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I have bought a phono-preamp stage from Audio Technica, and the output is quite low. (Using a MM cartridge.) I managed to retrieve a data-sheet which shows how the circuit is laid out. Phono Pre-Amp Circuit I'm not entirely sure how to proceed. The setup seems to be a fairly standard non-inverting OP-Amp setup. The Equalization appears to happen in the feeback. I initially thought R28/R18 were responsible for the gain but that doesn't seem to be the case.

I'm a bit stumped and would love to know if anyone knows if what I'm attempting to do is even possible.

Thanks!

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  • \$\begingroup\$ Any EE lesson is of course welcome. I'd love to learn more about how (and why) this works as it does, if that's not asking too much! \$\endgroup\$
    – Manuel
    Commented Dec 20, 2016 at 12:30
  • \$\begingroup\$ I would have looked at R6/R7 but at 1k0 its already low so at best you might only get another 3dB \$\endgroup\$
    – JIm Dearden
    Commented Dec 20, 2016 at 12:35
  • \$\begingroup\$ What do you mean 'the output is low'? \$\endgroup\$
    – Vladimir Cravero
    Commented Dec 20, 2016 at 12:43
  • \$\begingroup\$ There's nothing wrong with per say, it's just quite a bit lower than the rest of the line level input which leads to possible jump scares and possibly angry neighbors when I forget and switch sources. \$\endgroup\$
    – Manuel
    Commented Dec 20, 2016 at 12:44
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    \$\begingroup\$ I initially thought R28/R18 were responsible for the gain but that doesn't seem to be the case. No those are for biasing, roughly the gain is (R9+R10) / R6. But if you change any of those values then the EQ curve will also change. You would be better off adding an amplifier (without EQ) behind this circuit. \$\endgroup\$
    – Bimpelrekkie
    Commented Dec 20, 2016 at 12:49

1 Answer 1

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It looks like the answer was indeed R6/R7. These two resistors with the equalization network of R9 - R12 & C7 - C11 do indeed determine the gain.

As suggested, I simulated the circuit, and the default gain at 1kHz was 35db. Halving R6/R7 have raised the gain to 43db. Instead I chose a 1.3kOhm resistor in parallel via switch, giving about 600 Ohm and yielding an output of ~40db.

I also checked the RIAA equalization in the simulation for all three scenarios, and it's true that the equalization is slightly affected, the low-frequencies which are slightly more attenuated (less amplified) if the gain is increased. For 390 Ohms the deviation at the 20Hz end is about -3db. The equalization of frequencies from 1kHz and up isn't affected for some reason.

I picked 600 Ohm as final value since the deviation at 20Hz is about -1db VS the ideal RIAA-equalization curve and this is probably inaudible, especially since my speakers top out at around 40Hz anyway.

Upon implementing the above discussed changes (with a toggle switch) it seemed to bear the simulation results out.

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  • \$\begingroup\$ It's good practice to accept your own answer so future users can easily recognise this question has been correctly answered. \$\endgroup\$
    – Enric Blanco
    Commented Apr 11, 2017 at 12:53

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