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Old 22nd November 2002, 02:06 PM   #1
Jeff R is offline Jeff R  United States
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Default Use Power Amp Itself as Active Cross Over

I am just getting back into audio design from not being active since the mid 80's, when I used to read Audio Amateur, Speaker Builder, Audio and even the Journel of the AES. Back then, active crossovers were just coming into fashion, and I still have (somewhere) one of the first articles in SB about designing active crossovers using op amps.

Here are the basics of active crossovers as I understand it in the case of, say a 2 way speaker. The output of the preamp is directed to two active filters, one a low pass for the woofer, and one a high pass for the midrange/tweeter. The filters can be fabricated from op amps or discrete components. The outputs of the each filter drives seperate power amps, which then individually drive the speakers.

My question - why do we need to use external active filters if we are biamping? As we know, a power amplifier, at least the modern SS amplifiers such as the Leach Amp 3 I use, are nothing more than high power op amps and, as such, we can set them up to be high pass and low pass filters just by changing the feeback. So, if we are biamping with a pre-active crossover stage, why not eliminate the crossover stage and incorporate the desired filtering into the feedback path of the power amp?

If we use the power amp itself as a crossover filter, we eliminate extra wiring, connectors, power supply, noise, etc. Anyone ever try this?

Jeff
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Old 22nd November 2002, 02:26 PM   #2
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Default Good Question.....

There are plenty of old designs of integrated amplifiers with the tone controls incorporated into the feedback loop - Panasonic etc... .
I never liked the highs out of most of these amplifiers(smashy and hard sounding), so that may be your answer.
As a technique for a lows amplifier, this ought to be the go, but for highs maybe a problem with a high gain, high NFB power amplifier stage.
Correct load compensation across each driver would very likely help.
It is worth a try with modern power amp ICs, which are pretty much ideal power opamps, and reasonably economical - TDA1875 (gainclone) etc...

Eric.
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Old 22nd November 2002, 02:29 PM   #3
tiroth is offline tiroth  United States
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Building the active cross into the amp is definitely the way to go, but sometimes doing this as its own stage is preferable to modifying the amp. Most amps can be configured as HP or LP through a variety of means that depend on the topology. It isn't as simple as using an opamp though, because most amplifiers are NOT operational amplifiers (although some are).

You can be clever and use things like the input coupling cap (if it exists), or rely on changing the feedback, or miller capacitance, but basically I think you need to have a very good idea of how the amp you are modifying works, because there are tradeoffs to making these changes. Some may affect stability, especially going from resistive feedback to something that will decrease your phase margin.

Building a discrete xover into the front-end means you gain the benefit of less cables and no ground loops to chase, and you also don't have to worry about affecting the operation of the amp. It does add some extra circuitry to the signal path, but I would evaluate that in the context of how easily modified the amp itself is.
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Old 22nd November 2002, 06:46 PM   #4
Jeff R is offline Jeff R  United States
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I think the amp (i.e. the Leach Amp) would work great as a low pass filter for the woofer, as it is indeed one big old op amp, but as I think about it more, I am concerned about its use as a high pass filter. The gain bandwidth product of the amp is a lot lower than for low power op amps (integrated or discrete), so the amount of feedback in this low feedback design may be insufficient to implement a good high pass filter for a cross over. I have posed this question to Dr. Leach and I will let you all know his response if he replies. I may run some of my own calculations, too. Even so, eliminating the low pass active cross over is a good thing, even if the high pass cross over still has to be discrete.

Thanks for the comments thus far.

Jeff
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Old 22nd November 2002, 07:04 PM   #5
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This is a very good approach if you are willing to go to
the effort. Non-inverting power amps with a fixed gain
make excellent active elements in a filter up to 3 poles
in my experience. You have to make the input impedance
of the amp part of the model, and you have to raise the
impedance of feedback elements in the filter to compensate
for the high gain of the amplifier. I suggest simulating it
using a standard filter circuit but with the gain of the op
amp in the simulation set as high as the power amp.

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