[morsel]

Gonzo: I don't think SMD will be necessary for this project.

Voodoochile and Nisbeth: thanks for your detailed replies on casing and shaft extensions.

Elma Series N stepped attenuators come populated with low inductance SMD metal film resistors. No soldering required. Sound more appealing now?

If you are feeling rich you can go with the HeadRoom stepped attenuator instead, which uses the Elma mechanism with proprietary circuit boards that look really nice, but do not have board mount pins.

 

[Voodoochile]

I did not know you could buy Headroom's without an amp. Interesting.

Also, good points, Cameron. I meant to comment on that earlier.

If the op-amp is approximately centered along the fron-to-rear dimension, then I'd prefer the rear or center for the pot as opposed to the leading edge. If that makes sense!

Gonzo: that is true. Why not just go crazy and install some Shallco 45-position ladders!

 

[drewd]

Quote:

Originally Posted by morsel Gonzo: I was also thinking that custom smaller PAR-Metal cases might be good, as long as we don't have to get into complex group buy issues.

I bought a 6" wide type 20 case a few weeks ago for $50.00. Basically any custom work smaller than the smallest listed type 20 is $50.00. That was black anodized, but not countersunk.

Here's a picture. The one on the right is the standard 12" wide case.

-Drew

 

[amb]

Quote:

Originally Posted by drewd I bought a 6" wide type 20 case a few weeks ago for $50.00. Basically any custom work smaller than the smallest listed type 20 is $50.00. That was black anodized, but not countersunk. Here's a picture. The one on the right is the standard 12" wide case. -Drew

That 6" wide case is very nice. If we could make the main PCB fit in a case like that it would be ideal. The PSU will have to be external, which is fine. Those who want to put the PSU inside could just go with the 12" wide case. If it doesn't cost that much more to have Par Metal make a 6"x8"x2.5" case then I think it would be a good target to aim for. It would be especially nice if we could get the PCB layout such that the MOSFET heat sinks sit directly below the ventilation slots on the top cover.

As for shaft extensions for the volume control pot or stepped attenuator, is the panel bearing a standard part that vendors like Digikey/Mouser/Newark carry? The extension tube seems like it's going to have to be custom (because the length would vary depending on the specific pot or stepped attenuator used, as well as the distance to the case front panel). How would people feel about having to fab this themselves?

-Ti

 

[morsel]

If there is enough interest in board mounted Elma Series N stepped attenuators, perhaps they could be ordered with a shaft of the proper length and sold along with the M³ pcb and panel bearings.

 

[Nisbeth]

Quote:

Originally Posted by amb As for shaft extensions for the volume control pot or stepped attenuator, is the panel bearing a standard part that vendors like Digikey/Mouser/Newark carry? The extension tube seems like it's going to have to be custom (because the length would vary depending on the specific pot or stepped attenuator used, as well as the distance to the case front panel). How would people feel about having to fab this themselves?

The shaft extender can be any old piece of 6 mm. or 1/4" aluminium/steel plastic tube or rod. Mine are cut from a 6 mm. aluminium tube bought at the local version of home depot using a small hacksaw. This should be manageable for most people

The panel bearings I use are these (click the 2 in the bottom for a sketch)


/U.

 

[jamont]

Quote:

Originally Posted by amb How would people feel about having to fab this themselves?

I'd rather not

 

[Voodoochile]

You definitely only need the coupler and bearing.

The shaft is sold in 4' lengths for about $3. at an Ace Hardware store or home center. The Welborne kit is almost $20. The Handmade Electronics kit is about $4. You end up with a pot with a 10" shaft you trim to length.

Doing the shaft yourself is easy... finding a coupling and bearing is a little harder, and the kit is a better choice. If I can find a place that sells the individual parts in bulk, it would be cheap to make little shaft kits.

I'm afraid that a custom order from Dact would be big money or big minimum, but perhaps not. It would be the cleanest solution. Of course trimming the stainless shaft is not quite as easy as trimming the aluminum one.

 

[Earwax]

Quote:

Originally Posted by jamont I'd rather not

Me neither. I'd much rather solder SMD parts than have to build a shaft assembly. ( I know it's not an either/or question )

 

[DCameronMauch]

I'm not understanding people's resistance to the shaft extension. It sounds like the parts are more than easy enough to acquire. And if you can't handle cutting a little pipe (I use the one for cutting plumbing copper pipes, makes nice perfect cuts), then you probably can't handle the soldering either. The biggest challenge is the measurements. But I see that as pretty straight forward too. ???

