Moving power supply outside micpre schematic / idea check

Help Support GroupDIY:

mkiijam

Well-known member
Joined
Aug 25, 2017
Messages
67
I've included a quick drawing of what I mean. I have build the thing and it seems okay. Just trying to get any opinions on AC vs DC "transmission" from separate power supply. i.e. I could build the entire DC circuit in the separate PSU and send the +/- 18v - 12v - 48v. Or, just send the 20vac - 0 - 20vac of the tranx secondary, which is what I have done.

So the drawing is this:

Fused AC IN, AC DPDT switch, Ground bolt, 18Vac (2) / CT transformer, and then a 4-pin XLR out. I don't need 4 pin but it keeps somebody from plugging in an audio cable. ALL in a separate PSU box

about 3 feet of cable that carries the secondary of the transformer, i.e. the two 20Vac lines AND a grounded CT line of the transformer.

Inside of the micpre just has the AC input into the bridge rectifier and the CT/ground into the micpre chassis and DC power supply ground.

I hope this makes sense.

My question(s) is:

1. Is this okay?
2. It seems like the power supply current return is the transformer and that has been moved further away?
2a. Is #2 an issue?
3. Would it be better to build the DC circuit in the separate PSU box?
4. It works fine so far, very quiet of course. Should I be happy?
5. Why is every other supply (most at least) DC output and not local DC supply?
 

Attachments

  • PSU_test - Schematic.pdf
    38.9 KB · Views: 9

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
2,452
Location
New Jersey, USA
mkiijam said:
1. Is this okay?
2. It seems like the power supply current return is the transformer and that has been moved further away?
2a. Is #2 an issue?
3. Would it be better to build the DC circuit in the separate PSU box?
4. It works fine so far, very quiet of course. Should I be happy?
5. Why is every other supply (most at least) DC output and not local DC supply?
Sure. This is pretty much like your typical "wall wart". If you're trying to minimize noise, then you should really put the rest of the supply in the external enclosure. But if you're not throwing a lot of current around, I'm not sure it would matter either way. If you are, I would be weary about dropping 6 volts across that 7812. Not sure what you mean in question 5 but nowadays the most common supply arrangement for consumer electronic devices is to have a low voltage SMPS ACDC converter external and then internally there are one or more DCDC converted to make whatever voltages are required.
 

mkiijam

Well-known member
Joined
Aug 25, 2017
Messages
67
They main reason for just moving the AC was I already build the entire pre / power supply in the pre chassis. In just moving the transformer I don't have to build they entire DC section again, if that makes sense.

Good point, it is just like a "wall wart"

hmmm... hey waity, that's what it is, is just a big wall wart! :)
 

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
2,892
Theres quite a few smaller good quality mixers use the remote transformer and do the rectification /regulation in the mixer housing. I'd say if your happy with the noise performance keep things as they are .
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
20,251
Location
Hickory, MS
Are you dealing with a specific noise problem or just doing a what if?

I have used external PS inside their own chassis for large consoles, and did one phono preamp using an external wall wart.

In broad stokes moving the transformer away from sensitive circuitry should reduce magnetic field noise pick up, but good design can generally accomplish that with less heroic efforts.

Beginning with the premise of an external PS, rectifying and filtering the external PS so you are passing DC reduces some more noise. I made one console with pre-regulated DC in the external supply.

This stage is where some experiment and measurements could be useful. Try it and see how it works. If not that good try something else.

JR 
 

Latest posts

Top