advice for making a small passive adjustable stereo to dual mono mixer - Page 1
Forum section: electronics / beginners

author: ultrageranium
author_rank: Contributor
timestamp: January 17, 2022, 01:43:52 pm
content: Hello,I'm trying to make a simple passive circuit that would sit in a box between an Amiga and a pro audio mixer. The purpose is to control the amount of crosstalk between the L and R audio line out line before they enter the line in of the mixer. I have attached what I've done so far- switch:- pos 1: crosstalk amount control, from near hard pan to near dual mono- pos 2: dual mono- pos 3: untouched/hardpanI do have the following questions:- Do the resistor value make sense? It's a device that's sitting between an Amiga and a pro mixer, the line level is near a "pro" line level (+4dBu), up to 1.7V (peak). The line input of the mixer have an impedance of 10k. I'm asking because I've read that when mixing two signals, essentially creating a short between two outputs, it's recommended to put resistors. I've put 10ohms, to prevent the signal amplitude to drop too much. But is it enough? What could go wrong?- On this pagehttps://www.vintageisthenewold.com/amiga-audio-separation-solutionthere is a similar circuit where bi-polar caps have been added everywhere. Is this really necessary? I don't understand what this would add/prevent in this case.Thanks!

author: themadhippy
author_rank: Super Contributor
timestamp: January 17, 2022, 02:07:58 pm
content: why not just take the source directly into 2 mono channels on the desk and use the pan pots and if available sub groups.

author: ultrageranium
author_rank: Contributor
timestamp: January 17, 2022, 02:52:20 pm
content: All the other mono channels are taken and used in a way that can't be moved to a stereo line input (configured with inserts, etc). So I need this little gadget. It's also a nice excuse to learn a few things.

author: magic
author_rank: Super Contributor
timestamp: January 17, 2022, 04:03:54 pm
content: I suppose you could take a dual gang linear potentiometer, connect each track to L and R in opposite directions and take the signal from the wipers. You would go from normal, through mono, all the way to swapped. With two extra resistors equal in value to pot tracks, the "swap" side could be eliminated.Problem: output impedance is high and varying - not good going into 10kΩ without some buffering. But all passive solutions are likely to suffer from that.The problem with low resistances is that two channels of the source will be joined by low resistance and sourcing/sinking high current into each other. May be OK if they are capable of driving headphone like loads, or not OK otherwise.

author: mcz
author_rank: Contributor
timestamp: January 17, 2022, 05:38:44 pm
content: Yeah I would wonder if the Amiga's audio output is fine with driving a 20 ohm load. I would start with 100 ohms each. Also I assume those 10k resistors in your schematic are the inputs of your mixing console or whatever you are using. The amount of crossfeed you get with those 10 ohms compared to the 500 ohm potentiometer depends highly on the output impedance of your amiga sound thingy (because it is probably not considerably lower than those 10 ohm resistors) -> again, I'd start with 100 ohms instead of 10. However if the Amiga sound output is only capable of driving a line input, meaning it has a higher (1kOhm or so) output impedance, this circuit might not work as intended.These additional bipolar caps everywhere on the page you linked just provide some way of getting rid of any DC bias. Not strictly necessary since your mixing thingy probably has input coupling capacitors anyway.Some further reference on crossfeed (but unrelated to Amiga stuff):https://diyaudioheaven.wordpress.com/tutorials/crossfeed/

author: ultrageranium
author_rank: Contributor
timestamp: January 17, 2022, 07:44:52 pm
content: Oh dear, I think I'm starting to understand something.So, basically this is the load rule right? In the circuit design Rload should be bigger than Rinternal? Is this correct?Is it also correct that figuring out the Thévenin equivalent R of the last voltage divider on the audio out of the Amiga (it is literally just before the cinch connector) should give me an Rinternal I can work with? If that's the case this Rth would be 600Ohms... meaning that I should at the very least not go below this value for the R in series in my circuit?

author: ultrageranium
author_rank: Contributor
timestamp: January 17, 2022, 07:49:08 pm
content: Quote from: magic on January 17, 2022, 04:03:54 pmHmm I think I will need to draw this one to understand how that would work.Quote from: magic on January 17, 2022, 04:03:54 pmWhat is "not OK" in this case, there will be too much current drawn from the Amiga followed by magic smoke? Sorry if it's an obvious question...

author: magic
author_rank: Super Contributor
timestamp: January 17, 2022, 08:01:10 pm
content: Possibly up to magic smoke if the output is really lousy and unprotected.Basically, you have two voltage sources, which could be as far apart as ± full output swing. You connect some resistor between them. Current starts to flow.Similar problem as shorting the output to ground, or loading with low impedance like headphones.If you are lucky, it will work fine. If you are medium-lucky, you may experience loss of volume and/or bass due to output impedance.

author: ultrageranium
author_rank: Contributor
timestamp: January 17, 2022, 10:03:56 pm
content: Quote from: magic on January 17, 2022, 08:01:10 pmThanks for the explanation, I don't think I want to risk it or experiment with it to be honest. Following this and as I wrote earlier, the R thévenin equivalent of the last voltage divider before each of the cinch output inside the amiga is 600 ohms, so am right to think that in this case as long as each line out, L and R, are followed by at the very least a 600 ohms resistor this should be safe or safer?