Audio Research VT-100 Mark 3 SE/BAL switch

The VT-100 has a switch on the back that is used to bring pin 2 to ground when using a balanced IC. It also seems to put resistance into the circuit, accounting for the 100K ohms for single ended versus 200K for balanced. When I used single ended connections, if I switched from SE to BAL, the sound dropped about 6 dB. I was using a passive setup so this drop was obvious.

Now I have an active preamp, Ref 5SE, and so I bought balanced interconnects (mostly to get to use the 12 dB gain of the preamp instead of the 6 dB in single ended). I tried the SE/BAL switch in SE to hear if there was a sound difference, which there was (attenuated highs), but did I burn out one hot lead in the balanced IC by doing this? When I tried another balanced IC using the switch properly in BAL position, the sound was about 6 dB louder than the original IC. My question is if it is possible to damage a balanced IC by crossing one hot lead to ground.

I tried asking Kalvin Dahl at ARC about this but he said I was wasting his time on a conceptual question.

No, you did not damage the interconnect by shorting one of its signal leads to ground. The output impedance of the Ref5SE, which is more than 300 ohms for each leg of the balanced signal pair according to John Atkinson’s measurements in Stereophile, would have limited the current into a short circuit to no more than a few milliamperes, which can be safely conducted even by wires that are vastly narrower in gauge than the wires that are used in nearly all interconnects.

If anything would have been damaged, it would have been the output stage of the Ref5SE, but that seems very unlikely also. And per your comments the Ref5SE works fine with the different balanced interconnect.

But if you are saying that the original balanced interconnect now sounds 6 db lower than the replacement, with the switch in the balanced position in both cases, I don’t know how to explain that. If you are sure that is the case, all I can suggest is that if you have a multimeter check for continuity between each of its three pins at one end and the corresponding pin at the other end. And check that no continuity exists between each of the three pins and the other two (doing that at just one end is sufficient).

Some clarifications to your first paragraph: The reason the input impedance is 200K balanced and 100K unbalanced is simply that the 200K reflects the sum of the input impedances of each of the two balanced signal legs (XLR pins 2 and 3), while the 100K is the input impedance of just the one leg that is used to receive a signal in unbalanced mode. The reason for the 6 db difference in volume you mentioned in that paragraph, if I’m understanding what you wrote correctly, is that in balanced mode the amp responds to the difference in voltage between the two input signal legs, and when one of those legs is grounded (i.e., connected to 0 volts) that difference is only half as much as it would normally be. Finally, re your first sentence, on USA equipment it is most likely pin 3 that is grounded by the switch, not pin 2, and ground would be applied when an unbalanced signal is being provided, not a balanced pair of signals.

-- Al


Thanks very much for answering all my questions. In the case of the one IC having lower throughput, it could be the case that I bought a damaged IC. I bought both IC's recently on Audiogon to accommodate using an active preamp. I shipped the suspect one to Tara Labs, so will post what I find out from them.

The circuit diagram I have for the amp shows pin 1 as ground, jumped to pin 2 upper left for SE operation. Pin 3 is shown lower left, circuit going from left to right through the amp.

Thanks again.



Mystery solved. Tara got the IC's and determined that the grounds had originally been terminated on the wrong pins, so they reterminated all four ends on warranty...that's a conclusion I did not expect!


Wow!  Someone must have been having a particularly bad day when those cables were assembled.  Or perhaps multiple someones, if (as one would hope) their QC function for a given cable is not performed by the same person who did the assembly.

Anyway, glad that the issue has been resolved.  And thanks for letting us know.

BTW, you may want to consider purchasing a multimeter at some point.  Among other benefits, it would make it very easy to check a cable for opens, shorts, and miswires.

Enjoy!  Regards,
-- Al