Bi-amping with Chipamps

[Gopher]

I have bi-wirable speakers and I was thinking about bi-amping them with chipamps:

Bass/mid - LM3886, possibly bridged, with high capacitance regulated power supply.

Treble - LM1875 or LM3875, not bridged, low capacitance unregulated supply.

From what I've read, hi-cap regulated supplies have a good bass but the treble suffers, whereas the low cap (1000-2200uF) unregulated amp is best for the treble but not good for bass. My thinking is this arrangement allows me to get the best out of the chipamps for the bass and treble ranges.

Any thoughts? Preferences for the treble amp - LM1875 or LM3875 remembering that it will probably only require a few watts? Would I need to run the bass/mid and treble amps at the same voltage gain?

[sreten]

Hi,

No you do not need to run the same gain, using a lower gain
on the treble with a higher sensitivity tweeter is very typical.

Note that by treating your amps as large op-amps you can
implement the active filter functions around them. For unity
gain filter circuits, you use the - input point as your unity
gain point rather than the amplifiers output.

Also note active filters do not make designing a speaker any
easier, especially if you want to correctly implement lower
order c/o acoustic filter functions for good transient response.

All the modelling procedures for passive design
are applicable to active speaker design - see
FRD Consortium tools guide

/Sreten.

[Gopher]

Since they're bi-amped, I'll have to split the source cable to feed both amps. If I don't run them at the same gain won't the treble be reproduced at a different level compared to a single amp feeding both the bass and treble speaker drivers?

[tangmonster]

What drivers are you going to use? do you have the speakers specs?

[sreten]

Originally Posted by Gopher

Since they're bi-amped, I'll have to split the source cable to feed both amps. If I don't run them at the same gain won't the treble be reproduced at a different level compared to a single amp feeding both the bass and treble speaker drivers?

Hi,

Generally no. Many speakers include an atennuator in the c/o for the
tweeter. FWIW if you cannot design a passive speaker you cannot
design an active one. In that case go for bi-amping a normal
speaker and here you do need the same gain in both amps.

Its a good idea for the treble amp in biamping to massively reduce
the input capacitor so the bass cut-off is around 500Hz or so.

/Sreten.

[sangram]

You have to gain-match the amps if the sensitivity of the speakers is the same. Since this is unusual, the tweeter amp is usually run at a lower gain (about 3dB, depending on tweeter sensitivity difference).

If you do run the amps at the exact same gain, you will need an L-pad on the tweeter just like a passive crossover. Kind of defeats the purpose. I would still keep a large cap (~20uF) on the tweeter for DC protection, but resistors would not be so great.

[AndrewT]

if you plan to bi-amp an existing bi-wireable speaker then both sets of terminals expect to see exactly the same voltages.

The amplifiers driving those terminals must be capable of driving those terminals to the same maximum voltage, if both drivers can take similar transient peaks without damage.

The gain of the two amplifiers must be the same to allow the above to happen.

In designing the amplifiers for this duty you and the other contributors have made many very sensible suggestions.

1.) both amplifiers run from the same voltage of mains transformer.
2.) both amplifiers have the same gain.
3.) each amplifier can be optimised for the passband they must deliver.

Find out if both drivers after the passive crossover are of similar impedance.
This will determine the respective current outputs of the two, now dissimilar, amplifiers.

I would suggest that the bass/mid amplifier be optimised for the bass/mid driver impedance and use a high capacity smoothing bank and a very low frequency passband down to maybe 1Hz to 4Hz F-3dB. The RF filter could be dropped from 160kHz down to 25kHz.
The treble amp could be designed along the lines of a Peter Daniel implementation, but in addition the LF passband can be raised to 1/10 of the treble's LF turnover frequency, maybe around 250Hz for a 2500Hz crossover and keep the RF filter up around 160kHz.

Further, I would use a dual secondary transformer with dual rectifiers. An duplicate the dual rectifiers for the two channels.
That will keep the best isolation between the Audio Grounds and allow you to decide where, exactly, you need to join them.

[Gopher]

Thanks Andrew - that confirms what I thought. Maybe I've not made myself clear to the others. The speakers are two-way Wharfedale Diamond 9.1. The crossover is inside the speaker but they have separate binding posts for bass/mid and treble.

I want to run each driver with a separate amp, so I expect the voltage gain of each amp to be the same.

I will have a passive low pass filter on the bass amp to limit the treble frequencies it has to reproduce and a high pass filter on the treble amp to limit bass frequencies.

[AndrewT]

Originally Posted by Gopher

I will have a passive low pass filter on the bass amp to limit the treble frequencies it has to reproduce and a high pass filter on the treble amp to limit bass frequencies.

The crossover inside the Diamond already does that.
You must not interfere with the passive crossover roll-off characteristics.
The drivers and crossover have been designed to work together to produce the Diamond sound.

If you add electronic filtering in front of the Power Amplifiers then the slopes of the active filter must be far enough away from the passive slopes to not allow the two filters to interfere.

That is why my example showed a 10:1 ratio of filter turn over frequencies.
This is probably the reason that Sreten advised a 500Hz limit for the treble amplifier.

BTW,
your posts were clear enough.
It's different usage across the pond that causes all these problems.
We consider passive bi-amping as "bi-amping".

The US considers Bi-amping as applying only to active speakers. They assume one could never want to use multiple amplifiers on a multi driver passive crossover speaker set up.

[Gopher]

Andrew, nothing's ever simple is it? Could I just feed each pair of terminals with a conventional amp that isn't bandwidth limited in the way you suggest and let the crossover do the filtering

You say tomato I say tomatoe eh?