 

[ppl]

Quote:

Originally Posted by DCameronMauch I'm not understanding people's resistance to the shaft extension. It sounds like the parts are more than easy enough to acquire. And if you can't handle cutting a little pipe (I use the one for cutting plumbing copper pipes, makes nice perfect cuts), then you probably can't handle the soldering either. The biggest challenge is the measurements. But I see that as pretty straight forward too. ???

Locating the volume control at the rear has its advantages and disadvantages. Most of the disadvantages disappear if some sort of impedance transformation or Buffer is used. This implies an additional active stage or transformer and this option is something to not suitable to this application at this level of anticipated complexity, or shall I say lack of expected complexity.

An eligent solution lies in the layout and adopts what is often called localized circuit structure. This simply implies locating related circuits in as closet proximity to each other as possible yet can be somewhat detached from other connected circuits however of a different function. The Input and voltage gain stage can be one localized Block. The output stage and its power supply another localized circuit however connected to the previous localized circuit via feedback and signing lines. The power supplies for each of these localized Blocks can be remote for the Input and gain stages yet ideally the output and its supply should not be broken into separate localized Blocks.

The Bottom line of all the above babble is that yes a rear mounted pot and even relay operated input selection if desired. This will only require DC wires to connect to the front panel and the actual signal switched right at the source into the volume control and well you get the idea.

So Moving on to shaft coupler’s and related hardware the least expensive however quite use able is the couplers intended t convert a multi-turn pot into a panel mount. This takes care of your front panel bushing now get some ¼” rod of your choice of metals me stainless for long life lube the bussing with some silicon spray lube in the bushing. Use this in sparing amounts in bushing to maintain smooth rotation.
The required Bushing is available from Newark
http://www.newark.com/NewarkWebComme...jsp?id=44F7418
Less than $4 Dollar folks.

You might also want to throw in some of these they are pretty cool and only about three dollars http://www.apmhexseal.com/switchboot...00.pdf#page=11

Moving on to shafts and shaft couplers check these Guys out for the perfect complete package. http://www.oselectronics.com/ose_p100.htm

Possible small mismatches between metric and English measure can result in small imbalances so check shaft size the above hardware is for a true 1/4" shaft and that excludes the Alps Blue because it is a close metric equivalent however still slightly smaller than a true 1/4" and that is wy some Knobs made for 1/4" shafts sit so funny on the Alps

 

[Glassman]

what about this kind of heatsink?

 

[Nisbeth]

Quote:

Originally Posted by ppl Possible small mismatches between metric and English measure can result in small imbalances so check shaft size the above hardware is for a true 1/4" shaft and that excludes the Alps Blue because it is a close metric equivalent however still slightly smaller than a true 1/4" and that is wy some Knobs made for 1/4" shafts sit so funny on the Alps

PPL, a single layer of electricians tape on the shaft of the Alps is usually enough to convert the shaft from 6 mm to 1/4".

/U.

 

[DCameronMauch]

glassman: Those do look promising. But they need to be much taller. Because they are so short, their thermal resistance is too high. For four mosfets at say 1.5W each, it would need to be around 2.5C/W. Do they make any with the same dimensions but taller? The target case should allow for 2" high heatsinks.

 

[aos]

Quote:

I'm not understanding people's resistance to the shaft extension. It sounds like the parts are more than easy enough to acquire. And if you can't handle cutting a little pipe (I use the one for cutting plumbing copper pipes, makes nice perfect cuts), then you probably can't handle the soldering either. The biggest challenge is the measurements. But I see that as pretty straight forward too. ???

You obviously haven't been in electronics for very long. There is an aversion towards doing any kind of mechanical work.

Yes, I can easily cut a pipe with my Dremel or hacksaw. But, lots of us live in apartments, we don't have fancy workshops and even workdesks in our living room where we can hack and cut at will. The other problem is getting ahold of this stuff. Just because your local home hardware has it doesn't mean that even the same chain in Canada carries it, let alone the rest of the world. Not to mention that finding stuff in those stores is a challenge. Anyone counting on a big, popular project like this would do well to provide such stuff to DIYers (especially if it's easy for them to do so). Tangent for
example is doing a splendid work in this regard - teflon insulator sheets, SOIC to DIP sockets, standoffs and so on. That's one of the reasons why projects he provides parts for are so popular (or rather why so many DIYers actually manage to GET THEM FINISHED).