One of our loyal readers has a tech question which was inspired by our earlier TTOTD on phantom power and line level equipment (See TTOTD 3/19/98):
“After reading the tip on phantom power with line level gear I am concerned about my bass amp. It has a balanced DI out that I normally connect to a balanced input on a snake, which would then go to the mixer balanced (mic) input. I fear one of these days someone is going to turn on the phantom power accidentally and fry my DI circuitry. What is the best solution for this situation since my mixer doesn’t have a balanced line input?”
According to our problem solving guru, Mark (McGyver) Phillips, there are two solution paths for you. He says:
- Use a 1:1 600 ohm isolation transformer in line. This will isolate the amp from the phantom voltage (see WFTD archive phantom power). WARNING, the first weakness of most transformers is saturation distortion at low frequencies! A transformer designed for microphones will fall flat on its face with a bass at line levels. So buy a good one; you get what you pay for.
- Some manufacturers actually live in the real world. The amp you are using may already have its DI out transformer coupled. But that’s a very important “MAY”. If the output is capacitor coupled, as many are, the voltage rating of the capacitor is critical. Too much voltage and the cap blows. The voltage rating must be greater than the 48 volts of the phantom power. Substituting a cap, with the original capacitance value, but a 63 VDC rating (50V just isn’t enough overhead) will protect the amp’s circuitry from the phantom voltage and you won’t need to carry the extra transformer around. This is a novice tech type upgrade, but it is not for a non-tech. The voltages inside any amp, especially a tube amp, are very dangerous.