Are thermistors noisy?

[GregH2]

Hi all,

I am about to complete an amplifier and am contemplating the various kinds of ground loop breakers. I like the elegant simplicity of using a thermistor ala Nelson Pass. I'm just interested to know if thermistors are particularly noisy devices? If so, perhaps having them tied to ground is not such a good idea??

Are they just a temperature dependent resistor or do they have a forward voltage like a diode that could cause a noisy switching action?

Any thoughts on this?

Cheers,

Greg.

[AndrewT]

The Safety Earth connection is for SAFETY only.

The Audio Ground does not need to be connected to Mains PE to allow the equipment to operate correctly and well.

Why?

Because the Safety Earth connection is only required to pass Fault Current to PE to force the mains fuse to blow in event of catastrophic internal failure. This connection does not carry any Audio Current.

If there is no Audio current passing to PE then there can be no Audio noise due to current passing through any intervening resistance/impedance.
Using Thermistor instead of a diode bridge will make no Audio noise contribution.


All connections of Audio Ground to Mains PE can introduce mains contamination onto the Main Audio Ground (MAG). I reckon that the Disconnecting Network is only used to minimise the Mains contamination.

[GregH2]

Thanks Andrew,

I hadn't really considered that, but you make good points. Thermistor it is!

Cheers,

Greg.

[AndrewT]

Any one else want to put forward a contrary view?

[GregH2]

While we're on the topic, could they be a source of noise when used as an inrush current supressor?

[AndrewT]

Source of noise when used as a soft start. I don't think so.
Source of PSU output modulation when not relay bypassed. Yes, I have seen it on test. I don't like any unbypassed soft start impedance on the mains side of the audio transformer.

[sofaspud]

Any one else want to put forward a contrary view?

No here, but just to answer the questions, thermistors don't have a Vf like diodes, but are they known to be particularly noisy devices? I wouldn't think so, but I have no knowledge one way or the other.

[juma]

Originally Posted by swordfishy

....interested to know if thermistors are particularly noisy devices?...

Originally Posted by sofaspud

...are they known to be particularly noisy devices? ...

Next thing you know, poor little thermistors will be banned without ever knowing why ...

Guys, think a little before starting dangerous audiophool rumors - for example FirstWatt F2 is one of the quietest amps on this planet (20uV unweighted) and it has thermistors inside...

[GregH2]

Who was starting any rumors? I was asking a question. If people are dumb enough to interpret a question as a statement and/or fact then they shouldn't be building amplifiers in the first place.

I asked, not stated:

1) do thermistors have a switching action?

2) if so, can this switching action create noise?

There was no statement, no suggestion that I knew what I was talking about and no attempt to start a rumor, nor did I for one second argue that my question was likely or even possible.

What is the point of these forums if beginners like myself can't ask questions?

[Conrad Hoffman]

I think thermistors are basically some kind of non-metal-film resistor; I don't know the precise composition. Thus, they probably have higher thermal noise levels than metal film or wire wound resistors, but when you consider the applications where huge gains are used with the things, no one ever mentions noise. I've built very high gain temperature control circuits and have never seen noise that could be attributed to the thermistor. I probably wouldn't use one for the input resistor of a phono stage, but most anywhere else should be fine- if you want a temperature sensitive resistor.