General Guitar FX Design Question

[Paul Marossy]

Is there a certain dB output level that is generally accepted as convention? I'm designing a little tube preamp thing, but I don't really know how to determine just what the maximum volume level should be...

[yano]

Well, most guitars put out much less than 1V (maybe around 500mV), and most amps can handle at least 10V (some correct me if I'm wrong on this) on their input jack.

You can calculate the db of gain your pre amp provides by this equation:

Gain [in dB] = 20 x log10 ( Gain [ as ratio ] )

(that's log base 10)

So for instance if your preamp accepts a 1V signal, and for some value of your level puts out 2V, then the ratio of gain is 2:1...or 2.

So you get
20 * log(2)

which equals ~6db of gain.

Hope that helps.

[csj]

Paul,
Personally, anything less than a 3db gain increment is lost on me. My ears are pretty tired. I was going to work on upping the power of an 18 watter to around something in the mid 20 watt range and when I ran the numbers like Yano just did I figured why bother... I probably wouldn't hear the difference anyway.

[Transmogrifox]

Many pro-sound and audio applications measure levels based on dBu, which is the formula given by Yano based on 0 dBu ~ 792 mV.  I can't remember the exact number, but to get what is considered a "line level" input, keep the signal max around .7 V to 1V amplitude.

If you're using it for audio mixers, computer cards, etc., don't exceed 1V RMS, which for a guitar signal is probably relatively near 1V amplitude as well  (EDIT: Assuming moderately to severely distorted guitar. Clean is probably near 500 to 700 mVrms).  Designing for 750 mV amplitude is a good safe level for most mixers, and certainly enough to deal with in the pre-gain section so that you aren't dealing with a low-level signal and bad Signal/Noise ratio.

If only for guitar and power amps, then you can probably get away with using 4-5 V amplitude.

Here's a helpful link:

http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/dbconvert/dbconvert.html

[Paul Marossy]

Thanks all.  

I think what I meant to say was dBu...

Anyhow, that gives a rough idea of what to shoot for.