# Stereo-to-Mono Converter



## Tomo

Hey,

 I just got A first AUDIOPHILE speaker in my whole life for my office. Note the "A". I got ONE. Anyways, according to what I hear, if my space is mediocre having stereo isn't much greater than having mono. So I thought I would just build myself a mono amp or preamp (they are lending me their power amp). 

 However, I don't know how to convert stereo-to-mono. Do I use an opamp adder? 

 Thanks for listening to somewhat mediocre question for my mediocre mind.

 Happy listening to ye all, 
	

	
	
		
		

		
			





 Tomo


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## amb

An opamp "summing amplifier" would work well. Basically, an opamp wired in an inverting configuration with two inputs.

http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm


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## rickcr42

No way would I use an active opamp ciruit for such a simple function,one where not only is a passive device as good but in reality _way better_ since it is impossible to add any circuit or AC noise except in an active device (resistor noise aside and in this case a non-issue).Dynamic range ? Bass and treble ? Bandwidth ? All terms used with active stages and again have zero to do with passive unless by design intent.

 what you need to think is "signal mixer" and not stero-to-mono as much which to many means a simple SPST Mono switch on their preamp (also dead wrong in method and why highs end up muted)

 absolute best method is to use a Line Level Combiner which is a dual input line level transformer.This can get a tad pricey if using Jensens/cinemags/Sowters/Lundahls but for a cheapy solution this guy is not too shabby and at $20 even cheaper than any opamp+power supply solution :

http://www.edcorusa.com/sound/matchers/s2m.htm

 Second best,and the way just about all subwoofers combine the left and right channels for a single sub is the resistor mixer.No more than three 10K resistors and with no signal losses since you are combining two signals (adding or sum) which means a signal _increase_ which is then divided by 2 hence unity as the end result

 LEFT CH To one end of a 10K resistor,RIGHT CH To one end of another 10K resistor,other end of both tied together,a third 10K resistor with one end to the junction of R1 and R2 with the other end to circuit ground,MONO output from this three resistor junction to the output jack.....done ! And all for about $5


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## amb

Not trying to argue, but with a properly implemented opamp circuit using a low noise device, the resistor noise dominates, so a "passive" solution is not necessarily any less noisy. An active opamp circuit also has the advantage of low output impedance which may or may not be an important factor here.

 All that said, perhaps a simple two-resistor passive summer would indeed suffice for what Tomo wants.


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## rickcr42

Quote:


 Not trying to argue, but with a properly implemented opamp circuit using a low noise device, the resistor noise dominates, so a "passive" solution is not necessarily any less noisy. An active opamp circuit also has the advantage of low output impedance which may or may not be an important factor here. 
 

also not looking to argue but have to state my side.The "why" of my advice to be taken or not,no matter either way 
	

	
	
		
		

		
		
	


	




 it is just not rational to me personally to use an active device where one is not only not need but is in fact the less technically meritorious method when the entirety of what you are attempting is weighed.That "attempt" is to combine two signals into one.

 Using an opamp for a simple stereo-to-mono mix and with that simple ciruit a power supply,line operated so AC line noise suppression,RFI protection at the entry points to the device (power supply pins,input pins,output pin,feedback loop) in place of even a simple two resistor mixer is like using an opamp as an "anti-gain" device because you have too much gain when a resistor pad or pot would do the same job or like using a X5 opamp stage going to a X5 opamp stage because you need X10 gain.

 Downsides ? possible comb filtering from mixing the signals 1:1 but the added resistor to ground loading helps here a bit by adding a "preload" split so a path to grund for each at a load potential of 10K.The transformer totally eliminates this by actually using 100% isoltated inputs for each leg which are then combined without actual physical contact of those two signals into one single audio output signal.

 Upside is obviously sonic purity because ALL active stages no matter if just a single transistor have an audible impact or we would not use various opamps/topologies to tweek system sound and way more audible than any single resistor would or could be.
 Any active stage also has a higher cost factor and that not a small one. Circuit cost+power supply cost before even thinkng about connectors and enclosure or chassis.My opinion is even a simple power transformer from the cheapest outlet will cost more than the three resistors (and likely more than the cased trafos combiner).

