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.
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.
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.
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.
Thanks Andrew,
I hadn't really considered that, but you make good points. Thermistor it is!
Cheers,
Greg.
I hadn't really considered that, but you make good points. Thermistor it is!
Cheers,
Greg.
While we're on the topic, could they be a source of noise when used as an inrush current supressor?
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.
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.
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.
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...
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?
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?
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. 😉
PE=Protective Earth - the potential to which all exposed metalwork in the building is connected.
I don't know whether thermistors are noisy. They will certainly have normal thermal noise, just like any resistance. Whether they have excess noise too and/or 1/f noise I don't know, but I would not be particularly surprised if they had. Generally they are used in circuit positions where a little noise does no harm, so people don't need to think about noise from them. Noise may depend on the type of thermistor: NTC or PTC, and the particular metal oxides used.
I don't know whether thermistors are noisy. They will certainly have normal thermal noise, just like any resistance. Whether they have excess noise too and/or 1/f noise I don't know, but I would not be particularly surprised if they had. Generally they are used in circuit positions where a little noise does no harm, so people don't need to think about noise from them. Noise may depend on the type of thermistor: NTC or PTC, and the particular metal oxides used.
Possibly, but I think PE is a particular term used in UK practice. It may refer to a ground connection, or a connection (somewhere) to the supply neutral. In some cases several adjacent buildings can share a PE. In this case a supply fault can raise PE well above true ground potential but provided the bonding has been done correctly the situation is still safe.
Do the electricians in America use the term ground instead of earth, as we mostly use it in electronics? Like ''safety ground'' ''power ground''.
Yes, ground is the universal term here, along with line and neutral. Neutral, the ground potential AC wire, is generally connected to ground only at the breaker box. Earth is uncommon, though I've heard mains used.
The forward voltage drop of a diode doesn't cause a "noisy switching action." You're apparently thinking of reverse recovery which can cause RF (and audio) noise in a power supply when a rectifier switches from forward to reverse current, and the diode (especially a common 1N400x rectifier) has a slight delay in turning off, causing a fast-opening-switch-with-current-flowing problem in the secondary winding, which causes the noise (not just the diode itself). We've had at least one thread on this phenomenon in the last couple of months.
A thermistor may or may not generate more noise than a fixed resistor at the same resistance and temperature, but it definitely doesn't generate the noise a rectifier diode can.
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