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Old 11th May 2004, 07:59 PM   #1
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Question Opamp Parallelling & Crossovers

I've run into a problem and I can't seem to find anything on the net about it. I need to use an inverting-configuration in my preamp and want to add a listen source buffer. Now, the problem is I don't know how to parallel the opamps. I've got trouble with the crossovers too. The only things I find are about non-inverting configurations.

This is what I want to do:
- Buffer to record out (gain = 1)
- Opamp drives a PGA2310
- Input is from an SSM2404 (that's why I need to inverting configuration)
- Passive high-pass crossover @ 15~20Hz
- Passive low-pass crossover @ 25k~30kHz

How would I do this? It seems to me that the low-pass will change the input impedance and that paralleling the amps like I've done will change it too. I think this will also create problems because the record out buffer opamp will have a gain of 1.25 (it needs to make up for the 0.8 gain of the BUF634 in bandwidth mode).

Help ?

Here's the part of the schematic I've got trouble with:
The input resistor is NOT shown here! The 5k resistor in series with the inverting input is part of the passive crossover. The input resistor is in front of the SSM2404 which I've not shown here. It's also 5k.
Click the image to open in full size.
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Old 12th May 2004, 07:15 PM   #2
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Ah, c'mon, I'm sure some of you people know the answer to my question .
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Old 12th May 2004, 07:47 PM   #3
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I don't really get what your doing, or what the problem is other that it seems your making things really complicated by trying to make them simple.

Why not have the op-amp after the buffer.
And Inverting, non-inverting, no big deal, if you need to flip polarity just stick an inverting, unity gain opamp at the end.

OR, maybe this is miles above my head.
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Old 12th May 2004, 09:42 PM   #4
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No, look, I need to use the inverting input, because I'm using an SSM2404 for input switching. It's better if it's ouput is driven into an inverting input.
The lower opamp's output goes to a PGA2310, etc which forms the real preamplifier part.

The problem I have is that I don't know how to parallel that opamp together with a record out buffer, without changing the 5k input impedance and 1x gain. Also, I'm not sure what the passive input filters would do to the impedance.
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Old 12th May 2004, 10:43 PM   #5
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Why not feed from the SSM2404 into a unity gain inverted opamp, and then from there do your parallel connections?

No need to worry about input impedance, and you can keep your buffer on the inverted feedback like shown. And you can do all your crossovers noninverted, (or not, what ever is easier)
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Old 13th May 2004, 03:33 PM   #6
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Quote:
Originally posted by officeboy
Why not feed from the SSM2404 into a unity gain inverted opamp, and then from there do your parallel connections?

No need to worry about input impedance, and you can keep your buffer on the inverted feedback like shown. And you can do all your crossovers noninverted, (or not, what ever is easier)
Thanks for the info, I think I understand a part of it .
So, I'd only connect the bottom opamp to the SSM2404 output, and then feed that signal into the PGA & the BUF634 (+ OPA627) record out buffer?

How would I use the crossovers on the non-inverting input? I'd just connect them from the non-inverting input to ground while feeding the signal into the inverting input?

Is it to much to ask to draw/copy&paste/whatever what you mean? I'd be very gratefull .
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Old 14th May 2004, 06:53 PM   #7
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Please ?

* Yes, I shouldn't do all this bumping, but I can't find the answer to my question anywhere and I don't totally understand what officeboy means.
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Old 14th May 2004, 07:36 PM   #8
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Hopefully this will explain, ignore all values, I just ripped this and hacked it up from an active crossover i was designing.

http://home.comcast.net/~0fficeboy/misc/dh.gif

And hopefully someone with a bit more op-amp and crossover experience will step in and help out too.
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Old 14th May 2004, 10:31 PM   #9
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Ok, so I route the signal into the inverting input of the OPA627, then at the output I divide it into a buffer section & the PGA2310. I can see very piece (eg. buffer, pga input, ...) as a separate thing and add them all together, right?

OK, thanks! That helps a lot! Now only the crossover problem remains . You say that I could add it to the non-inverting side? How would I do this? I just connect all the crossover parts to the non-inverting side, although no signal is going in there?
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Old 14th May 2004, 10:52 PM   #10
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Well my example shows the basic layout of a second order high and low pass, the lowpass is the only part that is setup right (it's the middle section), and at those values it will crossover at about 115hz.

There is info on doing this found here. http://www.t-linespeakers.org/tech/f...allen-Key.html
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