I am building a tube phono preamp and I am thinking of putting a PSU in an external box. That means that I will transfer 300V DC to a preamp through a cable. I am concerned with a safety of this approach. Can anyone please advise or point me to a right source of how this approach can be done in a safe way? A related question - does the Main's ground need to be connected to the 300VDC in the PSU box or in the preamp box?
Protective earth ground should be connected directly to the chassis. The 300V would usually be grounded sometimes through a ground lift network to the protective earth ground in the PSU.
(My ground lift networks generally consist of a pair of anti-parallel 1N5408 diodes connected in parallel with a 4.7 - 10 ohm 3W metal oxide resistor. Diodes should be sized to comfortably handle the maximum fault current that can flow based on the local protective fusing. This is good to about 3A.)
I use CPC connectors purchased from Digikey or Mouser for applications to 400V or so without problem.
Make sure that there are no exposed contacts with high voltage on them when disconnected. (Use resistors in the pre-amp to bleed down any filter caps located there)
(My ground lift networks generally consist of a pair of anti-parallel 1N5408 diodes connected in parallel with a 4.7 - 10 ohm 3W metal oxide resistor. Diodes should be sized to comfortably handle the maximum fault current that can flow based on the local protective fusing. This is good to about 3A.)
I use CPC connectors purchased from Digikey or Mouser for applications to 400V or so without problem.
Make sure that there are no exposed contacts with high voltage on them when disconnected. (Use resistors in the pre-amp to bleed down any filter caps located there)
A few years ago, Fender Corp issued one of their "Pawnshop Series" valve guitar amps that had a similar configuration; there were two metal chassis inside one speaker cab, one housing the power amp section, the other, the preamp.
The (high voltage) power supply was inside the power amp section, so they had the same issue of running high-voltage wiring inside an open-backed cab, to the preamp.
Fender ended up running flexible metal conduit from the power amp chassis to the preamp chassis. The high-voltage wires were inside the metal conduit. The conduit was grounded as well. This way, even a lunatic user wielding a pocket-knife inside the guitar amp cabinet would not be able to get a shock from the wire, protected inside it's metal armour.
I don't know whether this solution would work for you - but at least it was safe enough to pass electrical certification requirements.
-Gnobuddy
The (high voltage) power supply was inside the power amp section, so they had the same issue of running high-voltage wiring inside an open-backed cab, to the preamp.
Fender ended up running flexible metal conduit from the power amp chassis to the preamp chassis. The high-voltage wires were inside the metal conduit. The conduit was grounded as well. This way, even a lunatic user wielding a pocket-knife inside the guitar amp cabinet would not be able to get a shock from the wire, protected inside it's metal armour.
I don't know whether this solution would work for you - but at least it was safe enough to pass electrical certification requirements.
-Gnobuddy
This means a female chassis mount socket on the power supply, and a male chassis mount
connector on the preamp.
And Bulgin connectors are well-suited for this purpose. (Mouser and DigiKey carry)
I don't know if they are difficult to find -- you could purchase chassis mount male octal connectors -- we used 'em on ham radio stuff decades ago for this exact purpose.
I have used XLR's but they aren't really rated for HV.
I found this
This might imply that 'ground first' only applies to 3-pin connectors.
on the Bulgin website.
This might imply that 'ground first' only applies to 3-pin connectors.
This should not be an issue as the power umbilical should never, ever be disconnected with the power applied.
The CPC connectors I mentioned are locking and are relatively inexpensive.
Neutrik makes some very nice, but not so inexpensive locking connectors.
This stuff is available from either Mouser and Digikey.
Bulgin stuff while very nice is not common in the U.S. although some lines are available.
Here is what I hope should work. Comes at around $50 on Mouser.
1. PN 211398-1 Reverse Sex RECEPTACLE 7 POS shell size 13
2. PN 211400-1 Reverse Sex PLUG 7 PIN shell size 13
3. PN 211401-1 Standard Sex RECEPTACLE 7 POS shell size 13
4. PN 211399-1 Standard Sex PLUG 7 PIN shell size 13
5. PN 206966-7 STD CABLE CLAMP 13
6. PN 164163-2 Grounding Pin
7. PN 66180-1 Connectors III PIN SOLDER
8. PN 66181-1 Connectors III SOCKET SOLDER
1. PN 211398-1 Reverse Sex RECEPTACLE 7 POS shell size 13
2. PN 211400-1 Reverse Sex PLUG 7 PIN shell size 13
3. PN 211401-1 Standard Sex RECEPTACLE 7 POS shell size 13
4. PN 211399-1 Standard Sex PLUG 7 PIN shell size 13
5. PN 206966-7 STD CABLE CLAMP 13
6. PN 164163-2 Grounding Pin
7. PN 66180-1 Connectors III PIN SOLDER
8. PN 66181-1 Connectors III SOCKET SOLDER
the mains Fault Current can exceed 1kA.
Take a typical mains input cable with a 10milliohms resistance. Add on about 5milliohms for the short internal wiring. Double that total for the return route back to the mains distribution board.
Any 220/240Vac supplied dwelling has around 250Vac (358Vpk) to drive the Fault Current around the 30milliohms circuit and the peak current that blows the mains fuse is around 12kApk
Well there is inductance in the route and some resistance in the fuse, so the peak current will be less than when assuming resistance of the cables only.
You'll find that the MCBs in the distirbition board are rated for between 5kA and 10kA in domestic dwellings. This lower rating presumably takes account of the source impedance of the mains transformer and the mains cables feeding the house.
I would not dream of using 3A diodes unless I had tested them first, to survive longer than the fuse.
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