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Old 9th May 2008, 09:10 AM   #1
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Default Chu Moy Headphone amp troubles

Hi all,

I soldered up a Cmoy headphone amp the other day, roughly based upon the Tangent layout. However, I need some help in fault finding, because it has some rather show stopping issues!

I'm using an OPA2134, and 18V as the power source, for reference.

The two obvious problems are:
a) There's no sound from the output unless the source is supplying a very strong signal, and even then it is just occasional transients. This appears to be due/related t to a -2V DC offset that appears on both channels.

b) This is most likely related, in some way, to the wonky behaviour of the rail splitter, which is splitting something like 15V / -3V. Which, while the resistive rail splitter is rather dodgy, is very, very bad.

Can anyone suggest further action to help with tracking this one down? Photo's can be supplied on request.

Thanks!
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Old 9th May 2008, 10:49 AM   #2
ratza is offline ratza  Romania
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Did you put the electolitics in the right way? I had the same issue after connecting one cap with wrong polarity.
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Old 9th May 2008, 11:11 AM   #3
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Have you done exactly as Tangent's instructions? Do you have a single power supply voltage or is it dual?
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Old 9th May 2008, 12:31 PM   #4
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Hi,

Supply is a single 18V supply (2 9V batteries in series, and yes, they meter at 18V, I did check), connected to the "power supply" described in Tangents (and the original Chu Moy) instructions - that is, a resistive rail splitter with a tank cap for each rail (or at least, I think that's all they do.) The caps are inserted observing correct polarity. There's no problem with the resistor values, I tested those, so it seems it must be a wiring fault I've made, but I can't seem to track it down.
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Old 9th May 2008, 03:48 PM   #5
Minion is offline Minion  Canada
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If you are going to use 2x9v batteries anyways what not run it off of a dual supply (+/-9v) this will get rid of the Rail splitter and could get rid of the DC offset because the supplies will be more Symetrical and stable than a virtual earth supply.....

I built my CMoy with a Dual regulated 15v suppl;y and it works great and sounds very clear, it just doesn"t have enough output current to properly drive my headphones to really high Volume...


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Old 9th May 2008, 04:53 PM   #6
gareth is offline gareth  Wales
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Sorry to jump in here guys but I too have recently built the Chu Moy. It works fine when running from a split battery supply (after reducing the gain) but when I use it with a split regulated supply there is a lot of noise in there and the sound quality is totally different, it is almost like I am listening to music in a tunnel.....Any ideas?????

Thanks
Gareth
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Old 9th May 2008, 05:57 PM   #7
Minion is offline Minion  Canada
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Quote:
Originally posted by gareth
Sorry to jump in here guys but I too have recently built the Chu Moy. It works fine when running from a split battery supply (after reducing the gain) but when I use it with a split regulated supply there is a lot of noise in there and the sound quality is totally different, it is almost like I am listening to music in a tunnel.....Any ideas?????

Thanks
Gareth

Maybe you need more filtering on your supply lines...Did you put 0.1uF caps close to the IC pins?? are you useing noisy regulators?? I am useing noisy LM78/7915 regulators with 100uF before the regs and 100uF after the regs with 10uF and 0.1uF close to the IC Pins and it is not noisy at all... did you seperate your audio and Power grounds and use a Star grounding scheme??

Just a few suggestions...

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Old 9th May 2008, 07:19 PM   #8
gareth is offline gareth  Wales
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Quote:
Originally posted by Minion



Maybe you need more filtering on your supply lines...Did you put 0.1uF caps close to the IC pins?? are you useing noisy regulators?? I am useing noisy LM78/7915 regulators with 100uF before the regs and 100uF after the regs with 10uF and 0.1uF close to the IC Pins and it is not noisy at all... did you seperate your audio and Power grounds and use a Star grounding scheme??

Just a few suggestions...


Just like you I am using 78/79 regulators (I wanted cheapos as this is my first build). I have +-12v supplies, 1n4001 diode rectifier, 0.047uf RF blocking caps, Panasonic 3300uF FC's (all I had available at the time), wima 0.47uF bypass into the regulators. Perhaps I need to revise the design???
Also I did not use the 0.1uF caps close to the IC...

Gareth
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Old 9th May 2008, 07:31 PM   #9
Minion is offline Minion  Canada
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The 0.1uf caps close to the IC are pretty much a must with most any opamp design were low noise is a priority, they help with high frequency oscilation which can show up as a hum or whine...You can solder them directly to the Copper side of the PCB directly to the IC pins and ground ...It is an easy fix so you should try that and see if it helps....

If you haven"t allready you could also put a small (100pf-220pf) cap in the feedback loop which will help filter out frequencies that are out of the audio band....

Generally poeple say that it is best to have the big cap before the regulator and a small cap after so if your 3300uF is before the reg then that is good but if the 0.47 is after the reg you should then also increase it"s Value.....


Cheers
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Old 9th May 2008, 07:37 PM   #10
gareth is offline gareth  Wales
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Quote:
Originally posted by Minion
The 0.1uf caps close to the IC are pretty much a must with most any opamp design were low noise is a priority, they help with high frequency oscilation which can show up as a hum or whine...You can solder them directly to the Copper side of the PCB directly to the IC pins and ground ...It is an easy fix so you should try that and see if it helps....

If you haven"t allready you could also put a small (100pf-220pf) cap in the feedback loop which will help filter out frequencies that are out of the audio band....

Generally poeple say that it is best to have the big cap before the regulator and a small cap after so if your 3300uF is before the reg then that is good but if the 0.47 is after the reg you should then also increase it"s Value.....


Cheers

Thanks minion,
So I connect the 0.1uF's from supply pins to earth(ground)? Do these need to be polarised?
The feedback ceramic I can put in as it sounds as if it will do a lot of good.
Yes the 3300uF are before the regulators so I would need to increase the 0.47's to what value?
At the moment, both the amp and power supply are built on perfboards and through another forum on the site I am starting to learn Diptrace and I hope to produce some boards soon!!!!!!!

Gareth

Edit..the 0.47's I have in parallel with the 3300's and be honest I hadn't thought of putting anything after the reg's. Also the amp is in it's own case and the power supply will (in future) have it's own case too! I have braided the supply cables into the amp.
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