Volume Box for HRDX hum

[Zouto]

Hi, I just built a Volume Box for a Fender Hotrod Dx as described here http://stinkfoot.se/archives/2159, using a small diecast enclosure.
Plugged into the ampīs fx loop, it works but with an annoying hum like a ground loop. Moving it away from the FX loop jacks to the Input side on top of the amp the hum disappears. It seems that the Box is picking up hum from the ampīs power transformer. Is there any cure to avoid that hum? Any experiences with similar volume boxes on Fender HRDXs?
Thank for hints, help and advice
Zouto

[Zouto]

Volume Box for HRDX humming

Hi, I just built a Volume Box for a Fender Hotrod Dx as described here http://stinkfoot.se/archives/2159, using a small diecast enclosure.
Plugged into the ampīs fx loop, it works but with an annoying hum like a ground loop. Moving it away from the FX loop jacks to the Input side on top of the amp the hum disappears. It seems that the Box is picking up hum from the ampīs power transformer. Is there any cure to avoid that hum? Any experiences with similar volume boxes on Fender HRDXs?
Thank for hints, help and advice
Zouto

[Jazz P Bass]

Is the enclosure itself attached to circuit ground?
It should be.

[Zouto]

[ATTACH=CONFIG]55378[/ATTACH]

Is the enclosure itself attached to circuit ground?
It should be.

Yes it is, by the jacks‘ threads, nuts and washers, the potīs ground lug is connected to the input jackīs ground. Meanwhile I found out, that a short patch cable to „PWR Amp In“ reduces the hum significantly, so that seems to be the way to go. The length of the „send“ cable doesn‘t matter.

[Helmholtz]

Yes it is, by the jacks‘ threads, nuts and washers, the potīs ground lug is connected to the input jackīs ground. Meanwhile I found out, that a short patch cable to „PWR Amp In“ reduces the hum significantly, so that seems to be the way to go. The length of the „send“ cable doesn‘t matter.

The problem is strange. When the pot is fully up you have the same signal level at input and output cables. Also ground is the same, so there shouldn't a difference in hum sensitivity between the cables. You might actually have a ground loop problem. You may try to minimize the loop area by running the input and output cables closely in parallel.
Or better insulate the output jack from ground and do not connect the output cable's shield to the box's ground (to open the loop) or ground it via a 10 Ohm resistor.

[Enzo]

Is the box completely metal? And are your jacks grounded to it? It sounds like your box is not shielding its innards.

[Helmholtz]

Why do we have 2 threads?

[Enzo]

I answered the one I saw first. This forum board occasionally gets into a state where it doubles your post. You know it is happening when it pops up the dialog asking if you want to exit the page.

[Zouto]

I answered the one I saw first. This forum board occasionally gets into a state where it doubles your post. You know it is happening when it pops up the dialog asking if you want to exit the page.

Thanks Enzo, exactly this happened, there was no way of stopping it. Is there a way to combine both threads?

[Enzo]

Don't bother, just stop posting in this one. My post was covered by someone else in the other thread.

This thread will then fade on down the page into oblivion on its own.

[Zouto]

I replaced the output jack with an isolated ReAn plastic model with just a signal connection, but no connection to the ground of the metal box. The result was a much louder hum like an open ground. So I went back to my old version with all metal jacks and a short „Return“ cable. There seems to be no better way. I‘m really interested to know, how other users of such a volume box dealt with hum issues.

[Enzo]

I have a small aluminum box with a 250k pot on it, and a couple metal bushing jacks. A handy level adjuster in my shop. I have never had any hum problems with it at all.

Is your box wired exactly like the circuit Jazz posted? If so there is no reason you should have any trouble.

[Pedro Vecino]

What happens if you insert a jack-jack cable into the send/return? It could be a problem of the loop itself that goes unnoticed when you don't use it.

[nosaj]

What happens if you insert a jack-jack cable into the send/return? It could be a problem of the loop itself that goes unnoticed when you don't use it.

I see no one has mentioned solder joints. Bad ones can hum. Regarding picking up hum from PT. If that's the case moving box in a different location can alleviate that as mag fields from PT come out in specific directions.
Nosaj

Bad jumper cables can too. Is jumper an instrument or speaker cable?

[Pedro Vecino]

The loop circuit in that amplifier incorporates two active elements (the two halves of a 4558 if I remember correctly). Apart from possible mechanical problems with some contacts in the jacks, the circuit itself may not work well.

I once made two boxes for that (to have two selectable volumes also) and they worked perfectly. I added a small selector with three positions to incorporate two filters of different values in order to give linearity depending on the attenuation. The tendency with large attenuations is the loss of treble and filters can treat it.

If I remember correctly those values were for a Crate V33. The HR Deluxe needed higher values (x 2 or 2,5 probably).