Phono Preamp Tube Rush

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

Whether or not you have "tube rush", in my opinion, is much more a function of the topology of the phono circuit than it is a matter of buying super low noise tubes. RAM and Kevin are completely reputable, but any tube will become noisier as it ages, and such aging can occur at different rates for different samples of the same tube type. So even SLN tubes are only SLN on the day they were measured. And one SLN may become only LN, for example, in a matter of several hours of use, while another may remain SLN for a much longer period.
I own both an Atma-sphere MP1 and a Manley Steelhead. The MP1 uses a dual differential cascode input voltage amplifier in the phono section, and the Steelhead uses a hybrid (solid state and tube) version of a cascode. (It’s single-ended.) Neither of these devices ever produces any tube rush at useable settings of the attenuator. The Steelhead, in fact, is silent for practical purposes (meaning you may hear a very faint noise at the sitting position if you are not playing an LP, with the volume control cranked all the way over).
It seems to me that phono stages deriving most of phono gain from the use of 12AX7 tubes at the input might be more likely to exhibit noise at high volume control settings.

Almarg, What formula did you use to calculate the Johnson noise of a phono cartridge? I was prompted by your earlier post to do some reading on it, too, and I would have guessed the noise of a phono cartridge per se would be much lower than what you quote. but I am not sure what are the correct values to plug into any of several equations I found on the internet.
Mijostyn, You can really achieve 110db SPL in your home audio system? I hope you don’t live in an apartment.

Almarg, What formula did you use to calculate the Johnson noise of a phono cartridge?

@lewm

Hi Lew,

I performed that calculation in two different ways, and got the same result both ways:

Method 1: The section of the Wikipedia writeup on Johnson noise that is sub-titled "Noise Voltage and Power" contains an equation for rms noise voltage which makes clear that rms noise voltage over a given bandwidth at a given temperature is proportional to the square root of the product of that bandwidth and the resistance which is responsible for the noise. That section also states as follows:

For a 1 kΩ resistor at room temperature and a 10 kHz bandwidth, the RMS noise voltage is 400 nV.[6]

Footnote 6 indicates that a more exact result is about 403.6 nanovolts, or 0.4036 uV (microvolts).

I then extrapolated from that number to the result corresponding to the 12 ohm resistance of my cartridge, over a 20 kHz bandwidth:

0.4036 x [square root [(12 ohms/1000 ohms) x (20 kHz/10 kHz)]] = 0.0625 uV

0.0625 uV is 78 db less than the 500 uV rating of my cartridge, since:

20 x log(0.0625/500) = -78 db

Method 2: I started with the following paper, although it pertains to microphone amplifiers:

https://www.sounddevices.com/microphone-preamp-noise/

The paper states that:

The noise generated by a 10k ohm resistor (based on the thermal noise formula above) is around 1.8 uV (-114 dBV).

I assumed they were referring to a 20 kHz bandwidth, or at least to something of that order of magnitude. I then extrapolated from the 1.8 uV/10K numbers to the 12 ohm resistance of my cartridge in a manner similar to Method 1 above, and after converting to db relative to the 500 uV rating of my cartridge I got the same 78 db result.

Best regards,
-- Al

Thanks. I can’t argue with your logic, and won’t.I might wonder whether one can model a phono cartridge based solely on its internal resistance, in that calculation.  But I have no better idea.

I might wonder whether one can model a phono cartridge based solely on its internal resistance, in that calculation.

@lewm, that’s a good point. But the inductance of my cartridge is spec’d as 25 uH at 1 kHz. If we assume its inductance is also around 25 uH at 20 kHz, its inductive reactance at 20 kHz would only be about 3 ohms. And it would be progressively less than that at lower frequencies, of course. So it would appear that the cartridge’s impedance is primarily resistive throughout the audible frequency range.

Best regards,
-- Al