EMI TG Summing Amp/Mixer - Need Help

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Active member
Apr 17, 2009
New York, NY
Years ago, I traded some stuff with a forum member in return for an EMI TG Summing Amp. The person I got it from didn't know too much about it and the info I received with it was sparse. I was left feeling like there were a few gaping holes I wish I could fill in. Oh and just to put things into perspective… I know next to nothing about the EE theory behind summing other than that the more channels one has, the more gain keeps falling to the wayside. Their function I'm well clear on. I just have no idea how they work "under the hood". I decided to write this thread to get some help from those more in the know about such circuits because ideally, I'd like to build the blasted thing!!  8)

OK, here are the basics…
My TG "kit" consists of:
- 2 x large PCBs (at least 8 x 12" each). Each PCB has 8 medium-sized EMI transformers mounted to them.
- 2 x Amp Output Cards (typical TG 2N4898 + 2N4910 circuit).
- 8 x Dual-Gang Potentiometers.

As far as documentation:
- I have the original EMI schematic for the large PCBs with the xformers mounted on them.
- Diagram of Amp Output Card (one marked "R" the other "L" indicating which pins are, "Audio In", "Audio Out", "-V", "+V" and 
  Below this diagram is a hastily written note about what voltages one should be able to run it on (that is if I read it right!  ;))
- The last doc is a hand-drawn hook-up diagram between the Output Cards, Pan Pot and Transformer (presumably on one of the 
  massive PCBs), in that order. In the interest of being meticulous, there is a 3k3 resistor between the Card's Output and the Pot.

So the question is, how on earth do I string all this into a functioning summing amp? I contacted someone named Guillame Sauvageot (sorry, forgot where I even got the idea to contact this person. That's how long I've had this thing) and he gave me some useful information;
"The output stage you have is an amplifier with a fixed gain of 14 dB.
It's a generic output stage used in the EMI desks (group output and insert output).
You could use it as a makeup amplifier in a passive mixing network.
With 8 inputs, the summing loss in a passive mixer is 18dB."

From what Guillame says it seems like I need some sort of makeup gain right before the outputs, right? He says that my output stage is 14dB and 8 inputs has a summing loss (in a passive mixer) of 18dB. Is the 18dB a summation from all 8 channels, or do each of the 8 channels lose 18dB? If the total summing loss of all 8 channels is 18dB that means I'm 4 dB short. Do I need a makeup amp that will supply exactly 4 dB, or does that not matter?

Lastly, I've seen a lot of people add things like Neve or API-style makeup amps to their summing projects. Since this TG Summing box isn't exactly common I'd like to keep the sound all "TG". By that I mean that I'd prefer that makeup amp to be EMI TG by nature. Don't really know how as there aren't many EMI documents overloading the internet.  :-\ Ideas, anyone?
I contacted Chandler thinking if anyone would know how to help me sort this it would be them. In reply I got a very snappy one sentence reply that simply said, "No. We can't help you with this". No suggestions, referrals, nothing. You'd like to think that the "passion" that Wade (owner of Chandler) has for EMI gear would translate to a little courteous banter in the very least! I guess that's asking too much.  ::)

My only idea came from a transformer on Sowter's website that looked like it might fit the bill:
1365 EMI B35A for TG12345 Output Amp R
"100% Mumetal output transformer ratio 1:1.77. Total DCR referred to secondary 160 Ohms aprox. Max level +18dBu at 20 Hz".
There's also a schematic, rather simple. Can someone chime in? I'd appreciate any and all help that comes my way.

Here are the only pictures of the project that are on my hard drive:





14dB vs 18dB means if you have only one channel signal you will get the same signal 4dB attenuated at the output (plus some noise intrinsic of the summing amp, but don't worry about it) Many times a loss is wanted at this point, how much will depend on the content but if you have all uncorrelated inputs you will have 9dB higher signal at the output, this depends on the material so 9dB is just a random number (for random inputs, easy joke, sorry) so probably having 4dB loss in there isn't a real problem. Probably you are going to some other equipment where you can trim the input sensibility and that would just adjust for what you are looking from.

The other thing to do if you really want the extra 4dB would be changing a single resistor, which shouldn't impact much in the thing, probably much less than adding another transformer.