Stereo to Mono Circuit

[swinginguitar]

Looking for a simple but good quality way to sum a stereo signal to mono. What do you guys recommend?

If I use an op amp summing circuit, what R values do I need for the feedback resistor and input resistors to get unity gain on the output?

[Barcode80]

Really, there would be absolutely no problem and no ill effects to simply splicing the two together passively. Why the need for a circuit of any kind?

[swinginguitar]

Really, there would be absolutely no problem and no ill effects to simply splicing the two together passively. Why the need for a circuit of any kind?

Asking 'cause I don't know :-)

Passive splicing - would the output have a tendency to clip (sum of 2 signals)?

[Barcode80]

What exactly is the application? Like what are you ultimately trying to accomplish by combining the signals?

[Penthode]

Take a look here http://www.extremecircuits.net/2009/07/stereo-to-mono-converter-based-on-fet.html

[swinginguitar]

What exactly is the application? Like what are you ultimately trying to accomplish by combining the signals?

audio interface box - a buffered send return loop for a pedal chain that returns in stereo, but at the flip of a switch sums the L and R signals into one output (to avoid re-cabling when switching from mono to stereo setups)

[swinginguitar]

Take a look here http://www.extremecircuits.net/2009/07/stereo-to-mono-converter-based-on-fet.html

that's about what i'm looking for, but would rather not deal with pots - think it would work with fixed R's right there, or are the pots needed to "trim" for different input devices?

[Barcode80]

I dunno, seems very "Rube Goldberg" to sum to mono using electronics. Wire can't "clip", as it were, so if you are concerned with clipping then introducing electronics into the mix would make it more likely. Like building a 2 ton machine to flip on a light switch while standing next to it, if you get my drift.

If you were summing two separate[/] sources into one signal, that would be different because you are dealing with different impedance. But if both the left and right are coming from the same device, a passive splitter would work just as well...

If you are going to go that route though, I'd keep the pots in place for the exact reason you mentioned, although you could use trimpots inside the case if you don't use a lot of different equipment.

[swinginguitar]

wire doesn't clip, but would the input of the next device clip?

for example, if the signal is hitting the input of an amp, if the left signal is normally going to the amp by itself, then I throw the switch that sums left and right, wouldn't the amp now see a hotter signal (L + R) since i'm passivley combining them?

[Fender3D]

audio interface box - a buffered send return loop for a pedal chain that returns in stereo, but at the flip of a switch sums the L and R signals into one output (to avoid re-cabling when switching from mono to stereo setups)

If you're using this with pedals, you should wisely check what pedals you will connect. Most choruses and flangers (and phasers) usually just invert wet signal's phase to achieve a fake stereo effect, if you sum this 2 outputs you'll dim/cancel (by summing 2 out-of-phase signals) the effect.
In this case, you'd better use one output only...

[Barcode80]

wire doesn't clip, but would the input of the next device clip?

for example, if the signal is hitting the input of an amp, if the left signal is normally going to the amp by itself, then I throw the switch that sums left and right, wouldn't the amp now see a hotter signal (L + R) since i'm passivley combining them?

While the logic is sound, it's unlikely. But it would depend on what device you are running into it. Generally, if you are cranking the volume while you have just the left channel running, it could happen. But that isn't going to be solved by just throwing some electronics in there, unless you built in some sort of dampening (like series resistance) on the same switch you use to swap between mono and stereo.

Also, it depends on your arrangement. If you are summing stereo to mono, which would literally be taking the full signal from each channel into one channel,  the mono channel (being the combined signal of both channels) is going to be the same output level as the combined volume of the two stereo channels, so there would be no issue. Which output you hear the two channels out of is not going to change their level. You aren't adding or taking away signal, simply splitting it or combining it. Either way, the same signal is there in the same volume, just with pieces of it either coming out of different jacks, or all of it coming from one place.

But if you are talking about only using the left channel by itself then putting the right channel on a switch, then yes, your volume is lower for mono than stereo and that will need to be accounted for. That's not really summing though, and just utilizing the left channel as opposed to both is not the same as mono vs. stereo. When something has a stereo out, the left channel generally contains all the signal that would go to both channels, but a portion of the signal (wet, dry, off-cycle, whatever, however it is arranged) is split off to the right channel when a cable is inserted via a switching jack. But there is no signal added or taken away by using the effect in either state.

