Slight hum from turntable source

Easy Runnin

I recently noticed a slight hum when my preamp is on the phono source. I only notice it when music is not playing. The higher the volume the louder it gets. It still occurs when the TT is completely disconnected and It only occurs on the TT source. I removed all sources and it still occurs. The preamp is a B&K MC-101. It has an internal MM/MC switch. I have the switch set to MC. The TT is a Technics SL-Q3 with a Denon DL-103.

Any thoughts?

BlueMDPicker

Does it also hum when set to MM? If not, I'd guess the culprit could be the extra gain needed for MCs.

SCompRacer

It is not uncommon to have noise with a phono stage due to the high amplification. Are we talking you hear it at your listening position or do you have to get close to your speakers? The rule of thumb for a quiet phono stage is listen at around 85dB SPL and lift the needle. If you don't hear any hiss/hum at your listening position, you are golden.

After trying what Mike suggested, there are a couple of other things. If you have a DMM or digital multi-meter, set to DC and measure for DC on your pre outputs to amp. Possible to have a leaking PS or DC blocking cap. Turn your gear off before unplugging or connecting anything.

I'm not sure if you bought it new or used. If used, check to see if they installed capacitors in the circuit board. From the factory it shipped with no capacitors for loading, but users could add capacitors to change loading. B&K felt the IC cables would add enough capacitance. IIRC the loading resistor could also be user changed.

Loading is all covered in the manual.

Easy Runnin

85db is way too loud for me. I used a spl on my iPhone and the volume around 75db. With the needle off the record and the switch set for MC the hum was noticeable with the Denon MC cartridge. With the switched set to MM and an AT120e MM cartridge the hum was barely noticeable.

I'm not the original owner. The pre was shipped from the factory with no capacitor and with a 100 kohm resistor. It doesn't look like anyone changed it.




As for the leakage test I assume you do it with the preamp powered on and the black probe on the outside and the red probe on the inside on both the left and right outputs. Doing that I got readings of 0.001

BlueMDPicker

I know Rich can speak to the possibility of adding a capacitive load, and I'll let him address that. A couple of questions and suggestions:

Is there an electromagnetic field from another component's power supply, lighting, etc in close proximity to the turntable? If so, do you have the flexibility to relocate components?

Are all components plugged into the same circuit? If not, can you rearrange connections to put them on the same circuit for testing?

Do the phono leads from turntable to preamp include a grounding conductor? If so, try lifting the ground between the turntable and preamp. If not, replace leads with set that includes ground. Make sure phono leads are not in close proximity to an electrical field.

Hum still present? Try lifting the ground on the turntable power connection with a 3 to 2 prong adapter.

Hum still present? Try lifting the ground on the preamp power connection with a 3 to 2 prong adapter.

Hum still present? Try lifting the ground on the power amp power connection with a 3 to 2 prong adapter.

Hum still present? Is the hum audible on both channels? If not, swap input connections and determine if hum moves to other channel. If hum moves to other channel, the problem may be in the preamp circuit components for the channel. If hum remains in original channel, check (or swap out) cables from pre to power amp. Hum still present? Check speaker cables. Hum still present? Problem likely in power amp channel circuitry.

I hope something in the above helps you find solution. Good luck!