OK, then try decreasing the cap. Remember, you're going to have a stage following this which has its own input capacitance. That will throw off your quest for standard values. I suspect it's a futile quest- almost everybody who has built a successful RIAA stage has used trim components or parallel combinations to hit RIAA accurately. Like I said, try seeing the effect of a 10-20% change in plate resistance to get an idea of what you're up against.
IME, it's nice to have RIAA within a quarter dB or so of absolute, but channel matching to be as painfully tight as possible, 0.1dB or better. So you're going to be trimming after building if you want accuracy and best performance, it's inevitable. Jack's suggestion is a good one. I often parallel caps with small NP0 or film trimmers, and manually trim things into submission with the help of a function generator and an accurate AC voltmeter.
Phono stages are not easy if you want high performance!
IME, it's nice to have RIAA within a quarter dB or so of absolute, but channel matching to be as painfully tight as possible, 0.1dB or better. So you're going to be trimming after building if you want accuracy and best performance, it's inevitable. Jack's suggestion is a good one. I often parallel caps with small NP0 or film trimmers, and manually trim things into submission with the help of a function generator and an accurate AC voltmeter.
Phono stages are not easy if you want high performance!
You probably already have it, but just in case you dont, the forums over at Intact Audio discuss a lot of LTspice models of riaa stages, often with tubes and LCR equalization, but usefully in the Sim City section there is a great inverse riaa signal generator you can use in your models.
Best of luck
Best of luck
Just as an observation == couldn't you take the difference values you observe in an actual circuit (with an accurate inverse RIAA filter) and derive Laplace values with which to compensate for the parasitic reactive components?
When I had the "Masters Noise" PCB's burned I specifically allowed for tweaking the values -- you could use an IC socket, I just used machined socket pins. You can hook up the variable cap with some #24 solid wire, or plug in various values. It's a very nice idea to have an impedance bridge or accurate LCR meter to know what you're working with before hand because the values can stray more than a few percent from what's stated on the capacitor case.
Note that if you are using a variable cap, it's going to be "hot" so put a plastic knob on the twiddling end and follow the usual precautions.
When I had the "Masters Noise" PCB's burned I specifically allowed for tweaking the values -- you could use an IC socket, I just used machined socket pins. You can hook up the variable cap with some #24 solid wire, or plug in various values. It's a very nice idea to have an impedance bridge or accurate LCR meter to know what you're working with before hand because the values can stray more than a few percent from what's stated on the capacitor case.
An externally hosted image should be here but it was not working when we last tested it.
Note that if you are using a variable cap, it's going to be "hot" so put a plastic knob on the twiddling end and follow the usual precautions.
A project for the future is a phono preamp graphic equalizer. I not only listen to LPs but 78rpm records, whose characteristic curves are not standard.
The idea is to have a 10 band equalizer (more if necessary) with digital potentiometers to dial in the appropriate eq curve for the record.
I think I will build a line stage first and use the equalizer for the phono preamp.
What do you think of this idea?
The idea is to have a 10 band equalizer (more if necessary) with digital potentiometers to dial in the appropriate eq curve for the record.
I think I will build a line stage first and use the equalizer for the phono preamp.
What do you think of this idea?
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