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Old 28th June 2012, 07:43 PM   #1
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Default Newb Going Down CMOY Blvd Hits Big Roadblock

Hi Everyone,

Iím a newb and am putting together a CMOY headphone amp for the first time. Iím using Tangentís instructions found here: http://tangentsoft.net/audio/cmoy-tutorial/. I just started soldering a few weeks ago and donít even know how to properly use a multimeter Ė at least in terms of measuring/testing circuits or connections as recommended in some of the troubleshooting threads out there. The only experience I have with mods is successfully desoldering the opamp out of an Onkyo CD player, soldering in a socket for opamp rolling, then inserting a new opamp into the socket. Since I was very pleased with those results, I thought Iíd step it up a notch and try to build the CMOY.

I got through stages zero and one as described by Tangent and all was well. I executed the test at the end of stage one and the multimeter showed that I had positive DC voltage at the positive cap and negative voltage at the negative cap. Magnitudes were practically identical and the LED turned on.

I just finished stage 2 and running the test for this stage revealed a problem. As instructed, I hooked a source up to the amp then the amp to headphones via alligator jumpers. The good news is I get no heat but the bad is that when I turn the sourceís volume (my iphone) up, at around half way, the sound starts to become really distorted and unlistenable. The higher I go with the volume the worse it gets. Iím not sure if this is clipping or distortion or what. All I know is that the sound breaks up once I reach half way on the volume adjust.

Does anyone know what I might have done wrong? Youíll find pictures of both the top of the board to show component placement and the bottom to show my soldering. Also included a pick of my alligator clip placement at the test phase, just in case. Hopefully that will help with figuring out where I goofed. I really thought I was being super diligent about making sure I placed components in the right holes and generally following Tangentís instructions so Iím not quite sure what the issue is. It doesnít really help that I have no electronics knowledge either.

One thing that I suspect may be at least part of the problem is the fact that I deviated from Tangentís instructions in one place. Instead of using OPA2132PA for the opamp, I used a NJM5532D opamp. It is the old opamp from the CD player that I modded (as mentioned in the first paragraph above). I figured it was just sitting around so why not use it? Do you guys think that could be causing the distortion and a quick fix would be to replace it with Tangentís recommended OPA2132PA?

Many thanks for your help with yet another newb trying to make a CMOY amp.

B
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Old 29th June 2012, 12:32 AM   #2
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I would change the chip the 5532 is not a FET input opamp and it will be dragging the virtual ground off centre causing distortion, it would work better with a TLE2426 rail splitter but not with the simple resistor divider
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Old 29th June 2012, 09:13 AM   #3
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Input bias current shouldn't cause an input offset of more than a few 10 mV. A few hundred nanoamps would hardly be expected to throw the voltage divider off by a large amount either.

One should check the DC voltages on the noninverting and inverting input pins (R2 and R3, respectively) and the output.
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Old 2nd July 2012, 09:12 PM   #4
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Do you get noise with the source in pause?
I had to lower the gain from 11 to 5 but I'm using the OPA2132PA.
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