Can anyone tell me what this JFET does in this circuit?

[JohnRoberts]

Incidentally, I'd like to talk a bit about resistor noise. All resistors, regardless of their construction and materials, make the exactly the same amount of thermal agitation noise based on their resistance.

Johnson noise.

But there is another mechanism for noise in a resistor that does depend on its construction and materials. It's called excess noise, but it only occurs when current flows (when there's voltage across it). This means it's unnecessary to use exotic low-noise resistors where no DC voltage is applied. This includes the resistor across the secondary of a transformer (damping of resonances) and resistor feedback networks around op-amps, etc. But you do need low-noise resistors in locations like biasing resistors for transistors, especially emitter and collector load resistors. This difference between thermal and excess noise is very widely misunderstood.

I recall or think I recall reading a white paper from back in the 70s from one resistor brand claiming that their carbon composition resistors expressed lower excess noise than other types of resistors. This is contrary to popular wisdom about resistor media (metal film or foil is considered better). Excess noise voltage is caused by granularity or lack of smoothness inside the conductor. Higher power resistors could be better than low power resistors of the same value.

I tried to calculate the significance of excess noise in typical phantom powered mic preamps where the 6.81k phantom resistors can have ten to twenty volts dropped across them in use. My judgement back then was that excess was not significant compared to the other noise sources involved. But I am not sure I calculated the uV/volt/decade calculus.

I can imagine this being a concern in vacuum tube designs with significant bias voltage across resistors. I am not a tube guy so don't know.

JR

 

[CJ]

Bill what is the purpose of reverse winding a section of an audio transformer?

Is it strictly for noise rejection or does it improve response?

Can it affect inductance?

Can transformer insulation types affect sound?

Thanks!