 In audio less really is more bcause everything adds a sonic imprint so the less you have the less imprint.

 Just my personal opinion but just like most here would use an opamp as a simple first order filter i would use a single cap and a single resistor to accomplish the same thing and have better sonics at 1/4 or less the price of the iffy active stage. 

 BTW-I personally if attempting the same thing would shoot right to Jensen and buy one of their raw line splitting trafos but that would totally toss any cost savings in the active vs .passive argument straight to the trash can.
 The splitting transformer would whip _any_ active stage butt hands down on the sonics but cheap ? _Not_


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## Tomo

Hey,

 Thanks, Rick and Amb. I think I will go with a Summer. I need some gain to drive this amp (and the speaker). ... Or I could just build a GClone with high gain or something.

 Hey Rick, can you post how's transformer summer wired?

 Tomo

 P.S. Just recalled your BIO. With an R on your chest, Rick? ... What resistance do you have? ;p


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## rickcr42

Quote:


 I need some gain to drive this amp (and the speaker). ... Or I could just build a GClone with high gain or something. 
 

If extra gain is what you need then active is probably the best choice all things considered though even this can be done passive at a cost 
	

	
	
		
		

		
		
	


	





  Quote:


 Hey Rick, can you post how's transformer summer wired? 
 

absolutely Tomohiko 

http://www.jensen-transformers.com/as/as078.pdf

  Quote:


 P.S. Just recalled your BIO. With an R on your chest, Rick? ... What resistance do you have? ;p 
 

I try very hard to _resist_ temptation but that battle is usually a losing one 
	

	
	
		
		

		
		
	


	




 cheers man


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## Tomo

Hi,

 I built a resistor-voltage-divider Stereo-to-Mono Converter. I wanted to check it out before I design myself an opamp version.

 three 10kohm resistors connect at a single point. One end to ground. The other two to L and R. I built it into connectors. 

 I am listening to it now. I don't mind MONO. Kinda easy to listen to. Sometimes, stereo sounds can be uneasy. (Especially when you are exhausted.) 

 I am going to build me another one for my home with spiffy chasis and counteract with opamp.

 Tomo


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## ezhezh

Hello rickcr42.
  
 You wrote:
  
 =====
  
 Second best,and the way just about all subwoofers combine the left and right channels for a single sub is the resistor mixer.No more than three 10K resistors and with no signal losses since you are combining two signals (adding or sum) which means a signal _increase_ which is then divided by 2 hence unity as the end result

 LEFT CH To one end of a 10K resistor,RIGHT CH To one end of another 10K resistor,other end of both tied together,a third 10K resistor with one end to the junction of R1 and R2 with the other end to circuit ground,MONO output from this three resistor junction to the output jack.....done ! And all for about $5
  
 ======
  
 I have a situation where I want to combine two inputs into one ... except the inputs are outputs from mikes.  Will the circuit you describe here work correctly?
  
 (My situation: our accordion player has built in mikes.  The output from his intrument is stereo (1/4" stereo plug).  I want o attach him to a mono wireless, so he can move around freely, without a cord.)
  
 I think I don't need a balance control on the "combiner" ... he has knobs for the two sides.   But If I do, do I just make the two input resistors pots?
  
 Thanks very much indeed


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## tomb

ezhezh said:


> Hello rickcr42.
> 
> You wrote:
> 
> ...


 
  
 I'm very sorry to say that rickcr42 passed away in 2010.  Perhaps someone else will see this and respond with an answer to your question.


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## Speedskater

Mikes often need mike pre-amplifiers, but the outputs of the two pre-amplifiers cannot be combined.
  
 See the summing parts:
  
*RaneNote*
  
*Why Not Wye?*
  

Splitting Signals
Subwoofing in Mono
Unbalanced Summing
Balanced Summing
Output Impedances
  
 http://www.rane.com/note109.html


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