Modding Mackie power supply to solve missing TRX

[mkiijam]

I would like to replace the existing E-I lam transformer with a toroid that I have. The original transformer has windings for "48v" phantom power. I was thinking perhaps I could just tap the +/- 18vac windings and add the caps to make a voltage doubling circuit? And then not have to change anything else?

 

[user 130681]

You have a ground issue with your idea. Take a close look at your wiring.

 

[mkiijam]

Don't see the issue. Can you indicate where?

 

[Bo Deadly]

Your shorting out the -18V winding through the 0V of +-16V and 48V.

Just get a 48V torroid.

Or skip it and just use it without 48V. I assume this is a small format mixer.

If this is an important piece of gear for some reason, then just get 3 SMPS, stack them like this:



remove the diodes and reduce the filter caps to whatever the SMPS can handle.

 

[mkiijam]

This is the circuit that seems to work fine that I am trying to borrow from. I still don't see the grounding issue. :-(

 

[thomasdf]

Your shorting out the -18V winding through the 0V of +-16V and 48V.

Just get a 48V torroid.

Or skip it and just use it without 48V. I assume this is a small format mixer.

If this is an important piece of gear for some reason, then just get 3 SMPS, stack them like this:

View attachment 98540

remove the diodes and reduce the filter caps to whatever the SMPS can handle.

Does this really work ? I almost looks too beautiful to be true ! What's the value of the resistors ?

 

[Bo Deadly]

Does this really work ? I almost looks too beautiful to be true ! What's the value of the resistors ?

Sure. Those are supposed to be chokes. But if you're regulating down, you don't need them. And in hindsight I would use capacitance multipliers instead of chokes anyway.

However, there are a number of rules that have to be satisfied to use SMPS effectively. The most important being that you have to select the right size SMPS to keep it from modulating the output (which can cause whistling noise in the analog circuitry) or going into "hiccup" mode. This is actually a fairly difficult issue for 48V because that load can vary greatly depending on how many devices are using it. One solution would be to use a shunt regulator but that is a non-trival exercise.

 

[mkiijam]

Imagine current coming out of the transformer center tap, through 0V of the +-16V supply circuit, down to the +48V 0V, through D5, through your 470uF and back into -18V if the transormer. There is no load. That is an AC short of that winding.

Yeah, I see this...

Here is the full schematic of the working system. Perhaps there is a load on the transformer and it just is not too much as this is working?
Isn't this effectively the same as the proposed mackie mod? Or no?

 

[Bo Deadly]

Actually looking at this more, I think you're circuit might work. I have deleted my post about the ground issue. I'm not sure you're going to get anywhere near 48V though. I would simulate this in LTSpice.

 

[mad.ax]

JLM Audio (and others) have been successfully using this trick for years...

Without firing LTSpice, 36 X 1,2 = 43,2V. Substract whatever dropout the regulator needs, and you'll end up with less than 48V...
Solution is to reference the 48V bridge minus pin to the +18V bridge instead of ground, as shown here:
http://www.jlmaudio.com/ACDCVer2sch.pdf

Axel