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In Reply to: RE: fool's bi-amp ? posted by Presto on February 23, 2012 at 15:28:41
It is foolish as long as you leave the passive xovers series inductor in the circuit as it makes a complete mockery of the bass amps DF (read: control) and passives still produce around 3dB or more of insertion loss.
There is nothing stopping you from using any type of amp fully active and since active xovers usually have gain control it is a lot easier to match amps. No L-pads needed anywhere.
Also a good active xover is a fair bit cheaper than a passive using boutique components.
In my case the saving was easily enough to pay for the extra amp(s).
Follow Ups:
I am well aware of the benefits of active crossovers (and typically use them exclusively). But this is not a "from scratch" DIY effort. This is an existing passive design with a specific sound, and specific voicing. Even the impedance related 'errors' are part of the final voice of the speaker. Since the final sound is not really 'erroneous', then these errors are actually part of the equation.
Let's say you have 4th order LR (electric) passive crossovers. You replace them with 4th order active. Guess what, the sonics just changed because the impedance of each driver in the passive network was not perfectly flat, which interacted with the crossover impedance, resulting in a very specific transfer function. So when people speak of "fools biamping", one could argue just as fast that an arbitrary acoustic response from an 'active replacement' is also foolish, if the intent was to preserve the original sonics of the speaker in question.
You can get the added benefits of active, but the "changeout" is far far more complex than saying "Okay - replace LR2 passive with LR2 active, there, I fixed it."
If I could offer a customer identical acoustic response with a given crossover, I would consider calling the effort a "passive to active conversion". But if the sonics change, it's really a new speaker at that point and not an improved version of the original.
Building from scratch is a whole other kettle of fish.
Cheers,
Presto
It is however similar to me converting my Tannoys (Little Red Monitors) to active operation.
The original low pass was 12dB/oct electrical, while the tweeter was an odd combination of 6 and 12dB/oct which created the needed cd horn boost and also included a notch filter.
This was replaced by a very standard active L-R 24 and a parametric eq for the boost and notch.
The result still clearly sounds like a Tannoy pepperpot DC but with a remarkably cleaner, tighter bass and a slightly smoother treble. Obviously the tweeter needed de-inverting.
I also got them to within +-2dB over their entire range.
The one main difference is that without the passive xover between it and the amp the woofers power handling went down from 100w to 50w (the driver itself was clearly marked as such internally while the 100w figure came from the tech manual which states power handling as 125Wrms, 100Wrms for woofer and 25Wrms for the tweeter part).
A friend of mine activated his later 215DMT (officially the crossover is an 'overdamped second order low frequency, first order high frequency', same as mine really) and Tannoys own engineers recommended fourth order L-R but that was some time after I did mine.
I didn't bother asking Tannoy, I just went with my previous experiences of turning passives to active. Usually it is a good idea to use steeper slopes active than passive.
None of the advantages of active/multi-amping are being questioned though. What I am talking about is a couple assumptions that are being made when suggesting the active conversion is the way to go:
1. Guys like the OP are willing to do a passive to active conversion at all.
2. Guys like the OP have the skillset required. (i.e. +/- 2db).
Some people are fanatical about the voicing of their "brand name" speakers and they want the voicing that the original designed intended. Some guys claim they can hear +/- 0.1 db. Or +/- 0.5 db. (And some can't hear a reverse null... go figure).
The point is, I would not call a bi-amper a "fool" (fools bi-amping) if his intent was to use dedicated amps for low and highpass sections. In fact, I would call him ingenious if his high-pass amp was lower in gain (i.e. single-ended tube stuff) so he removed a simple L-pad out of the original circuit to compensate and not waste amp power on heating up resistors for a gain drop he no longer needs.
All that said, I think it's possible to do a passive to active conversion that is very close. With a DIY effort, BOTH final products are DIY, so there is no "carved in stone response". But with a "precious" high end design, the final response is a signature - once you muck with it, they're no longer "that speaker" but really just a speaker with all of the same physical parts, sans crossover.
SO! (gasp for air). Did you measure impulse response with your Tannoys?
I like the idea of concentrics very much, which is probably for many of the same reasons I became a MTM and WMTMW fan.
Cheers,
Presto
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