CMoy casing hints?
Oct 19, 2008 at 8:31 AM Thread Starter Post #1 of 5

jinschoi

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Hello. I just finished my first CMoy using tangent's very clear instructions. I've been listening to it uncased, and it sounds great. Now I'd like to throw it in a case and have something a little less exposed. I'd like to ask a few questions on some casing details I haven't found any discussions on.

1. The LED. How is it "mounted"? Drill a likely-sized hole and stick it out from the inside? Feed the leads through from the outside? What keeps it in place? And by the way, what gauge do you guys normally use for this sort of thing? The (22 gauge?) wire I'm using from the Radio Shack jumper wire set is quite stiff and doesn't like to flex.

2. Grounding. Tangent's notes on grounding indicate that if you're wiring panel elements that ground to different levels in a metal enclosure, you're going to need to insulate one set. What's the easiest way to do this? I've read about insulating washers (where can I get these?), and using electrical tape (really? how is that done?). What's the best way to keep the solder joints and snipped leads on the bottom from grounding against the case? Can I just use paper, or is there something more suitable?

3. DC power. I'm planning to put in a DC power jack at some point using the diode OR bridge. Would it be safe to use this method to connect 8 NiMH AAs, or is that dangerous (AA voltage could drop below the 9V cutoff point)? Also, the original Cmoy article mentions wiring up a cable for an 8 AA battery holder, and he uses a third wire grounded on the case side but not connected on the battery pack as a "shield" wire. What function does that serve? Also, has anyone here designed a DC-DC converter circuit to run a Cmoy off AAs?

Any help greatly appreciated.
 
Oct 19, 2008 at 3:28 PM Post #2 of 5

Pars

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Quote:

Originally Posted by jinschoi /img/forum/go_quote.gif
Hello. I just finished my first CMoy using tangent's very clear instructions. I've been listening to it uncased, and it sounds great. Now I'd like to throw it in a case and have something a little less exposed. I'd like to ask a few questions on some casing details I haven't found any discussions on.


Congrats!

Quote:

Originally Posted by jinschoi /img/forum/go_quote.gif
1. The LED. How is it "mounted"? Drill a likely-sized hole and stick it out from the inside? Feed the leads through from the outside? What keeps it in place? And by the way, what gauge do you guys normally use for this sort of thing? The (22 gauge?) wire I'm using from the Radio Shack jumper wire set is quite stiff and doesn't like to flex.


Most use an LED bezel. Even ratshack carries some, such as this chrome one or these plastic ones which are the most commonly used for Cmoys. The LED goes into the bezel from the inside. 24ga wire is most commonly used, and if you are going to do much DIY, look at ebay for seller navships and buy some Teflon jacketed silver plated copper (SPC). Teflon won't melt when you solder it, and this is very reasonably priced.

Quote:

Originally Posted by jinschoi /img/forum/go_quote.gif
2. Grounding. Tangent's notes on grounding indicate that if you're wiring panel elements that ground to different levels in a metal enclosure, you're going to need to insulate one set. What's the easiest way to do this? I've read about insulating washers (where can I get these?), and using electrical tape (really? how is that done?). What's the best way to keep the solder joints and snipped leads on the bottom from grounding against the case? Can I just use paper, or is there something more suitable?


You won't need to worry about grounding issues if you are only battery powering the amp. You do need to worry about insulating the case however from the bottom of the board. I use vinyl tape (3M) that I get at Ace hardware... it is fairly thick. But you could do all sorts of things from electrical tape, cardboard cutouts, etc. to line the case with. I would use something heavier than paper because some of the lead ends can be sharp and poke thru.

Quote:

Originally Posted by jinschoi /img/forum/go_quote.gif
3. DC power. I'm planning to put in a DC power jack at some point using the diode OR bridge. Would it be safe to use this method to connect 8 NiMH AAs, or is that dangerous (AA voltage could drop below the 9V cutoff point)? Also, the original Cmoy article mentions wiring up a cable for an 8 AA battery holder, and he uses a third wire grounded on the case side but not connected on the battery pack as a "shield" wire. What function does that serve? Also, has anyone here designed a DC-DC converter circuit to run a Cmoy off AAs?

Any help greatly appreciated.



You should be fine with the AAs without any DC_DC converter, etc. the only voltage restriction in the Cmoy would be the opamp's max voltage, and the electrolytic caps. Look for tomb's Pimeta trickle charging circuit which also has the diode OR bridge on it. Found it: http://www.head-fi.org/forums/f6/tri...argers-185377/

You will need an isolated DC socket for this... the one he uses in that thread is from Mouser electronics and is good.
 
