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Old 2nd April 2002, 11:50 PM   #1
Ren is offline Ren  United States
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Question Pearl phono preamp questions

Hi,

I am interested to hear from all who have built and/or listened to the Pearl phono preamp before I start gathering parts. Thoughts on parts selection? Construction experience, advice, or cautions? How does it sound in your system? Did you use the suggested ps design or something else?

Thanks!
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Old 7th May 2002, 03:44 PM   #2
Ren is offline Ren  United States
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Default Pearl grounding questions...

I've ordered all the parts (I think) to build the Pearl phono preamp, and I have a question about grounding for those more knowledgable about electronics than I am.

I plan to put an IEC AC connector on the power supply module that Wayne suggested and connect that ps chassis to "earth" ground (3rd wire). Then, I am planning to put a copper mesh shield over the DC umbilical cord and connecting that mesh shield to the "earth" ground at the ps chassis. Wayne recommended bringing the two DC grounds together at the signal output, and connecting them to chassis ground at that point.

My question is: do I attach the umbilical shield to the phono pre chassis as well?

a) If not (to my understanding) that would leave the phono pre chassis floating with respect to earth ground.

b) If so, wouldn't noise on the "earth" ground find its way into the signal output from the Pearl?

I am concerned with safety first and then sound; I welcome any thoughts / advice on the subject.
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Old 7th May 2002, 06:11 PM   #3
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I'd be more concerned about shielding the phono leads, themselves.

Grey
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Old 7th May 2002, 06:18 PM   #4
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Thumbs up Grounding

We connect the earth ground to the circuit through a power thermistor, this gives several ohms resistance; effectively breaking ground loops while still protecting against electrical faults.

SAFETY FIRST! It matters little how good your system sounds if its lethal.

Cyclotronguy
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Old 7th May 2002, 06:27 PM   #5
jam is offline jam  United States
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Cyclotronguy,

Yeah! It could be a shocking experience.

Jam
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Old 7th May 2002, 07:29 PM   #6
Ren is offline Ren  United States
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Default Thermistor value?

Thanks.

I searched this forum and found some references that talk about using inrush limiters as safety devices as you mentioned. Generally everything is a trade-off, but what is a reasonable value thermistor to use for this purpose? I assume I want one that will go to ~ zero ohms fairly quickly in a failure situation, so maybe a 5 or 10 ohm thermistor?

Regarding shielding the phono leads, I am not planning anything other than keeping the leads short... maybe 1 inch. Since there isn't any AC in the phono preamp chassis I am not expecting problems. But that's the advantage of ignorance... I won't ever expect problems.

Ren
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Old 7th May 2002, 08:29 PM   #7
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five to ten ohms when cold sounds about right
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Old 7th May 2002, 09:14 PM   #8
Evaas is offline Evaas  Canada
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Default please explain

How does a thermistor protect you from an electrical fault? If such a fault existst, you want want to shunt it to ground through the thermistor. But then the thermistor gets hot and the resistance quickly increases until the time when you touch the metal case and get a nice shock because there's no longer a good ground shunt, and the thermistor didn't let enough current through to blow the fuse. What am I missing?

Evan
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Old 7th May 2002, 09:22 PM   #9
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Two types of thermistor - you're thinking of the wrong one. Resistance goes down with increase in temp for the right one.

Jake
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Old 7th May 2002, 09:25 PM   #10
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Default Thermistor

"Thermistors:
The term "Thermistor" is used to describe a range of electronic components whose principle characteristic is that their electrical resistance changes in response to changes in their temperature.
The word "Thermistor" derives from the description "thermally sensitive resistor".
Thermistors are further classified as "Positive Temperature Coefficient" devices (PTC devices) or "Negative Temperature Coefficient" devices (NTC devices).
PTC devices are devices whose resistance increases as their temperature increases.
NTC devices are devices whose resistance decreases as their temperature increases.
NTC thermistors are manufactured from proprietary formulations of ceramic materials based on transition metal oxides."

The idea is to use a NTC type to have a low resistance during a fault condition and a higher resistance during normal operation when you don't want your signal ground closely coupled to earth ground.

H.H.
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