NewYorkDave
Well-known member
Brad: No, you'd leave it at 4.7K.
Svart: No. Reverse log would only be down 1dB at center.
Svart: No. Reverse log would only be down 1dB at center.
If you're talking about simply replacing the continuous-track 20K dual linear pot with a 20K stepped dual linear pot, made up of strings of 2K resistors, then that would work fine. You'll still need to keep R1, R2 and R3 as shown on the schematic.
NewYorkDave said:Well, here's the problem: every part of the circuit is highly interactive.
The input impedance of the mixer should be around 5K at a minimum in order to work with a wide variety of equipment. Lower than that is no problem for most true professional equipment, but most definitely a problem for consumer and "prosumer" stuff. And the output of the mixer needs to have a level and source impedance compatible with the input of a mic preamp.
The input impedance of the panpot circuit is fairly constant at about 50% of the value of the pot--and this is much better than the "usual" dual-lin panpot circuit, which has an input Z that's all over the place. The panpot needs to be fed from a source impedance of not more than 20% of the pot value. That, along with the minimum input impedance requirement, sets the limit on the value of the level pot.
The panpot needs to be loaded by a resistance equal to the pot value; this consists of the mixing resistor and the buss impedance in series. A 200-Ohm buss impedance is negligible compared to the value of the mixing resistor in the case of this design, so we can ignore it and use a 10K mixing resistor for a 10K panpot, 20K for a 20K pot and so on.
The impedance of the buss is all channels (mixing resistor in series with the panpot source impedance, which varies from zero to about 15% of the pot value) in parallel, which is forced down to the desired value by Rshunt.
If we scale everything up to allow a 50K dual for the pan--which means ~50K mixing resistors-- the output impedance will be too high. If we adjust the shunt on the output to compensate, the total attenuation will be ridiculous, more than twice the already-substantial figure of the circuit as I drew it. You'd be lucky to achieve anything approaching an acceptable signal-to-noise ratio. If we scale it down to use a 10K panpot, the input impedance will be too low to be acceptable for "general purpose" use.
The input impedance of the mixer should be around 5K at a minimum in order to work with a wide variety of equipment. Lower than that is no problem for most true professional equipment, but most definitely a problem for consumer and "prosumer" stuff. And the output of the mixer needs to have a level and source impedance compatible with the input of a mic preamp.
if you scale everything down a factor of 1/2... the input Z will be the 4.7K pot in parallel with the wiper load (2.4k in series with perhaps another 4k). So not quite 2.5k, but perhaps low for consumer gear.letterbeacon said:I'm planning to build a 16 channel summing mixer as designed by NYD in this thread (schematic attached).
Back in the thread it was asked if you could use a different sized pot for the pan pot and I was also wondering if I could use a 10k dual linear pot, seeing as CAPI do one with a centre detent fairly cheaply.
NYD said no because...
Say if I halved the values of all the resistors and pots allowing 10k pot to be used for pan and a 4.7K pot for the input attenuator. Will this mean that the input impedance will be 2.5k? If so, I know that's low and as NYD says:
I am not a fan of passive mixers so good luck.But seeing as I'm building this for my small project studio, and will be mainly connecting a Fireface 400 and Behringer ADA8200 to it, an impedance of 2.5k should be ok? I.e. I'm not going to be connecting a wide variety of consumer equipment to it.
It's bad design if I was making a mixer to be used by anyone anywhere, but would it suit me? The Fireface has an output impedance of 75r and the Behringer is 1k, I think.
Would this also halve the insertion loss from 46dB to 23dB?
Thanks for your help.
Crosstalk in a passive mixer depends on two factors - bus loss and return loss.bluebird said:Its the impedance of the summing amp input thats going to allow cross talk as well as the size of the summing resistors.
Its been done on large format consoles. I believe the old 80 series Neve desk's have a summing amp with a transformer input. Probably something like the 1272.
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