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Old 15th May 2007, 03:46 PM   #11
Nrik is offline Nrik  Denmark
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I would say no. 30kohm is even worse.

If the frequency-determining resistors in your filter was say 3kohm you would shift the attack frequency 10%. If your resistors are of higher values (wich is not that hard to imagine) it would be far worse. And your attack frequencies will have dips because of the parallel-resistor in the input of the opamp is causing dividing networks in the lowpass, and a lower Q in the high pass filter section.

I realise that many people are fans of the sound made by NE5534, LM4562 and other bipolar input opamps, used as an amplifier, buffer, I/V converter etc.
But for filter circuits like these you really need the perfect opamp, with infinite input impedance and zero ohms output impedance, and that is just for the cicuit to operate properly.
We are not discussing sound quality here, just basic function.

It is not for fun that Rod Elliot recommends the TL072 (the preferred FET input opamp back in the days).

But hey: To find out yourself if I am right or wrong, use sockets, and try out the different opamps. Then you could also swap for even better types in the future.


OPA2134 datasheet
http://focus.ti.com/lit/ds/symlink/opa2134.pdf
Input impedance (both common mode and diff mode) 10^13 ohm or 10.000Mohms
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Old 15th May 2007, 04:04 PM   #12
ssmith is offline ssmith  France
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Nrik,
Huge thanks.
I think in my case I will take your advice and stick with the OPA2134 for the filter parts, and perhaps throughout... limiting experimentation to the buffer stages only.
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Old 15th May 2007, 04:15 PM   #13
Nrik is offline Nrik  Denmark
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You're welcome.

The OPA2134 sounds in my ears as almost nothing. It does not add or subtract much. So you can even use it as output buffer/amp.

If you want to go nuts and spend some more money, the OPA2604 sounds even more neutral.

But use sockets, and you are ready to find your own preferred opamps.
If you experiment, then use any opamp that:
- Have high input impedance
- Are unity gain stable
- Uses voltage feedback (not current feedback like some)

That's it. Good luck.
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Old 15th May 2007, 06:08 PM   #14
Tim__x is offline Tim__x  Canada
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That's 300k differential input impedance. In normal use the voltage across (AKA the differential voltage on) the input is but a tiny fraction of the input signal. The differential input impedance is bootstrapped by the amplifier so that it appears enourmous to the preceding stage.
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Old 15th May 2007, 06:27 PM   #15
Nrik is offline Nrik  Denmark
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Okay Tim_X can you explain it some more?

The diff input impedance is then multiplied by what? The open loop gain or?

(great to learn new stuff but OPA 2143 still works great and sounds better than NE5532 in my ears)
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Old 15th May 2007, 06:50 PM   #16
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These are such great responses, thank you very much!

I have already doubled my knowladge of Op-amps.

I am used to tubes, where every little tiny part has a profound impact on the sound. are there any Op-amps that measure poorly, but everone loves the sound of?


by the way
10^7 would be 10 Megaohms
10^13 would be 10 Teraohms
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Old 15th May 2007, 07:18 PM   #17
Nrik is offline Nrik  Denmark
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Quote:
Originally posted by alexmoose
are there any Op-amps that measure poorly, but everone loves the sound of?
....would you really want that?

Well you can't really say that any opamps have really bad specs. Only the really old ones, type 741/1458/324 are a bit slow, but no-one really likes the sound of those.

...Except for use in distortion-pedals for guitars. I have substituted some decent audio opamps for these poor sounding ones on request from my guitarplaying coworker, and he loves it. There are webpages outthere for guitarfreaks explaning those freaks who wants to 'upgrade' their pedals, to find scrapped ghettoblasters to find these opamps.

And thanx for the whats-in-a-number lesson.
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Old 15th May 2007, 09:19 PM   #18
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sounds great! I can't wait to start construction!
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Old 16th May 2007, 12:23 AM   #19
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Hi, everybody

A few days ago, I made a PCB for this proyect, what do you thing about?
Cheers
Iván
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Old 16th May 2007, 12:27 AM   #20
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The pictures:
Attached Images
File Type: jpg 2 vias linkwitz mono buffer y buffer componentes baja.jpg (34.2 KB, 516 views)
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