# Converting stereo to mono in amplifier circuit

#### tjohnson

Joined Dec 23, 2014
611
As I mentioned in a recent thread, I built a USB powered speaker (from this Instructable):

My audio input source is a stereo headphone jack plugged into my computer, but the circuit only handles mono sound. The Instructable says to twist the two stereo wires together, but I read in this thread:
You do not know if the music source will blow up if you short the channels together. I would make a mixer with two series 3.3k resistors.
Is it really necessary to add resistors?

#### kubeek

Joined Sep 20, 2005
5,736
Better safe than sorry. So yes, it is preferred to join the two channels through the resistors, unless you are willing to go and repair the signal source if it gets burnt out. 3k3 might be a bit on the high side compared to that 10k potentiometer and I would prefer somwhere in the 470 to 1K range, but the value is really not that critical.

#### tjohnson

Joined Dec 23, 2014
611
Does a potentiometer have resistance between two adjoining terminals, or only between the two outermost ones? If the first is true, could I just connect one wire to the left terminal and the other to the middle terminal in order to separate the two channels?

#### kubeek

Joined Sep 20, 2005
5,736
There will allways be 10K between the top and bottom terminals of the pot. The wiper chooses to which part of that 10K it is connected, so close to the bottom side of the pot there will be 1k between the wiper and the bottom terminal and 9k to the top terminal. This basically divides the input voltage, so all the way to the top you get 100%, and all the way to the bottom you get 0%, and all the numbers in between. If you don´t need the variable loudness then you can simply leave the pot out and connect the two resistors straight to the pin 3 of the LM386.

#### MrChips

Joined Oct 2, 2009
22,552
The proper way would be to connect one channel to the left terminal, the other to the right terminal.
The center terminal goes to the input of the amplifier. Now you have a BALANCE control.

#### tjohnson

Joined Dec 23, 2014
611
Thanks, but I'm using the potentiometer for volume control, not balance control. I'm wondering if connecting one channel to the left terminal and the other to the center terminal (which is directly connected to pin 3 of the LM386) would eliminate the need to use two separate resistors?

#### kubeek

Joined Sep 20, 2005
5,736
No, that wouldn´t really work. Are two resistors such a problem?

#### tjohnson

Joined Dec 23, 2014
611
No, that wouldn´t really work. Are two resistors such a problem?
No, I was just curious. Thanks for answering my question.

It seems like it's fairly standard to use 1kΩ resistors to bridge stereo to mono, so I think that's what I'll do. Out of curiosity, why aren't lower value resistors (like 100Ω) typically used so that the volume is reduced less?

#### crutschow

Joined Mar 14, 2008
26,057
Thanks, but I'm using the potentiometer for volume control, not balance control. I'm wondering if connecting one channel to the left terminal and the other to the center terminal (which is directly connected to pin 3 of the LM386) would eliminate the need to use two separate resistors?
No. The two channels would have different levels.
And if adjust the pot to the maximum volume position the two inputs would be shorted, which is what you are trying to avoid.

#### kubeek

Joined Sep 20, 2005
5,736
With low values like 100 ohm you are getting close to drawing too much current from the source. The source will have some inherent resistance in series with the output (say 20-50 ohms), so too low a resistance will cause more reduction in volume as well. And such low resistance would draw too much current from the source, which might burn the output stage.

#### AnalogKid

Joined Aug 1, 2013
8,823
The smaller the two summing resistors, the more each output will "see" the other. Since outputs such as these frequently are designed to drive ear buds that have a nominal impedance of 60 ohms or more, 10 times that is a good starting point. If the two summing resistors were 10 ohms each, each output would see a load of 20 ohms. this might cause overheating in the output amps, depending on their design.

ak

#### MrChips

Joined Oct 2, 2009
22,552
Here is a simple guide to follow.
You want the resistor value to be greater that the resistance of the source in order to prevent excessive loading.
You also want the resistor value to be less than that of the next input stage for the same reason.
Your input stage is a 10k pot. Hence the resistor should be some where between 100Ω and 5kΩ.
1kΩ seems to fit nicely.

#### tjohnson

Joined Dec 23, 2014
611
It wouldn't be a good idea to plug a mono audio cable (like this one that I bought) into my computer's stereo audio port, would it? I assume doing so would still pose the danger of shorting the channels.

#### bertus

Joined Apr 5, 2008
21,054
Hello,

When you use a mono plug in a stereo socket, you will most likely short one of the channels.

Bertus

#### tjohnson

Joined Dec 23, 2014
611
Hello,

When you use a mono plug in a stereo socket, you will most likely short one of the channels.

Bertus
I haven't used a mono plug in my computer's audio socket or changed any volume settings recently, but in the past few days I've noticed that my PC's audio seems noticeably louder than it was before. Previously, I couldn't hear sound coming from my headphones when I held them away from my head, but now I can.

Do you think it sounds like a limiting resistor got shorted inside the headphone socket? I wonder if something might have happened when I plugged in my speaker circuit to test it and didn't have it wired correctly. Or maybe it's just because I cleaned wax out of my ears recently.

Also, what are mono audio cables used for? All the audio sockets I've ever seen I think were stereo.

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