Advice needed re bi-amping using stereo amp per channel

[simon g]

paulf-2007 wrote:

There is NO advantage to passive bi-amping unless your singleamplifier is REALLY poor. The LF and HF current draw is separated by the crossover, and given that the amplifier you're using is a voltage source with very low intermodulation, there is no measurable or audible benefit.

There can be real benefit, however, from removing the passive crossover and using an electronic crossover (preferably DSP based) before the power amplifiers. Benefits are:

Much higher output level as each amplifier is handling a limited frequency range and consequently headroom does not need to be reserved for frequencies outside the amplifier's band.

Much better control of the crossover points and slopes than is possible withpassive RLC crossovers.

With a DSP-based crossover, it is much easier to provideeq and room correction, more difficult with an analogue electronic crossover and impossible with passive crossovers.

No loss of power through a passive crossover, although with well designed Xovers, this would in any event be small, but for high-power loudspeakers, not having a passive Xover is worth while.

Better "damping" of back emf by not having the resistance of a passive Xover in the way, but how much this is audible considering that the bulk of the reistance involved is the speech coil resistance is a moot point. Nevertheless the first four points are sufficient to make iactive operation using an electronic Xover well worthwhile.

S.

The above post from the much missed Serge (who definitely knows of what he speaks) is all you need to know on this subject.

 

[mac72]

The above post from the much missed Serge (who definitely knows of what he speaks) is all you need to know on this subject.

Most of above is correct but there is an advantage of passive biamping if you use separate amplifier to drive bass drivers with applied correction only downside is processing delay disturbing crossover frequencies but "standard" passive biamping is just waste of money with no real benefit

 

[MGTOW]

Put simply...

Passive. Pre-amp > power amp > (passive) crossover > speakers.

Active. Pre-amp > (powered) electronic crossover > multiple power amplifier > individual speaker drive units.

It does not matter where any of the components are located, passive crossovers are usually in the speaker enclosure but there are examples where the crossover is outboard, often in it's own box. Similarly the multiple power amplifiers can be outboard, as in a Naim active setup or onboard as with say, ATC. Bear in mind that passive crossovers operate at speaker signal level, electronic crossovers at line level and that passive crossovers are always 'lossy', ie some signal is lost whereas active crossovers are not.

Confusion comes in the hifi world with powered speakers, conventional passive speakers with a power amplifier onboard, sometimes one in each speaker but often a regular stereo power amplifier in one speaker, the master, linking to the second slave speaker via a regular length of speaker cable. Such speakers are 'powered passive', not 'active'.

Marketing language rarely differentiates between powered and active, the only 100% certain way is to count the number of individual amplifier channels each speaker has, a 'two way' speaker must have at least two channels of amplification, a 'three way' will need at least three channels etc. My own active speakers are two way but have two bass drivers, the manufacturer has chosen to use a power amplifier for each drive unit, so three channels of amplification in a two way speaker.

 

[Mr.Ian]

I played about with passive bi amping and passive tri amping, with matched amps. No benefits just a lot more complicated and expensive to wire in.

The area you need to focus on with the passive approach is the pre amp output impedance and the power amp input impedance. Connecting extra power amps to some pre amps will reduce the input impedance seen by the pre amp, which could lead to frequency roll off

 

[MGTOW]

I played about with passive bi amping and passive tri amping, with matched amps. No benefits just a lot more complicated and expensive to wire in.

The area you need to focus on with the passive approach is the pre amp output impedance and the power amp input impedance. Connecting extra power amps to some pre amps will reduce the input impedance seen by the pre amp, which could lead to frequency roll off

Passive bi-amping will change the impedance loading on the output of the power amps too. In a speaker with a proper 'split' crossover each amplifier will only see part of the crossover/speaker, ie the amplifier driving the high frequencies will only 'see' the high pass filter and the HF units, similarly with the bass section.

These 'half crossovers' may offer a different load to that load presented to a single power amplifier by the 'full crossover', so the amplifier may react differently, altering the sound.

