please educate my newb ass on increasing an opamp stages gain(clone theory )

[pinkjimiphoton]

so, like a while ago, i built lar's (cathexis's) vero of the eh clone theory.

it works, but was substantially less than unity gain when engaged...wasn't noticeable back in the pre-true bypass days, but it is blatant now.

so anyways, i tried adding a stereo make-up gain stage based on a pair of lpb1's on the output before the switch, but it not only amplified the ticking of the lfo way too much, was bleeding horrid amounts of nasty into the power supply for my pedalboard. i built it with 15v regulators, and run it with an 18v brick ps, but i'm gonna rehouse it to hopefully shield it better and eliminate some of the noise issues.

anyways, i'm figuring the best bet is to increase the gain to the first stage of the preamp...and (forgive me, i'm new and stupid as a stump still) i see there's a voltage divider  (circled) of two 300k resistors...

if i were to replace those with two 50k resistors connected to the outer pins of a 500k linear pot, with the wiper going to pin 3 of IC 1A, would that let me adjust the gain going to the first stage of the preamp?

i've looked at stuff til my widdle bwain is all beat up, and can't seem to figure this out...i understand i need to lower the values of resistors to increase gain...

but man, i am LOST... l'il help? i'll pay it forward!!

thanks

[R.G.]

anyways, i'm figuring the best bet is to increase the gain to the first stage of the preamp...and (forgive me, i'm new and stupid as a stump still) i see there's a voltage divider  (circled) of two 300k resistors...

if i were to replace those with two 50k resistors connected to the outer pins of a 500k linear pot, with the wiper going to pin 3 of IC 1A, would that let me adjust the gain going to the first stage of the preamp?

So you started your EE degree late. So what?


Good quess, but in this case, that voltage divider just sets the operating DC level. It's the "Vbias" generator for the first stage opamp.

The resistors you want for increasing gain are R3 and R4. The AC gain of IC1A is equal to R4/R3 or 390k/100K, or 3.9. You can increase gain by increasing R4, or by decreasing R3. Decreasing R3 is a problem to the original signal, because it is one load that the incoming guitar signal sees. Right now, at 100k, it's going to cause some small but perceptible amount of treble loss if what's plugged into the input is a guitar, not another pedal or a buffer. Decreasing R3 for more gain makes this worse.

So increasing R4 makes sense. R4 can be raised to standard values of 470K, 510K, 680K, 750K, 820K, 910K, 1M, etc. 750K would give you about a doubling of gain, which is a slight increase in audible level. Going above 1M starts to get noisy from the thermal noise of the resistors.

[pinkjimiphoton]

bless you R.G.!!!

i completely actually understood that!! maybe it is starting to sink in a little bit. i'll start with doubling, or use a 1meg pot til i can dial it in right.
at the moment, it's noticeably quieter when the effect is engaged.. and like i said, trying to put a gain stage AFTER the effect turned out to be a really bad idea...
it not only picked up lfo ticking, it polluted the whole power supply...i couldn't even use my fuzzface, and to me, that's like trying to play guitar with no thumbs....bizzarre radio interference, so much leakage it sounded like a motorboating amp...and the way the clone "bends" the note,  ...lets just say it was about the worst leakage you could imagine, which is weird, cuz without the add on it's way quieter than the original!

i'm gonna get on this tomorrow nite...thanks my friend!!

[Mark Hammer]

Start with DEVICE issue 1 here: http://hammer.ampage.org/?cmd=lt&xid=&fid=&ex=&pg=12

[Colonel Angus]

nice link. Looks like a lot more reading is about to happen 

[Mark Hammer]

I've had that stuff up for years, and people regularly comment on how useful that material is.  That's why Anderton is the man.

[Colonel Angus]

Agreed, EPFM paved the way for my stompboxing. I got the book when I was 18 and never had enough money for a hobby until now, and it is very useful. I look forward to reading some more of Anderton's stuff.

[pinkjimiphoton]

thanks mark! i've got that whole series downloaded i think. knowledge is power!!

sadly, i'm a dullard. 

[pinkjimiphoton]

anyways, i'm figuring the best bet is to increase the gain to the first stage of the preamp...and (forgive me, i'm new and stupid as a stump still) i see there's a voltage divider  (circled) of two 300k resistors...

if i were to replace those with two 50k resistors connected to the outer pins of a 500k linear pot, with the wiper going to pin 3 of IC 1A, would that let me adjust the gain going to the first stage of the preamp?

So you started your EE degree late. So what?


Good quess, but in this case, that voltage divider just sets the operating DC level. It's the "Vbias" generator for the first stage opamp.

The resistors you want for increasing gain are R3 and R4. The AC gain of IC1A is equal to R4/R3 or 390k/100K, or 3.9. You can increase gain by increasing R4, or by decreasing R3. Decreasing R3 is a problem to the original signal, because it is one load that the incoming guitar signal sees. Right now, at 100k, it's going to cause some small but perceptible amount of treble loss if what's plugged into the input is a guitar, not another pedal or a buffer. Decreasing R3 for more gain makes this worse.

So increasing R4 makes sense. R4 can be raised to standard values of 470K, 510K, 680K, 750K, 820K, 910K, 1M, etc. 750K would give you about a doubling of gain, which is a slight increase in audible level. Going above 1M starts to get noisy from the thermal noise of the resistors.

r.g., if you're monitoring this still....

another dumb question!!

for the two input resistors, why not 1meg and 68k like on a guitar amp? wouldn't that effectively make whatever was before it "see" the same load as say the input of a fender amp?

i seem to remember reading there was a tonal reason for 68k, but kind of surprised that going below 100 would make the treble suffer...is it too "padded down" with the smaller resistor?

thanks again!!

[pinkjimiphoton]

well, i took the standy- uppy resistor (390k) and cut the long length of lead and spliced in a 1 meg trimmer (with one leg soldered to the wiper) and it did the trick!! unity actually seems to be right about 1 meg.

thanks RG!!

took out the pesky extra gain stage, played with the trimmers some, and got my clone theory back. i have to try it today with my normal amp at normal (ridiculously loud) volume...

but it's a LOT quieter, noise wise.

also undid the stereo out...i never used it, and i didn't like the "stereo" thing...the original was just hard wired to the input jack. used insulated wah-style output, THAT made a HUGE difference in noise level.

having a hard time getting it to bias correctly...it sounds best with both trimmers clocked full blast opposite each other, you can really tame the effect down and make it polite, or almost make it sound like it's spitting formanats at you...glurrrgg glurrrg... it's pretty sick.

thanks!!!