Biamping with tubes and SS: Input sensitivity key?

Eldartford, sorry to be dense, but is this done with the amps idling or doesn't it matter? Does voltage change with resitance, eg if one is feeding a tweeter and the other a high excursion woofer, while measuring? Sorry, but pretend you're writing a munual for a technical newbie:)

Bob - adjusting the amps by ear is the same as EQ.

Springbok - Matching the gain doesn't help if the output power levels are different. There are three unknowns, so two knowns are necessary to figure it out: gain and output power, or, gain and input sensitivity. Gain is dependant on the type of output device whereas sensitivity and power are dependent on the number of output devices (roughly). These are two different mechanisms and both have to be taken into account. For example, there are 5W amps with high gain and 200W amps with low gain.

By the way, if you still want to measure output voltages, you would need to measure while playing a test tone back at moderately high levels to make sure the voltage potential is high enough for your meter to give you accurate measurements. Measure right at the speaker binding posts, putting black on black and red on red. To calculate the voltage difference in dB, you need to take the two readings (one for each amp), divide the larger by the smaller, take the log base 10, and multiply by 20.

My comments above applied to horizontal biamping only. In this case, the 800Hz is an issue because if you want an amp to power the bass and one to do the highs, they need to have a similar style/voicing since this crossover point is effectively not only bass.... Bass becomes directional around 100Hz so by 200Hz, you start to enter midrange frequencies. But by 800Hz, you will have the lower mids produced by the bass amp and the rest of the midrange produced by an entirely different amp. This may be an audible issue. If you want an amp to power the bass, it needs to only power the bass - thus <200Hz or so. This effectively decouples the voicing issue since a bass driver can't sound like a midrange driver. But in your case, your bass driver does both. That's the problem.

However, if you vertically biamp (which is what I recommended to you and still do), all these problems go away. You use one amp to power each speaker which will increase headroom considerably. The advantage of vertical over horizontal is that each amp sees an impedance that is much higher since you aren't paralleling bass drivers anymore and the impedance in the highs is generally high and the drivers are smaller and lighter so less power is needed for them. Each amp sees a "good" impedance and a "bad" one instead of two bad ones on one amp. This will make a huge difference in terms of lightening the load on your amps. I think you would like this vertical arrangement - and it bypasses all the issues and intricacies involved with horizontal mode.

Arthur

Arthur,
Done deal. You have sold me. Vertical biamping with 2 identical amps. If the TAD-60s dont cut it, I'll look for 2 more pwerful tube amps.
Thanks again, all, for your help.

I'm ovbviously coming to this late but it's an issue also of interest to me. I have a BAT VK 500 that I am exploring using in a biamp mode with a suitable BAT tube amp. After asking BAT directly, I was told by them (as others have pointed out here) that GAIN matching rather than input sensitivity was the key to making this work for a horizontal biamp set-up. The other point raised was to consider matching the general power output of both, that is, going for a powerful tube amp to match the powerful VK 500.

BAT also mentioned that inline attenuators could be employed if there was any perceived inbalance in the sound but I was told that this was probably not necessary using two BAT amps.