Whether this is audible or significant in any way will have to be determined by listening.

 

[mac72]

Passive bi-amping will change the impedance loading on the output of the power amps too. In a speaker with a proper 'split' crossover each amplifier will only see part of the crossover/speaker, ie the amplifier driving the high frequencies will only 'see' the high pass filter and the HF units, similarly with the bass section.

These 'half crossovers' may offer a different load to that load presented to a single power amplifier by the 'full crossover', so the amplifier may react differently, altering the sound.

Whether this is audible or significant in any way will have to be determined by listening.

Such a amplifier would struggle with any "real" life speaker and wouldn't have flat frequency response as a function of impedance vs frequency isn't constant and could vary inbetween 2ohm and above 8ohm hence I wouldn't use one for audio purposes , what you've been possibly experiencing was a minimal or not difference in the output/gain

 

[rabski]

I brought this up earlier. The suggestion is that the reactive load of a speaker's different sections will also affect an amplifier differently. I agree it's theoretically possible, but in real life, either section(s) of any passive speaker/crossover will still be a reactive load with varying impedance, so any difference is likely to be negligeable.

 

[MGTOW]

Such a amplifier would struggle with any "real" life speaker and wouldn't have flat frequency response as a function of impedance vs frequency isn't constant and could vary inbetween 2ohm and above 8ohm hence I wouldn't use one for audio purposes , what you've been possibly experiencing was a minimal or not difference in the output/gain

You are probably correct but it is a bit beyond my competence in electronics. Just exploring the possibility that the change of loading might explain the differences that some users report when trying out bi-amping.

I have tried passive bi-amping on a number of occasions with all kinds of different kit, really never found much in it myself.

 

[mac72]

With passive bi-amping biggest and most audible issue are the crossover points where mismatch would be quite evident creating quite big deeps , only advantage of doing so I can see is ability to treat separately low frequencies

 

[mac72]

I brought this up earlier. The suggestion is that the reactive load of a speaker's different sections will also affect an amplifier differently. I agree it's theoretically possible, but in real life, either section(s) of any passive speaker/crossover will still be a reactive load with varying impedance, so any difference is likely to be negligeable.

Very likely when it comes to tube amplifiers where load impedance is being reflected back to the transformer primary

Edit: I should have said "possible" but it would be sort of negligible , first I'd start with some proper measurements to see what happens ,

 

[DomT]

Why would they introduce it? The cynical reply is that it has the potential to sell more stuff. You'll find plenty of threads with the usual 'night and day improvement' sort of comments, so people want to try it. The related answer is that as people are potentially going to try it, it makes sense to offer the correct connections to try and stop people wiring it the wrong way and breaking things.

Sometimes more power overall is an improvement, because it gives more 'headroom'. However, common sense suggests that is only going to be advantageous if the original setup is somewhat marginal in terms of power requirements. The bass driver(s) in any cone speaker are going to require a shed-load more power than the treble. It gets a little more complex, in that the impedance curve means the load differs in some ways, but nevertheless, only a small fraction of the total power to drive a speaker is going to the treble units in a conventional cone-speaker setup. There are potential related benefits due to splitting the reactive load and its impact on an amplifier, but the extent of that is questionable.

The sensible advice is obviously to beg or borrow a second amp to try it, unless you aleady have two. And then to switch back after a while. Overall, if there is any improvement at all (and an improvement that is still apparent when you swap back to the original setup), the big question remains... You're going to have the cost of two amplifiers. How much more of an improvement would there be if you spent that total on a single, better one?

Thanks Rab. In my very specific situation there are hardly any ARC SS amps for sale and many have unobtainium transformers. Fortunately mine isn’t one of them. I love the sound of my ARCs with ATCs but the ATC boys say POWER in a Jeremy Clarkson accent and so I am/was curious.

 

[kneecap]

In the early noughties, horizontally bi amped a pair of Ruark Icons with Arcam Alpha 9/9p and it was definitely worth it.