Oct 19, 2008 at 5:53 PM Post #3 of 5

jinschoi

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Thanks! That is very helpful, regarding the LED bezel.

I will be eventually looking into DC powering the amp, so I think I do need to address grounding at this point. Still trying to figure out what's a good way to insulate the DC jack.

Oh, and the DC-DC converter I mentioned would be to run the Cmoy from 1 or 2 AAs, not 8. 4-5 times the amp hours at about 80% efficiency. I'm thinking of something like the MAX631.
 
Oct 19, 2008 at 7:47 PM Post #4 of 5

tangent

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Quote:

Originally Posted by jinschoi /img/forum/go_quote.gif
Still trying to figure out what's a good way to insulate the DC jack.


The best way is to use a plastic jack.

The only other thing I'd recommend is nylon washers, which can be hard to find in exactly the size you'd like for the purpose. In the past, I've bought RCA jacks with isolation washers, which happen to have the same inside diameter as my DC jack. I haven't found comparable washers available separately: the ones for RCA jacks tend to be a lot thinner, which is fine because we're not dealing with high voltages or massive amounts of torque in mounting.

As for electrical tape and paper, it's too easy for these to tear, and it only takes contact of a sliver to ruin your insulation attempt.

Quote:

MAX631


There's no free lunch.

You'll need to use at least 2xAA in series to ensure you don't fall below the converter's 2.0 V minimum at minimum useful battery voltage. If you choose the 12 V output model, this is as much as a 6x jump in voltage, which requires 6x as much current on the input side as it puts out, if you can get 100% efficiency. At 80% efficiency, the factor is 7.2. At least with rechargeables, the mAh ratio between a 9V and AAs is about 1:10, so you haven't completely wiped out your amp-hours advantage. Still, it's not the clear-cut 4x advantage you were expecting.

You can do better with the 5 V output model, but that severely limits your choice of op-amps.

On top of that, you run the risk of introducing an audible buzz into the headphone output. This converter is somewhat rare in having a switching frequency above the audio band, but still, I wouldn't be surprised if there were sub-harmonics or other noise sources that would cause audible problems.
 
Oct 20, 2008 at 2:39 PM Post #5 of 5

jinschoi

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Quote:

Originally Posted by tangent /img/forum/go_quote.gif
The best way is to use a plastic jack.

The only other thing I'd recommend is nylon washers, which can be hard to find in exactly the size you'd like for the purpose. In the past, I've bought RCA jacks with isolation washers, which happen to have the same inside diameter as my DC jack. I haven't found comparable washers available separately: the ones for RCA jacks tend to be a lot thinner, which is fine because we're not dealing with high voltages or massive amounts of torque in mounting.

As for electrical tape and paper, it's too easy for these to tear, and it only takes contact of a sliver to ruin your insulation attempt.



I may just give up on the tin and use a somewhat larger plastic enclosure. Less to deal with, and more room for error.

Quote:

You'll need to use at least 2xAA in series to ensure you don't fall below the converter's 2.0 V minimum at minimum useful battery voltage. If you choose the 12 V output model, this is as much as a 6x jump in voltage, which requires 6x as much current on the input side as it puts out, if you can get 100% efficiency. At 80% efficiency, the factor is 7.2. At least with rechargeables, the mAh ratio between a 9V and AAs is about 1:10, so you haven't completely wiped out your amp-hours advantage. Still, it's not the clear-cut 4x advantage you were expecting.

On top of that, you run the risk of introducing an audible buzz into the headphone output. This converter is somewhat rare in having a switching frequency above the audio band, but still, I wouldn't be surprised if there were sub-harmonics or other noise sources that would cause audible problems.


So for 2 AAs, a marginal improvement. I don't think there's any room left on half a protoboard for another IC and associated parts, though, so probably not feasible for a small Cmoy build.

But, go up to 4 AAs and now we're only talking about 3x more current for 15V and NiMH cells. Compared to 2 9V, you should get 2.5-3x more runtime, but more bulk and only 15V. I hate buying 9V alkalines, though, and I only have an AA charger, so maybe I will look into this as a possible external power source next.

The MAX761 looks like it might be a better choice, better efficiency and 300 kHz operating frequency.

Thanks for your excellent site, by the way, it's very informative and useful. Now, where's my ALPS pot?
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