Cmoy headphone amp + DC Jack = HOT opamp... - diyAudio
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Old 4th August 2013, 10:41 PM   #1
tim79b is offline tim79b  Australia
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Default Cmoy headphone amp + DC Jack = HOT opamp...

Hi all
First post here, but I’ve been lurking for the last few weeks digesting information…

I am very new to the DIY audio thing, and I have just finished building a cMoy headphone amp from the instructions at http://www.tangentsoft.net, and yesterday tried to add a little tweak by putting on a DC power jack.

I have added the jack by using a SPDT switch which, in one direction closes the battery powered circuit, and in the other direction is either off, or on wall power if the AC-DC Adaptor is plugged in, as shown in the attached circuit sketch (only the power section of the circuit is shown). The adaptor I am using I this one - http://www.jaycar.com.au/productView.asp?ID=MP3147

The problem is that, whenever I have the DC adaptor plugged in, I get no sound from the amp (but the power LED does light up), and the opamp chip gets really hot. If I switch to battery power and the adaptor is plugged in, I still get no sound, but the opamp runs at normal temp.

I finished late last night, so haven’t had a chance to do any testing yet (beyond checking that the polarities on the dc jack are correct), but wanted to see if anyone could think of any obvious causes, or had any specific advice on what I should be checking…

Thanks in advance for your help.
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Last edited by tim79b; 5th August 2013 at 01:34 AM. Reason: Tidy up formatting
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Old 5th August 2013, 11:18 AM   #2
davym is offline davym  Scotland
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Go over all your connections several times comparing them with the circuit diagram, check your p/s cap voltages are sufficient and that your not causing your virtual ground to collapse/short to one of the power rails. That would definitely cause an overheat! Folk on here with much more knowledge than me will probably fire loads of questions at you, prepare for brain ache! My money is on a connection error, i'd remove the power socket from circuit and check it works on batterys. Maybe diodes instead of a switched socket would be better.
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Old 5th August 2013, 11:41 AM   #3
Mooly is offline Mooly  United Kingdom
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If the opamp gets really hot then its frazzled... replace it and make sure it all works again on battery power. I don't know what opamp you are using but you can always test with cheapo types and then when its all OK fit whatever you want.

If this happened when trying your AC adapter then I would have suspected the polarity is reversed, but you say you have checked that and the LED lighting points to it being correct. Could the polarity reverse momentarily while you are actually connecting the plug ? with a return via some grounding of other equipment etc.
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Old 5th August 2013, 02:19 PM   #4
davym is offline davym  Scotland
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Source to amp grounding can be a problem with the Cmoy, it's why I run mine off rechargeable battery's when connected to my dac.
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