Thermistors and the F5

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This past weekend I spoke with a well regarded turntable manufacturer and he got on the subject of NTC thermistors. He came right out and said it, 'Thermistors are terrible for audio.'. He suggested using a DPDT On-On switch to 'by-pass the thermistor once the power supply transient subsides'. Not his exact words. I'm paraphrasing.

So...I believe that power supplies are important to audio circuits. Considering people spend $40 on hi-end fuses and hundreds and even thousands of dollars on power cords, maybe there is something to this?
Can this one component in the power supply effect the quality of the overall sound on the F5?

I have 5 CL-60s in my dual-mono F5. They were also incorporated in the second F5 that I built. One option is to remove them. Not a good idea, I think. Another is to use a relay circuit. Probably has it's own issues.
Is the suggested DPDT by-pass a good option? Sounded simple enough when mentioned, but are there any pitfalls?

Like to hear your thoughts on this.

For what it's worth, I think thermistors are a simple, inexpensive solution to a big problem. They save us big time.

Thanks,

Vince
 
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... 'Thermistors are terrible for audio.'. ...

It's just plain silly, unfounded and insulting to fellow man's intelligence - a well known "audiophool's" talk :rolleyes:

Run away as fast as you can from anyone who is proclaiming such an "absolute truth, known only to sacred circle of those blessed with a true knowledge" :yuck: IME, that's how you recognise the genuine snake-oil salesman...
 
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that's how you recognise the genuine snake-oil salesman...

You're more right than you know, because of the conversations I had with others afterward. I like to get more fact before judging anyone, but unfortuantely you might have hit it on the head first try.

Anyway, by-passing the thermistors wouldn't have any noticable effect in your opinion.
Juma, I'll take your earlier comments as a 'no, they don't make a difference.'.

Thanks,

V~
 
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I did the exact thing when I built SOZ, switched it out of the circuit after a good 3 second count after power on. Poor (lazy) man's soft start....worked great. Just need to remember to turn the soft start switch back to original position after power off. Now I just use a variac, less hassle.

See post #3
http://www.diyaudio.com/forums/pass-labs/69911-my-new-space-heater.html
 
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I believe the only influence a thermistor in series with the primary of a transformer will have, besides the inrush-current limiting, is that it adds a tiny(!) amount of Jhonson-noise, as it's residual resistance will be greater than that of a wire or relay.
I also believe that there are bigger fish to fry.
 
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I believe the only influence a thermistor in series with the primary of a transformer will have, besides the inrush-current limiting, is that it adds a tiny(!) amount of Jhonson-noise, as it's residual resistance will be greater than that of a wire or relay.
I also believe that there are bigger fish to fry.

It also obviously adds some resistance, which I've learned isnt always good in power supplies, especially non class A amps.

russellc
 
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Not too sure about that one. In class A the power draw is not influenced by the music output, so from the mains' point of view the amplifier is a fairly constant load. Therefore the thermistor (NTC-resistor) in the primary shouldn't alter the PSU's output-impedance.
In class B the power draw varies with the output. But that didn't keep McIntosh from using thermistors in their tubeamps.

Also, with 240VAC line voltage in Europe, the negative effect of the NTC will only be half as bad, making me worry even less ;-)

EDIT: Have to add that i overlooked the 'non' in the last sentence. Therefore I'll have probably have to agree with you.
 
I've got a number of amps that use thermisters in the start-up and then bypass them after a few seconds. Agree that thermisters appear to be perfect for this application, excellent technology! Resistors would burn if for example the relay failed but thermisters would be safe and happy. And they are small and can work at high temperatures.

My issue is whether or not a CL-60 (10 ohms, 5 amps) is sufficient for a large turn on inrush. I'm now using two in series in my amps but did notice that there are some much better (higher current) rated NTCs at digikey so I may change to them.

The F5 uses a CRC power supply so one would think that the effect of the thermister R is actually much smaller than the R in the CRC.

I have never seen anyone measure the 'hot' resistance of a thermister in class A mode amp. Wonder what it would be? My guess is it would be higher than you think.

I have noticed that in my big Class A amps with over 1000 VA in combined transformers and 100,000 uF in the PS, the 10 ohm CL-60s don't drop much in resistance in 10 seconds (I usually switch them out in 3-5 seconds). So after switching them out you get a second small (softer, gentler) inrush.
 
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Mega-amp,

Since you have used the by-pass, did you hear any difference with the thermistor in and out of the circuit?

Thanks,

Vince

Sometimes I'd forget to switch the soft start out, so I guess I unintentionally was testing which way it sounded better. After soft start cut-out I usually gained between 500mA-1A of current draw; mind you the reading was taken with a clamp meter on L1 primary, so not entirely accurate. Sound-wise, I honestly couldn't really tell a difference, but mentally that extra 500mA to 1A made all the difference in the world. In one of my 7 amp aleph builds, I forgot to turn off the soft start, the Cl-60 cracked up in flames after about an hour or so. So I guess one good thing about this simple soft-start, is that it extends the life of your in-rush thermistor, not that they're particularly expensive, just a hassle when you need to change one due to abuse.

john
 
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