How does gain work on a phono pre

Easy Runnin

I kind of understand Gain versus Volume on a guitar amp. Gain is how loud the input of the guitar is and impacts the tone. Whereas volume is the output of the amp and impacts loudness. If I turn up the gain too much I get distortion, if I turn up the volume the distortion get louder.

I guess it is the same thing with a phono pre. The gain controls the input from the cartridge. Too much gain leads to distortion. That distortion is passed to the preamp or receiver which has a volume control to make it louder. But what about things like sound stage, bass response and sibilance? Do these all deteriorate or suffer from to too much gain?

[Deleted User]

I believe what you are adjusting is the input sensitivity rather than the gain. Think of gain as a magnification factor: 1 volt comes in and 10 volts go out and usually is determined by the audio circuit's design. On the other hand if the input sensitivity were set too low then a spike in the music would overload the preamp and cause distortion. This distortion would be magnified by the rest of the circuit and would be louder or softer depending upon the volume setting.

jdjohn

A phono pre-amp does two things: 1) adds gain to the signal from the cartridge, and 2) de-emphasizes the RIAA equalization curve that was applied when recording/cutting (literally) the record.

The amount of gain applied should correspond to the type of cartridge being used. A moving magnet (MM) cartridge typically puts out around 4mV of signal, and normal amounts of gain would be 40-45db to boost it up to line-level. Moving coil (MC) cartridges (low-output versions) put out around 0.4mV, and so gain of about 60db is needed to boost it up. These are all general figures just as an example.

De-emphasis of the RIAA equalization curve (a standard for recording on vinyl medium) also has to be applied, or it just won't sound right. The phonostage equalizes the slope of the RIAA curve. Gain alone on a phono signal won't cut it.

mhardy6647

voltage gain works like this:

gain (dB) = 20 log (Vout/Vin)

Suppose one has a phono cartridge with nominal output* of 2.5 mV which one wishes to raise to 1 V (1000 mV) to feed to a line level input:

The gain needed = 20 * log (1000/2.5) = 20 * log (400) = 20 * 2.602 = 52 dB

That's how gain works.

The sensitivity and the dynamic range of the phono preamplifier are also important, because the signal isn't constant. Music is dynamic (more or less dynamic, depending on the music and the recording) so the dynamic range of the phono preamp has to accommodate the range of the input signal fed to it. This is why, at least in the good old days, the overload level of a phono preamp was also specified.

* The "nominal" output of a phono cartridge is (should be!) based on a specific condition of the recorded signal (groove!) that is being played.
Shure, for example, spec'd their phono cartridge output in millivolts RMS for a reproduced 1 lkHz signal cut at a (peak) velocity of 5 cm/sec


(from 2000 Shure catalog, via www.vinylengine.com)

A phono preamp should have an overload level of, say, better than a couple of hundred millivolts. This isn't really that much dynamic range, if you think about it -- a preamp with an overload level of 250 mV and a sensitivity of 2.5 mV would represent a dynamic range of 40 dB. "Digital audio" fans would scoff.

It's not really quite that bad, though, as the phono preamp should be able to linearly amplify an input signal well under the "sensitivity" spec -- but of course, at lower levels, signal to noise ratios will be worse than the "nominal" S/N.
In real life, of course the frequencies and levels change

The OTHER thing, of course, complicating the performance of a phono preamp, as stated above, is the EQ aspect.
To allow for the vagaries of trying to "compress" 3 decades of music frequencies (20 to 20k Hz) into a little wiggly groove, an equalization curve is used. Most rekkids that most normal people will encounter use a curve (standard) called "RIAA" (yes, that RIAA) -- and it is pretty extreme. The bass frequencies are diminished relative to the treble frequencies, with no alteration of level at 1 kHz upon cutting a record. An equal and opposite curve has to be applied during playback to result in a net "flat" response.

All of that is baked into the phono preamp specs.

Here are the RIAA record and playback curves

soundfreak1

As eluded too above, the gain you need is determined by the cart. you use. If you ever want to go to LOMC cart. get all the gain you Can afford ( allows more flexibility in choises). A just as important is the loading option the diff. pre''s offer.