My current headphone hybrid described here has significant flaw: it's amplification stage is DC coupled with output and it require manual adjustment of tube working point (via adjustable CCS) to center it's output.
I want to eliminate this by introducing DC servo circuit. With dual power supply amp's topology can be made fully DC coupled.
The first option is to use op amp output as cathode bias source directly. Like this (many values are arbitrary):
Is that a terrible idea? I've never seen this before.
I want to eliminate this by introducing DC servo circuit. With dual power supply amp's topology can be made fully DC coupled.
The first option is to use op amp output as cathode bias source directly. Like this (many values are arbitrary):
Is that a terrible idea? I've never seen this before.
Not terrible at all, if you get the loop stable.
C1-R2 and C2-R4 should be large enough not to let any signal couple through. As you use this in the input circuit. I did not check whether the given values are sufficient. Dc changes normally take several 10s of seconds to occur, thus you can make those values realy large.
Somewhat different; I have used such a topology in the bias circuit of output tubes in tube amplifiers. It provides fixed bias for the output tubes as well as giving the possibility of sensing over-current (to do emergency turn-off) as well as other compensations.
C1-R2 and C2-R4 should be large enough not to let any signal couple through. As you use this in the input circuit. I did not check whether the given values are sufficient. Dc changes normally take several 10s of seconds to occur, thus you can make those values realy large.
Somewhat different; I have used such a topology in the bias circuit of output tubes in tube amplifiers. It provides fixed bias for the output tubes as well as giving the possibility of sensing over-current (to do emergency turn-off) as well as other compensations.
@easphyx
I don't know about using the op-amp in the cathode circuit, but..
Yes, there is a well-regarded hi-end preamp that uses a TL072 / TL082 to control offset on the (differential) output line driver tube(s), which are 6SN7. It works by sensing any DC offset in the balanced output, then sending a proportional correcting voltage to one 6SN7 grid to null it. The second grid is fixed via a voltage divider, from the same PSU as the op-amp, at around -1.4V.
You should be able to do the same thing with an SE tube, if your op-amp is watching the offset between gnd and the output terminal.
Does that help at all?
I don't know about using the op-amp in the cathode circuit, but..
Yes, there is a well-regarded hi-end preamp that uses a TL072 / TL082 to control offset on the (differential) output line driver tube(s), which are 6SN7. It works by sensing any DC offset in the balanced output, then sending a proportional correcting voltage to one 6SN7 grid to null it. The second grid is fixed via a voltage divider, from the same PSU as the op-amp, at around -1.4V.
You should be able to do the same thing with an SE tube, if your op-amp is watching the offset between gnd and the output terminal.
Does that help at all?
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