Stereo To Mono For Subwoofer

[Fablab]

Hello, I have to connect the output of a stereo preamplifier to a mono (line input) subwoofer
I'd prefer to avoid to use the classic resistors summing circuit (I am not to sure it doesn't affect the stereo signal) so I have found this simple circuit with a NE5532

Part List
R1______150K
R2______920R
R3______150K
R4______920R
R5______920R
U1______NE5532


What do you think?

 

[JohnRoberts]

Bzzzt fail... That circuit will subtract the two stems leaving little of the bass which is mostly mono (in records).

You could sum two into the first opamp.  Or just use two resistors.

JR

 

[Fablab]

I am not sure if the resistors don't effect the stereo signal from the preamplifier ...
Another solution may be to use two transformers, but may be expensive

 

[JohnRoberts]

I am not sure if the resistors don't effect the stereo signal from the preamplifier ...
Another solution may be to use two transformers, but may be expensive

If resistors are going to load down your stereo feed, how will they drive transformers? 

Oh, well theres the water over there... drink it if you want.

JR

 

[Fablab]

The question is that I have read that the resistors create some crosstalk in the original stereo signal.
Using the transformers the left and right signals in the first stage remain indipendent (the summing is in the secondary) and there is no possibility to increase the crosstalk.

 

[Andy Peters]

The question is that I have read that the resistors create some crosstalk in the original stereo signal.

You do realize that the basis for electronic summing is two resistors?

If you are super-duper concerned about crosstalk, buffer the left and right signals with op-amps and then sum and buffer the output.

Since you're driving a subwoofer, perhaps a low-pass filter on the sum output is useful.

-a

 

[Fablab]

I am not sure about crosstalk using the resistors (I have never tried yet)
As I said I have read so in different articles
I have posted the circuit with the opamp because I have read that this method is better and similar to the one with transformers
A low-pass filter in the output may be a good thing!

 

[MHanson]

Bzzzt fail... That circuit will subtract the two stems leaving little of the bass which is mostly mono (in records).

You could sum two into the first opamp.  Or just use two resistors.

JR

Look closer at your schematic .... 

 

[Harpo]

Part List
R1______150K
R2______920R
R3______150K
R4______920R
R5______920R
U1______NE5532
What do you think?

Circuit is a differential line receiver with -44.25dB pad.

 

[PRR]

> resistors create some crosstalk

If there is a "zero" impedance at either end, no crosstalk.

If the preamp outputs are 50 ohms, and the resistors are 10K ohms, then there is about 50/(10K+10K) or about 0.0025 of the Left signal injected into the Right output. This is 52dB down. You can hardly get 30dB separation with speakers in a room. Many people would call this "negligible".

If 52dB offends you, add a "Virtual Earth" at the mixing junction (JR's first suggestion). With modern opamps we can easily get the "virtual earth" below 10 ohms. Now the sneak path is (10K/10) and (10K/50), or 112dB down. Well below the noise of most sources.

You can further polish-it-up with simple unity-gain buffers between the preamp outs and the mix resistors. The Out-to-In isolation of a buffer is never infinity, but very-very good.

 

[Fablab]

Great, thank you!

 

[gswan]

Does your subwoofer input also expect the signal to be filtered (ie LPF)?

 

[Fablab]

The subwoofer has only an input and a volume control

 

[gswan]

Then I would suggest that you need to incorporate a LPF into your summer.

 

[MagnetoSound]

The question is that I have read that the resistors create some crosstalk in the original stereo signal.
Using the transformers the left and right signals in the first stage remain indipendent (the summing is in the secondary) and there is no possibility to increase the crosstalk.

Another point missed ... how do you sum the transformer secondaries together anyway?

You're still going to need resistive summing (or buffers, preferably) on the secondaries if the mixed signal is not going to be reflected back through the transformers - transformers provide galvanic isolation, not signal separation.

 

[desol]

Great, thank you!

But don't forget that, John was right in his first suggestion?

Just sayin...

 

[Fablab]

At Edcor they produce some specific transformers for these applications, but I think you can use two line transformers as in the photo below

 

[gswan]

I built a simple stereo->mono summing amp for use with a mono monitor configuration. Details here:
http://www.axtsystems.com/index.php?option=com_content&view=article&id=92:articlemsm1&catid=54:catmsm1&Itemid=87

Not quite what you need, but it might give you some ideas or a starting point.

 

[Fablab]

Thank you Gswan!