Home Forums GuitarPCB Build Support opamp muff mids mod (answered)

Viewing 10 posts - 1 through 10 (of 10 total)
  • Author
    Posts
  • #3993
    Anonymous
    Inactive

    hi,

    i just built an opamp muff and installed the tone stack bypass and am quite satisfied with the sound in tone bypass. but i would also like to try modifying the tone stack some for more mids or perhaps even a mid control. i found tons of info on modifying a bmp tone stack but nothing on the opamp tone stack which looks a bit different. i’ve been messing around with the C7 and C9 values and put in a 25k pot in place of R12, this has been yielding some results. but since i’m a newbie and don’t really know much about circuits i thought i’d ask here if i’m in the right ballpark changing these values or if there are other capacitors and or resistors i should be messing with.

    here’s the link to the build which has the schematic https://guitarpcb.com/wp-content/uploads/2018/06/BD_BMP-OP-AMP-v2.pdf

    thanks, rick

    #3998
    Barry
    Keymaster

    The only way to boost any frequency is with an active tone control. Passive tone controls only cut frequency. In other words you would have to cut Bass and Highs to give the apparent boost in Mids.
    A better solution is to use an active baxandall circuit like our Tone TwEQ which will allow you to boost or cut frequency.

    #4001
    Anonymous
    Inactive

    hi barry,

    thanx for the response. i know about the tone tweq but it’s too big to fit in my build. i’m not necessarily looking to boost the mids, i just want flatter mids / less scoop. there are many mentions of mods for the classic bmp tone stack like this one here https://www.coda-effects.com/p/big-muff-tonestack-dealing-with-mids.html

    it’s just that the tone stack there looks a bit different than the one in the opamp, and i as i mentioned above i’m not sure if i’m changing the right capacitors/restistor (c7 and c9, r12 for mid resistor). is it not possible to do this type of mod with the opamp tone stack? either flat mids or a mid control? there’s a mid control in the jhs modded ehx reissue. i’ve tried it and it’s better than stock but the mids that you can dial in are quite low in frequency.

    thanx, rick

    #4010
    wilkie1
    Moderator

    Perhaps I can add a few thoughts to this discussion.

    The BMP Opamp tone section consists of 2 filters with a Tone pot that acts as a blend control that mixes the signals from both filters.  As such, it does provide the mid scoop response of a traditional muff. The low pass filter consists of R11 and C8 while the high pass filter is C9 and R12.  Adjusting these values will change the response of the filters .The article you referenced does a fine job of explaining these filters.

    As you observed, bypassing the tone section does flatten the mid response since it removes both filters. Barry is correct by saying that no passive filters can actually boost response.  If you want more mid response than the unity level you are achieving with the bypass, then you will need an active filter.

    The Tone TwEQ does provide an active mid channel filter that is centered about 1K.  If you do not have room for all of the pots, you could eliminate the Hi and Low components and just use the Mid section.  The PC board is small enough to mount vertically on its edge on the side on most enclosures.  I do this all the time to save space inside the enclosure.  Another alternative would be to build your own center active Baxandall filter on a piece of vero board.

    I hope this is “food for thought” in your search for the sound that you seek.

    #4015
    Anonymous
    Inactive

    hi wilkie,

    thanks, definitely food for thought in the tone i seek. ok, it looks like i was half in the right ballpark. i thought it would be C7 and C9 because those are the two you replace with a single 220nf to do the tone bypass mod. if it’s not too much to ask could someone figure out which C and R numbers in the opamp schematic correspond to which C and R’s in the duncan tone stack calculator? i just can’t figure it out completely. maybe then i could experiment there a bit with some values before doing it in the pedal.

    thanks,

    rick

    #4016
    wilkie1
    Moderator

    Try these:        Duncan Big Muff            BMP OA

    Hi Pass             C1    R2                        C9    R12

    LO Pass           C2    R1                         C8    R11

    #4018
    Barry
    Keymaster

    “there’s a mid control in the jhs modded ehx reissue. i’ve tried it and it’s better than stock but the mids that you can dial in are quite low in frequency.”

    I am curious to learn more about the JHS mod you tried and would like to see a schematic to see what is going on there. Since you say the MIDs that you can dial in are low in frequency perhaps between Wilkie and I that Mod can be “Modified” to better suit.

    I have seen a video of a “Pumpkin Patch Mod” but could not locate a schematic for it. Can you post a link please so we can examine that aspect as well?

    #4024
    Anonymous
    Inactive

    hi wilkie, excellent, thanks! and R3 in the duncan is P2 the tone pot and R4 is R13 right?

    hi Barry, the schematic for the mod is nowhere to be found, i don’t think they’re giving it out. i’ve opened up the pedal and it’s not visible without completely unassembling, which i don’t want to do because if it get the one i built to my satifisaction i will sell the ehx/jhs. i could visibly see the gate mod and did that. although it doesn’t totally work exactly like the jhs mod it’s to my satisfaction.

    i already started experimenting with the duncan tone stack last night but was waiting for confirmation if i’m changing the right C and R’s. now it’s just the question about R4 in the duncan. if it is R13 the change isn’t massive between 1Mohm and 220kohm, although 1M seems to have less loss in dB, but it didn’t seem to influence the frequencies. what i think i will try is changing C1 to 5.6nf and C2 to 10nf, this still leaves a bit of a scoop but in the upper mids around 2-4khz which is good. if i leave in the 25k pot in place of R2 i can dial in the amount of those frequencies i need.

    #4026
    wilkie1
    Moderator

    You are correct.  R3 is the tone pot.  R4 is the same as R13 which can be any value from 1M down to 220K without any affect.

    #4033
    Anonymous
    Inactive

    i wanna thank you guys for the help, such a great forum! the duncan tone stack definitely gives you a good reference starting point but in the end i’ve realized that the ears have to decide. probably haven’t found the final values yet but the capacitor values i’ve ended up using are somewhat higher than what i thought would be good from using the duncan calculator.

Viewing 10 posts - 1 through 10 (of 10 total)
  • You must be logged in to reply to this topic.