Summing - what is what

Hi all!

Here's some silly questions from a newbie:

My first projects has been the GSSL bus comp and racking 8 Telefunken V672 as mic preamps...so now the time would be right for building a summing mixer and at the moment I'm gathering info :razz:

My plan is to build a 16 channel balanced mixing network like the one New York Dave and others have shown us and then use two V672's as outputgainamp.

In my research I've come acroos a couple of things I don't really understand

What is the difference between an active and a passive mixer?
What is a summing amp like for instance the Siemens V275 or Neumann V475?

Would it be better to do the summing in "packs" of eight 8->4->2
instead of 16->2

Hope to hear from you wise people :thumb:

Morten

active summing usually refers to having pan pots as well as make up gain on the output. Now some active summers just have a make up gain ala tube tech. Passive stereo summers usually send the signal out 1 of 2 outputs and do not have a make up gain stage like a folcrum.

Summing amp as far as I know and I will most likely be corrected on this is the output gain amp on a mixer. In cases like the 762, It was modular design much like api...

As for 8 into 2 VS any other well thats up to you. Most large format consoles like SSL do each bucket of 8 into 2 and then take those outs and do it again so for example say you have48 inputs it's not a 48 into 2 as one would think. it's each bucket if 8 into 2 which would be 6 buckets. then it's the stereo out of those 6 buckets into 2. If you ask me I believe people have outboard summing all wrong. Most people are doing stems then sending those stems out to the summing mixer. in this case they are still summing in the box I say.
hope that helps...

Passive mixing is just a bunch of resistors. (Or mix-coils, but you don't see those today.)

No matter how you do it, a passive mixer always leaks signals from one input to all other inputs. If the sources only drive the mixer, no real problem; but if you try to tap, say, the drum channel for monitoring, you will hear the guitar leaking in. Also, the cheap ways to add mix-level and panpot controls will interact as the knobs are moved: the guitar level may rise or fall a bit as the drums are faded up or down.

All simple/cheap passive mixers waste signal power. If you want the mixed output to be nearly as strong as the sources, you must have an amplifier after the mix-bus. So in that sense, there are no purely passive mixers. You have to have more active devices somewhere to make-up the mixer loss.

The active mixer uses a dedicated amplifier, connected to force its input voltage (the mix bus) as low as possible. With near-zero voltage on the mix bus, leakage and interaction are reduced to near-zero. Calculations and calibrations are simplified.

> mixing network like the one New York Dave and others have shown us and then use two V672's as outputgainamp

That's classic passive mixing. It works very well. It is more suitable for simple DIY than a large active mixer.

> Would it be better to do the summing in "packs" of eight 8->4->2 instead of 16->2

There are many ways to skin cats, and there are interesting aspects of multi-stage mixing. But multi-stage demands either many more make-up amps, or much higher gain in your final make-up amp. For a console that is sold in 16, 32, 48... 120-input configurations as wide as 16 feet, multi-stage may be needed for practical reasons. For 16 inputs in 2 feet, just dump them all on one bus.

Of course there is a reason to sub-mix: you may want the drum-set, the horn-section, and other musical units controllable both individually and as a group. You might want a knob to turn-down the trumpet, and a knob to turn-up the trump/sax/tromb/flute group together for the bridge. Then you need sub-mixes, and more make-up amps, and it all gets rather complicated quick. Modern fashion is to do the group-mixes in a PC/Mac, so you have the relative level of trump sax tromb flute all settled, then bring out one signal to an analog mixer where you mix the whole horn-section against the whole drum-set, bass, lead gitar, singer.

Fantastic - thank you very much Puncho and PRR for your answers. The plan is to use my project as DAW summing mixer for my computer system. I think I will start by settting up a simple 8 channel mixer and then experiment from there...thanks again guys :thumb:

Morten

Hi Morten,

afaik the V672 is designed as a summing amp (with 0-Ohm/current input) that also works as a mic pre etc.. The datasheets at kubarth.de show how to hook it up as a mixer, this might make more sense than using a passive mixing network in front of a summing amp used as a mic pre :wink:

;Matthias

P.S. If you happen to need a new input transformer for the V672, email me...

Okay, I tried to use Google, and came up with nothing.

What's a mix coil?

Thanks!

Hi Matthias...yes you are right and I will not set it up as a mic preamp. I'm working on reading the german documents (my german is little bit rusty) on how to set it up. :grin: But as far as I can see you still need some kind of network before the amp....right?

Morten

Darren,

I know the subject had come up before, so I dug around and found one of my old posts. I wrote:

The principle of operation is essentially the same as the "hybrid coil" that has been used for many decades to separate send/receive audio from a two-wire telephone circuit... except in the case of the mixing transformer, it's used the other way around, to combine two signals with minimum interaction. If you have access to a copy of Tremaine's Audio Cyclopedia, second edition, there's several pages in the Transformer and Sound Mixer chapters that go into some detail on mixing transformers. There's also a ton of references about hybrid coils on the web.

The main attraction of hybrid coil mixing, at least circa late '30s and early '40s, is that it can be arranged to have half the loss of conventional resistive mixing (e.g., 3dB of loss from summing two inputs as opposed to 6dB). When gain was more readily available, as amplifiers and tubes became more affordable, coil mixing ceased to be cost-effective. As early as 1953, the Radiotron Designer's Handbook, 4th edition, offered only a very brief explanation of transformer mixing and commented, "coil mixing appears to be very little used." I can verify from my own research that coil mixing is not often seen in mixing console designs even from 50 years ago.

Click to expand...

There is good reading out there if you look for it. Here's one example --http://www.forsselltech.com/summing buss.htm

Hi Morten,

the summing works pretty much like the active opamp summing, just symmetrical and through the transformer, which is a bit confusing .

This is the principle of operation, the yellow box is the V672 .

R_i is the desired input impedance each channel sees (divided into two resistors, as it is symetrical), the minimum overall source impedance the input wants to see must be at least 80Ohm. The external NFB resistor (RG in the datasheets) sets the overall gain and interacts with the input resistor values, so you can get different gain for each channel by using different R_i values. I don't feel like figuring out the math for the gain resistor today , the datasheet is not too detailed on the summing amp configuration.

HTH
;Matthias