Behringer Amps and Isolated Jacks

[kfaria]

Hello, all.
I've searched here and no matches popped up so here goes...
Have two Behringer amps, GM110 and GMX212.
Both have input jacks with plastic bushings that have become stripped.
Would like to replace them with Switchcraft metal bushing jacks.
Question is: does the sleeve really need to be chassis isolated?
Without schematics, I cannot tell.
Could trim the mounting hole and use isolating shoulder washers, but would like to know if isolation is really necessary.
Thank for any revelations on this matter .....

[R.G.]

If the amps were working properly and did not have noticeable hum with the isolated jacks, chances are they need to be isolated.

But the engineers' creed is "if it works, do it". I'd say put in metal bushing jacks. If the hum level doesn't come up, great, you're golden. If the noise or hum levels are worse, you're going to have to isolate the jacks to get hum and noise back down.

And the knowledge only costs you the price of an extra pair of jacks. What a bargain!

[Gtr_tech]

You can use metal bushing jacks and still have it isolated. Use shoulder washers....old trick there. You may need to enlarge the hole in the panel a touch...

http://3.bp.blogspot.com/_1_6LVu8pkp...der-washer.jpg

[kfaria]

Thanks for your replies!

@R.G.: Was asking before replacing; do not want to release any Magic Smoke ^_^
Guess I'll tear into it and inspect the circuit, since I cannot locate a schematic.

@Gtr_tech: Yes. As stated in the OP, will most likely use isolating shoulder washer plus front side washer to be safe.
All depends on what I see in the circuit.

[Gtr_tech]

No chance of smoking anything (unless its a switching jack that has DC on the sleeve, and I've seen a few that are set up that way). Its just to preserve the grounding. If it was designed to be lifted off the chassis, its to avoid ground loop noise. So it would still work chassis grounded, but not as intended.

Std practice is to use a shoulder washer in the back and a fiber or nylon washer on the front (non shoulder type). works fine...I do it all the time....

[kfaria]

OK, Gtr_tech, that's the way I'll go, metal bushing with isolating washers.
Thanks for the confirmation.
Have done quite a bit of analog design meself, and prefer not putting any signal common to chassis ground for signal integrity.
So that makes sense.
Thanks again!

[drewl]

You could also use metal jacks that are stereo and use the ring connection for ground or signal low, and it will be isolated from the chassis.

[kfaria]

@drewl: That's a good thought there.
While I'm "inside" will check if that jack is a true stereo one.
Have a feeling it is mounted directly to a PCB, which might complicate the ring connection.
I'll have a look see.
Thanks again for another option!

(Edit) Just want to add a note here.
Have been tumbling this jack thing over in my head for over a year.
Came across this site purely by accident,
and within a few hours found some excellent answers.
Thanks y'all!!!!!

[52 Bill]

You could also use metal jacks that are stereo and use the ring connection for ground or signal low, and it will be isolated from the chassis.

It will be isolated until you plug a mono plug in the jack.

[kfaria]

Yes, Bill 52, so I'm going with my OP option: isolating washers with a metal sleeve.
{deity} I love this site!

[Enzo]

Then you will have to make larger holes in the panel.

So you have a couple broken jacks, but is it really necessary to use metal? Look, it will not HURT anything to make the change. It MIGHT cause increased hum. But why not TRY it as RG suggested. It might work fine for you. WHy hack up your panel only to find out you didn't need to. You have to pull the old jacks out anyway, and you will be soldering in your new jacks anyway, so trying it first is a matter of slipping the bushings through the hole and firing up the amp. 10 seconds later you'll know. Then if you still have to make big holes, go ahead, but if not, you saved the time effort and damage.

Yes, the jacks are on a circuit board. They are the square black ones with the angled corner type.

The schematic says nothing about where the circuit common is grounded to chassis.

[Mick Bailey]

You can test for hum before you make any change;

Insert your lead and connect up your guitar. Turn the guitar tone/volume up and have the amp set to a low-ish volume. Now, touch a piece of wire between the metal jack plug body and something on the amp that is connected to the chassis - a screw, pot nut or whatever. If it hums that's what you'll get with non-isolated sockets. If there's no change then it won't matter if the sockets aren't isolated.

[drewl]

It will be isolated until you plug a mono plug in the jack.

DOH!

[Gtr_tech]

You don't even need to turn up the guitar vol to check for gnd loop hum. It will make itself evident once you bridge the 2 gnd points. You do need the master up a little tho so you can hear the hum.

[Mick Bailey]

Turning up the guitar volume ensures that the amp is being tested under normal operating conditions.