OK, here’s where things get interesting:
Tuning the front end of a phono stage.
I used these two websites as my reference:
http://www.hagtech.com/loading.html
http://sound.whsites.net/articles/cartridge-loading.html
From there we see that the capacitance of the interconnect wires and inductance of the phono stage itself are key to proper tuning of the front end. As many of the phono cartridges are different, it’s crucial we get this right.
First, I measured the capacitance of the interconnects with my capacitance meter kit (got mine from: https://www.sparkfun.com/products/9485 ) Now when I got that, I also got, completely on a whim, a signal generator kit ( https://www.sparkfun.com/products/11394 ) I mean, you never know when you need a triangle wave, right? (Oh, by the way, if you get one, make sure you solder the switch buttons flat on the PWB, otherwise the buttons will physically stick on the cover plate)
OK, so the capacitance meter indication was bouncing around quite a bit, but it did seem to center around 100 pF. OK, that’s a good starting point. Now, how do we measure the coil’s inductance? Well, it turns out, that’s where the signal generator comes in. Set the signal generator at 10 Hz with a 1V RMS signal (2.7 V peak-to-peak) per the procedure in the second link, and connect to one channel of the cartridge (hopefully they’re both the same). Measure the voltage across the coil. In the case of the Grace, I measured 22 mV. Now, increase the frequency of the signal generator until the voltage measurement increases by the 3 dB (in my case up to 31.1 mV). Write down this frequency. (for the Grace, this was 450 Hz) Disconnect the signal generator and measure the resistance of the coil. Using the formula in the second link, we find the inductance is equal to the XL ( in this case the resistance measured) divided by the quantity ( 2 pi times the frequency measured at the 3 dB point). In my case the Grace inductance was 231 mH.
So now that we know the inductance and the capacitance, we go back to the first link and plug these values into the calculator, and we get the resonant frequency of 33 kHz for the cartridge system, which means the optimum resistor value would be 48k ohm for the 231 mH and 100 pF.
So, after all that, in my case the optimum load setting is no capacitance and use the 47 k ohm switch position on the input, and I should be good to go.
