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Old 1st July 2004, 10:35 AM   #1
gene57 is offline gene57  United States
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Exclamation Amp NFB network - can be used as crossover for bi-amp ?

I'm trying to use a minimalist approach to design a line level crossover for a bi-amp system.

I need to drive a midrange+tweeter (with traditional passive crossover), line level crossed with a woofer, let say at 400 Hz, 12dB/oct. The 2 amplifier modules are located inside each loudspeaker cabinet.

I will use a simple volume pot (may be followed by a unity gain buffer) as preamp unit, distant about 6 feet from the loudspeakers, and I'm tempted to avoid the additional line stages required for the crossover filters. Using a passive line level crossover could be possible, but I don't like the non optimal impedance load.

So I'm working on the following idea: why don't use the amp itself to implement LP-HP filters ?

One first 400Hz 6dB RC section can be used at the very input of the amplifiers, and a second 400Hz 6dB cut can be implemented into the feedback network. May be also the miller cap of the woofer amp should be increased.

Am I missing something? There are negative effects limiting the bandwith of the amps ? I'm using a classic single input diff + vas + driver + EF topology, class AB, NFB.

Let me have your comments, and thanks in advance.

Gianluca
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Old 1st July 2004, 01:54 PM   #2
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Have been thinking about it myself
Awaiting for a responce !

If i have time i''' start simulating on my amp.

grtz

Simon
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Old 1st July 2004, 02:29 PM   #3
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I have seen this used commercially on at least one occasion, and it should work. Some amplifier designs can get a little unstable when operating with very low closed loop gain (high levels of NFB) which would be the case outside your passband. Suggest you try it and see.

Cheers
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Old 1st July 2004, 02:33 PM   #4
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Quote:
Amp NFB network - can be used as crossover for bi-amp ?
The answer is: It depends !!!


It depends on the amp itself, i.e. if it remains stable when you mess around with it's feedback. But it is basically a good idea and a minimalistic one as well.

Regards

Charles
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Old 1st July 2004, 03:08 PM   #5
sreten is offline sreten  United Kingdom
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Hmmm......

The high pass cascaded 1st order would not be a problem.

The low pass input is not a problem either but the feedback loop
is. You need to increase the compensation for stability into the
lowest gain, for a simple first order filter = unity gain.

This will likely crush the slew rate of the power amplifier,
but with a 400Hz c/o frequency things should be fine.

Note as described these are not 2nd order filters with adjustable Q.



sreten.
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Old 1st July 2004, 04:01 PM   #6
sam9 is offline sam9  United States
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Assuming stability is not an issue, i think the following question should be asked (I and I've no idea what the answer is):

In the bands outside the XO region, which scheme (line level XO preceeding the amp vs. an XO inserted in the feedback loop) produces less noise and non-linearities? I would want to confirm this after the fact with actual measurement.

A lesser but important question would be: Can you sucessfully implement the desired XO slopes when inserted in the FB loop? That is: supposse the XO you desire is a 24dB L-R XO -- is that what you actually get or are actual results different from predicted?

The reason I bring this up, is that I tried vaguely analogous six months ago with a headphone amp and did not like the results. The roll-off of curves were not what I wanted/predicted and THD+N was unacceptably elevated across frequency range. No doubt, someone brighter than I could have made it work - but it was MY project not theirs. I concluded that even if I persevered until it worked, I was going to end up with something that was far lest minimal and elegant than simply keeping the two functions (filter vs. amplification) separate.

On the otherhand, if the amplifier you use resembles a discrete opamp maybe the whole thing is more straight forward -- just a random thought.
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Old 1st July 2004, 04:20 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by sam9
a)Assuming stability is not an issue, i think the following question should be asked (I and I've no idea what the answer is):

b)In the bands outside the XO region, which scheme (line level XO preceeding the amp vs. an XO inserted in the feedback loop) produces less noise and non-linearities? I would want to confirm this after the fact with actual measurement.

c) A lesser but important question would be: Can you sucessfully implement the desired XO slopes when inserted in the FB loop? That is: supposse the XO you desire is a 24dB L-R XO -- is that what you actually get or are actual results different from predicted?

d)The reason I bring this up, is that I tried vaguely analogous six months ago with a headphone amp and did not like the results. The roll-off of curves were not what I wanted/predicted and THD+N was unacceptably elevated across frequency range. No doubt, someone brighter than I could have made it work - but it was MY project not theirs. I concluded that even if I persevered until it worked, I was going to end up with something that was far lest minimal and elegant than simply keeping the two functions (filter vs. amplification) separate.

On the otherhand, if the amplifier you use resembles a discrete opamp maybe the whole thing is more straight forward -- just a random thought.
a) for low pass functions stability is a main issue, and fundamentally
affects the re-compensated amplifiers performance.

b) by definition the XO in the feedback loop should have lower
noise and distortion in the stopband.

c) getting complicated -
you cannot insert a filter function (except 1st order) in the feedback loop.
(Edit : you can insert a 2nd order bandpass or notch filter but
you can't insert anything with higher than 1st order stopbands)

Using feedback topologies you can implement 2nd order functions with a gain
stage and adding a first order filter to the input means the maximum order
you can implement with a single gain stage is 3rd order.
A problem with the 2nd order topologies is gain is restricted.

d) Find this confusing - why would a headphone amplifier need filters ?

sreten.
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Old 1st July 2004, 05:33 PM   #8
sam9 is offline sam9  United States
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") Find this confusing - why would a headphone amplifier need filters "

1- Equalization with regard to a specific headphone

2- Same as above but for sharp anomolies appearently the result of the interaction of specific headphones (even very good ones) and the ear canal. You can find details at the Linkwitz Lab websete.

3 (and most important to me)- Compensation for hearing loss of an elderly relative. I did this quite easily with an LM386, but that hasn't the greatest sonics. (This is non-trivial - my mother can now hear TV and radio again for the first time in 15 years! Sometimes even the best hearing aids don't work well in these situations.) I wanted to so something similar for someone else I know who misses high quality listening.


"b) by definition the XO in the feedback loop should have lower
noise and distortion in the stopband."

Won't argue with the principal, I just like to see such things confirmed with real harware.
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Old 1st July 2004, 05:46 PM   #9
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A basic problem with doing this type of thing in the feedback loop is that the gain can never become less than 1 (100% feedback). Suppose you have an amp with 20dB gain, you can then roll it off say at 20dB per octave for just one octave, then the gain is 1 and stays there. Normally in a xover you want the output to continue to decrease "forever".
All this is of course separate from the mentioned potential stability problems.

Jan Didden
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Old 1st July 2004, 05:55 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by janneman
A basic problem with doing this type of thing in the feedback loop is that the gain can never become less than 1 (100% feedback). Suppose you have an amp with 20dB gain, you can then roll it off say at 20dB per octave for just one octave, then the gain is 1 and stays there. Normally in a xover you want the output to continue to decrease "forever".
All this is of course separate from the mentioned potential stability problems.

Jan Didden
You are quite correct. I forgot to mention shunt
feedback is needed for a proper stopband roll-off.

As you say series feedback stops at 0dB cuttoff.

sreten.
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