Summing and Splitting, same bat time, same bat channel

> What I get with this arrangment is the summing I so desire, but the directs on the back of course are getting some of the other tracks bleeding in.

Draw it out and see why that happens.

Each source has a source resistance, not necessarily the same as the nominal load. For true-600 ohm gear of modern vintage, the source resistance may be 50-100 ohms, but for less professional stuff (and some stuff that claims to be pro, and most really old stuff) the source resistance may be 470, 600, or even 1,000 ohms.

I have assumed 1K ohms sources, 10K mix resistors, 12 inputs, infinite "direct" (bridged) loads, to make the math obvious. And I have napkin-drawn it unbalanced, though that really changes nothing in this analysis.



Say that Input 1 has a 1V loud signal, Input 2 has "the sweet sound of silence" (or a soft track) that you want to keep clean of the Input 1 signal.

The sneak path from Input 1 to Input 2 has two "split points": the mix network, and the output of Input 2. At each point the power can go several ways, and we can simplify each split into a 2-way split with tolerable accuracy.

Split 1: In round numbers, the mix network has loss of about 1/10. So the 1V on Input 1 lands at the mix bus at about 0.1V. This hits the "back" of all the mix resistors and leaks "toward" all the sources.

Split 2: Input 2 has source resistance of 1K. This with the 10K mix resistor gives loss of about 1/10.

So Input 1 arrives at Input 2 at (1/10)*(1/10)= 1/100 or -40dB of original strength. Assuming Input 1 is 1V, then 0.010V of Input 1 signal lands at the Input 2 jack and "direct" output.

What to do?

Suppose the source resistances were more like 100 ohms. We still have 1/10 loss in the mix network, but the back-feed path is the 10K mix resistor into the 100 ohm source, or 1/100 loss. Total loss is 1/1000 or -60dB down. In many situations that won't be a real problem.

But it can be. If you can get the source resistance down to 10 ohms we now have -80dB leakage.

> I can tolerate additional loss

There is another way. The 1/10 loss in the mix network "could" be higher. For illustration, a 100-input mix network or a reasonable network loaded in 100 ohms would have 1/100 loss. Combined with 100 ohm sources we get the additional 1/100 loss for total -80dB crosstalk, with 10 ohm sources we have -100dB crosstalk which really should be ample.

Simply loading the mix network reduces crosstalk by reducing signal. In many cases we can't afford much loss of signal because we are close to noise-level already.

This is really buying crosstalk the hard way: S/N is going down as fast as crosstalk. You have to be WAY above the universal noise to go for that path. NYDave's 10dB loading is a reasonable thing to try, and sure the cheapest quick-fix. But trying to get 20dB or 30dB less crosstalk that way will bite you like a snake (hissssss).

Active mixing has the indisputable advantage of reducing crosstalk without significantly reducing signal or S/N ratio. A common op-amp in the common summing-amp hook-up will show dynamic input impedance well under 100 ohms, under 10 ohms at lower frequencies (limited by non-infinite gain and bandwidth). Or to put it another way: mix-net crosstalk is now equal to the amount of feedback, not the number of inputs. Using 5534 or TL071 the crosstalk at 5KHz may be -60dB, plus whatever mix/source-resistance loss you have.

If you are stuck with a high-resistance source, high-resistance mix resistors help. They don't change the crosstalk in the mix-net, but increase the back-talk loss into your unavoidably high source resistance. A 10-in mixer, 1K sources, and 1Meg mixing will have 1/10,000 or -80dB crosstalk. However 1Meg mix resistors have 10 times the noise voltage as 10K mix resistors. However, the 10K mixer has self-noise lower than a cheap amplifier (and lower even than 5534), so the increase of noise may not be bad.

If crosstalk is more important than minimalist design, the path is clear: put buffer amps everywhere. The source boxes have resistance partly for stability in long cables but also for short-protection. Put a TL07x input buffer on every channel. Feed your "direct" output through a few hundred ohms for safety; feed the mix network directly from the op-amp pin. The output impedance of common op-amps will be an Ohm or less over most of the audio band, so the back-talk from the mix-network is swamped 1/10,000 plus the mix-net loss. Adding active summing gets you to a computed -140dB (but we have not considered inductive or capacitive leakage or ground resistance; -100dB would be exceptionally good in real life).

[quote author="PRR"]Draw it out and see why that happens. [/quote]

I'm glad your crosstalk analysis yields the same result as mine. I always wonder if I hit all the right buttons on the calculator :green:

This is really buying crosstalk the hard way: S/N is going down as fast as crosstalk. You have to be WAY above the universal noise to go for that path. NYDave's 10dB loading is a reasonable thing to try, and sure the cheapest quick-fix. But trying to get 20dB or 30dB less crosstalk that way will bite you like a snake (hissssss).

Click to expand...

Yes, the brute-force approach has its limits. One good piece of advice I saw in an old audio text was to never attenuate a signal to below mic level (about -45 to -50dBM), and preferably, to stay at least 10dB above that. Some old mixers ran at surprisingly low internal levels (down in the minus 30s and 40s) before the first post-mixing booster amp.

