First time designing

[kytul88]

OK, so I'm in a bit of a pickle and finding the straight forward answers is becoming way too time consuming. I'm building a passive (maybe switching to active) summing unit, it started out simple, but has evolved in my mind.
I originally was going to get a capi sumbus, because the idea of a transformer and opamp on every channel was enticing. I'm not in love with the api sound, but I do like it so I was generally alright with it. But the cost is just so much and still have to put it together anyways. But there's also things I didn't need with it (switching between left/right/center, meters, a "b" bus, an amp for make-up gain as I like to change different pre's with the diyre SB2 I already have.

So, after seeing/reading the simple passive summing unit from diyre from years ago using 6.8k resistors on the stereo signals and 9.5k on the mono's I decided to make that as I wanted the a few mono channels the sb2 doesn't give me. But then I still wanted at least the transformers, and after a lot research decided to go with some 10k:600 ones. (Trying some edcor out first to see if I like them as a couple of users said they liked them and it would be MUCH cheaper of a build if I like how they sound). But since that would drop db down before it hit the 6.8k resistors I didn't want the signal getting TOO low and having the preamps have to run super high to make up everything. So, the thought came to run an opamp after the tranny to bring that db loss back to close to the starting level.

So, with 600ohm's from the tranny secondary to the input of the opamp (some 2520 footprint variation, I have a few already to try) am I right to assume with an inverting configuration to use a 2.4k ohm resistor for R2 and a 480ohm resistor for R3, with the tranny output making up the R1 value? Or with a non-inverting configuration using a 768ohm resistor for R1 and 2.75kohm resistor for R2 (values that would get me close enough and were available from mouser).

Also, am I able to run those directly to the 6.8k's for the the summing like I'm thinking would work, or are they, being in series with the outs of the opamp, going to mess up everything and I need to completely rethink everything?

I attached pictures of the schematics and where I got the math from for the R values.

 

[ruffrecords]

It looks like you are trying to impedance match which is something you do not need to do. The first thing to note is that the output impedance of a 10K:600 transformer most likely is not 600 ohm. Transformers do what they say on the tin - they transformer. They have no intrinsic impedance of their own. A 10K:600 transformer is designed to be loaded with 600 ohms and as a result will present an impedance of 10K over the entire audio band to anything plugged into the primary.

So the first thing you need to do is load the transformer secondary with 600 ohms. A 620 ohm resistor right across the secondary will be close enough. The you want an op amp gain make up stage that does not add and more load to the 620R resistor already there. So you us one in a non-inverting configuration because that has the highest input impedance. The schematics below show typical split rail and single rail versions. Adjust R1 for the gain you require. Of course you do not have to use a TL072
.

Cheers

Ian