DL304 cartridge and mains hum (dependent on loading)

[johnfromnorwich]

I've posted this elsewhere so apologies to those who may have already read this question but I've yet to get an answer...

I've just installed my first LOMC cartridge (DL304). My phono stage is a Trichord Dino and I've set it to 100ohm and whatever the aximum gain setting is. The arm is earthed. I'd been led to believe that the capacitance loading would make no difference to a LOMC but 100 pF gives me very intrusive (what sounds like) mains hum. 1nF is absolutely silent. Any idea why this might be?

 

[Werner]

The cap/hum thing makes no sense, but even for a 304
the Dino's maximum gain of 74dB is excessive. Even the next step
of 70dB is a bit high. 66-68dB would be ideal.

 

[zanash]

its likely to be the phono leads thats picking up hum from the mains ....? if hats what it is ...

try routing these cables clear of all stray elctrical feilds and mains cables etc...


allternatively introduce them to mains to check that there the cause ...and its not something associated with a loose earth or bad connection elsewhere

 

[johnfromnorwich]

its likely to be the phono leads thats picking up hum from the mains ....? if hats what it is ...

try routing these cables clear of all stray elctrical feilds and mains cables etc...


allternatively introduce them to mains to check that there the cause ...and its not something associated with a loose earth or bad connection elsewhere

That's what I figured, but it doesn't explain why the cap switch makes a difference (it's on/off - not subtle). All the leads are well cear of mains cables - and orthogonally routed. The hifi is on it's own dedicated spur and everything else in the room was switched off. May not be mains of course but that steady hummm sure sounds like it.

 

[zanash]

that could suggest an internal phono box source ?

switching in caps can effect the hum heard ....as it prevents the dc from crossing [I think] to the signal path .

 

[johnfromnorwich]

that could suggest an internal phono box source ?

switching in caps can effect the hum heard ....as it prevents the dc from crossing [I think] to the signal path .

Probably. The Dino doesn't look very well shielded (plastic!). It's as if the cap is acting as a filter. I'm not really bothered by it, since switching the cap makes no audiable difference to the sound but it's worth knowing the cause.

 

[Werner]

You have to ensure that the hum is mains related, i.e.
has its fundamental at 50Hz or 100Hz. If so there is no
mechanism that allows a 1nF capacitor in parallel to the
amplifier's input to affect this hum: 1nF amounts to 1.6 mega-Ohm
at 100Hz.

If it is not 50 or 100Hz, then what you are hearing is down-modulated junk caused by amplifier instability, which indeed
can be affected by 1nF at the input. But we wouldn't call this hum.

What at the next-lower gain setting?

 

[johnfromnorwich]

You have to ensure that the hum is mains related, i.e.
has its fundamental at 50Hz or 100Hz. If so there is no
mechanism that allows a 1nF capacitor in parallel to the
amplifier's input to affect this hum: 1nF amounts to 1.6 mega-Ohm
at 100Hz.

If it is not 50 or 100Hz, then what you are hearing is down-modulated junk caused by amplifier instability, which indeed
can be affected by 1nF at the input. But we wouldn't call this hum.

What at the next-lower gain setting?

I've fixed it. Turned out to be the wall-wart PS for my DAC. Although the DAC was switched off, it's battery powered so it was charging. Removed the DAC for unrelated reasons and bingo. No hum.