Audio Systems: TT hum, mobile fidelity, rca phono

Question
QUESTION: Hi- And thanks in advance for your time.

I have a TT hum.  The TT is an Avid Acutus Reference TT, with a Dynavector 507 mkII tonearm and a Mobile Fidelity Carbon Fiber, MC cartridge.  There is a Din connector connected to the tone arm and RCA's and a ground wire connected to a Goldmund 3.7 MC/MM phono stage, which is loaded at 47ohms.  The phono stage is plugged into a Goldmund srp2.3 pre amp, which is connected to Goldmund 18.4 monoblock amps, connected to normal dynamic drivers Proclaim Audio speakers (no electricty to speakers)

When the volume on the pre is increased the sound gets louder.  It is audible at listening position during silent passages or between songs.

When the cartridge is disconnected from the headshell, the system is dead quiet.  If the phono is connected to pre and amps with input, system is dead quiet.

Have completely disconnected all other electronics in the room.  Also have tried lifting grounds etc. to no avail.

There is no effect whether the ground is connected to pre or not.  I have tried every scenario of grounding lead coming off phono plug to phono drive motor power/supply, pre/amps etc.  NO EFFECT.

We have tried a different din to rca phono plug and even a different cartridge.  Same hum.

Measured with a frequency analyzer there is signal present from 20hz to 250hz.

Have tried moving cables and equipment, no efffect on the hum.

Have moved electrical connections so everything is on the same circuit, no effect.

I have several other systems in this room and I can connect a CD Player to the pre and system is dead quiet, hum is only present with TT input selected.

Appreciate any insight...thank you

Bob



ANSWER: Check this:  Place a short across the output of the MC cartridge and see if the hum goes away.

AND:  Does the hum level change as the tone arm moves toward the center of the album?

AND:  If you wrap your hand around the cartridge/shell does the hum level go up?

I am suspecting you may have a MC cartridge that has hum induction.

Let me know more.

C

PS: Nice setup.  Should be getting some awesome sound.



---------- FOLLOW-UP ----------

QUESTION: Hi- I tried wrapping hand around the cartridge and tested for changes as the arm moves towards the center of the album.  No effect.   How do you short the cartridge output?

I have discovered one thing.  If I remove the input cables to the phono from the tone arm output, and insert shorting plugs on the input of the phono section, the hum is still present.  I believe this tells me that the TT/arm/cartridge are all ok.  But the phono stage might have an issue.  I have another one coming to test out.  I will let  you know.  

Again, I'm not quite sure how you exactly mean to short out the output of the cartridge.  I'm thinking that you mean take a small wire and run it between....this is where I'm not sure.....which leads coming out of the cartridge do I short out....thanks

Answer
Yes, to short the terminals, just put a small jumper across the output terminals.

BUT: As you learned, shorting the inputs to the preamp does not remove the hum.  Then, I agree with you the problem is in the preamp. It could have a power supply problem or a ground loop problem within the circuitry of the preamp in which case I would refer the problem to the manufacturer and ask for a replacement.  

For example, the ripple filter may be defective or a ground circuit has cold solder joints or something like that.

And, as a note, moving coil cartridges are notorious for hum due to their very low voltage output.  That is why I started you on that leg of the journey.  But, now it seems the preamp is the culprit.  Let's hope the replacement preamp solves the problem.

Tks for the feedback.
C