6DJ8 preamp w/balanced input - strange schematic - diyAudio
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Old 18th November 2011, 11:57 PM   #1
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Default 6DJ8 preamp w/balanced input - strange schematic

Thumbing through some ebay pcbs and found this circuit (see attached) - can anyone suggest how it might sound? Looks to be a kind of tube-solid state hybrid linestage.....
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File Type: jpg Bal + Unbal input Tube preamp sch.jpg (451.6 KB, 505 views)
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Old 19th November 2011, 12:14 AM   #2
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Odd for the sake of odd. I wouldn't expect very good common mode rejection (the raison d'etre of balanced in).
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Old 19th November 2011, 12:22 AM   #3
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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...

Last edited by kenpeter; 19th November 2011 at 12:29 AM.
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Old 19th November 2011, 12:49 AM   #4
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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
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Old 19th November 2011, 02:39 AM   #5
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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....

Last edited by kenpeter; 19th November 2011 at 02:55 AM.
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Old 19th November 2011, 03:12 AM   #6
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Quote:
Originally Posted by lordearl View Post
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
Use a voltage doubler.
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Old 20th November 2011, 07:57 AM   #7
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Quote:
Originally Posted by kenpeter View Post
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....
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?
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Old 20th November 2011, 08:01 AM   #8
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Quote:
Originally Posted by dsavitsk View Post
Use a voltage doubler.
The seller appears to recommend 90v ac max:
Bal/unbal input tube preamplifier stereo PCB ! | eBay
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