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need advice with CMOY mods [crossposted on Headwize]

post #1 of 2
Thread Starter 
Hi there!


Inspired by some of the posts at HF and HW I'm thinking about tweaking my basic CMOY a little bit.
Since I'm not very experienced with this stuff
I'd be grateful for any feedback you could provide.

The first thing I'd like to do is removing the voltage divider resistors and using a TLE2426 railsplitter instead. I would use the amp to
drive Senns mostly, so I guess it should be able to deliver enough current (right?)I've read you can (and even should ?) use the electrolytic caps as a power reserve. How would I connect them? just
like in the basic cmoy?
(+) - cap - (ground) - cap - (-)

Then there are the input caps. Some people say you don't need them at all and that they degrade the sound. What are the chances of getting DC out of the line-out of a PCDP? And in the worst case, what damage could 4.5V DC do to my Senns?
Assuming I don't need the caps, what should I do with them? Should I use them to bypass the fat electrolytics? (I'm using polyester caps, couldn't get better ones)
And what about this compensating pin of the TLE2426? (anybody ever used it ?)

Finally, assuming the input caps are gone, would
the 100k resistance from the non-inverting input to ground still be the optimal value? What's the significance of this resistor? (FYI I'm using
a 4.7k feedback res and a 50k pot )


Ok, that's about it.
Thanks in advance for your replies.

Bye

Redwoood
post #2 of 2
Hello,

Rail Splitter would be a wonderful idea. To drive Senn adequately you need +-1~2V output. So in theory you should be able to simply use 9V battery with rail splitter to get +-4.5V power. However, it is highly recommended to have much higher voltage since peaks will be cut off in some rare occation. I would use two 9V batteries and a virtual ground driver to ensure low offset voltage.

Coupling capacitor "may" be removed from signal path. Note that this is not always true. Coupling capacitor is there to kill any DC offset the amp stage may have. But if there is no offset you can safely remove the capacitor. However, in CMOY amp, note your opamp will start to pick up a lot of noise. What I do is to place 1~5kohm resistor in the place of the coupling capacitor.

WARNING: The above can only be done if you can absolutely garrantee there is no or very low voltage offset at the source. Otherwise, you may get large voltage offset at output of the amp. This large voltage is not so healthy to your headphones. HOWEVER, most importantly this can destroy the input stages of the opamp chips rendering them inoperable.

I do not use coupling capacitors anymore. I use my film caps in power supply along the side with electrolytics.

You should be fine with 100kohms connecting non-inv input to GND. This resistor sets the input impedance to ~100kohms. This resistor is not particularly specific. This resistor is there to lower input impedance enough so that you do not pick up RF or noise. Clearly lower value is better but you should not go below 1k~10kohms. Too low value is not good either. In your case, you should have values larger than your pot total resistance. **

Cheers,

Tomo

** Note that having resistor beside the pot will interefere with behavior of the pot. It is recommended to calculate the behavior beforehand.
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