Cables and (inter)connections


Q: The UcD amplifier needs a balanced input signal. My preamp only has unbalanced RCA outputs. Do I just connect the ‘-‘ input to ground?
A: Yes, but only as described here: http://www.hypex.nl/docs/wiring.pdf

Q: I would like to switch from XLR to RCA and vice versa without the need of pulling the unused cable out of the amp.
A: That basically amounts to a source selector. Refer to http://www.hypex.nl/docs/wiring.pdf for information of how to wire an RCA connector to the UcD module. Install a DPDT switch to connect the hot/cold inputs of the UcD module to either pins 2/3 of the XLR connector or to the signal and shell contacts of the RCA connector. Note that pin 1 of the XLR connector is not switched, but remains hardwired to chassis. The RCA shell contact also remains hardwired to the chassis. This means that when the DPDT switch is set to RCA, the cold input of the UcD module is grounded via the RCA connector.

Q: I have no preamp. My source is unbalanced and I would like to use the “RCA to XLR” cable method. Can I add a potentiometer and retain some of the benefits of this connection method?
A: Indeed you can. There’s no good way of adding a potentiometer to a truly balanced connection, but in a “pseudo balanced” connection it is possible. You'll need to use microphone cable (shielded twisted pair). I'm calling the two wires red and white because that's the colours they usually have. There are 2 stretches of cable. Cable 1 goes from the RCA terminals to the pot. Cable 2 goes from the pot to the UcD input.
1) Cable 1 (RCA end): shield and white wire to RCA ground. Red wire to RCA signal.
2) Cable 1 (Potentiometer end): white to potentiometer "common". Red to potentiometer "input". Shield of cable one is connected to shield of cable 2 AND TO NOTHING ELSE!
3) Cable 2 (Potentiometer end): white to potentiometer "common". Red to potentiometer "output" (wiper). As said in 2) the shield connects to the shield of cable 1 and to nothing else. RESIST ANY URGE to connect the shield to the pot common. 4) Cable 2 (amplifier end): white to pin 1, red to pin 3, shield to pin 2. So that means that the shield of cable 1 continues as the shield of cable 2, so you get a contiguous shield all the way from the RCA to the amplifier. Any circulating currents that go through the shield will never mix with the audio ground because the audio ground is kept separate (white wire) and sensed differentially by the amplifier.

Q: Do UcD amplifiers work well with tube preamps?
A: Fundamentally there is no problem of course. However, most tube preamplifiers are designed alongside tube power amplifiers which are always AC coupled. As a result, many tube preamplifiers lack precautions against turn-on/turnoff transients. Some are liable to produce the full anode voltage at their outputs during turn-on and turn-off. We find that a large proportion of repairs are due to tube preamplifiers discharging their output caps into the UcD’s input circuit. Note that most solid state amps dislike such treatment, not just UcD.