Please explain gain

In simple terms what is gain and why is it important to match gain when bi-amping?

Does active bi-amping versus passive bi-amping make a difference?

How do you compensate for different gains?
Gain is a measure of the difference in amplitude ( " volume " ) between the input and the output signal of an amplifier. Not all amps have the same gain, that is, they don't all amplify as much.

When amplifying with two boxes, you want to process the signal the same way with each one. If not, you deform the final output--the highs are boosted with respect to the lows, for example, if your treble amp has more gain than the bass amp.

A competent technician can adjust one amp so that it has the same gain as the other. You have to decide if your electronics' quality makes it worth two expenses here: the modification itself, and the probable loss of resale value in the modded unit(s).
I think you'll find that a lot of line-level crossovers (which is what you need for "active" bi-amping) will have gain controls to provide the adjustments you need. You might also be able to simply put a stepped attenuator in line to adjust gain on one amp.
Gain is a measure of the output signal divided by the input signal. It can be a ratio of output/input voltage, current, or power, depending upon the application.

If the gains are not close when used for bi-amping, then the tweeter and midrange/woofer volume levels will vary relative to each other as the overall volume changes. I have never tried this but I would think it would be somewhat disconcerting to listen to. It should be OK if they are at least close.
I am also interested in bi-amping and found this topic and responses very informative,Thanks.
I appreciate Slv's clear definition of gain. Mine is less precise. Although I tried to simplify for a first-timer, I am not highly knowledgeable technically, and to simplify well, you have to have mastered something sophisticated first.

I think it worth mentioning that you can biamp actively, with an additional powered crossover box between the preamp and the amplifiers, or passively, using the speakers' own crossover. If your speakers are already set up for biwiring, they can be passively biamplified. (And of course, there is a possible upgrade path in going from passive to active, as promoted, most notably, by Linn.)

I would be wary of an active crossover with built-in gain controls. My hesitation has to do with the base line reference for the amplification. There is no way I can see for the crossover to know the amount of gain each amp is applying, so all adjustments have to be done by ear. The result could be good or bad. I would much prefer to know that each amp was producing signals at the same level before going to the crossover.

Also, the additional volume controls make for more signal processing, which is potentially degrading to the sound, as well as extra expense.

I would want to be sure these questions were answered before using an active crossover with gain controls to compensate for two dissimilar amps.
All of my speakers(3 sets) are capable of passive bi-wiring,they do provide HF-LF inputs.For now I will only look into passive.I like to go SS for LF,and TUBED for HF.
I have a Sonic Frontiers Power 2 Tube amp and it doesn't specifically state gain in the specifications. There is a spec for Negative Voltage Feedback that is expressed in dB. Is this the same as gain? Is there a formula for calculating gain?

By the way thanks for the information so far, it has helped me understand things much better.
Gain is measured in dB but it is a difference between input and output for the whole amplifier. Your Negative Voltage Feedback figure is a different animal, a measurement of the voltage in a part of the amplifier's circuitry.

(Negative feedback should have an effect on gain, of course, since it involves taking a part of the output, inverting it and bringing it back to the input to cancel out errors. But this is not the measurement you want.)

An amplifier's total gain should be one of its design specifications. The manufacturer ought to be able to supply the figure. In the case of Sonic Frontiers, which is unfortunately out of business, I would contact Parts Connexion. The owner, Chris Johnson, used to run Sonic Frontiers.

Parts Connexion home page

Got it, thanks