I've tried it with a cheap Behringer small-diaphram condenser, an sE 2200A large-diaphragm condenser, and a transformerless SM57 clone.ġ) With no mic connected, the preamp has a noticeable buzz. The most confusing part is that the preamp reacts differently with every mic I own. I've also tried using an external phantom supply with no improvement, so I've determined that the phantom power itself isn't the problem. Testing confirms that I have 47.5vdc at the input, and it has essentially no sag under load. The phantom power circuit is based on this design. The only real changes were the addition of switcheable phantom power, a switching jack that disconnets the mic input when a guitar input is inserted, DC-rectified filaments, and a 20db H-pad after the output transformer. Here's the schematic, which I followed pretty faithfully: amplifier and power supply. I've built 4-5 tube amps at this point, so even though I'm not an expert, I at least know what problems to expect and what to try to solve them. I'm building a clone of an old Altec 1566A mic preamp, and I'm having some issues that I don't know how to track down.
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