[sdiy] slightly OT: tube design of preamp

[JH.]

Hi,

this is only slighly related to synths (I don't even record my own music to
Vinyl (:->) ),
but here is my first design idea for a tube phono preamp.

I've been told from many sides that 12AX7 / ECC83 is no good here. I was
puzzled to see *most* commercial
consumer preamps being built with these very triodes nevertheless.

But I've taken the advice that this is not optimal, so I think of using
lower Mu triodes, and then 3 stages instead
of 2. I have made a circuit (on simulation only, so far!) with two 12AT7 and
one 12BH7, which should be capable
of driving a decent load. The circuit is here: (untested!):
http://home.debitel.net/user/jhaible/tube_hifi/jh_untested_phono_preamp_1.pdf
I'm running the tubes with rather high current (is this good?) and with
rather low anode voltage for stage 1 and 2.
(I read this is good for low noise. But I have no idea if the 12AT7 is a
good choice for low noise at all.)

I have simulated the frequency response, using Koren tube models, and a
frequency - dB table to get a precise
piecewise-linear representation of the RIAA recording courve. If the
simulation is right, I should be within
+/- 0.2dB from 30Hz to 15khz without using any odd capacitor or resistor
values. (I have no data for the RIAA
courve below 30Hz and above 15kHz.)

Simulated frequency response is here:
http://home.debitel.net/user/jhaible/tube_hifi/jh_untested_phono_preamp_simulation_1.pdf
also showing the level and response after each stage.

Detail view of the output frequency response is here:
http://home.debitel.net/user/jhaible/tube_hifi/jh_untested_phono_preamp_simulation_1detail.pdf

The coder model (for the recording courve) explains why the latter picture
has such ugly looking (but
small in height) ripples: It's just a table.
http://home.debitel.net/user/jhaible/tube_hifi/jh_riaa_coder_model.pdf

Please give me your comments / criticism on list or off list, and don't be
shy. Tube stuff is quite new to me, and
I don't want to end up building a bad sounding amp!

JH.