Since the kind of box Fum has built is typically used for summing of DAW output channels, and followed by one's favorite pair of mic preamps, a source impedance of about 200 ohms, and a mic-level output, is actually desirable. It's not the very best way to go as regards S/N and x-talk, but it's become a popular idea because it allows the flexibility to choose a desired "flavor" of booster amp.

A minimalist active implementation could use a single 5532 for each output channel, run as simple inverters (shown here in outline form):

> I always wonder if I hit all the right buttons on the calculator

Well, if you cheat and use a calculator, note that on my fingers (1K)/(1K+10K)== 1/10 or 0.1 or -20dB, where a calculator would say 1/11 or 0.909 or -20.828dB. And that still ignores small details like mix bus effective impedance. If you have to meet a test-spec, use the exact answers. For real life, we rarely know exactly to-the-dB how much crosstalk we can stand, so this on-fingers estimate is "good enuf".

> One good piece of advice I saw in an old audio text was to never attenuate a signal to below mic level (about -45 to -50dBM),

Generally a good starting place. There is some place in every system where you have to take the noise you get, usually the input; and everywhere else you can choose to stay way above noise. Gain+loss from Mic In to mix bus should be positive for any likely control setting, a different way of saying that the mix bus level should not be lower than the mike level.

I would point out that the rule really applies to systems where the mix-amp was another mike-amp, and mix impedance was similar to mike impedance. Because mike impedance is inconveniently low for transformerless gear, we often mix at higher impedance. 10K instead of 150 ohms. The noise voltage will be higher in the higher impedance, so you want some extra voltage gain. (What you really want to watch is signal POWER, but since we got away from standard impedances this is a lot of trouble.)

> and preferably, to stay at least 10dB above that.

If your mix bus runs at the same level as the mikes, and the same/similar impedance, and the mix amp is similar to the mike-amp, then you have 3dB more noise mike+mix than mike alone. Mixing 10dB up from mike level makes mix bus noise insignificant.

Note that in an Active mixer, you figure the mix bus noise as if it were a passive mixer. Ten 10K mix resistors and a 10K output resistor looks like a unity-gain mixer, and is for signal, but the noise-gain is 10. It is better to realize that this is still a passive mixer with loss of 1/10, plus an amp with gain of 10. At the same impedances, an active mixer has no noise advantage. (It can, if the reduced crosstalk allows using lower resistances than you could get away with in a passive mixer.)

Hi all,

Jumping in late here after vacation. I am also building a mixer based on an API output gain stage and wanted to throw the design by you all for constructive criticism... The goal is a 16 ch 1:1 summing mixer where odd channels are hard wired left and even channels are hard wired right.

Assuming all balanced inputs (50 ohm source impedances). No auxes.

-8 channels assigned to left buss
-8 channels assigned to right buss

-10Kohm resistors on both HI and Lo side of the balanced inputs feeding the mix buss.

-Each side (both left and right) have one opamp each where the sum of the hi signals feeds the non-inverting input and the the sum of the lo signals feeds the input of the inverting input.

-feedback resistors are also 10K (although this might change due to the API output transformer I'm going to use).

Any thoughts? All input is appreciated.

Thanks

Mike

Looks good, but why call it a passive mixer? That's a conventional "virtual ground" active mixer.

I'm having a similar problem with a line mixer- used for keyboards mostly. with a switch to choose bus A, B, or out.- This is based on the hamptone pre. But my my have I go a lot of crosstalk right now. :?

http://f2.pg.photos.yahoo.com/ph/[email protected]/detail?.dir=/d30d&.dnm=e665.jpg

Initially I experimented with quite a few different mix bus resistors and several different gain pot values, so at the moment everything is about right as far as my levels at various points, (i.e im no longer hitting the gain stages so hard that they go into major distortion anywhere except the first 10degrees of pot travel.

I've been trying some variations on New York Dave's idea with the 330 ohm resistor, but I'm in an unbalanced mode here. I already got the drop in level I needed with an additional 15k between mix bus and ground, but the crosstalk is just deadly.



if you look at the pics you can see that I'm sharing a ground plane on the two boards... I can cut this if it might help. you'll also notice a few 470uf decoupling caps on the boards.

I don't know, I'm just probing blindly at this point, would love a bit of guidance

Thanks
Sleeper

Thanks Butta.
I'll have to try sumptin like that.
Initially I was using the 150k s as spec'd in the hamptone docs with a single JFP stage... the gain was too low with the 150ks.
At the same time I had my power transformer inside the chassis, even though heavily shielded, there was way too much noise.

I added the second stage at the same time that I changed resistor values and moved the supply. set up a whole new round of problems. Classic bad habit, have to stop. but my bench is better situated now, so maybe I can try and be a bit more methodical.

here's a question: kind of obvious, or maybe not.
Any problems associated with putting the resistors in series?
heck of a lot easier to replace them that way.

thanks
KM