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Old 6th June 2004, 01:15 PM   #1
hacknet is offline hacknet  Singapore
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Default Summing Channels with opamps

i`ve done a simple non inverting buffer by shorting the output and the - input having the signal go into the + input. the input is clamped with a 100k resistor to ground. i am using an opa2132 opamp

because i want to sum the both stereo channels for my sub woofer, i put the 2 channels on 2 different opamps and integrate them with 2 100R resistors at the output.

i have experienced a great loss in signal. before implementing the buffer, i had more then enough gain. now i don`t. infact the loss is quite high. i`ve tried changing the 100R resistors to 10k but the situation does not improve. in addition to that. i have got humming now.

can anyone enlighten me to the cause of my misery?

Thank You in advance!!
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Old 6th June 2004, 02:10 PM   #2
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It would have been easier to sum them on the input of on opamp.

What is the gain of your opamp circuit???
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Old 7th June 2004, 04:19 AM   #3
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unity gain on the buffer and unity on the XO i think, i`ve built linkwitz 24db circuit.
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Old 7th June 2004, 04:52 AM   #4
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Put a 5k resistor at the output of each. Let these both join at the input of a third op amp's + input with 100k to ground and output shunted to - input. That would be one of the several "proper" ways to sum signals.

Some loss in signal could come from the fact that signals that are out of phase between the channels, will cancel when summed.

Now, most deep bass usually is recorded mono, therefore in phase between channels, and will add without loss. If you do experience great loss in the bass, that means one of your inputs likely is wired with inverted polarity, therefore 180 degrees out of phase.

I always wanted to know how to obtain a correct mono mix from two stereo channels, because in the higher frequencies, anything randomly out of phase will cancel in a summing amplifier. If you sum stereo to mono but out of phase, you recover the ambient signal, which is the operating principle of some surround processors. Concurrently, it is noticeable how much you lose in ambient signal when you sum things in mono the conventional way... any thoughts?
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Old 7th June 2004, 05:15 AM   #5
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i`ll take that into consideration. thanks!
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Old 7th June 2004, 06:45 AM   #6
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You might try attached image for summing.
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File Type: png sumamp.png (1.2 KB, 176 views)
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Old 7th June 2004, 07:57 AM   #7
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Quote:
Originally posted by PMA
You might try attached image for summing.
The inverting summer is more or less ideal. A non-inverting summer has losses, by nature.
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Old 7th June 2004, 10:17 AM   #8
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Peranders,

Quote:
A non-inverting summer has losses, by nature.
How does it have any more inherent losses than the noninverting one (other than ones easily corrected by the gain setting)?

I am naturally more inclined to inverted operation but here I don't see why the result would be much different.
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Old 7th June 2004, 10:40 AM   #9
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The virtual ground of inverting circuit makes nearly perfect separation of the summarized input signals (not influencing each other).
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Old 7th June 2004, 10:47 AM   #10
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OK, I follow so far and I also think the inverting one has more elegance to it.

But say, if one uses two noninverting unity gain buffers to drive a noninverting summing third op amp, through 5k isolation resistors, as I described earlier - are there going to be any real world interferences between the two source op amps?
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