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Old 20th June 2012, 10:40 PM   #1
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Default First build, CMOY

I read in a couple of places than a CMOY was a great 1st build, so I decided to give it a try.
Google helped me find the tangentsoft tutorial:
How to Build the CMoy Pocket Amplifier

And built it with the recommended parts (except the pot). In Mexico Radio Shack does not carry components, so the board is not available. I found that the Harden CL-003 and the Steren 405 have the same holes.
Click the image to open in full size.
Click the image to open in full size.

At this point it is functional but found 2 issues:

1) If I connect the headphones with no source or the source is off, I get a lot of noise.
2) With the source connected and turned on I get some low volume hissing. It is inaudible with music playing but still annoying when you get silence.

From the tangentsoft tutorial I see that R5 (47 Ω) should only be used if you get noise, so I'm planning to install it. What you ladys and gents think?

In any case, with music I do not hear the hissing and definitely improves the sound of my crappy headphones. Now I need to listen to good headphones with this amp
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Old 21st June 2012, 12:43 AM   #2
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Well, added R5 and the hiss got a lot lower (still present) but got a lot of distortion. So I removed R5 again. Perhaps I need to lower the gain?
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Old 21st June 2012, 12:49 AM   #3
agdr is offline agdr  United States
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Quote:
Originally Posted by silver1omo View Post

1) If I connect the headphones with no source or the source is off, I get a lot of noise.
2) With the source connected and turned on I get some low volume hissing. It is inaudible with music playing but still annoying when you get silence.
For (1) use an input jack for the source that grounds the inputs when the plug isn't inserted, like this one STX-3120-5B-284C Kycon | Mouser that RocketScientist used with the O2 headphone amplifier: https://docs.google.com/file/d/0B52A.../edit?hl=en_US (study how J2 is wired on that schematic) . Without grounding the inputs they act like an antenna for noise without the source. To test this just manually short one of your inputs to ground with a wire or test lead when the source isn't plugged in. That channel should get silent.

For (2), what are you using for your source? If your source has a very high output impedance you might be listening to the Johnson noise in that 100K R2 resistor. I don't see a pot (volume control) in your photos. You might consider adding a 10K dual-section pot to the input, as in Figure 2 in that original Chu Moy article, to get the input inpedance down to 10K.

Last edited by agdr; 21st June 2012 at 12:58 AM.
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Old 21st June 2012, 12:56 AM   #4
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Thanks! Next time I put an order in mouser I'll get those jacks
I'm using a sansa clip+. I'll test with my laptop.

Edit:
Well, the laptop is really noisy.. Also tested with my wife's ipod and the hiss is about the same than with the clip+
Apparently the jacks I'm using can be wired to ground the inputs, I will try that.

Edit 2:
Issue number 1 is solved, thanks a lot agdr. For Issue 2 I may try lowering the gain.

Last edited by silver1omo; 21st June 2012 at 01:19 AM.
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Old 21st June 2012, 01:26 AM   #5
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Quote:
Originally Posted by agdr View Post
I don't see a pot (volume control) in your photos. You might consider adding a 10K dual-section pot to the input, as in Figure 2 in that original Chu Moy article, to get the input inpedance down to 10K.
I decided I did not want 2 volume controls (source and amp) and neglected to consider the resistance of the pot.
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Old 21st June 2012, 01:30 AM   #6
agdr is offline agdr  United States
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Quote:
Originally Posted by silver1omo View Post
For Issue 2 I may try lowering the gain.
Yep, give that a try. If you are using the gain of 11 in the Chu Moy article that is probably going to be way too much. A gain of around 2-4 may be worth trying.
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Old 27th June 2012, 01:01 AM   #7
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I finally got the resistors to change the gain, I set it to 5 and, it rocks!
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Old 27th June 2012, 08:07 PM   #8
agdr is offline agdr  United States
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Quote:
Originally Posted by silver1omo View Post
I finally got the resistors to change the gain, I set it to 5 and, it rocks!
Hey that is good news! Congratulations on the build.
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Old 27th June 2012, 08:16 PM   #9
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Here it is, not exactly portable, but I really wanted to fit the 2x9v in there :P

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Last edited by silver1omo; 27th June 2012 at 08:24 PM.
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Old 27th June 2012, 08:27 PM   #10
rodolfo is offline rodolfo  Mexico
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nice job
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