Phono Preamp Tube Rush

Yes Ralph you are correct usually j fets I think and I do believe it is to reduce noise. My next phono amp Is going to be balanced. Can't wait to try that. I would like just a little more gain for ultra low output cartridges and I believe from what I have read that a balance design should have lower noise levels. Cartridges are balanced to begin with.

It's also possible that some phono amps "throttle down" gain with no signal present. That would give a false reading on the ear to speaker test.

noromance"It's also possible that some phono amps "throttle down" gain with no signal present"

That is ridiculous there is no such phono preamplifier in existence that is intended for Music Reproduction Systems.

I would like just a little more gain for ultra low output cartridges and I believe from what I have read that a balance design should have lower noise levels. Cartridges are balanced to begin with.

The reduced noise can be significant. For a given gain stage, if executed in the balanced domain you get a maximum of 6dB less noise. Now if you have two gain stages or more this adds up.

daveyf 11-11-2019
Anyone here actually own a tube phono stage that is so quiet that when you place your ear right up against the speaker and the phono stage is active...and at some mild gain, but with no music playing ( or even at no gain) they hear absolutely no tube noise/rush ( dead silence)...I doubt it.

I’m doubtful that is even theoretically possible with typical combinations of system gains, sensitivities, and volume control settings, even if a solid state phono stage is used.

Out of curiosity I did some quick calculations of the Johnson noise that would be produced by the resistance of my phono cartridge itself. As I mentioned earlier I use an AT-ART9, which is rated to produce 0.5 mv under the standard test conditions and has a specified resistance of 12 ohms.

I calculated that at room temperature and over a 20 kHz bandwidth the 12 ohm resistance would result in Johnson noise that is about 78 db below the cartridge’s rated output. If we assume for example that the volume control is set such that the cartridge’s rated output results in an SPL of 90 db at the listening position (which I think is a reasonable ballpark assumption), it would mean that the cartridge itself would be producing 12 db of noise at the listening position, within that 20 kHz bandwidth. That would almost certainly be inaudible at the listening position, of course, even when a record is not being played, due to the combination of RIAA equalization, A-weighting, ambient noise in the room, etc. But I would by no means assume that to be the case if the listener’s ear is placed up against the speaker, even if the phono stage, cabling, and the rest of the system add zero noise. My understanding is that an SPL of about 0 db is considered to be the nominal threshold of audibility, and 12 db at the listening position would correspond to a much higher volume right next to the speaker.

Regards,
-- Al