Odd for the sake of odd. I wouldn't expect very good common mode rejection (the raison d'etre of balanced in).
Fine, except its gonna take more than 110V on the plate to move any current.
Reason I figure, VGS threshold of 9610 is -4V, so there will be +4 on cathode.
Amperex curves for 6DJ8 at -4 on the gate hit zero at about 110 plate volts...
I see only 48V rectified, OK its a doubler to 135V, so just maybe barely could
work if the doubling doesn't lose anything in 40.3K resistance above the plate.
I think its all biased far too close to complete cutoff....
The pMOSFET is operating with only -4V drain to source, so its Miller is going
to be crazy high and non-linear... You need at least -10V here at all times.
If you gave it a negative rail, the pMOSFET drain could be made happier.
The big problem here (top and bottom) is just not enough Volts...
Its not a bad circuit otherwise...
Reason I figure, VGS threshold of 9610 is -4V, so there will be +4 on cathode.
Amperex curves for 6DJ8 at -4 on the gate hit zero at about 110 plate volts...
I see only 48V rectified, OK its a doubler to 135V, so just maybe barely could
work if the doubling doesn't lose anything in 40.3K resistance above the plate.
I think its all biased far too close to complete cutoff....
The pMOSFET is operating with only -4V drain to source, so its Miller is going
to be crazy high and non-linear... You need at least -10V here at all times.
If you gave it a negative rail, the pMOSFET drain could be made happier.
The big problem here (top and bottom) is just not enough Volts...
Its not a bad circuit otherwise...
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Only reason I thought of using it is I have a 60v & 6.3v r-core which would suit it perfectly....but according to kenpeter's post, may still not be enough volts 🙁
If you only need differential input and can live without voltage gain,
Broskie Cathode Follower or Differential White Cathode Follower
either choice should work fine with 6dj8 at those voltages...
Neither requires any pMOSFET to help it along....
Not that I have anything against pMOSFET, but you do have to keep
the drain well biased if its gonna behave nice for audio. Just cause
it can theoretically operate with near zero Volts on the drain, don't
mean Miller capacitance stays sane when you abuse em' like that.
Without a bias voltage upon the drain, ain't nothing to make that
capacitance any different or less than symmetry with gate to source,
which could be hugely more Miller than you would normally expect....
Broskie Cathode Follower or Differential White Cathode Follower
either choice should work fine with 6dj8 at those voltages...
Neither requires any pMOSFET to help it along....
Not that I have anything against pMOSFET, but you do have to keep
the drain well biased if its gonna behave nice for audio. Just cause
it can theoretically operate with near zero Volts on the drain, don't
mean Miller capacitance stays sane when you abuse em' like that.
Without a bias voltage upon the drain, ain't nothing to make that
capacitance any different or less than symmetry with gate to source,
which could be hugely more Miller than you would normally expect....
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Thanks KenPeter - unfortunately I am going to require some voltage gain, I'm using an 2x AD1865 in balanced mode with a passive i/v stage and the output is in mV only!
How about the Broskie "Unbalancer":
Unbalancer
Would this do the trick?
Hi, Built this a while ago. Tested fine with unbalanced in, very quite. I never used it in anger as the unbalancing pmosfet circuit looked weird (as already stated). Randomly stumbled on this thread and found the board. hooked it up and Measured voltages. 100v ac in (nothing went bang and caps within max voltages) gives aprox 200v dc at tube. Voltage between drain and source of pMosfet is 3.5v. Is this too low to give good performance? Looking to build an unbalancer with a little gain to front end a pass F5. I'm guessing this circuit would not be optimal in this role? As I have it completed with transformer is it worth increasing bias voltage and trying. How Could I do this? or just move on to some other circuit. Thanks in advance.
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