op amp big muff midrange

ode2no1 · April 02, 2009, 03:53:36 AM

ok so i know there's the amz presence mod and all, but the problem is i'm not knowledgeable enough to know which resistor in the op amp version is the 22k resistor in the tone stack on the amz site since there isn't a single 22k resistor in my build. i used the veroboard layout with tone bypass that's known around here. i'd rather not add another knob so i was thinking of using a switch to go from stock to boosted mids. assuming someone tells me which resistor value to mess with, will increasing or decreasing it add more midrange?

thanks in advance.

jefe · April 02, 2009, 08:55:32 AM

Can you please provide links for each of the items you're referring to? Mainly the "amz presence mod", the vero layout you used might be helpful, and especially the schematic for the circuit you're building. I could go searching for them myself... but that's your job.

Besides, it's better if you give us the exact links you're referring to, rather than us guessing.

ode2no1 · April 02, 2009, 12:42:24 PM

haha fair enough.

here is the mod, followed by the layout: http://www.muzique.com/lab/tone3.htm

by the way, thanks for being so friendly about it jefe

jefe · April 02, 2009, 01:02:55 PM

Quote from: ode2no1 on April 02, 2009, 12:42:24 PM
by the way, thanks for being so friendly about it jefe

heh heh... I try...

I still need a schematic to look at, though... just found this:

http://www.geocities.com/CollegePark/Library/1355/bigmuffopamp.gif

I was comparing it to the layout you posted, and the AMZ link, but I'm pressed for time now. Gotta run, I'll try to help you out more tonight... but in the meantime, see if you can make anything out of the schemo I posted vs. the vero layout vs. the AMZ mod - it's good practice!

MarcoMike · April 02, 2009, 01:34:00 PM

apparently the amz presence thing is for the transistor BM in which apparently low and hi freqs are not balanced (because of high/low impedence reasons...I have no idea)
in your BM there should be no need for that... (that=that particular circuit mod)

jefe · April 02, 2009, 01:55:43 PM

Yeah, but tweaking the tone stack is fun, and quite usefull. I don't know much about the op amp version, but if it's got that "mid scooped" thing going on, then this mod could be very handy.

I'm assuming that the op amp version has somewhat of a "traditional" big muff tone stack. BTW ode2no1, google the duncan tone stack calculator, there's a calc for the BMP in there that is very useful for making some sense of this stuff.

ode2no1 · April 02, 2009, 02:37:44 PM

ok well it would seem that the 1.2k resistor going to ground would be the one to mess with as far as i can tell from comparing it to the amz page. can someone explain to me why there is a 3.3k resistor AND a 25k pot in the amz mod though? is it just to get you up to 28.3k since a 50k pot would have too much unusable range? anyway it seems strange that the resistor values in the tone stack are so small in the op amp version. i guess i'll try to tinker around. i just don't really want to start soldering and de-soldering too much until i know what's going on. i'm finally getting fairly neat with the guts of my pedals so i don't want to ruin it haha.

oh and marcomike....it does have the tone bypass switch, so having more midrange is an option already, but i'd like the tone control to be usable too. i have a modded usa ri that i'm pretty sure uses the amz mod (i got it modded like 5 years ago and wasn't as capable with this stuff yet) and i'd like that level of midrange control available on this one too. i just want to use a fixed value though and make it switchable so i can still have the stock sound.

ode2no1 · April 02, 2009, 02:50:39 PM

ok i just touched the lead of another 1.2k resistor to the one that's in the tone stack, putting it in parallel, and the mids decreased, so that must be the resistor to mess with. i was thinking though, i can just put a pot in series with that resistor to increase the midrange when turned clockwise, but returning to the stock sound when turned fully counter-clockwise, right?

slacker · April 02, 2009, 05:48:56 PM

Yeah that should work.

ode2no1 · April 02, 2009, 06:15:02 PM

this very well may be one of the dumbest questions ever, but i was thinking about it...and ok, when a pot is turned fully counter clockwise does it basically act as a jumper? cause, and like i said...may be dumb, i was just concerned about the pot being turned counter clockwise and not allowing the 1.2k resistor to apply its resistance. but if the pot acts as a jumper when it's turned all the way back then it would be perfect. anyway can someone please give me the green light on this one? haha.

