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Old 13th December 2007, 10:14 PM   #1
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Default What d'you put in the PSU chassis, and what in the circuit chassis?

OK - here's the setup. You want to make an all-DHT SET in two boxes and eliminate hum as much as possible. typically we are talking three stages so 6 filament supplies, possibly all DC or maybe a mixture with AC on the output stage.

You decide to put the PSU in one box and the circuit itself in another box.

The PSU consists of:

1) HT supply with at least a couple of chokes that are sensitive to placement.

2) AC heater supply to a tube rectifier and tube rectifier.

3) Several filament supplies (4 or 6). These consist of:

a) Transformer
b) Rectification and initial smoothing - one big cap and maybe a voltage reg to follow it. For some a big choke
c) Final filament stage - current reg, common mode choke or whatever

What are the "clean" parts of the PSU that benefit from being put in the circuit box, and what are the "dirty" parts that should be kept away from the circuit?
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Old 13th December 2007, 11:33 PM   #2
7N7 is offline 7N7  United Kingdom
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"1) HT supply with at least a couple of chokes that are sensitive to placement.

2) AC heater supply to a tube rectifier and tube rectifier.

3) Several filament supplies (4 or 6). These consist of:

a) Transformer
b) Rectification and initial smoothing - one big cap and maybe a voltage reg to follow it. For some a big choke
c) Final filament stage - current reg, common mode choke or whatever"



All these are essentially "dirty" although for (C) you could put the common mode chokes in the "circuit box".

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Old 13th December 2007, 11:39 PM   #3
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Hello 7N7!

Surely the final stages of the HT supply and filament supplies are fairly clean - not liable to radiate hum inducing problems?

I'm wondering if it isn't better to remove these to the circuit box.

I'm also concerned about generating hum in the PSU box, e.g. by putting a HT choke too close to a filament transformer, especially if that's EI.

I guess you could put the transformers themselves into a box and run an umbilical consisting of just AC to a kind of halfway house PSU which would have the rest of the stuff in it.
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Old 13th December 2007, 11:50 PM   #4
7N7 is offline 7N7  United Kingdom
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Hello Andyjevans!

I think it's best to keep anything that might have elements of ripple well away from say, a pre-amp. For power amp stages this of course is not so critical.

You didn't mention the final stage of the HT supply in your original post; as for the final stages of the filament supply, remember that this carries considerable current in relative terms, and it is the current that can cause big problems with radiated hum etc. I consider it wise, even with DC to twist the wires tightly together.

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Old 14th December 2007, 09:09 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by andyjevans
Hello 7N7!

Surely the final stages of the HT supply and filament supplies are fairly clean - not liable to radiate hum inducing problems?

I'm wondering if it isn't better to remove these to the circuit box.

I'm also concerned about generating hum in the PSU box, e.g. by putting a HT choke too close to a filament transformer, especially if that's EI.

I guess you could put the transformers themselves into a box and run an umbilical consisting of just AC to a kind of halfway house PSU which would have the rest of the stuff in it.
In a dedicated power supply box you can pack things pretty closely together in most cases without much difficulty, just make sure to orient closely spaced magnetics for minimum interference with each other and in almost all cases you will be ok.

I build everything with outboard supplies, and I put all of the power supply circuitry in one chassis, and all of the audio circuitry in the other. DHT filament supplies I generally float at the supply end and ground in the audio chassis. Large common mode chokes can be used locally to provide isolation from the umbilical for RF, but I have not bothered to do this since I stopped building commercially - and I haven't had a problem with this either.
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Old 14th December 2007, 11:41 PM   #6
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I second what Kevin says. I put all of the power supply components in the power supply box. Each seperate supply (HV, regular B+, bias, and filament is independent and floating from each other. Each has a seperate hot and ground lead in the umbilical cord to the amplifier chassis. The only "power supply" component in the amplifier chassis is the supplemental power supply capacitor from the HV to ground. The negative terminal of this cap becomes the star ground point. All of the power supply grounds are tied together here. There is one fat wire from this ground point to the input connector which is the only ground for the chassis. The positive terminal has 4 connections, the HV wire from the supply, left and right OPT red wire, and the bleeder resistor.
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