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Old 3rd November 2006, 12:23 PM   #1
bdef is offline bdef  Denmark
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Default Compare of opampís

Iím building an active speaker and for the filte iím going to use a lot of opamps. Iím going to use one as input buffer with a gain on 6 dB and a input impedance of 1 Kohm. The other opamps input pins will se out into resistors between 10 and 220 Kohm.

I have looked at NE55xx, AD8620, AD826, AD812, AD825 and OPAx134, but maybe youíll know some types that are better for my use.

Iíd like to learn about your experiences on opamps.
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Old 3rd November 2006, 12:51 PM   #2
BobEllis is offline BobEllis  United States
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Here is one active filter builder's experience with opamps. You'll find people defending and promoting their preferences with near religeous fervor. Opamp choice seems a matter of personal preference, influenced by filter impedance.

Your relatively high impedances in the filters suggest that you will want a FET input amp due to the input current. Using 10K for my low pass sections I ended up with 35 mV offset using NE5532s. FET input opamps will need virtually no input current, so they are better suited to higher impedance filters.

If you need boards for your active crossovers, I have a group buy going that closes today. Details in the thread linked to above and on my www button below. I have a shopping cart set up if you decide to buy.
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Old 3rd November 2006, 01:20 PM   #3
AndrewT is offline AndrewT  Scotland
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your proposal to use 1k0 input impedance is unusually low for a domestic amplifier/crossover.
Not many sources are capable of driving 1k0 properly and those that have DCblock caps on their outputs will suffer from severe bass cut due to the filter action of CR acting as a high pass.

Commercial gear is often designed to drive 600ohm, could your's be such?
regards Andrew T.
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Old 6th November 2006, 07:19 AM   #4
bdef is offline bdef  Denmark
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My preamp is build over a 10 Kohm Alps, a AD812 and a god power supply. I have read somewhere that low impedances minimize the influence of the signal cable. Now Iíll try :-)

I think Iíll make it configurable so I can switch between 1 Kohm and 10 Kohm easily.

My power amplifier (a gain clone) has a DC block capacitor. Therefore if the NE55xx is the best sounding Iíll use it regardless of its DC offset. My biggest problem is that Iím building it using SMD components and therefore itís not that easy to change opamp just to try the sound of it.

Thanks for the link.
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Old 6th November 2006, 11:37 AM   #5
BobEllis is offline BobEllis  United States
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I think you may have misread the low impedance reference. Low impedance on the sending (preamp) end is what minimizes cable influence most, AFAIK. Of course if the receiving end impedance is too high you risk noise and RFI/EMI susceptibility.

You might try experimenting with placing your volume control before the final buffer in your line stage. While you are experimenting with line stages, use a socket and swap out opamps to see what [u]you[/] prefer. (keeping in mind the bypassing requirements of the higher speed varieties). You may find that the NE5532 does better than others driving low impedance loads, as this is what it was designed to do.
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Old 6th November 2006, 01:11 PM   #6
bdef is offline bdef  Denmark
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The AD812 are made to deliver into 150 ohm with very low distortion. My preamp have relays to select the audio source, sending the signal direct into the ALPS from where the AD812 gets the signals before sending it to the output.

Some years ago I talked with Peter Holstein from Holfi. He uses a 75 ohm output impendence from there preamps and 800 ohm input impedance in there power amplifiers, for the same reason, though they donít use opampís or anything with feedback at all.

The funny thing is that over all where I read the NE5532 gets mentioned, though itís old. I havenít found much on AD825, a little more in OPAx134/132 :-)

I have some links, but the text is in DanishÖ..

My preamp: http://www.hifi4all.dk/forum/forum_p...=42025&KW=bdef
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Old 9th November 2006, 07:22 PM   #7
JesseG is offline JesseG  Canada
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May I suggest you have a look at the new LM4562 from National.


I managed to get a few samples to play with. This is one great opamp. My first project with it is a MM Phono stage. It was dead easy to build and sounds better that any ss phono pre I have heard yet. Excellent detail and resolution, fabulous bass response and the PS requirements are trivial.

Good luck

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