting the emitter to the positive terminal and the base and collector to the negative terminal. Do that for all transistors and compare the read-outs. Mind you, a temperature difference of 1 degree Celsius already causes a voltage difference of about 2 mV.
No. Even high voltage low gain transistors work as current sources. Here the voltage on the base is fixed at 1.2V by the diodes. The Vbe drop on the transistor is around 0.6 V so this leaves 0.6V on the 120 ohm resistor, so there should be close to 5ma flowing through this current source.
