Standalone Phono PSU, cable to carry AC or DC?

I am having to build a standalone PSU to remove the transformer hum that has plagued a recent phono build since the start (I tried to pack too much into a small enclosure).

The standalone PSU will house a transformer which is connected to a XLR jack which in turn will connect to the PSU input on my phono stage enclosure.

The question:

Should the XLR cable carry AC (12V-0V-12V) and have the rectifying done inside the phono stage?

or

Should the AC be rectified in the PSU standalone enclosure and then sent over the XLR in the form of DC (+12V/-12V) to be filtered and regulated in the phono enclosure?

or

Does it matter?
 

rayma

Member
2011-04-29 8:37 pm
You think I need more in the PSU box other than a filter cap?

A rectifier and filter cap still has significant 120 Hz (and harmonics) of ripple. You want to keep that out of the RIAA box.
An RC or LC filter before leaving the power supply box can reduce that ripple by an order of magnitude, or more.
Remember how small the phono cartridge signal is.
 
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Also XLR connector is usually a microphone cable. Spend $1 and use a center pin recess DC connector, or an AMP mate-n-lock or something.
I got great hum improvement in my Herald disco mixer by abandoning the interal transformer right next to the high gain RIAA circuit, and buying a $1 wall transformer 18 vdc for a race car set from Salvation Army resale. The wall transformer is plugged in the power strip 8' from the mixer enclosure. There is still some residual hum; perhaps no CLC in the wall transformer after the rectifier is the reason. Could even be half wave rectification. Inside the mixer enclosure, which is grounded steel, I have a filter cap, then stacked 22r 8 v zener 8 v zener 22 r two filter caps on the zeners to make a split supply for the op amp circuits. No R in the middle of the C's.
 
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To max out the benefit, also have two floating DC voltages (from two separate, isolated transformer secondaries),
brought into the RIAA enclosure as 4 wires, before connecting the two commons together.

I have been lurking on diyAudio for the past few weeks after a nearly 20 year electronics/audio hiatus trying to regain knowledge long since forgotten.

I don't disagree that it would be beneficial to keep the two supplies separate until inside the RIAA enclosure before connecting the commons, but why is this?

Also, what would be the best way then of handling the mains earth. Presumably it would be connected to the PSU chassis, then as a 5th wire (or shield ?) to the RIAA enclosure where it, as well as the supply commons would be connected to the RIAA chassis?

Thanks,
Clive
 
The mains earth is covered by national electrical regulations, and rating agencies recommended practices. We are not allowed to discuss any variation here. The practices of 1961 or 1966 designed products I discussed earlier are no longer allowed.
I am confused, I am not too sure what that has to do with the internal connection of the mains earth wire or why we are not allowed to discuss any variations.
The mains earth enters the power supply enclosure via a 3 pin socket, along with 110v or 220V depending your location, and then is typically connected to the chassis. With a standalone remote power supply the earth should also then be connected to the amplifier being fed from the supply.
My question, is should the earth best be a fifth wire in the cable, or would it be preferable to have it connect via a shield around the other conductors?
 
In the USA, safety earthing wires must be rated to carry 20 amps, if I understand that part of UL practice from my appliance testing days. Both those earth suggestions quite likely would not. The common scheme involves making a low energy part of the circuit "double insulated", which requires a plastic case and lot of tricky business to exclude RF from ubiquitous cellphones and nearby broadcast stations. for these reasons I'm using a wall-transformer which is UL listed to power one of my analog-source projects. I shall not be discussing how I handled the ground wire of the mag-phono cartridge shell to the RIAA circuits, nor the final earth ground, less this entire thread be erased. It really cut the hum and squelched the CB broadcaster that thought dogs barking dixie continuously on CB band was funny, though. Drop by sometime for a little oral history and demonstration (not likely).
Bands handle the ground loop problem caused by area of the loops between equipment with transformer gadgets called "DI units". They are not cheap and the ones stocked in music supplies are not particularly high fidelity either, IMHO.
 
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