Headphone Amp - Reconfigure Stage 1

Posted by Mike C (A) on October 13, 2016 at 11:55:09

I have a headphone amp with a 12AX7 driving a parallel 6SN7 single ended output stage (transformers). It sounds good, with a fine CD player and AKG K812 phones; but it's not quite as good as my highly modified Elekit TU882. I have a fine 5751 i.l.o. 12AX7 and a pair of good NOS 6SN7s.

Stage 1 looks odd to me; B+ 255V, with the 12AX7 at about 1mA with only 22Kohm anode load. This looks more like how to load a pentode rather than a triode but I'm no expert (I mainly use active loads). I don't like the 12AX7 and I doubt its configuration here.

So I'm about to reconfigure stage 1 and wish to share ideas.
As it's on PCB I want to use a pin compatible valve instead of the 12AX7 so it's just a matter of changing bias and load resistors. Here are some possibles.

12AU7 at about 2.7mA with 56kohm anode load; valve anode voltage 80V. Valve Ri looks about 11kohm, which is 1/5 of the anode load, agreeing with a rule of thumb I think. Bias resistor just under 1kohm. I have some reasonable 12AU7 variants such as 5814.

For:
Reasonable current in the stage
Good ratio of RL to Ri
Low Zout - remember it has to drive parallel 6SN7 with quite a lot of miller cap of about 170pF if I got that right.
Against:
12AU7 is not a good sounding valve
12AU7 runs best at a higher current such as 5 or 6 mA or more.

Other possibility:
12AY7 / 6072, at various possible op points.
One is 1.2mA, 90V anode, load about 130kohm. Ri about 34kohm, giving RL about 4 x Ri which I don't think is too bad.
Points for:
12AY7 runs OK at low currents.
It is a good sounding valve
Points against:
Zout of about 30kohm may be a bit high to drive the parallel 6SN7 well.
1.2mA looks low to drive a substantial load

Or, 12AY7 at about 2mA, anode at 80V, anode load about 85kohm.
Ri is then about 26kohm I think if I read the curves right; RL is only a bit over 3 x Ri, perhaps not ideal but might be OK? Zout reduces to about 20kohm; my figures suggest, with 170pF, about 45kHz upper limit which looks good to me.
Points for:
Zout not too bad
Current not too bad to drive stage 2
A good sounding valve
Points against:
80V on the anode looks low to me; would it compromise the sound I wonder
Anode load less than rule of thumb of 5 x Ri.

Long term I plan to build a new chassis with a better driver stage.

I have considered better valves e.g. E80CC (I use it and like it) or 12BH7, but the extra heater current is a risk I don't want to take (having blown a mains TX in the past).

I may well build some or all of these variants.

Any comments welcomed.

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Topic - Headphone Amp - Reconfigure Stage 1 - Mike C 11:55:09 10/13/16 (0)

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