Mono sub-out from one trafo?

[Skorpio]

To prevent hum in my long connection to my active subwoofer, I am considering a galvanic seperation (trafo).

Could I use a single trafo (e.g. Lundahl 1517) with two primaries, to make the L-R summing and then use one secondary to sub-out?

Will this give any problems?

http://www.lundahl.se/pdfs/datash/1517.pdf

[Chris Hornbeck]

Originally Posted by Skorpio

To prevent hum in my long connection to my active subwoofer, I am considering a galvanic seperation (trafo).

Could I use a single trafo (e.g. Lundahl 1517) with two primaries, to make the L-R summing and then use one secondary to sub-out?

Will this give any problems?

http://www.lundahl.se/pdfs/datash/1517.pdf

If you can drive the transformer with a low source impedance to the left and right main channels and include "build-out" resistors in series with the transformer primaries, your plan will work out well. I'd recommend build-out resistors much larger than source impedance (for interchannel isolation) and paralleled secondaries.

Many years ago transformers were sold for the purpose of high-level (speaker level) mixing for a summed center channel speaker. These were used without any build-out resistors.

All good fortune,
Chris

[Skorpio]

Thank you for you reply...but there is something I don't understand:

The drive is Pass B1, with 270R in series with output. What are those "build-out" resistors? If placed in series with primary, the will increase drive resistance? Are they for seperating channels at pre-output? What values recommended?

I could parallel secondaries for 2:1 operation...subwoofer load/input is 33kOhm and 5 meters RCA wire...

[Chris Hornbeck]

Originally Posted by Skorpio

The drive is Pass B1, with 270R in series with output. What are those "build-out" resistors? If placed in series with primary, the will increase drive resistance? Are they for seperating channels at pre-output? What values recommended?

I could parallel secondaries for 2:1 operation...subwoofer load/input is 33kOhm and 5 meters RCA wire...

"Build out" is a name for series resistors used to isolate the two channels from each other. But, if you're only going 5 meters, I'd instead recommend running your sub's power cord through an extension cable back to the location of your preamp, dressing the wiring *together* (counterintuitive, but usually correct) and feeding signal from a simple resistive Y. Maybe about 27K series' ("build-out") to a single 270R shunt. Even *much* bigger resistances won't be a problem at sub frequencies. If the sub has stereo inputs, you won't even need the Y. Just a thought.

All good fortune,
Chris