Summing stereo to mono

[Karlsmart]

Hello Solderdude, I was redirected here by Claus after recieving a lot of help from another thread.

My problem is that i need to downmix a stereo signal to a dual channel mono signal. I plan on doing this between the DAC and the amp (correct me if I'm wrong). My initial thought was so buy a line level combiner, such as https://www.edcorusa.com/s2m-c or http://www.disconetto.dk/shop/stereo-mo ... 4364p.html.

I was then linked a schematic by Claus that showed two 4.7k ohm resistors summing the signal, and noticed the line level combiners had a 4.7k resistance aswell. I was just wondering if you know how the specific value 4.7 came about, why the 47k R3 is not needed, and maybe some information about how this will limit sound quality (if at all).

The gear i use is the ODAC and an O2 Amplifier with Hifiman HE-500 38 ohm headphones.

Sincerely Karlsmart

[Solderdude]

You can do this with other mixing resistors as well.

The input resistance of the O2 is 10k

The mixing resistors just need to be big enough to not 'load' the source too much for stereo signals.
As the output R of the ODAC is low a 1k resistor will do already.

A 'real' mixer circuit is actually different and is more of a 'summing' circuit.
In the circuit with the 2 resistors you just 'combine' left and right, they do not add up.

Still it'll work just fine.
Given the 10k input R I would use 1k resistors to 'sum' the signals.
So ground from ODAC and O2 connected as usual.
Connect the L and R input of the O2 together. (this lowers the input resistance to 5k.
a 1k between the left out of the ODAC and 'combined input' of the O2
Plus a 1k between the right out of the ODAC and 'combined input' of the O2
This will mix the signals just fine and won't load the ODAC too much.
You can also use 2 of those 4,7k resistors for summing but the input signal will drop in level a bit more.

No need to add the 47k though, the input R of the O2 is much lower and has the same function.

Because stereo contains high frequencies that determine positioning in a stereo image this means the phase of L and R signals could sometimes be shifted in phase or even almost counter phase.
In that case it is unavoidable that sometimes the amount of treble may be less than when stereo is heard via 2 speakers and 'mixes' acoustically in the room.

Similar as to listening to an FM radio stereo signal when you hit the 'mono' button.
So yes, sound quality might be affected but at least you will hear left and right signals combined.

The Monacor device will do the exact same thing but is already built into a nice enclosure.
However, it is easy to just solder a resistor in an RCA plug and make a short cable.

The other one has a small audio traffo inside and is not well suited for this task.

Let me know if you need help with this...

[Karlsmart]

Thank you this is extremely informative. I'm going to order some resistors and make the connector. No point paying 50€ for what can be done for <10 :)

I'll report back if i encounter any issues, or when I'm finished!

[Claus-DK]

Could the treble loss be EQed away or is it to complex ?
Been wondering about that with the FMradio for years, now I know and FM are alomst dying, well better late then never..

[Solderdude]

The treble that's gone is gone...
It disappears because of phase differences between L and R channel which gives clues about positioning of instruments.
It's not really phase differences but rather timing differences but effectively they become phase differences for certain frequencies.
A comb-filter effect.
When you mix the signals, or in case of FM, you don't separate them, certain higher frequencies 'cancel out' or reduce in amplitude.
Of course you can lift the remaining treble a bit to 'mask' this effect somewhat but it won't be the same.

[Ostebaronen]

Maybe with some machine learning you could predict the missing treble, but that would be awful lot of work to do so.

[Karlsmart]

The cable is done now and works perfectly. The treble loss is much less noticeable than the software solutions i've tried.

Thank you very much for your help.

[Solderdude]

What value summing resistors did you end up with ?

[Karlsmart]

I bought some 1k resistors and put them between the dac and the amp.