April 2018

Bi-Amp'ing Basics
Article By Jack Sharkey
KEF Direct

  Today, I'm going to talk to you about bi-amplification. I would like to do it in a straight-forward, easy to write, easy to read, fun and exciting manner. That's not possible. But here goes.

First, Let's Clear Up Some Confusion
Bi-wiring and bi-amp'ing are not the same thing. Also, what you probably think of as bi-amp'ing is probably not actual bi-amp'ing.

Multi-amp: All frequencies are run through the pre-amp and amplifier and are presented at the speaker terminal.

Bi-amp: LF and MF/HF (or LF/MF and HF) are split into two signal paths prior to the amplifier before presenting two separate signals (LF and MF/HF or LF/MF and HF) at the specified speaker terminal.

Tri-amp: LF, MF and HF are split into three signal paths prior to the amplifier before presenting three separate signals (LF, MF, and HF) at the specified speaker terminal.

An active subwoofer being sourced by a receiver's sub out is a basic form of bi-amp'ing.

Now that you're less confused, let's move on...

Actual Bi-Amp'ing
Bi-amp'ing (and tri-amp'ing) is pretty much a necessity in the pro-audio world. This is because the amount of power needed to make all of those lip-synching artists sound so loud in your local hockey arena could make your head explode (literally and figuratively).

Without bi- and tri-amp'ing that insane amount of power would be exponentially greater. This is because in multi-amp'ed systems passive crossovers are used and passive crossovers suck up an amazing amount of power and turn it into heat. Couple that with the need for concert audio systems to be somewhat modular to adjust for differences in hall and arena sizes (not to mention outdoor gigs), and the only feasible way to make an act sound decent without shutting down the local power grid is to tri-amp or bi-amp the audio.

This is accomplished through the use of active crossovers and people who know how to set them. Active crossovers are placed between the source and the amplifier and are individually adjusted for the speakers that make up the lows, mids and highs (think about the link between the amplifier and your CD player at home or the digital file reproducing your favorite artist's vocals while she dances and pretends to sing at that concert you just spent a fortune on).

If you are not using an active crossover between the source and the amplifier, you are not actually bi-amp'ing anything, but to make you feel better the audio gods have coined the term "passive bi-amp'ing." To truly bi-amp your home system, you will have to remove the passive crossovers from the speaker cabinets and use a line-level active crossover in the signal chain above the amps. But if you've spent a fortune on a good pair of speakers with well-designed passive crossovers, it seems kind of dumb to pull the crossovers out in order to save a few watts of power or reduce some theoretical intermodulation distortion (IM).

Also, keep this in mind: Unless you are pulling out the passive crossovers and replacing them with a well-tuned and highly stable active crossover network, and then tuning that active crossover properly with the proper test equipment, the power benefits you are gaining will probably not be worth the extra money.

In terms of intermodulation distortion, you are basically replacing one circuit that may or may not cause IM with another circuit that may or may not cause IM. If you've bought a decent pair of speakers with a well-designed passive crossover network you may not realize any noticeable gains, and in fact, unless you know how to set an active crossover, you are more than likely causing more harm to your overall sound quality than if you had just left your system alone.

What About Power?
A passive crossover uses inductors, resistors and capacitors. Capacitors not so much, but inductors and resistors take a lot of the power they receive and turn it into heat before it gets a chance to be turned into sound. This is why systems in commercial venues need to use active (electronic) crossovers to get the job done. So, if you're "bi-amp'ing" your home system but still using the factory installed passive crossovers, the gain in power efficiency you were looking for when you went to bi-amp in the first place will be negated because of all of those energy-to-heat-transferring resistors and inductors. Granted, the MF/HF may be powered with a lower power amp, therefore creating less chance for harmonic distortion, but if you're going to run that low power through a passive network, you're going to need to increase the output current to keep up with your low frequencies.

Of course, then you have to concern yourself with matching the volume of the HF with the volume of the LF so you can listen to your music as it was intended to be listened to. This factor is often overlooked, but to my ears it's a pretty major thing.

Also, if you are not splitting the frequencies before the power amp you are not reducing the stress of frequency-related loading within the amplifier circuit and power supply as the amps are receiving the full frequency range. This means that the amp is producing the full frequency range and presenting it to the speakers regardless of what you want it to do.

Horizontal And Vertical Bi-Amp'ing
In spite of all of this, you say you are still willing to see if a bi-amp'ed system is right for you. Okay. Are you going to horizontally or vertically bi-amp your system? And does it matter?

In a horizontally bi-amp'ed system, one stereo amp powers both (L & R) bass drivers and one stereo amp powers both (L & R) MF/HF drivers.

In a vertically bi-amp'ed system, one stereo amp is used for the MF/HF and LF (L channel) and another stereo amp is used for the MF/HF and LF (R channel).

In both cases, you can think of it in terms of having four separate amplifiers.

Theoretically, a vertical system will be more efficient because the heavier loading of the lower frequencies is split between two amps (and subsequently two power supplies). Meanwhile, some people like the horizontal system because they can use one type of amp that may work better with high frequencies (like a nice low-power tube amp) and one type of amp that works better with low frequencies (like a meatier high-power solid state amp). Unfortunately, like most things in life, this sounds really neat on paper but in the real world there are unintended consequences that may cause you more aural grief than sonic joy.

1) Getting the signal levels of the two different-type amps to match is extremely hard to do without the right measurement equipment. Therefore, you may not be listening to your music with the intended balance between the highs and lows.

2)  It is very difficult to get two different amplifiers with two separate sonic characteristics to play nice with each other in the midrange area (where most of the musical information resides).

So once again, if you are considering bi-amp'ing your system, tread carefully and do as much research as you possibly can before taking the plunge. You may be better off spending your money on a really good, clean, 250 Watts per channel amp, than buying two 125 Watts per channel amps.

There are still benefits to bi-amp'ing (if all of the above is taken into consideration). Those benefits are: 

1) Transient signals are less likely to be present within a frequency range and are therefore less likely to cause amp overloading and clipping.

2) Reduced intermodulation distortion (IM) (frequencies that are not harmonically related and that develop non-harmonic frequencies which translate to the ear as distortion).

3) Reduced interference from CEMF (counter electro-motive force). CEMF can be developed by a hard-working LF driver that theoretically "pushes" electrical energy back to the crossover or MF/HF drivers. Basically, as a driver moves back and forth, it produces a current in the voice coil. During a quiet passage immediately following a loud passage some of this CEMF may potentially interfere with other frequencies.

Caution! Opinion Below
Unless you are willing to spend seriously large amounts of money, and then you are willing to take out the crossovers that were designed for - and installed in - your speakers, and are then willing to take the time to properly set your active crossovers, I'd spend my money on a really good pair of speakers and a really good amplification/pre-amplification system and just go ahead and enjoy the music.

Can you benefit somewhat from even a passively bi-amp'ed system? To an extent, but the return on your investment isn't enough to convince me you shouldn't just spend your money upfront on a decent to really good multi-amp'ed system.

Another thing to consider: Our engineers spend an enormous amount of time and money getting our passive crossovers set precisely for the cabinet volume and driver configuration of our speakers so you don't have to.

There will be some who swear I am dead wrong here, but that's okay. A lot of this comes down to what we think we hear and there is nothing wrong with that, but what you've got now are the technical reasons for going one way or the other.

Take note here that monoblock systems are not bi-amp'ed systems and aren't covered by this article.

The opinions expressed in this article are the author's own and do not necessarily reflect those of KEF or its employees.

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Marlboro NJ, 07746

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