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Trying to find a resistor value for JFET RIAA Phono Preamplifier schematic
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MasterT:

--- Quote from: CatalinaWOW on December 04, 2017, 05:44:14 pm ---Since OP wants minimum parts count, does anyone understand the purpose of C3?  This is an audio preamp and just about any garden variety capacitor should work fine over the frequency range.  Doesn't seem necessary to have C2 and C3 of different capacitor technologies to cover the entire frequency range.

--- End quote ---
I think, they put two caps C2&C3 in order to get better approximation with RIAA time constants. 3% correction of C2 seems useless  if not to build a circuit with 1% tolerance components.
 Googling over wiki for RIAA standard, I discovered an error in R5 value, it has to be 22 k. Definitely not a 28k, and tacking output impedance of the first stage amplifier, about  20.5 k 
 There is a calculator
http://www.mh-audio.nl/calculateRIAA.asp

and outputs: R1=22000 R2=3198.958 C2=34.09nF C1=99.4nF

Since 34 nF is not available, it explains a necessity for C3.
CatalinaWOW:
The explanation for C3 makes sense from a math standpoint.  Since very few circuits in the time of LPs were built with 1% components I suspect the correction provided for C2 is a triumph of the calculator over the practical.  Remember this whole equalization scheme was invented when slide rules were king for calculation and thus 2 and possibly 3 digit accuracy was all that was normally achieved.

Perhaps someone will build a version of this with all Rs and Cs trimmed to their optimum 0.1% values and report on the perfection of its sound. 
Frankentronics:

--- Quote from: MasterT on December 04, 2017, 10:40:21 pm ---...I discovered an error in R5 value, it has to be 22 k. Definitely not a 28k...

--- End quote ---

Good catch...!

I think this is one of the things they addressed in rev.3 of this schematic. They changed that R5 to a value of 22.1k. I believe you were probably still looking at the rev.2 schematic (which is the first one I posed) when you caught that error. In one of my subsequent posts I posted rev.3, which I discovered at a later point.


--- Quote from: Benta on December 04, 2017, 09:45:46 pm ---Make space for the cap if you like, but don't mount it to start with. See how the circuit behaves first.

--- End quote ---

If you don't mind me asking, what kind of behavior from the circuit would I encounter in order to mount the cap?

Thanks...
Lightages:
As far as needing a cap, it depends on the cartridge. Someone has already pointed out that the capacitance needed by be already supplied by the cable from the cartridge to preamp. If you can measure the capacitance of the cable or have the specs and can calculate then you can determine if you need to add any or ignore it.  You said that this is just a fun project so you can ignore it, or if you want to play you can adjust it and see what happens.

Many phono preamps had dip switches to change the resistor and capacitance on the input. You can do the same by putting an IC scoket t the input and plug in different resistors and capacitors and play around to your liking. You should use metal film resistors and polypropylene caps as the best for low noise. Stop obsessing and have fun and experiment.
Frankentronics:

--- Quote from: Lightages on December 05, 2017, 04:41:24 am ---As far as needing a cap, it depends on the cartridge. Someone has already pointed out that the capacitance needed by be already supplied by the cable from the cartridge to preamp. If you can measure the capacitance of the cable or have the specs and can calculate then you can determine if you need to add any or ignore it.  You said that this is just a fun project so you can ignore it, or if you want to play you can adjust it and see what happens.

Many phono preamps had dip switches to change the resistor and capacitance on the input. You can do the same by putting an IC socket at the input and plug in different resistors and capacitors and play around to your liking. You should use metal film resistors and polypropylene caps as the best for low noise. Stop obsessing and have fun and experiment.

--- End quote ---
This is all useful info. Thanks. And at the right time, as I'm compiling the components through DigiKey.

I'm ordering 1% resistors. 1/4 W should be OK, right?
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