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Old 13th May 2002, 07:45 PM   #1
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Question Need help choosing op amp for active crossovers and preamp

I've seen so many posts on this topic here and elseware that I'm fairly consufed at this point.

I need a simple volume/balance control pre amp between a CD and active crossover setup. Thats my first op amp need.

Second, I will be building the active crossovers and presume that the op amps for it, if I want good sound, will have to be as good as those you would expect in a good pre amp. Is this a valid assumption, if so, which op amps?

Below is a list of ones that Ive seen referenced. Of the bunch, the AD825 and OPA627 seem favored. However, building two 3 way crossovers with the OPA627 would be quite expensive and I'm wordering if its worthwhile. I've also recently seen the AD Buf-04 which looks like it might be good for this.

Thanks in advance for your advice and suggestions!

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Old 13th May 2002, 10:06 PM   #2
jleaman is offline jleaman  Belgium
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Default LF-356

I'd use a LF356 if you need the schematic that i have and used for my active x-overs please let me know ill post it..

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Old 13th May 2002, 10:13 PM   #3
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The best way is to built your own discrete op amp. This is what I have been using in my crossovers. Although that circuit is from 1988 it is very similar to the active stage used in the current Pass Labs crossover (must have been designed by the same person . Beside the schematic I show the actual modules I've built. Very simple and compact. There is also additional info about D.I.Y. Op Amps at: http://passdiy.com/legacy.htm
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Old 13th May 2002, 10:42 PM   #4
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Default discreet opamp

Replace 2N5566 with 2SK389 and BJT transistors with 2SC2240 and 2SA972s and you will have a very good circuit. I would put 100 ohm gate resistors on the jfet. I have done this jfet swap for the 2N5566 with great improvement in sonics many times.

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Old 13th May 2002, 11:31 PM   #5
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Good info Harry. By replacing 2N5566 we will also be OK with the New Law since 2N5566 is in metal case.
Any further improvements? Maybe we could backengineer Pass Labs crossover instead of waiting for upcoming project.
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Old 13th May 2002, 11:48 PM   #6
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I agree with H. Haller and H. Potter, discrete op amps provide very superior quality of sound, and are not expensive. But good filtering an decoupling of power supplies are essential.

Regards, P.Lacombe.
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Old 14th May 2002, 12:00 AM   #7
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Default Power supply

This true with Op Amps and discrete designs. Especially with circuits with poor PSRR.

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Old 14th May 2002, 01:09 AM   #8
mlloyd1 is offline mlloyd1  United States
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The posted circuit with HarryHaller's suggestions sounds like a good bet. If you're looking for easy but good, the AD744+AD811 makes a good gain block, too.
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Old 14th May 2002, 01:13 AM   #9
Apogee is offline Apogee  United States
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Question Dumb Question????

Looking at the schematic vs the modules, I see four bjt devices on the modules yet the schematic only lists three...

Am I missing something?

Would the elders please enlighten....


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Old 14th May 2002, 02:04 AM   #10
rljones is offline rljones  United States
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I've built an active 4th order x/o using AD8610 using all SMT devices and with a very highly filtered power supply. I agree with a very clear power supply; the x/o uses a C-L-C-R-C configuration using double bridge recitifiers. The total capacitance is 132,000 microfarads (22k per cap; L = 10 mH; R = 1.2 ohms). This +/-22VDC supply (which is as quiet as a battery on my scope) is then locally regulated using precision zener shunts for a +/-13V supply for each channel. I am very happy with the sound.
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