Is equal gain OK for bi-amping?

From a theoretical standpoint, "passive biamping" is really only advantageous when using a pair of identical, direct-coupled solid-state amplifiers that have a low output impedance, especially when they're Class AB or B with conventional unregulated power supplies. In this case, the gains are matched precisely, and separating the high-current demands of the bass can significantly improve the linearity of the mid/high amp. These types of amplifiers also don't exhibit odd side-effects from a high-impedance load for a good chunk of the signal spectrum with which they're driven.

With your particular setup, the Class D bass amp could possibly have an increase in its ultrasonic noise output, as its output filter is load-dependent. And many tube amps will misbehave into a high-impedance load; if this occurs at low frequencies it could be prone to LF oscillation (motorboating) depending on its design.

Not that getting good results with such a setup is impossible, but you will definately want at least one of the amps to have a variable input sensitivity for fine-tuning, and accept that you have many unpredictable "tone-control" interactions going on that will make the performance rather quixotic and unpredictable.

Ngjockey,
IMHO biamping can only be ideal with two identical amps. Other than that I'd go active.

Before we start on amps, what are the speakers? If it's a 2-way, chances are biamping is a waste of time because the crossover is too high to be of any benefit. Even on a 3-way, anything over 300 Hz, is getting uncomfortably into the sensitive midrange. Personally, I wouldn't biamp over 150 Hz unless going active.

There are Endler EVS/Rothwell inline attenuators but they aren't designed for fine tuning as much as coarse volume control and the input impedance can be low for an already divided setup.

If the amps have 2 very different input impedances, say 20K and 60K and they have equal gain, won't the output volume be different also because of the impedance being un-equal?

No.

If the two preamp outputs are not separately buffered, the inputs of the two amps will see the same input voltages at all times, because they will be driven from a point that is the same from an electrical standpoint.

If the two preamp outputs are separately buffered, and both power amps have much higher input impedances than the preamp output impedance (which is the relationship that should be provided anyway, in any preamp-to-power amp interface), the difference in power amp input impedances will not result in a significant difference between the voltages that are input to the two amps. "Much higher" in this context would mean perhaps 20x or more, relative to the worst case (maximum) preamp output impedance across the audible spectrum.

Best regards,
-- Al

Al: very helpful thanks to all here.

One question though: If the amps have 2 very different input impedances, say 20K and 60K and they have equal gain, won't the output volume be different also because of the impedance being un-equal?

Minorl, your posts are always extremely knowledgeable and helpful, but in this case I'm not sure that you are correct regarding the need for sensitivities to match.

As I'm sure you realize, sensitivity is the input voltage that is required to drive the particular amplifier to its maximum rated output power, whatever that may be. I don't see how that relates to achieving a suitable match between amps in a passive bi-amp arrangement.

The goal would be that equal input voltages to the two amps result in equal output voltages. Assuming equal load impedances, that simply requires that the voltage gains of the two amps are equal.

What does concern me, though, is that the output voltage of the ss amp will be essentially constant as a function of load impedance, while the output voltage (and hence the gain) of the tube amp (with a higher output impedance) will not be. Depending on the speaker's impedance vs. frequency characteristics, that could result in a slight but conceivably significant deviation from frequency response flatness, especially if the tube amp's output impedance is high (i.e., its damping factor is low) and the speaker impedance at mid and high frequencies is low.

Also, the OP should keep in mind that in a passive bi-amp arrangement, if there is a large disparity in power capability between the two amps, much of the power capability of the ss amp will not be utilizable, because the volume level that can be utilized will be limited by the clipping point of the lower powered amp.

05-17-11: Koestner
Oops, seem like I forgot input impedance has to be the same also.

As long as the input impedances of both amps are much higher than the output impedance of the preamp, which they should be, that shouldn't matter. If the two outputs of the preamp are not separately buffered (as is usually but not always the case), the combined input impedances of the two amps should be at least 10x the maximum output impedance of the preamp across the audible frequency range. That combined input impedance is equal to the product (multiplication) of the two input impedances, divided by their sum.

Will I really be wasting the power the upper amp makes in bass notes if the speaker's crossover blocks them, doesn't the speakers crossover create a much higher impedance at bass frequencies thereby not alowing the upper amp to waste that energy?

In a passive bi-amp arrangement, the speaker's crossover will dramatically reduce the current demands on the upper frequency amp, but will not change the range of voltages that it needs to output. Its output voltage capability, however, may increase, at least a little bit, as a result of the reduced current demand.

Best regards,
-- Al

Oops, seem like I forgot input impedance has to be the same also. Well that makes it a lot harder to do. Maybe I will get an integrated tube amp and adjust the volume until it sounds right on both top and bottom.

Will I really be wasting the power the upper amp makes in bass notes if the speaker's crossover blocks them, doesn't the speakers crossover create a much higher impedance at bass frequencies thereby not alowing the upper amp to waste that energy?

Hi Koestner, I would second what Minorl said. I am running a Marchand active crossover and bi-amping with a valve amp on top and a SS Class A/B amp on the bottom. The advantage of doing this is that the valve amp does not "see" any bass frequencies, so its apparent power is higher.

The downside of using an active crossover is that the slope of the crossover will sum with the internal passive crossover of your speaker, resulting in a steeper slope and possibly introducing phase issues. Ultimately the best solution is to go fully active - bypass your speaker's internal crossover and connect the amp directly to the drivers with an external active crossover in place.

You can do this upgrade in stages (which is what I did). Start off by bi-amping, then add the active crossover later.

For the output signals of both amps to be the same level, the input sensitivity of both amps must be the same and the gains must match also. Other characteristics also factor into this, but these two are the important factors. If the gains are the same, but the input sensitivity is different, then the output levels will not match for the same signal input. You "May" hear a gap or hole between the upper registers and the lower bass. Borrow an amp from a friend or dealer to use for a week or so before buying one to see if this is something you want to do or if another amp will work. An active crossover works wonders in this circumstance, and they have input matching or gain controls also.

enjoy