Convert cartridge output voltage to db gain
My cartridge is low enough, about .35mV that to get it to sound the same as the CD player the volume has to be turned up. Paul Marks, the phono preamp creator said it was a matter of the gain of the phono preamp combined with the cartridge output that made it necessary. A slightly higher one, .65mV was mentioned, would obviate that necessity. So some part of it is the gain issue, but not the noise issue. |
Actually, the equation is: db = 20Log10(Av), where Av is the voltage gain (Vout/Vin). Vin is the input signal (your cartridge output of 0.24mv) to the phono stage. Ultimately you want a line level of 0.5 to 1.0v from the phono stage's output to match the same same level from the CD Player. If you are aiming for 0.5v output from the phono stage, Av = (0.5v / .24mv) = 2083.3 Plug this into the "db = " equation and you have a db value of 66db, needed by the phono stage to achieve that 0.5v signal into the line stage. |
So, by that calculation, he has adequate gain in the phono stage despite his protestations. As I said, it is the noise level, not the gain that needs to be addressed. Since he is turning up the gain (reducing attenuation) in the in the integrated amp to attain the same (subjective) output with the same phono stage, I suspect the input stage on the new integrated. Kal |
Ok, thank you for the responses. That does explain the volume difference to some extent. According to John's calculations, my phono stage has an output of 0.5V, while my CDP has an output of 2.5V. So, I'm guessing (since I can't find my old sceintific calculator) that I would need a cartridge with about a 2.0 mV output with 66 db of gain to make a closer match to the 2.5 V output of my CDP. Is this correct? Or as Kal suggest, I should try changing the preamp tubes in the integrated amp? (1 12AX7, 1 12AU7) Cheers, John |
A cartridge with 2.0mv output will need a higher input impedance than your .24mv cartridge, typically something from 10K-47K ohms. Can your phono stage provide that *without* switching to the MM inputs and reducing the gain you're trying to increase? Even if you can get a suitable impedance without losing gain, I'm not sure I'd replace the cartridge. The higher noise floor is in the new line stage, not the phono stage or cartridge, so address that. Your old (seperate) line stage had an acceptable noise floor with phono, the new (integrated) line stage does not. It's not really a gain issue, as already discussed. Any decent line stage should play with a similar noise floor at both 10 o'clock and 2 o'clock. Those are not extreme settings. It's worth trying different tubes. Which will work best depends on the circuit of course, but FWIW the best new production JJ 12AX7 and 12AU7 both have a low noise floor in my preamp, and they won't break the bank. If that doesn't help, the quality of the line stage circuit and/or the power supplies in this new integrated begin to become suspect. You haven't described just what this higher noise floor sounds like. That might help... |
