Need better RIAA circuit for my phono amp.

[voycieck]

I'm building phono amp. My RIAA circuit doesn't look right to me. If anyone knows better one, let me know. I marked those parts with "*". Would be better if I use active RIAA?

 

[Salas]

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[voycieck]

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Thank You for correcting my RIAA circuit. I will use Yours, and I change my cathode resistors to 470 ohms. As I made mistake with 12AX7's, do I need to recalculate cathode resistors for 6922's? I think, I could use 470 ohms for them also?

 

[Salas]

Don't know, depends on the spice models. Mine gave the 1ma you noted on 12AX7 using 470R for instance. 820R for 3.2mA and 6922, and 34dB gain at the output. I.e. takes a 5mV cartridge to give 250mV. Just measure the real life voltage drop on their resistors for both stages, divide by their values to calculate current, and adjust to your targets if off prediction. Don't make the inter-stage coupling capacitor C6 different than 10nF that I used. Else, FR tends to over grow in the lows due to the source resistance.

 

[voycieck]

Don't know, depends on the spice models. Mine gave the 1ma you noted on 12AX7 using 470R for instance. 820R for 3.2mA and 6922, and 34dB gain at the output. I.e. takes a 5mV cartridge to give 250mV. Just measure the real life voltage drop on their resistors for both stages, divide by their values to calculate current, and adjust to your targets if off prediction. Don't make the inter-stage coupling capacitor C6 different than 10nF that I used. Else, FR tends to over grow in the lows due to the source resistance.

Thanks for Your tips. I will build it without RIAA first, setup working points of the tubes and then I add RIAA. My cartridge is 4mV. If I don't get enough gain, I could add bypass caps to cathode resistors, which I don't want to do. Most likely I rewire heaters of 6922's and use 12AX7s instead. Your tips gave me some starting points and save me lots of guess work. Thank You again.