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May 24, 2019, 12:20:48 pm

Author Topic: Phono Stage Gain  (Read 706 times)

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Offline robomaster

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Phono Stage Gain
« on: June 22, 2018, 11:25:52 am »
Here is an article I found while browsing the internet.

http://www.theabsolutesound.com/articles/taking-the-guesswork-out-of-phonostage-gain-1/

It concerns phono gain vs cartridge output and interaction with a preamp line stage. It states that the output of a phono stage should ideally be about 1 Volt. There is a formula given to determine this value. For an example a cartridge output would have to be about 3.2 mv to drive the Eros (50 dB gain) to a one volt level output. The article also states that a typical digital device such as a CD player has about 2 volts output. A cartridge with 7 mv output (Rega Elys 2) would produce almost 2 Volts output through the Eros. Why is 2 volts too high for a phono stage output but OK for a CD player output? They both go to the same line level inputs. I guess preamp input overload comes into play here as well. Any thoughts on this?

Offline Paul Birkeland

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Re: Phono Stage Gain
« Reply #1 on: June 22, 2018, 11:57:41 am »
Why is 2 volts too high for a phono stage output but OK for a CD player output?
The article seems to suggest that 1V is more of a minimum.


 I guess preamp input overload comes into play here as well. Any thoughts on this?
Most preamps (but not all of them) have the attenuator at the input, so you could send 10V of signal into the preamp and just turn the level control down.

If, however, you have a 10mV moving magnet cartridge and you try to use it with the Eros, the EF86 gain stage will overdrive the 6922 gain stage and you'll end up with some undesirable performance problems.
Paul "PB" Birkeland

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Offline Doc B.

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Re: Phono Stage Gain
« Reply #2 on: June 22, 2018, 11:58:53 am »
"We've determined we need 1V output because that way we can set our volume knob at about 2 o'clock."

Ahh, the voltage-time-knob proportionality theorem, developed I believe by Jack Daniels and Dan K Kush. This superseded the commonly adopted "this one goes to 11" convention of the 80s.

Back in the day the completely arbitrary output value was only 0.5V. Obviously this has been adjusted for cosmic inflation.

Surely they meant "putting the guesswork back into phonostage gain".

I'll be here the whole week. Tip your waitress. Try the veal. You've been a great audience, goodnight everybody.
« Last Edit: June 22, 2018, 12:02:21 pm by Doc B. »
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Offline Paul Birkeland

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Re: Phono Stage Gain
« Reply #3 on: June 22, 2018, 12:01:48 pm »
Ahh, the voltage-time-knob proportionality theorem
This theorem is only true if you ignore speaker sensitivity though, right?
Paul "PB" Birkeland

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Offline Paul Joppa

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Re: Phono Stage Gain
« Reply #4 on: June 22, 2018, 01:54:05 pm »
Here's the technical deal. "It's complicated," as they say on facebook ...

In the old days (i.e. before digital) recording studios, radio stations, etc. used VU meters. Those meters showed the signal level averaged over something like 300 milliseconds. This reflected the perceived sound level fairly well. The instantaneous peak voltage, however, would be much larger - on good recordings of unamplified instrumental music, the instantaneous peak signal is about 14dB greater than the peak VU measurement. This difference was called "headroom." For tape and vinyl, the 0VU reference level was chosen to allow that headroom, and used to specify sensitivity for amplifiers etc. It was assumed that the device would handle the headroom gracefully.

Digital recording, however, has very hard limiting, and the standard level is the clipping level - in other words, there is zero headroom. Sometimes you will see dB(FS) indicating full scale. The 2vRMS specification for CD players is not really comparable to the 5cm/sec vinyl signal specification. If you look up measurements of cartridge overload, you will see that most cartridges will handle 25cm/sec signals, at least in the midrange, and some as much as 50cm/sec. That's a headroom of 14 to 20dB.

The bottom line is that a a phono preamp with 40dB gain (a factor of 100) will amplify a 5mV signal to 500mV, with instantaneous peaks of 2.5v. A standard CD, specified at 2.0v, cannot put out more than 2.0v.

It gets more complicated, especially when you get into highly compressed pop music, which may have as little as 1.5dB headroom. But I don't want to go there ...
Paul Joppa

Offline robomaster

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Re: Phono Stage Gain
« Reply #5 on: June 29, 2018, 02:29:50 pm »
Thanks for the info. That clears things up a bit for me. The 2mv spec of a digital output is a maximum output value. I get a louder output from my Eros phono section and cartridge than my CD and that explains why. This was not always the case with other setups I have had. Now I know why. I am wondering also what is the maximum output of a cartridge that should be used with the Eros? The highest I have seen is the Rega Elys 2 with 7.2 mv. I am outputting about half that now with the cartridge I have.
« Last Edit: June 29, 2018, 02:33:39 pm by robomaster »

Offline Paul Birkeland

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Re: Phono Stage Gain
« Reply #6 on: June 29, 2018, 03:55:15 pm »
I've used a Rega Super Elys (6.8mV) with the Eros circuit.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man