[Sinbios]

my cmoy in a mint tin is working great, except for one little quirk: when i use power from the dc jack, the sound is gone. when i wiggle it around, it comes back. i pulled out my trusty voltmeter and found that when the sound is there, one rail measures 8.3V and the other 7.6V (is this normal, btw? seems pretty unbalanced to me...). however, when the sound goes out, the virtual ground apparently is collapsed, and all 16V was concentrated on one rail. how did this happen, and how do i fix this? how come wiggling the jack/plug around helped?

 

[bg4533]

Are you using an isolated jack for DC input? I believe you need to be.

 

[Sinbios]

i'm not, the jack is the one recommended on tangent's guide, CP-6. maybe insulation is the issue here, but i don't get how wiggling it unconnects the case from the jack...

for the insulation, would plastic washers insulate it, or does the part that goes through the hole have to be insulated as well?

 

[aeriyn]

The power rails in a cMoy are going to be that unbalanced, moreso when music is playing. The resistive voltage divider is not perfect. If you want to keep the rails more even, replace the two resistors with a TLE2426 "rail splitter."

Oh, and yeah you need an isolated DC jack, or it will collapse the virtual ground. Isolating the CP-6 jack is a really big pain in the rear, I've done it before, but there's other jacks that are already isolated.

 

[Sinbios]

Quote:

Originally Posted by aeriyn The power rails in a cMoy are going to be that unbalanced, moreso when music is playing. The resistive voltage divider is not perfect. If you want to keep the rails more even, replace the two resistors with a TLE2426 "rail splitter."

i happen to have one on hand, but it's for my MINT. does 0.7V's worth of unbalance affect anything?

 

[aeriyn]

Quote:

Originally Posted by Sinbios i happen to have one on hand, but it's for my MINT. does 0.7V's worth of unbalance affect anything?

Probably not really. Most resistor dividers are off even more than that.

 

[tangent]

Quote:

the jack is the one recommended on tangent's guide

No part in those lists is recommended unconditionally. What's happening here is that the barrel of the power plug (V-) is being tied to the case, but you have virtual ground also tied to the case, probably through your I/O jacks. The virtual ground loses the fight for the case potential, so it goes to V-, dragging virtual ground down with it.

That jack would work just fine in a plastic case.

I did remove the CP-6 from my pages a few days ago, though, but it was because of the relative ease with which the switch inside those jacks breaks. Aeriyn pointed out that I don't actually like the jack, so why have it on the lists?

All that's beside the point here. The point is, if you use a metal-bodied jack, you need to pay attention to things like case grounding.

 

[Sinbios]

hm, then do you have any recommendations for insulated jacks from either radioshack or digikey?

 

[Sinbios]

ok, insulated the jack body with electrical tape, and now it's working great. initially i went all high-tech and tried to use teflon, but the damn thing is too soft and kept getting pierced by the rough edges of the mint tin.