measuring gain

[amonte]

I see a lot of booster schematics which claim to raise the level by "x"-dB.  Is there a way to measure this?  Also, is there a certain level of db that is not "safe" to run in front of an amp?

[R.G.]

I see a lot of booster schematics which claim to raise the level by "x"-dB. Is there a way to measure this?

The db is a unit of gain. It is a logarithmic expression of gain, which matches the human ear's logarithmic response to increasing loudness.

Gain is output divided by input, so something with a gain of 10 has an output that is ten times larger than the input.  The "bel" is the logarithm to base ten of the gain. That is, a gain of ten is ten to the first power, so a gain of ten is expressed in "bel"'s as 1 bel.  A gain of 100 is 10 to the second power, which would be 2 bels.

The bel is a large unit, as you see. It was more useful to express things in tenths of a bel. So a gain of 10 is 1 bel, but 10 deci-bel.

Strictly speaking, the bel is a unit of *power* gain. If you refer only to voltage gain, the figure is twice as high, or 20decibels per gain of 10. Strictly again, the input and output impedances must match, but hardly anyone follows the rules, only looking at the sizes of the voltage.

This works out to: 20db is a gain of ten, 40 is a gain of 100, 60 is a gain of 1000, and so on. 6db is a doubling of the voltage, gain of two.

DB's add where the gains are multiplied. A gain of 20 is 26db (x10 and x2).

To get db gain,  measure the arithmetic gain of a circuit as output voltage divided by input voltage. Take the common logarithm, and multiply by 20. If you're measuring an actual device, take care that the output is not clipped, as this would cause an artificially low apparent gain.

Also, is there a certain level of db that is not "safe" to run in front of an amp?

Depends.  Unsafe gain is a little like saying "Danger 1 million ohms!"

However, you obviously can't put 120Vac into the input of an amp without at least thinking through the results.

Amplifier inputs are designed to take a certain input range - each one a different range, but there is a limit to each one. If you over drive the amp input, it distorts. There is usually a range of overdrive that is between the onset of distortion, but  below the start of damage. It depends on how big the input signal is, not the gain that produced it.

That being said, the "typical" guitar signal is about 10mv to 100mv.  Tube amplifier inputs usually don't distortion at the **input** until the signal is over a volt or so. They're also pretty tough.  It's not necessarily good for a 12AX7 to put +/-50V on its input, but I'm not sure it would damage it in the short term.  Solid state amps are not nearly so robust, and will be damaged at such large voltages unless they have protection circuits on their inputs - which many of them do.

I guess the right way of looking at it is how big is the biggest signal the overdrive unit  can produce? If it's powered by a 9V battery, it usually can't make over 4.5V of signal, maybe even up to 9V peak. This level will be non-damaging for almost any amp.  I don't know of any overdriver unit that has a power supply big enough to put damaging voltage out to a following amp, but that doesn't say that some starry-eyed kid hasn't built one with a +/- 100V power supply that could damage an amp.

All in all, I wouldn't worry about it. Use all the gain you want as long as the power supply is a 9V battery or a 20V or less wall wart.

R.G.

[amonte]

RG, thanks so much for the informative post!  I have some questions, but I want to read it a few times and "digest" it a bit.

Thanks again.