Biamping with tubes and SS: Input sensitivity key?

When biamping a low sensitivity speaker (85db), 4 0hm impedance, with a 800 Hz crossover with a TAD-60 (input sensitivity given as 1 volt or more) and, eg, a Belles 150A reference, with an input sensitivity of 1.4 Volts, and a volume control on the TAD, will it be necessary to get an attenuator on the inputs of the SS amp to equate the 2, since adjusting the output of the TAD will still make it less than the higher powered amp? Is 60 WPC tube power enough to drive the major energies above 800 Hz, which is where mid-range and highs predominate? Is this correct or is there another way of attenuating the input of the higher powered amp? (Not interested in playing with additional or active crossovers. Only in "simple" remedies". Or does someone know of a comparable quality amp with an input sensitivity of 1 volt? I have the TAD-60 and am looking for a suitable SS amp with at least 150WPC with sound similar to the Belles. Unless you all tell me that the TAD-60 is not powerful enough to provide power above 800 Hz to such an insufficient system,then I will look further.
TAD-60 (input sensitivity given as 1 volt or more)
Well, which is it: 1v or more?

It;s not a matter of attenuating the higher power -- for now, just focus on getting the input sens that same for both amps. Or one will output more energy than the other, given the same signal.

You could do this via the xover -- but I wouldn;t recommend it.
You could do this via the volume control -- but only one point will be correct; not recommended.

Strongly recommended: KIS
For all you know, maybe the amps have identical input sens??
Sorry, Gregm, just established from Paul that the Input sensitivity of the TAD-60 is 1.25 volts.
>>KIS<< was supposed to have a question mark. What is KIS?
With volume control on the TAD, there should be no problem in matching the amps. Put the SS amp on the bass and the tube amp on the highs, adjust the volume for a balanced sound (use familiar music to do this) and voila, you should be done.

Check out my system page - I do the same thing with my JM Lab Electra 926 and I love it. My tube amp has never sounded better!

The TAD will have plenty of power for 800Hz on up. No problem. My 40Wpc tube amp can rock the house and it powers 200Hz on up. As for a good SS amp for the low end, I would highly recommend a McIntosh. It should mate well with the sound of the TAD. The MC7100 is a cheap bargain and has amazing bass with two-settings for input sensitivity - might be a great feature for you. If you can afford more, get a MC162. These are my recommendations.

However, 800Hz is a very high crossover for technically-accurate horizontal biamping. But you might get lucky and have good quality sound anyway. Try it and see.

Keep It Simple
The usual is "KISS". KIS, Stupid.
It's input sensitivity and gain of the amp that determine the output level
KIS: I just wanted to keep out the "stupid"
Interestingly, when you look on the websites of amp manufacturers, it is rare to find all of sensitivity, gain and input impedance. I'm sure a phone call to each of the ones of interest will provide this, though. Thanks for your input.
Isn't input sensitivity a measure of input volts needed to attain rated output? In that case, it is NOT the measurement needed for matching two amps for biamping, unless both amps have the same rated output. I think GAIN is the more relevant measurement.

It is a good idea to have, on the amp with higher gain, a volume control. That way you have precise control over the balance between the amps (in-line attenuators can be used if the "wrong" amp has the higher gain).
The best way is to compare input sensitivity at the same output level. You can usually calculate all the relevant data from the little bit that is given in the specs.

KISS is correct! The inefficient speakers still sound better driven with one amp of low power than biamping with a SS "D" amp. Coherence and fluidity is lost. I will try biamping with 2 TAD-60s - I've just found a used second one. Will biamp vertically - one amp to one speaker, identical tubes. If the bass is not tight enough, will use a TAD-1000 for LFs. At least they are the same amps, just more power. But then they would be biamped horizontally.
thanks for input. Numbers dont translate into coherence, so it still is probabaly correct that 2 identical lower powered amps will sound better than trying to match sensitivity, impedance and gain.
If you use an electronic xover for biamping. the issue of input sensitivities of the amps will not be that important since the levels can be adjusted via the xover.
Yes, Larryi is correct. Input sensitivity doesn't help. You want to match gain between the two amps. Regardless of their specs, you should provide both amps the same test tone and measure the output voltage of each. If they differ by more than say 0.5dB, you should add an attenuator.
I'm not a techie - exactly how do I measure the output voltage of each?
You'll need a voltmeter (digital true RMS would be best) and a reasonable test tone (like -20dBFS sine wave at 1KHz). Play some music to set the volume to a comfortable level. Then play the test tone on repeat. You'll have both amps wired from the preamp and connected to your speakers. Simply check the voltage at the "low" speaker binding posts and at the "mid/high" speaker binding posts. I would check both speakers just to verify channel matching. You are sending the exact same voltage to the input of each amp, you want the exact same voltage being sent to the speakers. Otherwise, you'll hear an imbalance.
Well, I'll tell you what. You can measure all you want to but that isn't the right way to do this. I do exactly what you seek and have messed with several combinations and have tried every way myself. This is assuming you are still going to try some horizontal biamping which it sounds like you might from your last post....

