# Bi-Amping matching amps

#### bulbous_blues

Hi I haven't used your forum before, but have read several posts. I hope this is a good place to ask this. I want to horizontally bi-amp a class AB amp (for highs) and class D amp (for lows). I understand that both amps must have matching gain and input sensitivity. The AB amp has a fixed 20db 3.2v input and the D amp has 28.6db 1.43v input. The class D has a variable gain so I can match the 20db, is there a way of working out what the input sensitivity will change to when lowering the gain down to 20db. If it makes a difference both amps are balanced and into a load of 8ohms, the AB is 130 watts per channel into 8ohms and the D is 185 watts per channel into 8ohms.

Please don't get to technical with me lol I'm more a dabbler than a technician.

Many Thanks

#### rayma

If you want to reduce the D amp by 8.6dB from its normal 1.43 volts, then the formula is:
( where V = new sensitivity in volts)

8.6dB = 20 x log ( V /1.43 )

so ( V / 1.43 ) = 10 ^ ( 8.6 /20 ) = 2.69

and V = 1.43 x 2.69 = 3.85 volts

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#### bulbous_blues

Awesome ty Rayma, thats very helpful. So do you think a mismatch of 3.2v and 3.85v would be unusable? How forgiving if at all.

#### steveu

The only time you would want to match the gains between bass and mid + high is if you keep the mid/tweeter speaker attenuators that a typical full range speaker box crossover uses. IE waste most of the high frequency amp power and headroom. The natural sensitivity of high frequency drivers is always much higher than that of a woofer, so you need more gain and power for the woofer.

But many things are not defined here, so you need to try and see what need correction.

#### bulbous_blues

HI thanks for your comment Steve. I will match the gain best I can and give it a whirl. I'm not expecting great results its just for fun, but you never know. Ive heard some people had success. I will post results, maybe someone will find it useful or interesting.

#### rayma

Awesome ty Rayma, thats very helpful. So do you think a mismatch of 3.2v and 3.85v would be unusable? How forgiving if at all.

You just won't be able to clip the D amp with the 3.2 volts, but that won't be a problem.
It's only 1.6dB difference.

#### jan.didden

Paid Member
Your matching is assuming that both the low and high speakers have exactly the same sensitivity.
Do they?

Jan

#### krivium

Paid Member
I'm not expecting great results its just for fun, but you never know. Ive heard some people had success.

Sure it is a successful way to approach things: 99% of 'powered' loudspeakers works that way!
As pointed you did not specify sensitivity of drivers used ( which will have much more importance than your power amp matching imo) but you did not specify either what kind of filtering you'll use too: dsp, line level, passive ( personally i don't see the point using passive with multiamping but ymmv)?

The kind of filtering will affect results too.

Some interesting article:

https://sound-au.com/bi-amp.htm
https://sound-au.com/bi-amp2.htm

#### bulbous_blues

OK well I'm just going to be passive bi-amping. I totally understand this is not the way to go for best results, unless vertically bi-amping with matching amps. Its just that I will have 3 amps in the house with different characteristics and two of them have variable gain. I'm not laying out money as if this is a project, its just an experiment. They are not separate speakers for high and low. Just speakers that are bi-wirable. I knew if I popped onto diyaudio someone would know how to work out the input sensitivity for me as that's u guys bread and butter.

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