Conversion of valve amp to accept balanced input

Dear Dr. Jon Wong,

I have a pair of Cary Audio SLM 100 MK 5 valve mono block power amplifier. As my BAT pre amplifier are using XLR(balanced inputs and outputs), I am thinking of modifying my power amplifier input to true XLR. Pleae advice if any one in singapore is able to do the job. I do not want shorted negative, I want true XLR. Thanks.

Naga

Hello Naga!

If you want to modify your Cary valve amp, my suggestion is you might not gain much. I do not have access to the Cary and BAT circuits, but here are my assumptions and analysis:

1. Balanced transmission is designed for better noise immunity using cancellation of positive and nagative signal noise. For short length of transmission cables (say below 3 meters), there might not be significant gain in using balanced mode. Also, for domestic environment there is no significant noise disturbances.

2. For power amplifier with an Operational Amplifier style circuitary, (usually found in transistor amplifiers), the conversion to balanced mode can be done by rewiring the positive and negative inputs. However, for vavle amplifiers, the input is usually a single ended class A. Cary uses the 6SN7 driver and phase splitter. The conversion in this case would mean an introduction of an auxiliary circuitary. That usually means using an integrated circuit to do the balanced input.

3. If you want to add a pure tube circuit as the balanced receiver, that will involve extensive work. The chasis might not even be able to take it.

4. Modification to any factory equipment always will void the warranty.

Having considered points 1 to 4, I suppose it is best using the current configuration you have.

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