RF Noise in Power Supply

[sandy]

I have made a preamp based on M7. I tried both valve and mosfet based power supply (B+).

There is however a strange problem, i observe in my house. Whenever a vehice goes near my house, the pre generates a hi-frequency noise for couple of seconds. However, when i tried the pre in my friends place...i did not face this problem. The same problem happens when i turn on a light in my house..

Please help..

Cheers
Sandy

[Rod Coleman]

Sandy, have you looked into your house wiring or Mains supply quality? Sounds like the emissions from vehicle ignition systems are being picked up by the wiring. Normally, the supply presents a low RF impedance (line-neutral & L/N to Earth) and does not disrupt audio supplies.

Maybe you can borrow the equipment to do an earth-loop impedance test and measure supply quality. It is quite possible that the street wiring has too much inductance back to the substation, and vehicle emissions are picked up in the street & relayed to you. In this case you'll need some mains conditioning to reduce common mode noise and some (suitably rated) capacitance L to N.

Sometimes house wiring is to blame: eg inadequate guage cable is used, earth not connected, loose terminal screws , tired fuse wire etc.

I assume you can perform this remedial work safely; if you're uncertain, be sure to get help from a friendly electrician!

[Sch3mat1c]

Shield the equipment better and use more bypass caps.

Tim