[swinginguitar]

Simple enough....

Guess I'm overthinking (as usual)!

I think what brought me to this point was, in my mind's eye if have, say, a 1 volt signal on one wire, and a 1 volt signal on yet another, and if I tie the 2 together to a common output, now I have a 2 volt signal to deal with at the input of the next stage....

[artifus]

try it. add a resistor/pot to each if it bothers you. it's a five minute experiment. it's unlikely you'll kill anyone.

[ashcat_lt]

Generally we don't sum two or more sources using straight wire connections.  The L and R outputs, being probably very low impedance and also likely identical in impedance, will load each other down, causing volume and probably tone loss.  Use a resistor in series with each.

Worth repeating, though, that many "stereo" effects have the exact same wet signal on each side, but with one of them inverting.  If you invert one of these signals before summing, you end up with the dry out of phase, and the wet in.  It's a kind of crazy way to turn a stereo chorus into a mono vibrato.

[artifus]

yeah, but what does it sound like?

experiment and trust your ears. cross bridges when you come to them, etc... all good advice above, but remember your goal.

[Barcode80]

Generally we don't sum two or more sources using straight wire connections.  The L and R outputs, being probably very low impedance and also likely identical in impedance, will load each other down, causing volume and probably tone loss.  Use a resistor in series with each.

Worth repeating, though, that many "stereo" effects have the exact same wet signal on each side, but with one of them inverting.  If you invert one of these signals before summing, you end up with the dry out of phase, and the wet in.  It's a kind of crazy way to turn a stereo chorus into a mono vibrato.

I'm going to have to disagree with you on that first point dude. The concept is the same as a headphone splitter, or those "sum to mono" Y adapters that let people who are deaf in one ear hear both channels. No circuitry is in those, and even audiophiles use them when needed. My EE theory is rudimentary to be sure, but I'm not seeing how joining two signals with identical impedance is any different than the single mono signal that contains both signals...

Good point about the phase though. But if the phase is correct, I don't see how it could possibly cause any tone loss or additional loading. Especially since, as I mentioned before, when the signal is not out of phase on the right channel, then in most cases the right channel signal is summed into the left channel for manual operation unless something is inserted into the right channel jack...

[Processaurus]

Generally we don't sum two or more sources using straight wire connections.  The L and R outputs, being probably very low impedance and also likely identical in impedance, will load each other down, causing volume and probably tone loss.  Use a resistor in series with each.

Yes, this is absolutely the way to do it, and for those reasons.  Last week I made a stereo 1/8" plug to mono 1/4" plug for hooking ipods, computers, etc to amps or PA's, and just used little 1/8 watt 2K resistors in series with the tip and the ring on the 1/8" plug side.

If you use a straight Y cable, the outputs being connected together will try to fight each other to the death, unless the designer of that particular piece of gear had the foresight to include a series resistor in between the output stage and the output jack on each channel.  It is sloppy to assume this is always the case.  Better to spend the 2 cents for the resistors on your end.

[swinginguitar]

Can you elaborate on the science of adding the resistors, howit alleviates loading, and how the values are dervied?

[Barcode80]

I actually just did some "homework," and rescind my previous arguments

resistor off each output would be the way to go, preferably into an opamp buffer.

[ashcat_lt]

Let's look at it from the perspective of the L output.  Let's take a worst case scenario - an opamp output with no series resistor after.  Output impedance of both L and R are extremely low, and identical.  The L sees a load equal to R||Input, as though the R itself is another input.  Whatever the input-Z of Input, the total parallel Z must be smaller than R alone, thus the "in-Z" is smaller than the out-Z.  Think of it as a voltage divider with in-Z as the bottom R and out-Z as the top.  You'll lose at least 6db.  On top of that, the current demand starts to approach infinity, and may quickly go beyond what the L opamp can safely source.  Ugly.

Good circuit design would have a series R after the opamp to set a minimum load Z, but you know what they say about assume.