summing… again.

[mylesgm]

So I'm looking to build a summing box with transformer inputs on each channel, different make up stages and transformer output.  I basically want everything hard wired except the make up amps which will be switchable between two topologies and a switched output gain.  All 16 inputs will be hard wired left/right.  Would also have two main outs (main and monitor).

I've got loads of transformers, a bunch of jensen 10k;600, trimax 1;1, bbc 10k:600, 20k:600 and I'm just not sure how I should go about this.

I want all the inputs to use the same txs and they are all excellent transformers but I'm not certain about the input side of the circuitry.  If I use 10k:600 how much gain will I lose across 16 tracks that I must make up with the summing amp stage?  one of the summing stages will most likely be the CAPI stage and the other either a neve stage or possibly germanium.  I'v also considered a 1:1 tx input feeding the stereotype board and then onto the CAPI stage but I'm unsure which way to go. 

Anybody want to hit me with some thoughts?

 

[crochambeau]

Your description of function sounds a lot like my Autogram AC8, except for switchable active electronics..  though I have two output sections that could conceivably have different modules installed. Mine's set up with a bunch of 1:1 transformers (Triad A67-J being the stock 600:600 ohm inputs) on input, and while I wouldn't expect anyone to express interest in cloning it (it's a huge beast of limited functionality), you might find some inspiration looking at the service data/schematic:

http://www.autogram.net/manuals/manuals/AC-8_Audio_Console.pdf

Sadly, I have no first hand experience with 10K to 600.

 

[ruffrecords]

I'll start by assuming you have 16 inputs total which means 8 routed left and 8 routed right.

10K:600 transformer will drop the level by 12dB so in theory you have 12dB more gain to make up.  With 8 signals to the bus the bus loss will be 8 times or 18dB so the make up gain needed is 18 + 12 = 30dB. However, you can now mix with 600 ohm resistors instead of 10K. SO the bus impedance will be 600/8 = 77 ohms. This is a pretty good source for a 600:10K transformer so you could add one to each bus and get 12dB of nearly noisr free gain before you hit the make up amp.

Even if there are 16 inputs per side the bus loss is only 6dB more so with the transformer the gain make up amp only needs to provide 24dB of gain.

Cheers

Ian

 

[mylesgm]

16 inputs maximum, possibly only 12 routed hard left/right in pairs.  The idea being that the only controls will be summing amp gain and summing amp selection.

Option 1
10k:600 drops the signal by 12db, feed this to a passive network of 600ohm resistors and then these feed a 600:10k transformer into the summing amp with a gain of roughly 24db depending on mix level.  That's definitely doable and the noise floor should be low.  What about adding a 600:20k or 600:15k on the make up side?

I've also quite a number of 600:600 transformers so I could put these on the input side and 600ohm passive network so a loss of 18db.  If I feed that into a 600:10k transformer I'd be just 6db shy of unity correct?  If that's the case then would the make up amp be contributing much colour?

Option 2. 
10k:600 txs on input side each feeding a DOA using the stereotype board so tx -> DOA -> resistor network -> CAPI summing amp.  Any thoughts on how that might work Ian?

Also thinking of input sources, D/A outs so very low impedance but also some DIY and commercial outboard like 1176 or pultec which may contribute to the needed input impedance.  I was thinking that using 10k inputs would essentially remove the question of possible mismatched impedances.

 

[ruffrecords]

There's no reason you could not use a 1:10 transformer at the input to the make up amp as I do with the my Twin Line Amp. You would need a secondary load of no less than 150K which is no problem for most amps these days.

600:600 transformers should be fine. You don't have to worry too much about source impedance because there are no mono channels to cause crosstalk. You just need to be sure your source gear can drive a 600 ohm load.

The only problem I can see with option 2 is noise from the input DOAs is always present on the bus. This may be only -100dBu but 8 of them will be 9dB higher or -101dBu. You signal is 12dB lower than normal, say -8dBu so the S/N drops to 93dB. May not be a problem but there is little point in adding noise if you don't have to. Alternatively you could add a mute button to each DOA output.

Cheers

Ian