Fader resistance & load.

Hey all

I'm in the process of changing the fader of my mixing desk.
The actual fader is 2K and is followed by DOA buffer amp.
I have a 10k TKD fader for test, to my ear it seem to work fine, but I don't do more measurement. (I will)
As "standard" i can also order 5k fader, will this be a better choices?

The fader is also motorized... so not cheap, I prefer to avoid 16 fader mistake order  :-X

I attach the actual schematic, fader connected at B3 B6 on top, with 33ohm series after wiper and 10k // (PFL bus) at the fader pcb

Zam

fader schemo

In theory the 2k or 5k ohm fader will have lower thermal noise, but I have used truck loads of 10k faders with no compromise to the audio path.

The non-inverting gain stage should not appreciably load the fader.

JR

So in theory it's better to have lower resistance right ?

Thermal noise! How it is with conductive plastic???
The TKD fader I buy for my proto is CP and ladder technologies,
and by the way I'm impressed by the dead silent sliding noise

Zam

Thermal noise has nothing to do with the actual material used, other noises are. For good resistive materials the noise will be pretty close to the thermal noise figure, as metal film or ww resistors. Conductive plastic is probably not so far from that. In other materials as carbon composite the noise will be much higher than the thermal noise, worse at LF than HF. They are fine for RF but we usually try to avoid them in audio unless we are looking for that vintage hiss.

As pointed by JR 10k pot will be fine, the load at the fader will be the 220k resistor, in a 10K pot the lower output impedance you will have will be 2k5 when it's at -6dB from full up. Considering that distortion introduce by the switching transistor is inversely proportional to the load and the noise will be dominated by other part of the circuit (mic preamp or summing amp most likely) I suggest going for the 10k.

JS

Tks to both of you
I will go for the 10K then

did some one ever try to  put some audio equipment in liquid nitrogen to reduce the thermal noise ??  :

Zam

zamproject said:

Tks to both of you
I will go for the 10K then

did some one ever try to  put some audio equipment in liquid nitrogen to reduce the thermal noise ??  :

Zam

Click to expand...

The liquid nitrogen is tough on the microphones and singers.

Not worth the effort...

JR

> put some audio equipment in liquid nitrogen to reduce the thermal noise ??

Radio astronomy is all about liquid gases.

In audio we can almost always hit the studio background noise without exotic temperatures.

Low noise amplifiers to handle deep space SETI antennae are routinely cooled down, as are CCD sensors for high end video cameras. This is usually done with Peltier effect devices (thermo-electric coolers), and to get really significant temperature reductions, two or three Peltier devices are stacked in series to get a much lower device temperature, since one junction alone can't do all that much.

Note that the Johnson noise of a resistor is proportional to the square root of the resistor temperature above absolute zero, so cooling something from 300ºK (room temp) to 270ºK (freezing water) isn't going to do much. One needs to get way down there, at least to dry ice temp, or maybe liquid nitrogen. A big problem is that you still need the circuit to work at those temperatures, and semiconductors can perform poorly at LN temperatures. I seem to remember that many FET-like devices seem to get extra transconductance around dry ice temperatures, as they also lose Johnson noise, so that's apparently the recipe for low noise CCD sensors and the like.

Usually the best low noise trick is to parallel multiple devices at room temperature and/or aggressively reduce circuit impedances so that the resistances can be lowered.