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Stereo-to-mono passive mini mixer


Davo
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Before I go out and buy parts to build something bulkier than I'd like, I figured I'd ask here. I'm looking for a compact gizmo, about the size of an AA cell, that takes two channels on a single stereo 1/4" jack and mixes them to a mono output. It's like a splitter but in reverse. Where can I find such a thing? Would it be better to just bolt a small passive mixer to the side of the bass amp I call a keyboard amp?

Edited by Davo
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I'm not sure on this, but consider the MB6582; it has 8 mono outputs that is being mixed to one master output using a bunch of 10K resistors. I bet the same thing could be done in your case - use a 10K resistor each for R and L before they go to the mono output.

The signal will be a bit attenuated ofcourse, but the resistors are important because of (insert fancypants electronics expressions here) :) a quick search on google gave me this

20080926174400!Passive_Mixer.jpg

Being pretty close to what i imagine you want to achieve - 2 channels summed into 1. Best thing is that it'll fit in a matchbox or the likes

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I was using this page as a model. Incidentally, it links to here, which is about a cheap, quick, and effective passive mixing box. Both those pages specify using 680 ohm resistors. How was the choice of 10K ohm made for the MB6582?

The urge to tinker got the better of me and I immediately headed out to a parts shop. I got a male stereo plug, a female mono on-cord jack, and about a foot of cable. There is enough space in the hood of the plug to stick two 1/4W resistors.

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How was the choice of 10K ohm made for the MB6582?

Dunno, i did ask Wilba once though ... if i could only remember what he told me :) I must start taking notes sometime soon

Here is a few other designs, similar but one uses 4K7 resistors.

I guess you could easily test out different resistor values to find one that suits you the best. The correct way would be to google some articles about passive mixers to see if there is a way to calculate the most correct resistor value. Actually pretty interesting, but i have to sleep now, so no more google for me today :)

I really wish that you will share your knowledge with us here if you find out something cool??

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Wilba has a small mention of it on the MB-6582 wiki page:

I used 10K resistors there because that's what I've seen before in passive mixer designs, but the output is very attenuated, and I am guessing that you could drop these to 1K or less, as the outputs of the audio buffers after each SID can probably handle that. Someone with more audio electronics (and SID!) knowledge can probably answer that question.

From what I have found while tinkering with my NES and the PowerPak, small changes in resistor values don't change the output all that much, and I suspect that is because we hear loudness logarithmicly. That said, I haven't really found the output of the MB-6582 to be soft really though I am curious to know what good values are as well since I will have to make my own passive mixer for my MB-SID most likely and might as well try to improve upon the passive mixer design. Actually, I will likely mix down to two stereo channels and feed that into a Cmoy style headphone / pre amp.

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Thanks m00dawg! I was sure i saw/heard something like that, but couldn't put my finger on the source :)

That said, I haven't really found the output of the MB-6582 to be soft really though I am curious to know what good values are as well since I will have to make my own passive mixer for my MB-SID most likely and might as well try to improve upon the passive mixer design.

The summed output of the mb6582 is the one that contains all of the audio outputs, the small jack connector. If you make sound with let's say 1 SID.. Isn't it's output stronger from the 'dedicated' output compared to the summed one?

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The reason I haven't noticed the soft audio is probably because I'm running it through a hardware mixer most of the time anyway. The only time I use the dedicated audio jacks are when I am doing recording or something like that. Otherwise, I don't have enough inputs on my computer or my mixer to be able to use the discrete inputs. I haven't had to really crank up the gain on the mixer though was my point. I've also used the passive audio jack with headphones and it sounded actually a bit loud, though I am sure that depends greatly on the headphones.

I suppose a nice benefit of using the Cmoy is I can just turn out the volume pot a bit higher if the audio seems a tad soft. I think 47k is probably a pretty good place to start though. I've seen that value used elsewhere as well, though not in a dedicated mixer design necessarily.

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