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Old 3rd February 2009, 09:16 AM   #1
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Question Attenuator BEFORE or AFTER active crossover?

Is there any benefit of having the volume control potentiometer after an active crossover?

My thought: Usually active crossovers are fed by a preamplifier, so the signal levels of source devices are attenuated if the volume control isnīt cranked up to stop collar. In that case the active crossover handles very low signals and SNR is not ideal.

Feeding an active crossover with full signal level would increase SNR. Only drawback is that a multi-ganged potentiometer (4-ganged for 2-way stereo, 6-ganged for 3-way stereo etc.) and equivalent number of output buffers are needed.

Do I suppose right?
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Old 3rd February 2009, 09:57 AM   #2
Eva is offline Eva  Spain
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Yes, but in practice it's not that bad to have the volume control before the active crossover.
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Old 3rd February 2009, 02:12 PM   #3
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I think so too in general, but how bad is 'not that bad'?
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Old 3rd February 2009, 04:32 PM   #4
AndrewT is offline AndrewT  Scotland
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I used a dual track volume pot before the pair of two way active crossovers.
I was surprised at the increase in noise level at the speakers.
I redid all the R & Cs to lower impedances, but found only a small reduction in noise.
I have since reduced the gain of those power amps and the noise improved yet again but still not satisfactory.
The active crossover was all unity gain using opa275.
Unacceptable in my view.
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Old 3rd February 2009, 05:50 PM   #5
adason is offline adason  United States
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Good question SmellOfPoo,
I am using active 2way crossover to split the signal for two subwoofers and the satelites. Since I am using two different amps, solid state for subwoofers and tube for satelites, volume control on each amp after the crossover is a neccasity, since the amps have different gain and speakers different sensitivity.

I played around with volumes on amps turned lower and master volume higher up ...vs one amp all the way up, the other one lower to compensate, with master volume much lower....well, I did not found much difference in noise performance overal, since my crossover is just two buffers and passive split.

I belive the difference will be audible if only a poor active crossover is used, like for car or something.
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Old 3rd February 2009, 11:45 PM   #6
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Quote:
Originally posted by SmellOfPoo
Is there any benefit of having the volume control potentiometer after an active crossover?
In PA/Pro crossovers, its common to have one at the input and one at the output.

The Input is meant to adjust your gain. You would want to adjust the signal running through the crossover to be as "hot" as possible without clipping.

At the Output, it is usually a Line driving amp. It can be Unity or with modest gain. This will allow the user to control the volume out to the amplifiers.

If your amplifiers have gain controls at the inputs, you may want to do away with the output volume at the crossover. Most Hi-fi amps do not, so there's no other way but to have volume controls at the outputs of the crossovers.


Quote:
Only drawback is that a multi-ganged potentiometer (4-ganged for 2-way stereo, 6-ganged for 3-way stereo etc.) and equivalent number of output buffers are needed.
Not sure I'm following you here. You only need 2-ganged pots in a stereo crossover. Doesn't matter whether its 2,3 or 4-way. Every output is 2-ganged. You don't control overall volume at the crossover. It's done either at the mixer or your Pre-amp.

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Old 3rd February 2009, 11:59 PM   #7
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Quote:
Originally posted by Andrew T
I used a dual track volume pot before the pair of two way active crossovers. I was surprised at the increase in noise level at the speakers. I redid all the R & Cs to lower impedances, but found only a small reduction in noise. I have since reduced the gain of those power amps and the noise improved yet again but still not satisfactory. The active crossover was all unity gain using opa275. Unacceptable in my view.
Hi Andrew

That's a bit strange. It should be very quiet if they are at unity gain.

Have you tried running the Preamp direct into the crossover and transfer the volume pots to the outputs (passive) followed by a Buffer?

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Old 4th February 2009, 08:33 AM   #8
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Maybe I should sketch my proposal: Iīm about building a sort of 'all in one monster receiver' because I dont want to have a rack full with stacks of cases any longer. Tuner, DAC, 2 3-way active crossovers and 6 mono amps combined in one enclosure, each with separate transformer and PSU. Yes, itīs gonna be huge, heavy and maybe beastly. Proper shielding and grounding maybe a challenge too.

My idea is that tuner, DAC and 3 miscellaneous input signals pass a selector switch, go into a buffer, then into 3-way crossover, followed by 6-ganged attenuator (3*2 channels), 6 (3*2) buffers and finally into the amps. Possible gain differences between the amps (SymAsym for low and mid, LM3875 GainClone for high) are equalized by simple voltage dividers on the amps inputs.

So the signals would run on full level all the way until the attenuator.

Quote:
You would want to adjust the signal running through the crossover to be as "hot" as possible without clipping.
Will I achieve that in the way I described? Forgive me if my exposure isnīt clear, Iīm no native english speaker.
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Old 4th February 2009, 10:17 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Michael Chua
That's a bit strange. It should be very quiet if they are at unity gain.

Have you tried running the Preamp direct into the crossover and transfer the volume pots to the outputs (passive) followed by a Buffer?
agreed a bit strange. I didn't expect using 100nF and 10k to cause a noise problem with +34dB amp and 86dB speakers. But it does. 200nF & 5k into +28dB amp and 86dB speakers is better but still not acceptable.
I haven't tried my 90dB or 94dB speakers in this arrangement.

I have not tried using discrete buffers either side of the crossovers/pots.

Dual track pots before the crossovers is much more convenient than 4 or 6track pots after the crossover. That convenience has a consequence.
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Old 4th February 2009, 10:49 AM   #10
forr is offline forr  France
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Here are some pictures of one of the best installations in France.
http://www.cinetson.org/phpBB3/haut-...ile-t9877.html
All to have an extremly high overall definition.

His owner, THXRD, has compared both solutions for the attenuator :

N/A converter of the digital source --> two channels attenuator --> A/N converter of the speaker processor --> N/A converter of the speaker processor --> power amplifiers

and

N of digital source --> N of the digital processor --> N/A converter of the digital processor --> six channels attenuator --> power amplifiers

He did'nt notice any change despite the more many stages included in the first scheme.


I did the same and did not agree until I got a much better N/A for the digital source. My speaker processor is a BSS366. The volume control before it is much more handy.
My 6 ways / 6 positions stepped attenuator (-10 to -60 dB) is still in the circuit at the -10 dB position. A -14 dB attenuation would be preferable I think, the voltage gain of the power amplifiers being about 30 dB.
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