• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Circuit for a hum sensing probe?

Jump to Latest
#21
trobbins said:
Detecting is fine, but that isn't the same as probing imho - where you may need to position a search coil in a particular location and the coil oriented in any of 3 orthogonal directions (x,y,z), and then relate that back to say a pickup loop (eg. an output transformer winding, or a circuit loop) that also has a nominal orientation and location (which you may be able to then adjust to minimise the transfer).
Click to expand...

+A. Finding the field is not the problem, it's finding the antenna.
 
#22
20to20 said:
Hence my suggestion, which seems to have been summarily dismissed, for using a cheap MM phono cartridge. Ever torn one of those apart to see how small the coils are? Get one of those $17 P-mount AT cartridges. I've had a MM cartridge pick up power transformer magnetic radiation from 2 feet away requiring TT repositioning to remove it. Yes grounded.
Click to expand...

Good idea, delicate work to extract a coil and solder it into a probe though, especially since the coil is designed to move over its magnet. It would be small and sensitive, but that loose magnet kind of scares me with my aging eyes.
 
#23
Windcrest77 said:
Good idea, delicate work to extract a coil and solder it into a probe though, especially since the coil is designed to move over its magnet. It would be small and sensitive, but that loose magnet kind of scares me with my aging eyes.
Click to expand...

I wouldn't open it for the coils. It may need the magnets attached to the stylus to produce signal. Those cartridges are small though and plastic covering will keep them out of trouble.
 
#24
Take a typical operating amplifier B+ power supply that is either CLC or CRC filtered.

Check the [3 inch wire] that connects the B+ secondary center tap to the negative
lead of the first filter cap.

Connect the magnetic sensor coil to an amplifier that has a speaker connected.
Put your sensor coil next to the 3 inch wire.
Rotate the sensor coil through 3 axis (90 degree rotations).
Wow, it is not just 100Hz or not just 120Hz (2 times mains line frequency).
The transient rectified current has many harmonics.

If you have not properly confined this transient current to a local ground loop, then
that sound will be on your amplifier output.
 
Last edited:
You must log in or register to reply here.
Top Bottom