[altie]

I recently finished putting my Cmoy amp together, only it doesn't work! The sound is very distorted even at normal listening levels. What's odd is that if I put my finger against C1 and bridge that to ground, it sounds great. I measured the gap across my finger with a multimeter and estimate it at 300K ohms, which would be equivalent to an R2 value of 75K ohms. Out of a mix of frustration and curiosity, I tried replacing R2 with a 68K ohm resistor that I pulled out of an old intercom. This produced even worse sound (not entirely unexpected). Any ideas what's wrong?

 

[eric343]

Here's a trick I've learned when you don't know what resistor to use: Hook a linear taper pot up(MAKE SURE THE POT IS CLAMPED IN A VISE!!! This keeps it from getting misadjusted while hooking up/etc) instead of the resistor, and adjust it until everything works fine. Now measure the value of the pot.

 

[altie]

Thanks for the tip, but I don't know if I'll be able to do that. Is it possible that I just need a different value resistor in there? Seems to me that everyone else has been able to put together a good, working amp without modifying the original plans.

Also, I tried something else. Since the plans said that C1 could be omitted if DC line voltage suppression isn't necessary, I went ahead and removed C1 and R2. The sound was fine, but too quiet. So I've got a few new questions: is DC line voltage something I need to worry about, and how should I go about upping the volume if it isn't?

 

[thomas]

C1 is used to block DC from the signal. Headphones do not like DC, even a few volts can damage them. If there is a small DC component in the signal, and you don't have the cap, it will be amplified by the amp, possibly frying your headphones. If you are sure that your source has absolutely no DC, then you can get rid of the cap...

The cap and the resistor will filter out the lowest frequencies (including DC), but should have little effect on the audiable frequencies... It certainly shouldn't have much of an effect on the volume...

Double check all the connections/solder joints throughout the amp, and check all componant values... Its probably just a simple problem... the original plans work very well without modification

 

[altie]

Ugh, I feel like a moron: I neglected to make the voltage divider and whatever else the instructions called for in the power supply. I've just been running straight off of a nine-volt with nothing in between its terminals and the v+/v- terminals on the opamp. Does this need to be fixed, or does it not matter?

 

[eric343]

Umm, if you're making a Cmoy the +/- of the batteries should be connected to the +/- terminals of the opamp. The power supply creates an artificial ground, so that positive-to-ground is 4.5v and negative-to-ground is 4.5v. Positive-to-negative should always be 9v.

 

[Apheared]

yea it matters. yea it needs to be fixed! The opamp will run fine, what comes out will be hard to call "audio" though.

You have to set a midway point between the rails; either a virtual ground at the power supply or you have to bias the signal inputs to halfway... it's _alot_ less parts to do it in the power supply stage.

This probably doesn't need to be said; please use junky headphones while building and testing.

 

[altie]

Thanks a ton guys. I ought to have this finished today, thanks to all of your help.