Adding Stereo audio signals, after a panpot circuit? - Page 1
Forum section: electronics / beginners

author: jwhitmore
author_rank: Regular Contributor
timestamp: August 19, 2020, 04:53:58 pm
content: My experiment converting mono signals to stereo audio and adding them did not go to plan, a whole bucket of fail. I think I can see my mistake, but not sure how to resolve it. I didn't think this through, but you live and learn. Hopefully someone cleverer then me can look at this and suggest a solution.For the moment I'm using a signal generator with two channels and working with a 100Hz Sine wave, 100mv and a 600Hz Triangle wave, 100mV. I've attached three scope traces, because as soon as I add the two mono signals it effects my inputs as the signals are simply connected together. The output of the two added is what I expected to see out of the simple mono addition of the signals. That's the third scope trace.So I had the 'clever' idea of converting the two mono signals to stereo and panning both left and right in the stereo field. So I found a circuit  which seemed fairly logical and simple and I implemented two of those circuits and simply connected the two 'left' and 'right' outputs together. So each of these circuits individually functions correctly, but connecting to two lefts and rights, it turns out, is totally wrong. Like I say I didn't think this through. I think my problem is that each circuit is potentially applying negative feedback of it's output to the other pan circuit's input so the output of left and right is cross contaminated and a mess. Nothing like the simple mono addition in the third scope trace.So I need something to stop cross contamination of output signals to feedback of opamp. It's kinda like all I need is a 'diode' like device and the negative feedback is connected before the diode whilst the outputs of the two circuits are added after the diode. I might be totally wrong but I don't think I can use a simple diode as this is an audio signal which traverses the zero voltage level. That would be too easy.On the face of it I think I need four more OpAmps which 'buffer' the outputs before they're physically connected together. So instead of a non-inverting panpot circuit I've used from  use an Inverting amp and then follow each of the 4 outputs with another unity gain inverting amp. After the inverting unity gain 'buffer' connect the 2 lefts and 2 rights together. Having said that the unity gain inverting amp is still applying negative feedback so I'd still have the same problem, of feeding one signal to the feedback of the other signal.I've been searching for circuits to add audo signals together and all I'm finding is summing amplifier circuits. I don't think that is helping me as the signals are simply connected together before the amp.https://cdn.shopify.com/s/files/1/0321/7609/files/Panpot.pdf?263

author: Renate
author_rank: Super Contributor
timestamp: August 19, 2020, 11:06:22 pm
content: Quote from: jwhitmore on August 19, 2020, 04:53:58 pmBut it does help.You need to have resistors on the signals into the summing amp.The summing amp input (when balanced) is a "virtual ground" so each of your inputs only sees a resistor to ground.

author: jwhitmore
author_rank: Regular Contributor
timestamp: August 20, 2020, 01:16:31 pm
content: Ahhhhh! Thank you so much for that, very helpful advice.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2020, 10:14:29 pm
content: Directly connecting them together adds the signals but also reduces their amplitude to 1/2 because the output impedances form a divider, so the net effect is to average them. Usually this is sufficient as you can get the same results as adding them by doubling their amplitude to make up for the attenuation.Look up "summing amplifier" which is an inverting amplifier with multiple inputs if you want to be able to add them without one signal interfering with the other.