I built an interface to go from an unbalanced RCA output on my preamp to a balanced input on my power amp. I was using an adapter cable before, and near as I can tell it worked fine.
But I'm pondering adding a MiniDSP Flex inline between the preamp and the power amp, and if I'm going to spend hundreds I want it to have balanced connections--I see that as the long-term endpoint for the stuff that I buy.
My preamp came with the option to have balance outputs, but I've never seen that particular preamp (a B&K MC-101) with the balanced optional output actually installed, and I can't find schematics to identify just how they did it. And maybe I do recall somebody's off-the-cuff comment that their balanced output wasn't all that well implemented, even though their amps also have balanced inputs.
I originally thought to build something inside the preamp enclosure, but couldn't find power that would work and didn't want to put a transformer inside a preamp whose main transformer is in a separate box. So, I ended up building a separate enclosure.
So, I bought a couple of the Sparkfun Outsmarts modules that use the THAT 1646 differential amplifiers, and a Meanwell +/- 15VDC power supply, and installed them in an enclosure:
The balanced output uses a TRS quarter-inch phone plug, and I bought cables with that plug on one end and an XLR on the other end to go to my amp. The ones I bought were too short--ideally I'll like to use very short RCA cables on the preamp end and longer balanced cables to get from there to the amp.
The Outsmarts takes the unbalanced signal and reamplifies it with reversed polarity to create the other side of the balance pair. Thus, it provides 6 dB of gain--twice the voltage swing compared to the unbalanced input. It's quite clean for use with vintage equipment, but it someone is wanting a dac with a SINAD of 115 or more dB to go into an amp with a SINAD that high, this device will bring it down to something around 100. Still effectively transparent to my ears, and still better than my old analog preamp. Still better than the CD's I listen to. So, not much of a compromise. But if that is intolerable, buy a DAC with properly implemented balanced outputs and forget this stuff.
As to what I can hear, the only effect that is apparent to me is the gain, and even that is a subtle effect. Playing the highly dynamic drum solo on the Chesky demonstration CD, I can still make the clipping indicators on my Buckeye NC502MP amp flash, but not quite as often and not quite as brightly (meaning: duration not as long). I cannot hear any audible clipping even when those indicators are flashing on the transient peaks, so I can assume they are fairly conservative.
I used the 15-VDC bipolar power supply because the THAT chip's voltage swing is limited to a range about 2 Volts less on each end as the rail voltage. So, that power supply gives me +/- 13 volts on the chip's output. That exceeds the "maximum" output voltage of my preamp (which is 12 volts, unbalanced). It vastly exceeds the input sensitivity of the amp, but I may not always use an amp with 26 dB of gain.
In practice, I can more often use the line-amp bypass feature of my B&K preamp. Even vintage analog sources that output, say, 500 mV instead of the 2 full volts a typical CD player will produce can drive the power amp to loud levels. That was not always the case before.
The only weird thing is that the Sparkfun modules will hiss on their outputs if their inputs are floating and undriven. My preamp doesn't have an on-off button, but rather uses a switch to change the output from the line output to the headphone output. "Off" is in the headphone output position. If the power amp is on, the hiss is objectionably loud. It vanishes when I switch the preamp so that the unbalanced input is being held to nominally zero volts instead of floating.
Unlike a DI box, these do not have extraneous controls--no volume control or anything like that. And they do not have musical-instrument preamplifiers in them that have to be attenuated, as would usually be the case with a DI box.
Rick "who will probably build more of these--they are handy" Denney