If you look at a typical frequency response of a system in a room, the bass can be very lumpy - like 5 or even 10dB humps and dips. Making sure the amps' outputs are matched to 0.5dB won't help when your response is many times worse. Besides, there are several ways you can calculate all this but it isn't clear that will help you either.

You will have to use your ears and you will need some volume control on one of the amps. This is the only way - trust me, I have been there and done that. This way you can find the right balance between bass and the rest for your room.

But you have more fundamental issues that need to be addressed first. First, your speakers are not the right candidates. It will be just too hard to find amps that will work with an 800Hz crossover point. IMO, but you are welcome to try your heart out!

Second, your speaker's crossover and sensitivity leads me to believe they are 2-way speakers. If so, horizontal or even vertical biamping won't give you as much benefit as it would with larger 3-ways.

I think your best bet is to find one nice amp you like and stick to that. If you recycle the money for the two amps into one very nice one, I bet you will be much better results far easier, provided the rest of your system is also up to par.

If you want to match the gains of two amps this mean that there is no difference. So the logical thing is to measure the difference. This would be the voltage between the hot output terminals of the two amps. If one amp has a gain control you can achieve a very accurate match simply by adjusting the gain until the meter reads zero. If you want to measure what the gain difference is using the (small)differential measurement along with the (large) output voltage measurement will be more accurate than comparing two large output measurements.
Arthur, with all due respect, trying to compensate for room modes by adjusting gain by-ear in a bi-amped system is simply ridiculous.

You make the effort to match gain to take the electronics out of the equation. Now, treat the room, apply EQ, adjust speaker placement, adjust listening position, etc.
It will be just too hard to find amps that will work with an 800Hz crossover point.

Will you explain this in some detail? I'm obviously missing something. Thanks.
Eldartford, that's an excellent suggestion -- measuring the voltage difference. Thanks.
Aball, I hear what you say. I certainly couldn't make the sound coherent using the gain control on the lower powered tube amp, which I suppose is not surprising. But does your reservation still apply to vertically biamping with 2 identical tube amps, which I now have, although one is being repaired? The speaker manufacturer strongly advise biamping because of the inefficiency (85 db) and the unique driver dispersion and energy requirements. They suggest "2 medium-powered amps, or bi-wired with one high-powered amplifier". I have listened with a powerful D amp bi-wired and one tube amp bi-wired and far, far prefer the tube sound. If you believe the vertical arrangement with identical amps wont work, please explain why. If they don't work, I'll probably look at the Moscode Hybrid 401, which I haven't heard, but all the reviews are pretty stellar.
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.

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.
Arthur, thanks for the reply.

Your comment --
Bob - adjusting the amps by ear is the same as EQ.
-- is true in the sense that you are cutting, but you are applying it across the entire band handled by one amp. I don't see that as a very useful EQ.

It seems to me that we are both trying to accomplish the same thing - having a level matched output from the woofer and tweeter. I'm suggesting voltage and a meter at the speaker inputs and you're suggesting sound pressure and ears in the room.

Springbok - Matching the gain doesn't help if the output power levels are different.

Unless I misunderstood, that's not true. The only thing regarding power is that either amp not be driven to clip. If 50W is enough for the tweeter and 100W is enough for the woofer, that's OK as long as the gain is the same. Active speakers are designed this way.

Your point about the 800Hz crossover point is a good one. Thanks for the clarification.

It's true that vertical biamping has some pluses as you point out. Another one is better channel separation, shorter speaker cables. The negative is that you lose the capability of matching an amp to a driver.
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.