osufishman stereo line level to mono - diyAudio
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Old 27th December 2010, 05:17 PM   #1
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Default osufishman stereo line level to mono

I would like to take a stereo line level and convert it to mono for a subwoofer amp without affecting the original line is as the same input also goes to a stereo amplifier.

The circuit is a buffer followed by an inverting summer and a unity gain inverter. The resistors can be adjusted as needed or add gain if you like. A couple dual opamp ICs of any flavor could be used.

What are you thoughts on this circuit?

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Old 27th December 2010, 05:49 PM   #2
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Here is another version, modified to use the extra opamp as a bandpass filter

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Last edited by osufishman; 27th December 2010 at 06:06 PM.
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Old 4th January 2011, 02:01 AM   #3
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I'm trying to do the same thing exactly. Will keep an eye on this thread, as so far my thinking matches exactly yours.

Btw which sub filter are you using? Just curious.

Good luck!
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Old 5th January 2011, 12:35 AM   #4
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Some progress,

The filters will just be 2nd order butterworth, sallen key. Only one quad opamp chip will be needed for the stereo to mono + bandpass filter I have selected the component values to give 1.1db of gain with -3db @ 28Hz and -3db @ 170Hz . I have a bit more tweaking to do with component values though they should be all 5% tolerance parts and I probably need add the gain earlier on than in the last opamp, but here is what it looks like so far. If anyone notices anything ridiculously wrong please hit me in the head, as this is my first audio project.

Probably going to use a chip amp of some sort to convert the mono+ to mono +/-

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Click the image to open in full size.

Last edited by osufishman; 5th January 2011 at 12:38 AM.
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Old 5th January 2011, 10:46 AM   #5
AndrewT is offline AndrewT  Scotland
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Change the filters to MFB (both inverting to restore phase).
The first MFB can be used as a summing stage and a 2pole filter. Then you can delete the two input buffers.
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Old 5th January 2011, 02:35 PM   #6
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I am not seeing what advantage the Multiple Feedback would have over Sallen-Key. I could see going down to 3 opamps for a summing bandpass but that would actually increase component count because one of the filters would have to apply to both left and right channel before summing.

If I switched to the MFB inverting filters wouldn't I still need the input buffers to isolated the left and right channel from each other before the first filter stage?
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Old 5th January 2011, 03:25 PM   #7
AndrewT is offline AndrewT  Scotland
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the MFB is an inverting topology.
It starts with a gain setting resistor just like an inverting opamp gain stage.

This allows two gain setting input resistors to be placed at the front of the MFB filter.
That first opamp in the MFB arrangement acts as a summing stage AND as a filter.

I said that earlier
Quote:
The first MFB can be used as a summing stage and a 2pole filter.
You can delete both the buffers.

The advantages of the MFB for the one extra component that it uses, is that Frequency and Q damping and Gain are all independently adjustable, whereas with a unity gain S&K the Frequency and Q are both dependent on the same component values. and Gain =1 (+0dB)
The gain setting or Equal Component Value S&K separates the Q damping from the Frequency setting component values, but Gain and Q are now dependent on the same component values.
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Old 5th January 2011, 06:50 PM   #8
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Thanks for the information, I will have to consider the advantages you have listed in the overall system.

I think what I will do is switch to MFB for both stages, so I will have an easy gain adjust later on. I still prefer to keep the input buffers on each channel rather than using an rc network into the first summing MFB filter.
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Old 5th January 2011, 08:30 PM   #9
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Update to inverting MFB topology, first stage Chebyshev, second stage butterworth is why the plot looks funky.

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Old 5th January 2011, 10:34 PM   #10
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Gain control will from a processor to a digital pot, resistive optocoupler, or voltage controlled mosfet to vary the impedence of the gain resistor. This approach will be used for every channel.

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Last edited by osufishman; 5th January 2011 at 10:36 PM.
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