Remote transformer for power amp

[Jamie F]

I am wanting to separate a transformer (1200VA) from the rest of my power amp. Three questions:

1) Can anyone recommend suitable connectors to connect the separated transformer (in its own frame) to the rest of the linear supply in the amp frame?

2) Best arrangement for interconnecting cable (twisted, shielded, doesn't matter, etc)?

3) Are there any issues to be wary of (e.g. reactance of link between secondary and bridge rectifier)?

Reasons for separation:
a) more space in amp frame
b) less weight
c) less noise
d) safer (no mains in amp frame)

Thanks

Jamie

[dhaen]

James,

What power is the amp?

What is the voltage? So we can determine the current.

How many supplies? Eg + G - G...

Cheers,

[Jamie F]

10x LM3886, fairly lightly loaded. Mains is 240V, rails +-30V & 0V, (24V secondaries), 2x 56000uF smoothing.

[dhaen]

Jamie,

Firstly, sorry about getting your name wrong before.

Next: This is not a trivial problem.
IMO Your amp chips would become unstable with a great length of cable.
So, you'll need the smoothing caps in the amp case.
But, then you'll get reverse recovery noise from the rectifiers radiated. The way round this would be to either split the smoothing betweent the PSU and Amp, or to put the rectifier also in the amp. You'd still need to take special precautions against recovery noise.

Not very positive really..sorry.

Best hope from a reply from someone who's actually done it. Rather than a "theoratician".

Cheers,

[paulb]

Put caps in both the power supply box and the amp box. The big pulses coming from the bridge rectifier to the first cap are best confined to the power supply box, rather than running through the cable that runs between the boxes.
Rod Elliot also recommended a small value of resistance between the 1st and 2nd set of capacitors. My thoughts were that the cable itself would provide some resistance, and may be enough.
I used old surplus military connectors for power connections between the two boxes. The ones we used to call 'infinite-turn'. They're ugly but reliable and can handle a lot of amperage.

[Jamie F]

Thanks Paul.

Are Rod's recommendations to be found on his site, or were these made to you / or a thread here. I would be interested to read more on this.

I had planned just to separate the transformer, as I felt the link from the transformer to the bridge was far less critical than having a break somewhere else.

However, I can see the benefit in isolating the bridge and first smoothing caps. That said, intuitively I would have thought you wanted to minimize resistance between caps, as this surely will increase charging time?

Also, is there any implication for the star ground point of such a separation. Obviously, the star would need to be taken off the join of the second set of caps. Is the resistance back to the join of the first set significant?

Thanks

Jamie

[jan.didden]

Jamie,

From my own experience: you DO want some resistance (or at least L) between the caps in the xformer box and the caps in the amp box. You want to increase charging time (just a little, of course) to decrease higher harmonics that would be radiated. The higher harmonics come from very short, high and sharp charging pulses. So, don't make those cables too fat.

The star ground is very important. Since you have just a single supply, run it up to the amp and make a star point where the loads return (speaker cold), and run separate earths from each amp to this point as well. How are your inputs configured? Separate sources, mixing console? Especially with multiple amps, there a lot of opportunity for ground loops.

Jan Didden

[paulb]

Originally posted by Jamie F

Are Rod's recommendations to be found on his site, or were these made to you / or a thread here. I would be interested to read more on this.

I had emailed Rod to ask him about this arrangement with his P3A amp. I have larger caps in the amp box:
"Try using a 0.1 ohm resistor between the "little" input caps and the big ones. Minimal voltage drop, but a surprising reduction in ripple and mains noise."

[Jamie F]

Thanks for the quote Paul.


Jan, thank you for your help. You are right that the star grounding is important. I've been having a lot of trouble with this. The power amp is being fed from active filters. To try to solve bad buzz on certain channels, I implemented ground-compensated interconnects. This didn't help much. Investigating the noise further (terminated inputs), I seemed to be getting something related to charging pulses on the noisy outputs. I also implemented Nelson Pass's suggested .1uF caps across the rectifier diodes - do these really make a difference when a 56000uF is sucking current to recharge? (I didn't notice any improvement).

I connected each input from the filters straight to each amp module, then a thick 0V back to the star point. Initially I had speaker returns going to each module, then changed this to have them going to the star. Each module - star 0V connection is the same length.

I don't have a mains ground loop. The electronics in the whole system is only connected to chassis & safety earth in the power amp, via a network. Disconnecting / shorting this makes no difference.

This is why I want to isolate the power supply, without compromising the low impedance of the rails.

Jamie

[jan.didden]

Jamie,

Just a gut feeling, really, but I doubt that remoting the transformer will solve it. Before heavily investing in cables & connectors, I would just try it out with a temporary set-up (unless you want to do it anyway of course).

Jan Didden