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Old 1st January 2011, 02:22 AM   #1
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Default Bi amplification

I have a few questions about this. First, can it be done successfully with 2 different stereo pairs of amps which have different gains? How would you set the gains properly so that the level of the tweeter is exactly matched with the bass?

And is there any reason why the amps should all be the same except the gain issue?

Is it invariably true that having the amp drive one drive unit (as opposed to the whole speaker) is an easier load?
I have heard arguments supporting this claim but I don't see why it is obvious.

thanks.
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Old 1st January 2011, 03:11 AM   #2
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Old 1st January 2011, 03:32 AM   #3
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so is there any benefit to the practice of driving the woofer with a separate amp to the tweeter? I dont mean directly driving the drive units, but i mean still driving them through the passive crossover?
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Old 1st January 2011, 03:51 AM   #4
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Hi Professor,

Here is a good place to read (Elliott Sound Products): about the power distribution of the amplifiers: BiAmp (Bi-Amplification - Not Quite Magic, But Close) - Part 1

I don't think this will directly address your question about the gain issue, but you may find the crossover is making compensation for different db ratings of the drivers already.

Can you adjust the gain of your amplifiers internally with switches/resistors?
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Old 1st January 2011, 11:30 AM   #5
AndrewT is offline AndrewT  Scotland
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Default Sorry to all for this long-winded reply

Hi,
let's start at the beginning.
We have a 4 terminal 2way speaker with internal passive crossover.
The manufacturer has split the crossover into two sections: Low Pass and High Pass.

We use one amplifier to drive this speaker.
Short the two sets of speakers terminals together. Run one pair of Flow and Return wires to the terminals.

The music comes out of the speakers.
The terminals see exactly the same voltages.

Now remove the shorts. Run two pair of Flow and Returns from one amplifier to the two sets of speaker terminals.
Music plays because the speaker terminals are receiving the same voltages, much as before. But not identical ! The circuit around either pair of cables carries different currents to the high and low pass halves of the crossover. These individual currents are different from the combined currents that flowed to and from the shorted speaker terminals.
Those different currents and thus different voltage drops will result in a tiny difference in voltages at the speaker terminals even though the voltages at the amplifier terminals is identical.
That explains part of the philosophy behind Bi-wiring .

Now replace the one amplifier with a two channel amplifier. connect one channel to the high pass speaker cables and connect the other channel to the low pass speaker cables.
Remember when in bi-wire mode the amplifier ends of the speaker cables had identical voltages applied to them.
In Bi-amplifying identical voltages must be applied to the two sets of cables for the equivalent voltages to appear at the speakers and come out as music. The gains of the two channels of the amplifier must be identical to ensure the applied voltages are identical.

In the Bi-wired mode the currents in the cables were different even though the applied voltage was the same.
It is similar in the Bi-amplified version. The output current of the amp feeding the low pass half is different from the output current of the high pass half.
This difference in current even though the applied voltages are the same is due to the different impedance of the two halves of the crossover.
In band the filters (HI & LO) have a nominal speaker impedance. out of band they rise in impedance sometimes to very high levels of impedance when more than a couple of octaves away from the in band signal.
The increased impedance of the out of band combined with the nominal impedance in band is a reduced loading on the amplifier. That reduced loading usually makes for better amplifier performance for the in band frequencies that the speakers need to reproduce.

But we are not finished yet.
Some amplifiers are good over the whole frequency range and maybe particularly good over a narrow range of frequencies.
If amplifiers were not identical and instead one chooses amplifiers that are particularly good over the frequency range required by the half of the speaker then maybe we could get improved overall performance.
Let's take a simple example.
All AC coupled amplifiers have a DC blocking capacitor somewhere at the input. A wideband amplifier must use a large capacitor value to allow the bass frequencies to pass as easily as the higher frequencies. If the amplifier is only to be used to feed the high pass half of the crossover, then the input capacitor can be much smaller, instead of a cheap MKT and/or electrolytic, a really high performance teflon & silver foil could be substituted. Would that allow the Treble to sound better? Maybe.
Finally two amplifiers located right beside the speaker terminals with 500mm of speaker cables will perform better than one amplifier located in another part of the room and connected with 6m of complex impedance (capacitive and inductive and resistive) cables.

Sumarising:
The amplifiers must have the same gain.
The amplifiers do not have to be identical.
The amplifiers could be selected on the basis of performance in the frequency band required of them.
Ability to use very short cables.

I think you can gather that I favour passive Bi-amping. It is simple, avoids the complexity of going to active speakers and produces good value results.
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Old 1st January 2011, 12:14 PM   #6
kvholio is offline kvholio  Netherlands
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Quote:
Originally Posted by Professor smith View Post
so is there any benefit to the practice of driving the woofer with a separate amp to the tweeter? I dont mean directly driving the drive units, but i mean still driving them through the passive crossover?
Having recently obtained a Lecson Quattra, i can only say Yes.
It's an integrated amp specifically designed for passive bi-amping,
having 4 separate power-amps inside.
Definition and clarity improve significantly compared to using just one amplifier per speaker.
The difference in performance can't be from reduced load on the powersupply
in the case of the Lecson, since it has a shared supply that feeds all four amps.

Happy New Year,

Klaas
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Old 1st January 2011, 02:21 PM   #7
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kvholio, what do you think the improvement is due to if not from reduced load?

Andrew, so what youyre saying is that biamping is usually better than not, but not quite as good as total biamping with active crossovers before the amps?

What are the gains though and are they significant enough to bother buying another amp to do this?
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Old 1st January 2011, 02:40 PM   #8
Bigun is offline Bigun  Canada
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It's helpful to know the cross-over frequency for your speaker. If, for example, you have a set of PMC DB1's the cross-over, I believe, is around 2kHz. This means that the vast bulk of the power (80% or so) is consumed by the bass-midrange driver. For the tweeter you could then consider the practicality of using a less powerful amplifier, even Class A.
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Old 1st January 2011, 04:42 PM   #9
kvholio is offline kvholio  Netherlands
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Quote:
Originally Posted by Professor smith View Post
kvholio, what do you think the improvement is due to if not from reduced load?

Andrew, so what youyre saying is that biamping is usually better than not, but not quite as good as total biamping with active crossovers before the amps?

What are the gains though and are they significant enough to bother buying another amp to do this?
Professor Smith, please note that i wrote "no reduced load on the powersupply", the load on the amplifier-circuitry will be reduced.
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Old 1st January 2011, 04:58 PM   #10
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Quote:
Originally Posted by Professor smith View Post
Andrew, so what youyre saying is that biamping is usually better than not, but not quite as good as total biamping with active crossovers before the amps?
Can't speak for Andrew but I think that's a fair assumption.

Quote:
What are the gains though and are they significant enough to bother buying another amp to do this?
If you are buying another amp buy the XO also. They're no more expensive than a good passive XO and much more user friendly.
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