transformer balanced summing mixer design

[eatlife]

Hi , I am constructing a 24 channel passive summing mixer. The design is basically the new york dave schematic , no level pot just the mbb pan switch.1/2 watt 1% metal film resistors throughout, the only change is I got a whole bunch of transformers from an mci console about 40 1:1 600 ohm and about 30 10k balanced to 50k unbalanced -8 db (or line in ) . My question is, I have tried passing audio with the 1:1 at summing mixer input feeding the pan with success, I just can't get the pan right (3db down at center) in the schematic it uses 4.3 k resistors to achieve center, is there a formula to calculate what I should use there? where do I find it? . also is using the 1:1 my best option?I would like to use the parts i have. My sydec ibox24 ch ad/da outputs are rated as variable /110 ohm nominal output impedance. thanks

[Matt Syson]

Hi
For a channel feeding 10K resistors to the bus, increasing to 14K3 is about 3dB reduction in level so should work fine.
Of course it is being considered balanced here so it is actually 20K normally with 28K6 when 'panned centre'.
Of course the are other discussions as to whether it should even be -3dB at centre or -6dB. Some go for a compromise at 4.5dB. The argument follows that the sound pressure level should remain constant which is not necessarily the same as constant power being emitted by the loudspeakers.
The formulas are simple maths excercises but you do need to have appreciation of 'log' powers.
Matt S

[eatlife]

Thanks Matt, I must have something wrong in the mbb switch because left and right work fine, When I switch to center the sound almost disappears.
What do you think about driving the 1:1 600 ohm xfrmer with the converter nominal output of 110 ohms ? can you suggest any improvements for my application? thanks for your time -Mo

[Matt Syson]

Hi
The transformer will be lightly loaded so you should perhaps check that the response is as flat as it should be but in reality it will probably be pretty good.
Thee is no such thing as 'perfect' when using transformers so if it sounds good to you then just go for it.
The fact that the transformer is designed to handle 600 Ohm loads is more to do with the resistance of the winding and unless you were to get some optimised for say 2K5 or whatever your mix resistor and level pots work out at (also being variable!) is a better solution than a 10 or 20K 'line in' transformer being 'overloaded'.
Matt S

[eatlife]

Thanks for the info Matt, I really appreciate it. I will get to building and post photos and specs upon completion .-mo