Connecting Multiple op amp inputs from a single op amp output - diyAudio
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Old 29th July 2012, 03:19 PM   #1
ZLyzen is offline ZLyzen  United States
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Default Connecting Multiple op amp inputs from a single op amp output

So I have an internal pre-amp section of a 2.1 system and I'm wondering if what I want to do will work.

I want to have 2 separate outputs (L+R) from 2 different outputs do 2 things:

1. Combine (Add) and be sent to a low pass op amp to go to the sub amp.

2. Each go to their own op amp for a high pass for the satellite speakers.

So the outputs would both go to the same location, and they would also have their own separate connections, I'm wondering, since the outputs would be hooked up directly would I run into problems trying to keep their signals (L+R) separate.

My solution, is to add a 2 gang potentiometer before the subwoofer, to control subwoofer level, this would also allow the signal to combine after the potentiometer.

Any thoughts?
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Old 29th July 2012, 03:37 PM   #2
paulb is offline paulb  Canada
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You will need an opamp circuit section as a summer - this will keep L & R isolated from one another but combine them for the sub output. There will be lots of circuit examples around as to how to do this.
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Old 29th July 2012, 08:39 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

1.) is an implicit feature of all active subwoofers,
it does not affect the separation of the channels.

rgds, sreten.

Both stereo line level and speaker level inputs are summed to mono.
For stereo subs you simply omit the other channel connection.
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Old 30th July 2012, 03:30 AM   #4
ZLyzen is offline ZLyzen  United States
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Okay so here is a drawn up example of what I am talking about, and I've noticed, though it isn't often, there is some bass that has a slight tendancy to lean towards one side or another in some music. Therefore I would really like to sum left and right for my subwoofer's input. I'm worried that summing the signal will affect channel separation for the satellite speakers..

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Old 30th July 2012, 01:39 PM   #5
Jay is offline Jay  Indonesia
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You can either build a mono or a stereo subwoofer. For stereo subwoofers you don't have to sum both channels. But for mono subwoofers you have no chance but to sum both channels at low level (e.g. at preamp output). 33k-100k resistors from each channel to a summing buffer is adequate.

You can either build a subwoofer system with only active low pass filter, or both high pass and low pass active filter.

With only active low pass filter, you rely on the natural roll-off of a bookshelf, which is not good because the roll-off slope is irregular (especially for vented system).

With both LP and HP present, it is about where you put the crossover frequency. If it is low enough, then there is no worry with channel separation because at low frequency the stereo information is minimal.

If the cut-off frequency is high like in PC subwoofers (or Bosse system) then you have to know that it is a cheap system, with some consequences. Even a good mono subwoofer crossed very low often cannot substitute a good stereo subwoofer.

If you worry too much about channel separation I think you can buffer each channel signal before summing them.
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Old 30th July 2012, 01:53 PM   #6
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Quote:
Originally Posted by ZLyzen View Post
Okay so here is a drawn up example of what I am talking about, and I've noticed, though it isn't often, there is some bass that has a slight tendancy to lean towards one side or another in some music. Therefore I would really like to sum left and right for my subwoofer's input. I'm worried that summing the signal will affect channel separation for the satellite speakers..

Click the image to open in full size.
That'll work in principle, but if your pots are at max you effectively short L to R.
Better to use two resistors of say 10k to the summer, and the pot for the sub after the summer. You can get away with a single pot, and no tracking problems.

Also, no need for U3A and U4A - just take the L & R from U1A and U2A.

jan didden
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Old 30th July 2012, 09:35 PM   #7
ZLyzen is offline ZLyzen  United States
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Thanks for all the replies!

and Jan, that's what I was worried about, glad the summer will keep the channels separate! as for u3a and u4a, those are my high pass filters for the satellites, so I just thew them in there to show that I need the left and right separate still
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Old 8th August 2012, 12:17 PM   #8
AndrewT is offline AndrewT  Scotland
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The summer (adder) is an inverting opamp.
The -IN pin is the input.
The +IN pin is connected to Audio Ground.
You connect as many summing resistors to the "summer" input as you have channels you want to add together.
For two input channels you have two input resistors. These would normally be ~ equal to the feedback resistor. The output of the summer is [Vleft / Rleft] + [Vright / Rright] * Rnfb If all three resistors are selected @ 10k then the output will be the sum of both Vleft+Vright. If you add that pot that Janneman suggested you can adjust the summed voltage downwards to your required attenuation. Alternatively select Rnfb for a lower value for lower output voltage or higher for a higher output voltage.

For minimum output offset (output DC) a resistor can be added into the connection from +IN to Audio Ground.
It's value for near zero DC is ~Rleft//Rright//Rnfb, for all three @ 10k then Rdc=3k3, for Rin=10k and Rnfb=5k1 then Rdc=2k4
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Last edited by AndrewT; 8th August 2012 at 12:21 PM.
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