Tube Preamp for ZEN
I would propose a Constant Current Draw stage as shown at the bottom of this link:
http://www.homestead.com/whaan/files/index.html
This circuit can be used to good advantage with the 6H30 which has a heater to cathode max voltage of 400. This eliminates the need for a second floating filament supply.
I would propose a Constant Current Draw stage as shown at the bottom of this link:
http://www.homestead.com/whaan/files/index.html
This circuit can be used to good advantage with the 6H30 which has a heater to cathode max voltage of 400. This eliminates the need for a second floating filament supply.
Certainly there is some challenge for tube circuits
to drive the Zen, but here are some items to consider:
1) The output impedance of the tube preamp is not
much of an issue since it ony reduces the gain, and in
fact will give lower distortion, all other things being equal.
2) The output capacitance of a tube preamp will induce
a low frequency roll off based on the output resistance plus
the input resistance. Most of the time we see 10 uF,
and with a 1000 ohm output and a 1000 input, the corner
frequency will be below 10 Hz. Not great, but do-able
in many cases, and you can always put in a bigger
cap.
3) If you want higher input impedance, just use higher
values for R10 and R11 (revised circuit). With 4.7K input,
we see some more distortion at 20KHz, but no worse than
the original Zen.
4) Output current of the tube circuit is the main thing, since
they usually have plenty of voltage gain to throw away.
To drive the Zen to full output, we are talking a tube output
stage which can cleanly deliver 1 mA for the original Zen
and 4 mA for the revised Zen. Cathode followers are called
for here, not outputs directly off the plate, although that, too,
is do-able.
5) Upcoming Zen project will feature a much higher input
impedance, and should be adaptable to previous versions.
to drive the Zen, but here are some items to consider:
1) The output impedance of the tube preamp is not
much of an issue since it ony reduces the gain, and in
fact will give lower distortion, all other things being equal.
2) The output capacitance of a tube preamp will induce
a low frequency roll off based on the output resistance plus
the input resistance. Most of the time we see 10 uF,
and with a 1000 ohm output and a 1000 input, the corner
frequency will be below 10 Hz. Not great, but do-able
in many cases, and you can always put in a bigger
cap.
3) If you want higher input impedance, just use higher
values for R10 and R11 (revised circuit). With 4.7K input,
we see some more distortion at 20KHz, but no worse than
the original Zen.
4) Output current of the tube circuit is the main thing, since
they usually have plenty of voltage gain to throw away.
To drive the Zen to full output, we are talking a tube output
stage which can cleanly deliver 1 mA for the original Zen
and 4 mA for the revised Zen. Cathode followers are called
for here, not outputs directly off the plate, although that, too,
is do-able.
5) Upcoming Zen project will feature a much higher input
impedance, and should be adaptable to previous versions.
Tube Pre Amp and Son of Zen
I had no trouble driving a Son of Zen at 50 watts or less using the single ended connectors of a Conrad Johnson PV9A pre amp that is over ten years old. Even the built in phono section driving a low .4 millivolt moving coil could do the job. And the phono uses 12AX7 tubes. What happens is the volume control gets turned up near the 12 o'clock position or slightly more. With your ear near the tweater you hear more tube hiss. Other wise nothing strange happens. It still has that classic full tube sound of older tube equipment. Off course I have changed the tubes once in ten years.
I had no trouble driving a Son of Zen at 50 watts or less using the single ended connectors of a Conrad Johnson PV9A pre amp that is over ten years old. Even the built in phono section driving a low .4 millivolt moving coil could do the job. And the phono uses 12AX7 tubes. What happens is the volume control gets turned up near the 12 o'clock position or slightly more. With your ear near the tweater you hear more tube hiss. Other wise nothing strange happens. It still has that classic full tube sound of older tube equipment. Off course I have changed the tubes once in ten years.
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