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Cmoy components/gain
May 7, 2013 at 10:19 AM Post #32 of 50
I shorted the out and in- pins and got gain 1. Is it OK to leave the other resistors like they were before? I get a nice 4mV DC offset on both channels.
If I convert it into a a47 (with unity gain), do I have to change anything except adding 4x 47ohm output resistors?

Thanks a lot for your answers, with the help of you guys I manager to build an amp which already sounds more awesome than I would ever have expected! :)
 
May 7, 2013 at 10:51 PM Post #35 of 50
What gain did you try to build it at? 
If you stuck with 2-3ish there is no real reason that the gain should be toooooo high. 
 
I would make sure that everything is (was) in the right way around. One of the best things about the Cmoy is that its a great platform to learn and gain experience *cheaply*. Better to figure this out and remind yourself to be more careful next time than to risk messing this up again. 
 
It is also totally possible that gain of 2-3 is just too much, but I have seen more experienced builders than myself build stuff with gain of 22 instead of 2. Yea. 
 
 
Quote:
unknownax said:
I shorted the out and in- pins and got gain 1. Is it OK to leave the other resistors like they were before? I get a nice 4mV DC offset on both channels.
If I convert it into a a47 (with unity gain), do I have to change anything except adding 4x 47ohm output resistors?
 
Click to expand...

 
As long as the output offset voltage is within sane limits (yours certainly is) and its not oscillating or behaving weirdly I would leave it. 
 
To convert to a CHA-47 what you are describing is pretty accurate, but you may find it easier to build a new amp from scratch than to modify a Cmoy. You will gain extra experience for the effort and get extra control over the layout.
 
If money is an issue sell the cmoy to recover the parts cost and move along to the next amp. Nothing wrong with that. 
 
May 8, 2013 at 11:43 AM Post #36 of 50
I first built it with a gain of 4 but changed to 1 because my headphones don't need an amp for volume, they sound a lot better with one, though. I.e I didn't mess up, I just don't want the amp to amplify :)

I built it on a breadboard and it's easy to convert it to a cha-47. Just a socket and a few jumpers needed.
I won't move on to another amp, as this one isn't even cased up. Neither do I know what the point of another amp would be. This one propably bottlenecks all "audio equipment" I have.
 
May 10, 2013 at 2:29 AM Post #37 of 50
I converted it to a cha-47 now, using 2x jrc4556ad. Now it sounds even better and the DC offset actually dropped to 2.8mV.
I've got a few (hopefully last) questions though:
1. Are the 47ohm output resistors crucial? Can I remove them? (I've heard that they keep the opamps from fighting each other)

2. I want to add a tone control like this:
http://www.41hz.com/forums/content.php?279-Passive-tone-control to my amp. What does the 1uf input cap at the tone control do? I've already got 4.7uf input caps at the amp.

Thanks a lot for your answers! :)
 
May 10, 2013 at 8:08 AM Post #38 of 50
The 47 ohm resistors should be used though the 47 ohm value is not critical.
It's just the value the original designer had laying around. A lower value
like 10 ohms would work better.
 
The 1uF input capacitor on the tone control prevents any DC coming from
the source from making the pots sound "scratchy" when adjusting them.
 
May 12, 2013 at 9:18 AM Post #41 of 50
I only have 4 of them...but I'd have some 2w 10ohms like this
2WDRAHT.png

They're "wire?" resistors afaik. Or maybe I should just leave it until I get some normal 10ohm resistors.
 
May 19, 2013 at 5:32 AM Post #42 of 50
Now the amp is cased and all I need is a linear power supply with 18vdc. I'll diy a simple one with a transformator, diode reticifier and 1 capacitor (2000-10000uf).
What kind of diodes should I use in the reticifier?
Do I need to add additional filtering? I don't want it to be noisy... :)

Thanks for your help!
 
May 19, 2013 at 10:22 AM Post #44 of 50
For filtering, something as simple as a 1500uF Panasonic FM followed by one of those 10 ohm, 2 watt resistors followed by another 1500 uF Panasonic FM
to form a CRC filter.
 
You may have some parts laying around that wold work
 
May 19, 2013 at 12:13 PM Post #45 of 50
I saw that kind of filter somewhere. It suggested using a pair of big >2000uF caps, 10ohm resistors and a pair of <200uF caps after them. The middle of the capacitor pairs was connected to a vground formed by 2x 0.5M resistors.

Should I just connect the cap(s?) to +, - and forget the virtual ground? I need to order the transformator anyways so what would be the "ideal" cap values before and after the resistors? :)
And do I use 1 or 2 resistors to form a crc filter?
Sorry about the questions but google is full of weird mess...
 
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