ronv,
Darn, can't get rid of me yet! But we are darn close.
Before I start soldering I wanted to check with you one more time.
I took your schematic of "dudes amp" and rearranged and it and re numbered the components so it is easier for me to understand. I also substituted a better OPA and increased the caps for better line filtering and better LF response. I am using the caps because I have them on hand and they do wonders for the low end FR (especially C3 on my schematic). I hope increasing them doesn't hurt anything. R6 in the schematic below is a trimmer. And the output will go to Line In on a sound card for speaker measurement. I will not be recording anything but would like the measurements to be as accurate as possible.
Before the cap and OPA changes, the circuit was down 3 dB at 20Hz and at 20K Hz. Now it is almost straight as a string from 20-20K Hz. Indeed, with the better OPA HF goes out to 6.5 meg Hz, makes about a 3 dB rise and then starts to fall. The Q of the HF rolloff is more than 1.
I found a 10pf cap across R6 eliminates the HF peak. Since the pot is variable, so is the effect of the 10pf cap. So when R6 is down to 5K the HF -3 dB point is about 2 meg Hz and with R6 at 100k it is about 130k HZ. If R6 goes significantly below 5k, the hump comes back with a vengeance. Is this hump anything to worry about? I hope that the circuit is OK without the 10 pf cap (fewer parts is better) cause the top end seems to be self limiting anyway.
Your "dudes amp" schematic has no current limiting resistor or cap on the output. And I show it the same way. If I use it that way should I be concerned about my M-Audio Transit audio interface (outboard USB sound card)? In other words, is it safe to use as shown? If not, suggestions?
Well, that is about it except for the pictures. They are attached.
BTW, would anyone want to see the finished product?
And THANK YOU for all your help.
Thanks.