[shieldst]

I'm building a Mini-CMoy pocket amp and I had a question about setting the gain:

The guide by tangentsoft.net (under "Tweaks") says that

"In an op-amp circuit, the resistors in the feedback loop (R3 and R4 in this case) set the gain: the amount by which the output voltage is multiplied compared to the input voltage. The formula for gain in this amp is G = (R4/R3) + 1."

The build calls for a gain of 11, which I've heard is excessive, so I want to lower this to 5 or 6. There are two sets of R4 and R3 resistors - which do I change to affect the gain? Can I safely change one of the resistors in both sets if I'm not sure?

Thanks.

 

[cobaltmute]

Yes, you can change just one of the resistors. I would lower the R4 resistor. I find, at least with my sources, gain 5 to be more than enough.

 

[Pars]

Each channel has its own set of R3/R4, so you need to change them both. Most people leave R3 alone (1K) and change R4. I've used 4.75K in the past (gain 5.75), but many go lower than this. 3.3K is a common resistor value.

 

[shieldst]

I'll go with a 4.75k resistor (that's all I have lying around). Thanks for the help - I just wasn't sure if I needed to change the resistors in both channels or just one.

 

[pictureguy]

Maybe it didn't do it right, but this is my first effort.  I already had the 10k resistor in.
So, I changed R3 to 2.2k......=gain 5.5     
Not best choice, but this'll work for now.   1st try at this amp.  
 
If I were doing this again, I'd use the 4.7k in R4.
The Tangent site says lower values in these 2 positions are best.