[strider]

I'm getting a hum through my speakers with the volume on my pre anywhere from about 10 o'clock and up. Just got done putting my Bugle into an enclosure I built from MDF, I have the input RCA jacks wired with shielded, twisted,  Teflon insulated wire; the outputs are wired with Cardas chassis wire. Moved the chassis ground for the phono cable to the back of the enclosure as well using a Cardas ground post. I can make the hum louder by putting my fingers nearer the input wires, gets much quieter if I touch either the ground post on the enclosure or the ground screw on the board.  The hum was not there at all before I mounted the Bugle in the enclosure. Any ideas on where to start looking?

Here's a picture, if it helps....

[Theo]

Things to try...

1.  Twist each pair (+ and - leads) of the output channel wires together.  This should reduce RFI.
2.  Replace the two pieces of wire on the power switch with two short pieces of jumper.  As it is now, they behave like antennas.  You can use two pieces of scrap resistor lead for this purpose.

[hagtech]

Try getting a ground between equipment.  The BUGLE and wiring is picking up electric fields.  I'm wondering if perhaps the glue or resin used in MDF is partially conductive.  Your fingers will point you the right direction.

jh

[strider]

Thanks for the suggestions. I'll try them tonight when I get home.

Doing research before I started building the enclosure, I read that the inputs were especially susceptible to picking up RFI. Could the amount of wire (3/4" or so) exposed from the shielding on the input wires be another possible source?

[Theo]

I read that the inputs were especially susceptible to picking up RFI. Could the amount of wire (3/4" or so) exposed from the shielding on the input wires be another possible source?

I don't think so.  The input wires in my Bugle are shielded Cardas 4x33awg tonearm cable.  Quite similar to what you're using and too have about 1" exposed wires there as well.  No excessive hum though.  Try twist all exposed +/- pairs where possible.  You may need to provide a ground as previously suggested.

[Theo]

This was what I did where I debugged grounding issues for my Bugle.

To diagnose, I wrapped one end of a wire on the ground post of another phono preamp (plugged into an outlet) that has no hum problem; the other end of the wire wrapped to the Bugle ground post.  Connect up your TT to the Bugle like you normally do (including the ground wire from your tonearm to the Bugle ground post).  If the hum diminished, you've at least found one part of your problem.

To fix it, I then connected the ground post in my Bugle to the IEC receptacle's ground pin with a piece of short wire to ensure that I have a ground.

[strider]

Just a quick update.

I stole some time Saturday night to try and nail down the cause. First I tried running an additional ground wire from the Bugle's ground to my preamp's phono ground. No change. Next I ran both the TT ground wire and the extra ground wire directly to the ground post on the Bugle board. No change.
Next I twisted each pair of output wires together, still no change.

I misplaced the resistor leads I was going to use to replace the wire loop I have completing the circuit for the power switch. I'm going to find something today and (hopefully) try to replace them tonight.

[strider]

I'm out of ideas. Nothing I've tried has gotten rid of the hum; it may be a bit reduced, but it could also be in my head. If it is reduced, it's only by a slight margin. First replaced the wires on the power switch with pieces of resistor lead. Next I removed the input wires, replaced one that had a slight nick in the jacket, stripped the other's ends back to new wire, and reinstalled them. After that I removed everything from the MDF base it was attached to, then I removed the input and output RCA wires and replaced them with the original RCA jacks that were soldered directly to the board. Essentially it's the same as before I began mounting it in the enclosure, only difference is the now present hum. Even in this state, I can put my fingers near the input RCA's on the board and the hum gets louder. Could I have damaged something on the board itself when I desoldered the RCA jacks and switch? I had a tough time getting the switch off the board. Any help is appreciated, I'm done for the night.

Thanks,

Ben

[hagtech]

Where are the batteries?  Sounds to me like pickup, perhaps getting onto one of the cables.  I still worry the mdf itself might be the problem.  Follow your fingers!  Move wires around.  Anything you touch and it gets quieter, that means it's not getting the right ground connection.

jh

[strider]

Thanks for sticking with me, Jim. The batteries are on the floor right now, moving the connecting wires doesn't effect anything hum-wise. I took the Bugle completely out/off the MDF base and had it perched on top of an Isoblock last night, no change.
I re-built the Bugle last night to it's pre-hum state (RCA jacks mounted directly to the board), with the exception of still omitting the power switch. It did get quieter, but only very slightly. Having the phono cable ground lug attached to the ground screw on the board, I can touch the screw and the hum all but disappears completely. Still gets louder when I have my fingers close to the input side of the board. From what you're saying, it sounds like the ground screw on the board is not providing the ground it should. Is there another point on the board I could use to provide the ground?

Where are the batteries?  Sounds to me like pickup, perhaps getting onto one of the cables.  I still worry the mdf itself might be the problem.  Follow your fingers!  Move wires around.  Anything you touch and it gets quieter, that means it's not getting the right ground connection.

jh

[hagtech]

It means that screw on the board needs a ground wire connection to the linestage.  Normally this is provided by the interconnects, but something might be odd here.

jh