the DAC output is 1mA on 100 ohm resistor.
plug this to the preamp (in the picture).
I measured the signal at the output in the preamplifier, I do not remember so much, but something in the 5 volt range output.
don't know the impedance of the power amp, but when i build it, the recommendation was that need a preamp with very low impedance and high voltage output.
the reason that i remove the 12ax7 cathode follower it's because i measure high 2nd harmonics and 3nd harmonics in the output.
and without follower i measure only 2nd harmonics, that i like.
plug this to the preamp (in the picture).
I measured the signal at the output in the preamplifier, I do not remember so much, but something in the 5 volt range output.
don't know the impedance of the power amp, but when i build it, the recommendation was that need a preamp with very low impedance and high voltage output.
the reason that i remove the 12ax7 cathode follower it's because i measure high 2nd harmonics and 3nd harmonics in the output.
and without follower i measure only 2nd harmonics, that i like.
This thing's output impedance is around 100kOhm. Not suitable for almost any power amp, not even remotly appropriate for driving interconnect cables of any length.
You should try to troubleshoot your cathode follower first before abandoning the whole idea. It might be just not optimally biased for the job and therefore adding distortion.
I would suggest using 2 valves and have a second stage cathode follower for lower output impedance.
If you need the high gain and a low output impedance you have 2 options: add a cathode follower after the 12AX7. You better use a 12AT7 in stead of a 12AX7 for this stage.
The other option is to use a different tube with high gain and low Rp, e.g. a EC86 or D3A (triode switched)
The other option is to use a different tube with high gain and low Rp, e.g. a EC86 or D3A (triode switched)
thanks
do you have schematic for this circuit?
i understand that the V+ and filament are same?
thanks
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I believe your best option is to forget the cathode follower, this tube of yours has low gm, it will be unsuitable for low distortion in that position. Use a FET follower, it will be transparent and preserve the sound you like so far.
You could DC-connect both stages by removing R11, R13 and C3, it will be simpler and better
No I don't have ready-to-go schematics but it is a very simple circuit. I've found: Hybrid Amplifier by Andrea Ciuffoli where you can get some ideas for the D3A.
Note that your demand of high gain (~50) is much higher than preamps typically deliver. In fact it is more like an input stage of an integrated amp. I am busy with a similar 'preamp' that should be able to drive a EL84 directly (~7V). I am thinking of using the D3A for it.
Thank you very much, an interesting idea.
maybe i have miscalculation:
DAC output is +-1mA on 100 ohm resistor, so 0.1v+-
i measure with scope, the power amplifier at full volume is 2.5Vptp
so i need 25 gain
need a new post: Which tube is suitable for 20 gain and low exit resistance?