ode2no1 · April 02, 2009, 11:45:09 PM

ok guys (if anyone cares haha) so i added a 10k pot in series with the 1.2k. unfortunately i only had an audio pot, but it works and it boosts the mids in a nice round way. it's weird...when the total resistance gets to about 8k you can turn the gain pot all the way down and it actually sounds like an overdrive. the downside is that with the gain up and the mid pot up pretty high you gotta turn the tone knob pretty much fully clockwise and even then it's still lacking high end. overall the pedal is pretty boomy though...the tone knob at noon without the mid pot engaged has massive low end...so i think i'm gonna socket the output cap. but yeah....can someone point me to which one that is either on the schematic or the vero layout? usually the output cap is something i don't have much trouble finding, but it's a bit confusing on this one.

Gila_Crisis · April 03, 2009, 03:57:09 AM

if you need more stuff about opamp big muffs just go here in my gallery:

http://www.aronnelson.com/gallery/main.php/v/Gila_Crisis/Big_Muff/Opamp_Big_Muff/

actually that vero layout it's my own design

jefe · April 03, 2009, 07:09:48 AM

ode2no1 - sorry I didn't help you any last night... I got caught up in one of my own builds

Sounds like you're doing ok on your own though. Truth is, I don't know that much about this circuit, but I was hoping to help you figure it out, and gain some knowledge for myself in the process. I would like to build it myself one of these days.

Another thing... I suck at vero, never used it before, so I'm not much help in that respect either... sorry... and good luck!

ode2no1 · April 03, 2009, 10:56:10 AM

Quote from: Gila_Crisis on April 03, 2009, 03:57:09 AM
if you need more stuff about opamp big muffs just go here in my gallery:

http://www.aronnelson.com/gallery/main.php/v/Gila_Crisis/Big_Muff/Opamp_Big_Muff/

actually that vero layout it's my own design

is it really? you know...i can't imagine having to sit there and figure out a vero layout. i imagine it takes a lot more patience than i probably have haha. anyway i'm glad you took the time to do it cause i really enjoyed building this. it is the highest parts count effect i've built to date. so i wonder if you'd be able to help me out here since you're obviously into the big muff. from staring at the schematic it seems that there is no definitive output cap that i could change to lower the overall low end. it looks like it's all in the tone stack, but i could be blind and not see that i'm looking for. can you lead me in the right direction? for some reason i'm obsessed with being able to have the stock sound at my fingertips, so if there is one single cap i can change to lower the bass response a little bit then i'm gonna go ahead and wire up a switch to go from stock to whatever new value. by the way...how does the sustain mod sound compared to stock? obviously i assume it has more sustain, but what else is different? i hadn't come across that vero layout when i started this.

and no problem jefe. i appreciate your trying to help me out. what're you building at the moment?

nelson · April 03, 2009, 05:35:52 PM

Quote from: Gila_Crisis on April 03, 2009, 03:57:09 AM
if you need more stuff about opamp big muffs just go here in my gallery:

http://www.aronnelson.com/gallery/main.php/v/Gila_Crisis/Big_Muff/Opamp_Big_Muff/

actually that vero layout it's my own design

Yeah, it's some good work.

I've personally committed the 77 version of your vero layout to enclosures twice(with various mods). Including my own PCB versions I now have four versions of the 1977/78 opamp muff with mods I think are worthwhile.

I prefer a rail to rail dual opamp in place of the 4558 and I like a LF251 in place of the 741, of course YMMV.

I'm curious, it's a pretty well done vero layout, were you looking at my project files when you did it?

For some reason I think you were, although I can't put my finger on why.

ode2no1 · April 03, 2009, 05:42:17 PM

hey nelson, seeing as you've built the thing a few times can you help me with reducing the low end? is there one cap i can lower or do i have to change a couple of components?

nelson · April 05, 2009, 04:51:27 PM

Try a 1n cap from lugs 3 to 2 of the sustain pot. It won't decrease bass, but increase treble which is basically the same thing.

Alternatively you could lower C13 + C5 on the schematic here which will cut the bass going into the clipping section.

http://electroconducive.googlepages.com/1977OpampMuffSustainMod.pdf

You could add a high pass filter on the output of hte sustain pot too. If you google for RC filter calculator the first hit should be an amz tool that's handy.

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op amp big muff midrange