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CMoy construction questions

post #1 of 13
Thread Starter 
Alright, so I want to build a cmoy to take with my to test a pair of headphones but I have a few questions before I begin...

First, since this'll be in an altoid's tin, can I solder all the "grounds" to the case?

Secondly, would I be able to you a higher ohm resistor for the LED do give me a dimmer light and allow for longer battery life?
post #2 of 13

Yes you could, but most people ground the amp with the 3.5mm jack screws


the led draws very little current so battery life is really not a factor but too bright a light is a pain so yes use a large resistor




post #3 of 13
Thread Starter 
Thanks FRED

Now that I'm more awake, I can answer properly. So my thinking with using the case as the ground is that it could act as a shield from EM interference, similar to the foil around RCA cables. I don't know if handling the case will cause problems if I do this though.
post #4 of 13

No, handling the enclosure won't be a problem. If you hear sounds while touching the case, it's because of some other problem, not because you have a metal box.


If you want a list things to worry about with mint tins, that can be provided:


  • If you don't carefully insulate the circuit board from the bottom of the case, you'll short the amp out
  • V- and virtual ground aren't the same thing. Might be obvious to you, but let me tell you, people do forget this. They'll take a perfectly working battery-powered amp, then install a DC power jack so it can run from wall power instead...and use a metal-bodied jack, which connects V- to the case, which you previously connected to virtual ground.
  • Metal shavings are more dangerous to your skin's integrity than plastic shavings. :)
post #5 of 13
Thread Starter 

Thanks for the advice Tangent. I had a question though, since I'm getting parts from Radioshack, how will using a 100k pot change the performance?

post #6 of 13
Since you're getting parts from Radioshack, why not get a 10k pot, like it says on the schematic.

Edit: discovered RS only stock 100k.mad.gif

A 100k pot will make very little perceptible difference. The SNR will be compromised slightly.


Oh, soldering all the grounds to the case may not be the best idea. Normally we use 'star grounding' (Google it) to get the best (noise) performance. The Cmoy is simple, and will probably not be compromised for that reason, but more complex designs can suffer as a result of inappropriate grounding schemes.
Edited by wakibaki - 2/15/14 at 10:51am
post #7 of 13
Originally Posted by JMikesell View Post

since I'm getting parts from Radioshack, how will using a 100k pot change the performance?


The main effect the resistance has is that it forces R2 higher, which increases the amp's noise floor. It requires R2 to be 1M+, plus it makes the input more sensitive to induced noise.


All that is minor next to the fact that you're proposing to put in a cheap, scratchy pot.  RS pots are not particularly high quality.

post #8 of 13
Thread Starter 

I have enough parts that a second one shouldn't be too expensive to make, this is my first and is mostly just for practice but also to demo some pawnshop headphones.


So I have the power section hooked up and am using an old battery, the problem is that going from positive to ground I get .6v and from v- I'm getting -6.7v. I checked the soldering and re-soldered a few places but no dice. Any suggestions?

post #9 of 13

Can you post pictures? 

No? Seriously? As early as 5 years ago you had to try to find a cell phone without a camera. Set it to maximum resolution. I want these pictures so big that they make my (fast) internet connection slow down. 


How far along in the build process are you? 


Did you follow Tangent's instructions? If not, whose instructions did you follow? Where did you deviate from the given instructions? 

post #10 of 13
Thread Starter 
Pics are coming

Yeah, I'm following Tangent's instructions and I'm at the end of Stage 1. My deviations include forgetting to include the power switch, using a larger 33k resistor for the LED and bridging the LED resistor to an empty hole and connecting the LED+ to an adjacent hole
post #11 of 13
Thread Starter 

Here's the top



And the bottom

post #12 of 13

I don't see the jumpers joining the 4th row of pads (where the rail caps connect) to the virtual ground strips.


Also, what is the wire going to the virtual ground strip, first row?

post #13 of 13
At least a couple of these solder joints look suspect. The solder should wick up onto the wire from the board forming a concave curve, and obviously wetting both the wire and the pad. When the solder sits up like a blob of mercury, and particularly when it tucks down round the wire, often leaving a dark ring all the way round the wire, this is an indication that the joint has not wetted correctly. It can be hard to understand that a wire sticking right up through a blob of solder is not actually making contact with the solder, but it can be the case that there is a layer of flux (sometimes burnt) completely preventing the wire and solder being in contact.

Of course sometimes joints that look like that are OK, but it's better to remake them, and without an excess of solder, so that you can see that the joint is good. I sometimes inspect my joints using a jewellers loupe, and of course you can always test with a meter to make sure there is continuity between the wire and solder or pad.

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