Summing a stereo signal - Page 1
Forum section: electronics / projects

author: richlooker
author_rank: Regular Contributor
timestamp: June 25, 2020, 10:47:53 am
content: [ Specified attachment is not available ]I am trying to find the best/correct solution to a well-known problem for which there exist several more or less working solutions, but none that are proven by theory.There is a range of small, affordable hardware synthesizers - the Korg Volca range - exemplified here by the Volca Bass:With a built-in sequencer, ribbon touch keyboard and a headphone output, these can be used standalone, but the real fun starts when you combine several of these units (9 different ones exist,) and use external MIDI controllers (eg. sequencers, keyboards and drum pads.) This requires that you hook them up to a mixer, and therein lies the problem. 6 of these 9 Volca synths produce a mono signal, but they all use the same stereo headphone amplifier chip (TI TPA6111A2) and a 1/8" stereo TRS socket (which is the only available signal output.) Here is the schematic of the headphone circuit:Mixer channels are a limited (and expensive) resource, so using two channels for what is essentially a mono signal is not an attractive option. As most mixers have a balanced mono 1/4" TRS inputs, these are some of the different approaches/attempts to connect these synths to mixers I have seen;1/8" TRS to 1/4" TRS - seems logical on the face of it, but this means the R and L channels are routed to the positive and negative rail of the balanced input, so what you get is the difference between L and R. Probably because of component tolerances, there is some difference, but the effective signal is very weak, and mixer gain controls must be turned way up before you can hear anything1/8" TS to 1/4" TS - also logical as we have a mono signal, but this shorts one of the output channels to ground, resulting in attenuation, compression and distortion of the other channel1/8" TRS to 1/4" TS with only one channel connected - works, but you lose some signal level (6dB?) compared to using both channels and a stereo input and compared to the true stereo Volcas1/8" TRS to 1/4" TS with a passive summing resistor network - my hypothesis is that this is the optimal solution, if the resistor values are chosen correctlyI have found this generic solution, which does not account for output and input impedances:I want to establish; how do I passively sum the two channels, without straining the Volca output, while getting the highest possible signal level? (The output impedance of the headphone amp chip is not stated in the data sheet. The input impedance of the mixer is 10K between each of +/- and ground.)Cheers, Richard

author: moffy
author_rank: Super Contributor
timestamp: June 25, 2020, 11:30:20 am
content: Since you have 2 outputs which are the same mono signal, you only need to use one of them, you don't need to mix them.

author: bateau020
author_rank: Frequent Contributor
timestamp: June 25, 2020, 11:36:33 am
content: How about making the output differential?Insert an inverting opamp circuit before or after C114, or (not better but a lot easier) connect the left side of C114 to IC16A pin 1? For the latter, IC16A has no gain (well, it has -1), so that should work.Then you have a balanced signal between T and R, and you gain 3db compared to if you would connect only 1 side (T to S or R to S), and it would be as the balanced mono 1/4" TRS destination inputs expect.edit:By the way, seen the input impedance, I would expect no difference in signal level between these 2:Quote from: richlooker on June 25, 2020, 10:47:53 am

author: richlooker
author_rank: Regular Contributor
timestamp: June 25, 2020, 12:12:03 pm
content: Quote from: moffy on June 25, 2020, 11:30:20 amI know, but I want the signal level to match that of the true stereo Volcas, such that the signals are equally loud when the mixer faders line up.Cheers, Richard

author: richlooker
author_rank: Regular Contributor
timestamp: June 25, 2020, 12:13:40 pm
content: Quote from: bateau020 on June 25, 2020, 11:36:33 amI could do that, but then the signal would sound really strange when I use the output for headphones. Plus, I don't want to mod the synth unless I have to.Cheers, Richard

author: themadhippy
author_rank: Super Contributor
timestamp: June 25, 2020, 12:55:27 pm
content: Quoteso adjust the head amp(input gain) on that channel of the mixer.

author: moffy
author_rank: Super Contributor
timestamp: June 25, 2020, 01:34:16 pm
content: Quote from: richlooker on June 25, 2020, 12:12:03 pmLet us say that the mono output is V1 (pin 1, U16). Then the output (pin 7, U16) is also V1, since they are identical. The output of your little mixer circuit is:Vo = V1*(20000)/(20000 + 475/2) or approximately V1. The slight reduction is because of the 20k resistor, otherwise it would be V1. So what is the problem with matching the faders?

author: Bassman59
author_rank: Super Contributor
timestamp: June 25, 2020, 07:44:54 pm
content: You're waaaaaaay overthinking this.Get a few ofthese cablesor similar. Get a Mackie 1402 or Mix12 mixer. Take the outputs of your Korg things, connect them to the ⅛" TRS plug on the cable and plug the ¼" sides of the cables into the mixer's ¼" line in jacks. The line-only inputs on the mixer are stereo, so connecting is simple.Twiddle the knobs on the mixer until the mix is what you want. Use the mixer headphone jack and solo switches for monitoring. Take the mixer main outs into whatever is next.

author: richlooker
author_rank: Regular Contributor
timestamp: June 26, 2020, 10:40:20 am
content: Quote from: Bassman59 on June 25, 2020, 07:44:54 pmI know; I have realized that after overthinking it even moreI wanted to establish the scientifically proven correct solution. My summing theory came about as I drew parallels from acoustics to electronics; if you place two speakers, each with it's own amplifier, playing the same signal, itt wil sound louder to the ear than if you use just one of them. Alas, summing an electrical signal doesn't work that way, as established by Ohm and KirchhoffConsidering the diagram above; if we assume that the two generators are equivalent with respect to level and phase, we can simplify it like this:Now it's obvious that the summing network will only ever attenuate - not amplify - the signal. The highest signal level is achieved when R1=0 and R2=∞Conclusion: The optimal solution is to connect one of the output channels to the mono mixer input, and leave the other one unconnected.Cheers, Richard

author: Jan Audio
author_rank: Frequent Contributor
timestamp: June 26, 2020, 04:04:04 pm
content: 1 : Like sayd : dont mix anything, not needed.2 : They are not cheap : if you buy 9 of these it still cost more then 1000,-3 : Building your own is more fun, and cheaper.4 : Dont start buying, you will only buy more.5 : A audio mixer is the most simple thing to build, just more wires and connections if you want bigger ( make without EQ is most simple ).

author: dmills
author_rank: Super Contributor
timestamp: June 26, 2020, 04:39:29 pm
content: 6dB? I don't see the issue, but to the extent there is one, the answer is to pad the stereo units down by 6dB to match the mono ones not the other way around.I would suggest a pair of 100 ohm resistors would make a suitable pad for each channel of each stereo synth, 200R (series) is largeish compared to most headphones, but 50 ohms (parallel) is small compared to the 10k of the mixers line input.