How 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 first proposed 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.
Original Idea
Final Implementation with buffered input and MFB topology. First stage Chebyshev, second stage butterworth. This gives a nice flat response. Based in standard value 5% res and caps.
Sub Volume control by 2k pot for R20:
The first proposed 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.
Original Idea
Final Implementation with buffered input and MFB topology. First stage Chebyshev, second stage butterworth. This gives a nice flat response. Based in standard value 5% res and caps.
Sub Volume control by 2k pot for R20:
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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 +/-
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 +/-
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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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?
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?
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
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.
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
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.
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.
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.
Hi there,
I'm really sorry to resurrect this old thread, but I really need help related to this stereo to mono issue.
I've build a switcher for a friend of mine to switch between PC and turnable table, as the PC sound card has its own 3 output L+R+SW I need to create a SW output for the turnable table. I came out with something like this, everything's working except the stereo signals get "back-tracked" through all those resistors so the L+R are not any more stereo output.
Is there a way to avoid this back-tracking? I'm quite organized to make PCB and have a good choise of component so no problem if something more complex, opamp, jfet or what else, is needed.
The system is already up and running, with pre-amp and power amp, the only problem is the switch.
Thanks in advance for your help
Armando
I'm really sorry to resurrect this old thread, but I really need help related to this stereo to mono issue.
I've build a switcher for a friend of mine to switch between PC and turnable table, as the PC sound card has its own 3 output L+R+SW I need to create a SW output for the turnable table. I came out with something like this, everything's working except the stereo signals get "back-tracked" through all those resistors so the L+R are not any more stereo output.
Is there a way to avoid this back-tracking? I'm quite organized to make PCB and have a good choise of component so no problem if something more complex, opamp, jfet or what else, is needed.
An externally hosted image should be here but it was not working when we last tested it.
The system is already up and running, with pre-amp and power amp, the only problem is the switch.
Thanks in advance for your help
Armando
sorry to to be a pita
Andrew, butlets say L+R channels of the turntable get doubled
- 2 wires(L+R) go to the switch
- 2 wires(L+R) get connected (one each) to the 2 paralleled restisors going into the neg input of the opamp
so you mean the 2 channels (L+R) going to the switch are still kept separate despite the 2 other wires get paralleled by the resitors?
(hope you can see this ASCII correctly...)
TT=Turntable
---------switch-R
I
TT-R --└----------------/\/\/---┐
I
I
┼----subwoofer
I
TT-L --┬---------------/\/\/----┴
I
---------switch-L
Thx!!!!
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