Biamping: how much do the woofers really need?

MartinLogan Audio Owners Forum

Help Support MartinLogan Audio Owners Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

JohnA

Well-known member
Joined
Jan 8, 2007
Messages
229
Reaction score
2
Location
London
I've been running my SL3s with the Musical fidelity XA-200 monoblocks and they were fine.
Then I decided to biamp them, and I used the XA-100 for the woofers. It delivers 200watts per channel at 4Ohms, and I thought that would be more than enough. After all they are just 10inch drivers with a low-pass filter, how hard could it be, right?

Now I've got another pair of XA-200s and tried them on. I didn't expect much difference really, from a point onwards it doesn't matter how much amp you throw at a speaker, it will still perform the same if everything else remains the same. After all they are just 10inch drivers with a low-pass filter, right?

Well, no.
Even the tonal balance has changed, to the point where now I've had to reconfigure everything around them (the 'helper' speakers I also use for rock, and both subs).
On their own the SL3s are now more 'adequate'. I get better frequency response with the settings at 'flat' now, while the smaller amp worked best at '-3db'.
This is surprising because nothing else has changed.The position/orientation of everything is exactly the same, and the volumes involved in the tests were not particularly high (80db).
Obviously all these amps have exactly the same voltage gain (30db)

So what gives?
I'd understand for the panels to sound different if I one was to swap a crappy little integrated with Krell monoblocks - panels need all the current they can have.
But the woofers?
These are quite a traditional load, and at 4 Ohms 200 watts shouldn't really be much different than 400 watts.
But they are:confused:

Edit: According to the table in this biamping article (scroll down a bit) the power distribution bass/panel is around 40/60% (the SL3 has Xover frequency at 250Hz).
Knowing the additional power demands of the electrostatic panels, I'd be inclined to modify this figure closer to 30%/70%.
This makes it even more strange that more power to the woofers makes such a profound difference.
 
Last edited:
JohnA,

I think what you are experiencing is the difference in GAIN not the difference in POWER between the two amps.

All devices in an audio chain have a level of gain / attenuation. That is, they will either multiply the input signal by some factor, or will attenuate (cut) it by some percentage (up to 100%, which means no signal).

It sounds to me like the two amps you’ve tried have quite different gain structures for a really noticeable change (usually on the order of 2 to 3dB is required for ‘Noticeable’).

I looked at the MF site, and they do not seem to publish specs for gain on each model. Not even in the owner’s manual. But even if the manual stated same gain, the actual units could have a variance.

This is why it’s vitally important when bi-amping to able to individually control gain of each amp, or at a minimum, gain on one of the amps.
 
As for ‘How much power does the woofer really need’ question, my take is: Enough to bottom the woofer driver. :devil:

Which for most woofers in the ML line (8, 10 and 12” single voice coil drivers), my estimate is that anything beyond 200w is enough to either bottom out the driver, or put it in serious thermal jeopardy after a couple of hours.
Once you’ve reached the point of running out of physical travel on the driver, any more power is wasted.

A good 100wpc SS amp will do very nicely in most situations, but might not have the clean, undistorted muscle to push a 12” acoustic suspension aligned driver to its max.

Much of this rests on the Crossover as well, as it can ‘waste’ a good bit of power with overly complex topologies or weird impedance curves.
If just driving the raw woofer (as I do in my Monoliths) 200w will not only bottom out the driver, it will fry the voice coil in no time if not careful.
 
...I think what you are experiencing is the difference in GAIN not the difference in POWER between the two amps....

Jon, they all have 30db gain, precisely to make biamping easy among the range;)
 
Jon, they all have 30db gain, precisely to make biamping easy among the range;)


John, but do they really?

If you are hearing a difference, then there is one. Might want to pull out a volt-meter and test the actual outputs of the amps playing a steady test tone.
 
..Might want to pull out a volt-meter and test the actual outputs of the amps playing a steady test tone.
Hmmm, may I should do this at some point.

Come to think of it, my 'panel' pair of X-A200s is a special one, originally built for the owner of MF who had electrostatics at the time. Each monoblock has got an extra pair of output transistors - at least that's what the chief tech told me when I got them back after accidentally burning an output stage:eek:

Can't ever sell them, I see them like Rachel in Blade Runner:angel: , an experimental one-off replicant with no expiration date

The bottom line is that their gain might be different to the rest - so it has to be measured, yes.

But this doesn't change the fact that the previous amp was an XA-100 and this new pair of X-A200s is standard, hell the XA-100 wouldn't be 3db out of spec, would it? Needs to be measured.:think:
 
Perhaps what you are hearing is just better distortion characteristics from the larger and potentially better-designed amplifier. Do you know if the circuit topologies, parts, and measurements are the same between the two? Watts is certainly not the only thing that matters as to how an amp will perform with a speaker, even just a woofer. Has one amp got a larger power supply, better capacitors, more energy storage, etc. etc.? Whatever the reason, at least now you know that it can and does make a difference. I would be curious what the difference in sound would be from your former setup to switching around and running the XA-100's on the panels and the XA-200's on the woofers. But it sounds like running the 200's on both is working pretty well for you.
 
... Do you know if the circuit topologies, parts, and measurements are the same between the two? ..

They can't be far off because they come from the same era of Musical Fidelity.
The XA-100 cost 2/3 of the price of a XA-200 pair at the time, it's not like one was bottom of the range and the other at the top.

It's clear to me now that the bottom part of the SL3 needs more power than I originally thought.

You know what? A Krell FPB600 went for £1500 the other day on eBay, and I now regret for not going for it.:(
1200watts at 4Ohms and (potentially) 2400 at 2Ohms wouldn't go amiss with the ML black hole of power:p
No need for biamping either, I think that one doubles again at 1Ohm (not sure though)
 

Latest posts

Back
Top