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Pioneer DJM700 phonos Humming
Old 15th June 2019
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Pioneer DJM700 phonos Humming


My DJM700 mixer is giving me a hum from all three phono channels, even when there is nothing at all connected to any channel inputs.

My line and CD inputs are fine, no humming there.

Ive checked the C5 capacitor and it seems fine. But given that it's only on phono would suggest its not a mains power input issue. More an issue with grounding the phonos?

Any suggestions would be very helpful.
Thanks in advance
Old 16th June 2019
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Richard Crowley's Avatar

A phono preamp is a high-impedance, high-gain circuit. It is the very definition of the kind of circuit that collects as much hum as possible from everywhere.

For that reason, it is NOT VALID to evaluate or test a phono (or microphone or guitar) input which is not terminated. The typical practice is to use a shorting plug when evaluating, testing or working on a high-impedance (or high-gain) input.

Short the input(s) of all high-impedance, high-gain circuits. THEN evaluate or measure the circuits. If the SNR (Signal-to Noise Ratio) is still unacceptable, THEN you can start troubleshooting (BrEnglish: "fault-finding").
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