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garysch

Hum elimination - proper DI box

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In one of the other threads it was noted that the Privia line has an unbalanced audio output. When feeding professional amplified speakers, mixers, etc., balanced output and balanced cable allows for very long cable runs without hum or change in sonic quality. I own a Radial Pro passive DI box, but that just didn't seem to work right, and I also wanted a dual channel capability because I generally use two amps for the piano or need to feed a monitor and PA sytem.

The piano has plenty of output so the matter is strictly that of converting the unbalanced piano outputs to balanced. While there are loads of DI boxes, the lack of info regarding impedance transformation or ratios was disconcerting. My Radial Pro box reduced the levels. Instead, I opted to build my own DI box which proved to be an inexpensive and one evening project. The result is a dual channel box that has eliminated hum, ground loops while converting the outputs as required.

The heart of the unit is a pair of Edcor model PC600/600 transformers. http://www.edcorusa.com These are small 600 ohm to 600 ohm transformers that can be wired unbalanced on the input to balanced on the output. They are designed for this use, have a flat frequency response and very low distortion. The transformers are mounted in a 4 x 3 x 1.5" plastic box strapped vertically on opposite sides with a wire tie. Two standard 3 connector 1/4" phone jacks are mounted on each end. The unbalanced inputs are wired: transformer pin 1 to tip, pin 3 to sleeve (pin 2 on the transformer and the ring on the jack are not used). The output or balanced jack is wired: transformer pin 5 to ring, pin 6 to sleeve, pin 7 to tip. In a dual channel box do NOT connect the sleeves of both channels together. If a metal enclosure is used, you must use fully insulated 1/4" jacks.

Costs: The transformers are $6.37 each, but with shipping and handling my cost was $28. The four jacks and box will cost you around 15-$18, so for under $50 you have a dual channel matching DI box. One option is to use XLR connectors. That would work in the same size box if you place the transformers on the short sides. The net result is complete hum elimination and the ability to use balanced TRS cables to the powered speakers and the PA. Contact me if you need any other details.

Gary

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