Transformer and fader question for passive mixer

rotheu · Mar 13, 2011

Hi everyone,

I know it's a beaten up topic, but this case is slightly different because of the transformers and faders available to me. I have a bunch of 4:1 transformers, which can be used in revers, and 10KA faders from trident 80B desk. I'm planning to build a 24 channel mixer, but trying to avoid any active circuitry on the inputs. This is the basic idea utilizing NewYorkDave's pan circuit. My concern here is low input impedance if I use 4:1 step down and resulting high loss in the pan circuitry, reflected input impedance with 10K fader on the secondary and number of channels (24). Maybe some modification to the component values in the pan circuit could be made? Also summing resistor values. I would like to ask for your help on this. Do you think it's doable? Thanks a lot.

ruffrecords · Mar 14, 2011

I think your problem is going to be noise. You lose 12dB of signal in the input transformer, then let's say you have 10dB in hand in the fader plus 4dB loss in the pan gives -26dB feeding the mix bus. With 24 inputs the bus loss will be 24 times or nearly 28dB so the bus signal level will be -54dB (relative to whatever standard input level you choose). Assuming the nominal input level is +4dBu then the bus signal is -50dBu. Now, the pan pot resistors are 50K so Rx should no be less than 50K so the bus impedance will be 50/24 or about 2K. The noise in 2K is about -122dBu so you even with a noiseless gain make up amp you signal to noise will never be better than 122 - 50 = 72dB.

Cheers

Ian

rotheu · Mar 14, 2011

Ian,

thank you very much for your reply. I decided to try a step-up on the input instead of step-down. Wasn't sure the soundcard outputs will be able to drive 600 Ohm reflected impedance on the primary of the input transformer. So I built the "channel", used 47K summing resistors and loaded with 2K bus impedance (case of 24 channels). Frequency response is reasonably flat with only about 1db roll-off at 20KHz (0.5db at 16KHz) and totally flat low end. And I only need about 32db of make-up gain with fader at 0db attenuation, or 42db to allow extra 10db for the fader. Since the summing bus has to see at least 10 times higher impedance or 20K, I will go directly to the grids of the tubes in the gain make-up amplifiers bypassing the transformers shown on the schematic. According to your post, noise level should drop to around 90db, which is not perfect but far better than 72db. Seems like it might work, but I still have to work out all the details. Thanks again.

ruffrecords · Mar 15, 2011

OK, that will improve the noise a lot. What you need to watch now is the maximum level the transformer can handle. If you have +4dBu going in you will have +18dBu on the secondary. That is probably OK but the difference between that and the maximum output the transformer can do into 10K will determine your headroom.

Cheers

Ian

Transformer and fader question for passive mixer

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