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RIAA stage problem ...
RIAA stage problem ...
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Old 13th March 2014, 11:50 PM   #21
loepke72 is offline loepke72  United States
diyAudio Member
Join Date: Jun 2011
Location: Arlington TX
I noticed you redrew the schematic to place the 15K resistor in series with the load instead of the RIAA network. I ran a simulation with that resistor removed from the feedback network of the original circuit to recalculate the resistor in the compensating pole on the output; try changing the 353 ohm resistor to 331 ohms. The overall response is fairly flat with the given RIAA network values, about +/- 1.0dB 20-20kHz.

As far as the simulated cartridge L/R values, that only models the electrical response of the cartridge. The electrical values very well have been chosen to work with whatever mechanical response the cartridge/stylus has, which we have no real way to know. However, as you found out, changing the parallel C at the input has a great effect on frequency response. I'm not sure why so many published designs have a 100-150pF capacitor there as most cartridges work well into a total of about that amount. You need to add in the ~100pF or so of cable capacitance as well, so that gives you 200-250pF of capacitance which is correct for most Shure cartridges and not much else. For the most part, just looking over cartridge specs from different manufacturers, most seem to be happy with the cable capacitance and whatever input capacitance there is at the input minus any physical capacitors that are there in the circuit. With too much capacitance at the input, the response will have a peak at HF followed by a sharp rolloff.
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Old 14th March 2014, 08:03 AM   #22
jiiteepee is offline jiiteepee  Finland
diyAudio Member
Join Date: Mar 2007
Thank you very much guys for helping with the issue! I got it working well now.

I need the R112 to get suitable gain (15K brings +30dB) for flat output so I can use this device as a pre-amp for digital riaa as well. Probably I replace the C4 with 470uF as suggested (I have a very steep digital HP filter @20-40Hz in recording/listening chain).
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