[Subsonic]

I just recieved my Alps in the mail today and found that the pot's case is isolated from the signal pins. If I'm going to use a metal case for my Meta42 project, do I still need to ground pot's case to PG? This amp will be strictly for home use and I'm planning on using an Elpac Wall Wart to power the amp so wouldn't I let the Elpac take the case & pot to ground instead of to virtual ground or should it be the other way around (amp take the case to ground)?

What got me thinking about this is that I saw Tangent's new Meta that he built for himself and it looks like he grounded his alps to PG even though he's using a Hammond chassis (metal). Or is he just using an insulated DC jack?

 

[morsel]

My recommendation is to tie case and pot to signal ground and isolate the power jack. You don't want V+ or V- on the case, and that is what you would be doing if you tied the power connector to the case, since the META42 generates a virtual ground via the TLE2426. This virtual ground, which is 1/2 way between V+ and V-, is tied to signal ground. This is why you should isolate your power connector from the case, and also why you can't use power supplies that tie one of their rails to AC ground with an amp such as the META42 which generate a virtual ground - to do so would short ground to one of the power rails (probably V-) if an AC grounded source is connected to the amp.

 

[Subsonic]

I understand your point clearly as I have thouroughly read tangent's articles, but I still don't see how V+ and V- will touch the chassis if I use insulated panel components (RCA and mini jacks). I guess I should have been a little clearer before hand, I meant that the metal shaft (case) of the ALPs is isolated from the signal pins hence it being isolated from V+ and V-. If I were to go that route I would have to insulate the RCA jacks and Mini jack. Correct? But the more I think about it, I think insulating just the DC jack would be easier than insulating everything else.

 

[rickcr42]

what you are doing when you ground the case of the pot is to isolate it from interference in theory.
I personally never ground the case but others do and report beneficial results

 

[antness]

I have always noticed benefits in noise when grounding the Alps Blue case. A very simple way to do this is to unscrew the bottom-right screw on the rear of the case and wrap a thin wire (24AWG works) around the screw shaft, and then solder it in place. Screw the screw back in completely to the chassis. Then solder the other end of the wire to the Alps Blue ground pins which just happen to be right next to that screw

 

[tangent]

In the case of the ALPS Blue Velvet, grounding its chassis is not an option -- if you don't, you will get hum when you touch the shaft. You can cheat and use a plastic knob to get around this, but....

In the picture you saw, the pot is isolated from the case with the same type of insulating washers that come with many insulating RCA jacks. This works because both parts use 5/16" holes -- you can use some of your spare washers as-is, if you have them.

Also in that picture, the jack is indeed isolated, but it wasn't necessary. I did it for looks, not for electrical value. (Notice the black border around the jack.) By isolating the pot anyway, I ensure that the pot chassis is only connected to virtual ground. I have personally had problems when grounding the pot's chassis to V- in a META42. Since a suitable alternative is available, I haven't tried to find out why this problem exists.

As I said, I put insulating washers on the DC jack only for aesthetic reasons. It's really dodgy electrically because I had to make those washers myself. They have no lip on the inner edge that prevents the jack from sliding around and touching the edge of the panel hole. I suppose there might be some commercial ones available with protective inner lips, but I don't know where to get them. Having been through this, I would now prefer to either get an insulated jack, or to simply make chassis ground equal to power ground.

Regarding using chassis ground as signal ground -- that looks attractive, but it can lead to ground loops and such. It's better to simply ensure that all ground connections go straight to the board and have no other path, since the board's ground situation is a known quantity. That is, we know it works. We have no idea how it will interact with your case, so we don't recommend that you throw that variable into the equation, even though you might possibly get away with it.

 

[Subsonic]

Thanks everyone

. It seems you all have different approaches when it comes to grounding the pot and chassis. I decided to play it safe, at least for this project, and use tangent's method of grounding where he insulates all panel components (including the DC jack) from virtual ground.

Tangent, I notice that digi-key sells those nylon bushings but not in 5/16" size. I just ordered a few of those bushings for my mini jacks, but I'll also have to make my own for the DC jack and Pot (don't have extras). I assume I can just use 2 nylon (1 thick, 1 narrow)washers and krazy glue 'em.

Oh, a few last question for tonight. What is the function of the pin that's on the left side of the Alps shaft? It looks like it is there to prevent the pot from rotating. If so, how do I ground that? Should I use a thick washer on the shaft to separate it from the chassis?

Thanks again.

 

[tangent]

Those nylon bushings you're looking at are probably something else. (Give a part number if you want us to take a look.) They're probably more like spacers. What you want are washers with a lip on their inner edge so that they fall into the hole a bit and won't let the pot or jack touch the edges of the hole.

What you might want to do is get some extra RCA jacks with the right kind of insulating washers on them already. Then you can swipe one set of washers for the pot, and use that jack on a plastic-cased amp later, where insulation doesn't matter.

I wouldn't try to insulate the DC jack. As long as you're only using chassis ground for one thing, you're fine. Power ground is just as good as any use, and it's a better use than using it for signal ground.

Quote:

What is the function of the pin that's on the left side of the Alps shaft? It looks like it is there to prevent the pot from rotating. If so, how do I ground that?

You don't ground it -- you grind it.

Grind that silly thing right off, because it gets in the way. The way you're supposed to use it is to drill a shallow hole in the back of your panel for it to rest in -- the purpose is indeed to prevent it from rotating in its hole, but it's a fairly silly feature, really. If someone's cranking so hard on the knob that they start rotating everything inside the amp, they deserve to lose, IMHO.

 

[Subsonic]

Quote:

You don't ground it -- you grind it.

Maybe I should have gotten a few of those generic stepped Alps instead

. All well, if it must be done, it must be done.

I'm looking at the spec sheet for the bushing and they do look like the ones that are on my RCA jacks except that they don't list a 5/16" one.

 

[tangent]

The pseudo-ALPS steppers have the nubbin on them, too. Don't worry about it -- a few quick swipes with a hacksaw or a few seconds with a Dremel and an emery wheel and it comes right off.

I see the terminology problem here -- the "bushings" are the ones with the long shafts that come out from the washer part. The "shoulder washers" are the ones you're interested in. I'd have known what you meant if you'd called them that... And indeed, there are no shoulder washers with an inner diameter of 5/16" and an outer shoulder diameter of 3/8", like insulators for the larger types of RCA jacks. Surely they must exist somewhere, though. Still, I wonder how much effort it's worth going to -- getting an extra pair of RCA jacks isn't all that expensive, unless you're going for Cardas or exotica like that.

 

[antness]

I always drill the hole for the little piece that prevents the pot from rotating. Better to be safe than to eventually have the pot begin to rotate, and have to fix them all if you're a manufacturer. Quality potentiometers have that feature and it's good to use it

 

[aos]

Of course, if you solder the pot to the board, there is no need for anti-rotation pin at all. Yet even the PCB mount version probably has it.

 

[Subsonic]

Quote:

Originally posted by aos Of course, if you solder the pot to the board, there is no need for anti-rotation pin at all. Yet even the PCB mount version probably has it.

The alps isn't going to be mounted in this project because the enclosure is too narrow and I wouldn't be able to fit the Neutrik jack. So I'm thinking I might have to drill the extra hole for that pin and find some way to ground it.