I am still experimenting with different phono preamp topologies. This is the one I intend to build next (and after that -- a shunt feedback RIAA phono stage).
Brief description:
The highest "normal" signal from an MM cartridge that we should ever see is about 166mV RMS or 235mV peak (Self, 2018), although this value is presented as an outlier, and it is suggested that most records played with a normal hi-fi cartridge should only present levels up to 60mV RMS (~85mV peak). Finding the expected value of spurious signals (i.e. clicks and pops) was a bit more difficult, some searching (including various discussions here) suggested that it could be as high as 400mV peak (Clappers et al., 2020). The gains of the two stages were selected such that the first stage never clips before the second.
The 75us corner of the RIAA de-emphasis curve is implemented passively after the 1st stage using R19 and C8 in the hopes that this would be enough to prevent the high-frequency content of pops and clicks from overloading the 2nd stage. This and the other two poles were calculated using Peter Baxandall's equations (Baxandall & Lipshitz, 1981). The component values were selected to give reasonable values of gain resistors R2 and R4 (they are relatively small to minimise their noise contribution, but not so small as to strain the gain stages' ability to drive the feedback network). Another design objective was to avoid the use of many different capacitor values for the RIAA EQ. As presented, four capacitors of the same value are used (C10 is three 33nf capacitors in parallel). The values of RIAA components were tweaked a bit after simulating the circuit in LTSpice: for example, the best standard value for R4 is 390 rather than 360Ohm, but I reduced it to slightly increase the total gain. Theoretically this impacts the low frequency response, but in this case any aberrations are irrelevant since the circuit includes some infrasonic filtering.
The values of C7/R20 and C2/R3 were selected to give a useful degree of infrasonic attenuation without impinging on the musical content too much. We're down 1.4dB at 20Hz and 0.6dB at 30Hz, and between 42Hz and 200kHz we're within +/-0.25dB of the target curve (assuming that all components are bang on their nominal value). The degree of infrasonic attenuation ranges from 16dB at 4Hz to 53dB at 0.55Hz -- the region between 0.55Hz and 4Hz is considered the most problematic when it comes to artifacts produced by warped discs, etc (Self, 2018). While this is definitely not the best infrasonic filter out there, it is still much better than no infrasonic filter at all, and I did not want to include another gain stage.
R21 and C9 are the "Marcel damper" proposed by @MarcelvdG which can be useful if this phono preamp is implemented using opamps from the OPA165x family, or in general for filtering RFI collected by the tonearm cable.
I modeled this circuit using LT1115 because the model is already available in LTSpice, however it can be implemented using many different gain devices. I want to implement it using two different discrete opamps:
https://www.diyaudio.com/community/threads/discrete-opamp-with-jfet-cascode-input.427760/ - first stage
https://www.diyaudio.com/community/...e-or-going-crazy-with-discrete-opamps.427868/ - second stage
Bibliography
Baxandall, P. and Lipshitz, S., 1981. Comments on "On RIAA Equalization Networks" and Author's Reply. Journal of the Audio Engineering Society, 29(1/2), pp.47-53.
Clappers, M. et al., 2020. Overload margin in Phono pre-amps. Available at: https://www.diyaudio.com/community/threads/overload-margin-in-phono-pre-amp.356112/ (Accessed 2025/05/20.)
Self, D., 2018. Electronics for Vinyl. New York: Routledge.
Brief description:
The highest "normal" signal from an MM cartridge that we should ever see is about 166mV RMS or 235mV peak (Self, 2018), although this value is presented as an outlier, and it is suggested that most records played with a normal hi-fi cartridge should only present levels up to 60mV RMS (~85mV peak). Finding the expected value of spurious signals (i.e. clicks and pops) was a bit more difficult, some searching (including various discussions here) suggested that it could be as high as 400mV peak (Clappers et al., 2020). The gains of the two stages were selected such that the first stage never clips before the second.
The 75us corner of the RIAA de-emphasis curve is implemented passively after the 1st stage using R19 and C8 in the hopes that this would be enough to prevent the high-frequency content of pops and clicks from overloading the 2nd stage. This and the other two poles were calculated using Peter Baxandall's equations (Baxandall & Lipshitz, 1981). The component values were selected to give reasonable values of gain resistors R2 and R4 (they are relatively small to minimise their noise contribution, but not so small as to strain the gain stages' ability to drive the feedback network). Another design objective was to avoid the use of many different capacitor values for the RIAA EQ. As presented, four capacitors of the same value are used (C10 is three 33nf capacitors in parallel). The values of RIAA components were tweaked a bit after simulating the circuit in LTSpice: for example, the best standard value for R4 is 390 rather than 360Ohm, but I reduced it to slightly increase the total gain. Theoretically this impacts the low frequency response, but in this case any aberrations are irrelevant since the circuit includes some infrasonic filtering.
The values of C7/R20 and C2/R3 were selected to give a useful degree of infrasonic attenuation without impinging on the musical content too much. We're down 1.4dB at 20Hz and 0.6dB at 30Hz, and between 42Hz and 200kHz we're within +/-0.25dB of the target curve (assuming that all components are bang on their nominal value). The degree of infrasonic attenuation ranges from 16dB at 4Hz to 53dB at 0.55Hz -- the region between 0.55Hz and 4Hz is considered the most problematic when it comes to artifacts produced by warped discs, etc (Self, 2018). While this is definitely not the best infrasonic filter out there, it is still much better than no infrasonic filter at all, and I did not want to include another gain stage.
R21 and C9 are the "Marcel damper" proposed by @MarcelvdG which can be useful if this phono preamp is implemented using opamps from the OPA165x family, or in general for filtering RFI collected by the tonearm cable.
I modeled this circuit using LT1115 because the model is already available in LTSpice, however it can be implemented using many different gain devices. I want to implement it using two different discrete opamps:
https://www.diyaudio.com/community/threads/discrete-opamp-with-jfet-cascode-input.427760/ - first stage
https://www.diyaudio.com/community/...e-or-going-crazy-with-discrete-opamps.427868/ - second stage
Bibliography
Baxandall, P. and Lipshitz, S., 1981. Comments on "On RIAA Equalization Networks" and Author's Reply. Journal of the Audio Engineering Society, 29(1/2), pp.47-53.
Clappers, M. et al., 2020. Overload margin in Phono pre-amps. Available at: https://www.diyaudio.com/community/threads/overload-margin-in-phono-pre-amp.356112/ (Accessed 2025/05/20.)
Self, D., 2018. Electronics for Vinyl. New York: Routledge.
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