When I engage the PH12 phono board, however, there is a considerable increase in noise. I have always thought this was normal since every phono amp I've ever heard had this same sort of rushing noise, but I thought it would make for interesting discussion.
I figure the best way to discuss noise would be to show some measurements rather than try to describe it.
Below are two frequency spectrum measurements captured with REW software and a calibrated mic. I think this set was captured with the mic close to the woofer of one speaker as I was focusing on hum at the time. The input signal is full spectrum pink noise. Don't let the big numbers scare you, these are actually very quiet. As I mentioned, the line stage alone is almost dead silent to the ear. But these plots show the spectral content of the little bit of noise that does exist.
I don't have an absolute reference level to give meaning to the dB scale shown, so they are really only useful in a relative sense.
The first shows the SP13 line stage alone with my VTA ST70. In this configuration the aux input is selected and the volume is set to 0dB. The 0dB position was determined beforehand as the volume knob position at which the bench measured output signal is exactly the same voltage as the input signal. Physically, this happens to be very close to the 12 O'clock position of the volume control in my unit, which is a nice design (Thanks Roy!)
As can be seen, the noise is dominated by 60Hz and all its harmonics, i.e.: 120, 180, 240, 300, (360 missing), 420... These are all clearly evident. But as I mentioned, you really can't hear it much.
The second plot slows what happens when the PH12 phono board is engaged. The volume control is still at the 0dB point. The noise is now significantly louder in the speakers.
The third plot shows an overlay of the first two plots for comparison. You can see that with the PH12 added the noise floor has increased roughly 7-10dB in the frequency range from 70Hz to 500Hz. I am just wondering if this is normal. I'm guessing this is actually pretty good performance considering we just added 40dB more gain to the mix. Still, 10x more noise definitely moves it into the realm of easily audible. Is this just a fact of life for all phono stages?
Clearly the spikes are due to 60HZ power line noise leaking into the circuit, and that could possibly be improved but I have not had any success so far. (Any suggestions welcome!) But the residual noise below this (thermal?) is also increasing by the same amount in this frequency band. I believe this is the "rush" component.
In both cases the inputs (Aux and Phono) were grounded at the RCA jacks on the back of the preamp during testing.
Overlay of both SP12 alone and SP12+PH12