Audio Mixing Matrix - Page 1
Forum section: electronics / projects

author: E
author_rank: Contributor
timestamp: October 10, 2013, 12:02:57 am
content: Hi! I'm trying to build a 6x6 audio mixing matrix. It's supposed to be able to mix six audio line signals onto six line outputs in any possible combination using digital switches.For this I created a smaller 3x3 matrix to test some circuit designs and finally came up with this one:(see attachment)The three inputs are on the left, simulated with 0.7V sine waves at different audio frequencies. For switching I plan to use a 1.2kOhm analog switching chip, which I emulated here by using a 1.2k resistor and a manual switch. I want to prevent crosstalk between the channels so I used virtual earth mixers.These alone still had the problem that enabling / disabling channels on different outputs changed the levels at the other outputs. To handle that I added the opamps on the left.Unfortunately opamps still confuse me a lot and the circuit simulator isn't always the most reliable, so I'm not sure if this design is any good or if I may be doing something really weird and unnecessary or lacking something. The resistor values btw. are not calculated but guesstimations.I would greatly appreciate some comments for the circuit. Do you think that it will work (it does in the simulator)? Are the resistor values good? Should I add coupling caps?Thanks!E

author: david77
author_rank: Frequent Contributor
timestamp: October 10, 2013, 12:41:13 am
content: That will work. I've built a 12x6 audio switching matrix not that long ago.Some suggestions:1. The input buffers don't have to be virtual earth types. That is the reason why you're experiencing level shifts when switching multiple inputs on one output.Use a non-inverting configuration for the input buffers, the advantage is much higher output impedance and no - or less - variation in output level.2. Do you need so much amplification? A high value resistor introduces more noise than a lower value. I'd suggest a max. of 22k instead of the 63k. Depending on what you want you'll need to change the 3k9 resistors as well.I'd go for a v=1 buffer on the input. I'll post my circuit as an attachment, it's hand drawn and in German but it should be clear enough anyway. There is no amplification in there, v= -1. So it inverts the signal by 180°. If that is not desireable you can add another inverter stage to get the signal back the way you want it.3. Yes, I'd add coupling caps after the switch matrix before going into the inverting amps. Also a resistor on the input of the buffer stage will help keeping DC levels around the GND mark, see attached circuit.4. What kind of analogue switches are you planning on using? They can be quite expensive.I've recently done a series of tests on how different topologies and types of switches behave in audio circuits. The result was that the ordinary 4066 bilateral switch performs very well when used in a virtual earth amp arrangement. That's because the 4066's bad habits mainly (Ron) vary by the voltage that's applied to the switches. Basically the lower the voltage across the switch the less trouble the varying Ron causes. In a VE type of arrangment there is basically no voltage present on the bilateral switch. Result is Ron does not vary, no non-linearities.Hard to beat for the price. They can be supplied by +/-5V anf the enable inputs are then TTL compatible, +5V is ON and 0V is off.

author: David_AVD
author_rank: Super Contributor
timestamp: October 10, 2013, 02:35:36 am
content: I'd be using higher value resistors for the virtual earth mixing. Maybe 47K.

author: E
author_rank: Contributor
timestamp: October 10, 2013, 10:33:10 am
content: Hey Davids,thanks for the help. German is perfectly fine for me, since I'm also member of Dave's big german fanbase2.) nope, my problem rather is that I have too much amplification, which is why my resistor values were so small. Using your schematics, I might still have to scale it down since the combined signals go way above 0.7. I'll have to take a closer look at my input signals to find out, they probably already are a sixth of 0 dB.4.) I was planing to use74HC_HCT4066from NXP.I see you're using NE5532 opamps. Would you recommend them to me as well? What should I be looking for? I haven't used opamps in two years or so, but I remember there being a lot of different types with many specific properties.CheersE

author: Hideki
author_rank: Frequent Contributor
timestamp: October 10, 2013, 11:02:40 am
content: Although it's old, NE5532 is still a good choice for general audio use, and the price is right. Modern chinese-sourced designs tend to use the NJM4580 a lot, but it's not necessarily any better.

author: david77
author_rank: Frequent Contributor
timestamp: October 10, 2013, 07:52:52 pm
content: Hey E,yes I can fully recommend the NE5532 or NE5534, they've been around for decades and there certainly are newer better OP's but they perform very well and are cheap.I've built my switch matrix using mechanical switches as the input part of a custom mixing desk for my own use. In the stages that follow that input part I use the 4066 extensively for signal routing. There I also use a fair few DG409's which are far superior to the 4066, much easier to handle because they don't need to be in a VE type arrangement but they're also far more expensive. Nearly 5,- EUR a piece from Reichelt.The project's not finished yet but so far it looks promising. There are about 60 NE5532 in there now.