[mfsoa]

I have some questions regarding the matching of amps or amp levels in a biamp configuration. Please let me know if my assumptions go wrong-

I have two amps with the following mfgr specs:
VAC PA100/100: 100 watts/8 ohms; 0.775V/full power, tube amp w/ taps for 2, 4 and 8 ohm speakers.
Sunfire Signature: 625 watts/8 ohms, 1250/4; 2.5V/full power.

So I calculate that for a 1V input, (in theory) the VAC puts out 133 watts and the Sunfire makes 250 watts into 8 ohms. The Sunfire would make 500 into 4 ohms. So on paper the Sunfire would be the more powerful of the two amps and would be the louder of the two amps.

But, when I hook a voltmeter up the the amp terminals and play track 21 of the SPhile test CD (-20 db , 1K tone I think)(same preamp volume setting, speaker cables disconnected) I get:
VAC L channel, 8 ohm tap = 2.836V  R= 2.834V  w/ 4ohm tap= 1.998V  2 ohm= 1.413
Sunfire R channel (voltage tap) = 1.877V   L= 1.844V

Why is the V lower on the Sunfire? Does this voltmeter test even tell me anything meaningful? I'd assume the Sunfire would have higher voltage all things being equal.

When I listen to this setup I hear just a bit too much bass (Sunfire is used <= 200 hz, speakers are rated 4 ohm in this range) but overall the balance isn't too far off.

I think the ear is the only way I'll really be able to match the levels if I biamp. There are just so many variables (2, 4 and 8 ohm taps on the VAC; Sunfire doubling into 4 ohms; speakers rated at 8 ohms above 200 hz where the VAC is doing the work and 4 ohms below 200; Voltage measurements that seem to contradict the mfgr sensitivity ratings)

It doesn't seem possible to do this in a reliable way on paper 

Thanks

-Mike

[pbrstreetgang]

Its because you are using the same pre amp volume setting- hence the lower input sensitivity of the VAC is going to be louder than using the same pre amp volume setting to the less sensitive Sunfire.

[mfsoa]

But if I put 0.25 V from a preamp into the VAC, I calculate that I should get 0.25V x (100W/0.775V) = 32 watts.

0.25 V into the Sunfire should give me 0.25V x (625W/2.5V) = 63 watts. (8 ohms)

I'm still confused as to why the measured voltage on the VAC was higher than on the Sunfire.

-Mike

[SET Man]

Hey!

   Mike, to bi-amp with two very difference amps. In this type of system a separate volume will be needed for each amp.

   I have to admit that I don't have exprience with this type of system. But I think that trying to calculate or matching it with numbers won't get you the absolute answer. Keep in mind that each amp will also react differently to the speaker driver that connected to it.

  I would say that your best bet is if the bass level is too high put a volume on the Sunfire of which I assumed that you driving the bass section with. This is the easy one. Use your ears to fine tune the sound until you feel that is good to you. Than use SPL meter to take some measure and see if it somewhat confirm what  you are hearing.

  Good luck and let me know what you ended up doing to get the system to sound right.

Take care,
Buddy

[pbrstreetgang]

But if I put 0.25 V from a preamp into the VAC, I calculate that I should get 0.25V x (100W/0.775V) = 32 watts.

0.25 V into the Sunfire should give me 0.25V x (625W/2.5V) = 63 watts. (8 ohms)

I'm still confused as to why the measured voltage on the VAC was higher than on the Sunfire.

-Mike

the input impedences into the amps are also the same?

[JoshK]

Mike,

You are close but you are getting yourself a little mixed up.  The easiest way to match amp levels when vertically biamping like this is to put some reference voltage into one amp hooked up to the drivers you want to drive and measure the SPL with a mic, then do the same for the second amp into its drivers and measure its SPL.  You don't even have to know the absolute SPL, the comparison of the two is good enough to figure how much extra attenuation on one amp (the more sensitive one, in your case the tube amp) in order to match levels.

In your case, it gets confusing because you have many moving parts.  It sounds like the tube amp is driving 8 ohm drivers and the Sunfire driving 4ohms.  So the Sunfire is going to put out twice the watts it would otherwise put into 8 ohm drivers for the same voltage input.

Your VAC has an input sensitivity of .775V and the Sunfire has 2.5V.  This means the VAC is the more sensitive amp.  It will take much less voltage input for the VAC to reach full output. 

Off the top of my head your algebra looks off...

But if I put 0.25 V from a preamp into the VAC, I calculate that I should get 0.25V x (100W/0.775V) = 32 watts.

0.25 V into the Sunfire should give me 0.25V x (625W/2.5V) = 63 watts. (8 ohms)

I'd have to look it up to be sure, since I tend to mix SPL formulas up, but I don't think wattage scales linearly with voltage, I think it is with the square of voltage



But then to complicate matter more, the sensitivity of your drivers are different in all likelihood. So for each watt, the drivers make different levels of SPL.  Then we get into baffle step, where the drivers playing at a lower frequency will have less SPL on axis below the baffle step because the waves are radiating into a sphere instead of half a sphere (forward only).  You can do the math for all of this but it isn't as simple as you were first attempting.

Again, this all is a lot simpler to measure the input voltage into the amp and output SPL from the drivers you want to drive with that amp.  Compare the two SPLs and make adjustment to the input voltage (or attenuate the louder amp). 

[jeffreybehr]

Mike, this is easier than you're trying to make it.  The easy way to do it is to calculate the Voltage gain of each amp, expressed in dB.  The standard way to do that is to use power into 8 Ohms, so the VAC is driving 28.3VAC into 8 Ohms = 100 Watts while the Sunfire is driving 70.7VAC into 8 Ohms = 625 Watts.  The VAC has 31.2 "dB" of gain while the SF has 29 "dB" of gain.  (I use quotes around dB because this is technically an incorrect use of the term deciBel*, but it's still useful in this case).  So IF--IF--IF your speaker's 2 'ends' are the same sensitivity and the 2 amps are driven with the same Voltage and there are no gain controls inserted into this mess, the bass level will be a couple dB shy of the midrange level.  This is easily solved by adding a gain control on the higher-gain amp, the VAC.

Voltage gain is the key.  BTW, you'll lose 3dB of Voltage gain by using the VAC's 4-Ohms taps.  BINGO--problem solved without gain controls!

Try it; you might love it.

* since a deciBel is a measurement of POWER, not Voltage, ratios

[mfsoa]

OK, I see that it's not linear, thanks.

-Mike

[blownrx7]

OK,
I get how to attenuate regular RCA connections.  How do I attenuate an amp that has only balanced (XLR) connections?
TIA