[sdiy] slightly OT: tube design of preamp

[Theo]

Are you using the old or "new" Riaa curve.
The old one is a straight line down from and to infinity, the "new" curve
has a maximum around (think its was )20hz

Theo


----- Original Message -----
From: JH. <jhaible at debitel.net>
To: diy <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, August 18, 2004 10:24 PM
Subject: [sdiy] slightly OT: tube design of preamp


> 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.pd
f
> 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_simu
lation_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_simu
lation_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.
>