Hi, the MXR Blue Box is a great all-in-one octave/fuzz but unfortunately the glitch of the octave might not be required in certain playing styles. I have built the Mantle Fuzz and am currently trying to reduce the glitch by changing the feedback resistors of the Schmitt Trigger (R7 and R8). Any other thoughts on this?
Mantle Fuzz / MXR Blue Box - Reduce Glitch
- Thread starter lukegalea
- Start date
Feral Feline
Well-known member
Embrace the glitch, choose a different octave-fuzz when the Mantle's inherent nature is not required.
RG Keen suggests the Anderton Rocktave as a more suitable candidate for octave-down:
RG KEEN: "I've listened to both a Blue Box and a Rocktave, and the Rocktave front end does a much better job of producing a waveform that can be tracked and used by digital stuff later. It's output also has much fewer sputters, crackles and mis-starts."
Not to dissuade you from your endeavour, I am merely here to try to learn more about what makes this circuit tick — I really like my Blue Box clone.
Keen has a pretty good explanation in the DIYSB thread above of what's doing what (you'll have to translate R15/C8/Q2 etc from the GGG schematic to the Mantle's).
Here's a few things I've come across about the circuit:
Mark Hammer, in the same thread above (IIRC, I didn't mark it, but it's in my notes right below RG Keen's description of how the circuit works):
MARK HAMMER "IC1b serves two purposes. The principal purpose is to output a square wave that reliably exceeds the minimum needed to trigger the flip-flop into division, for much of the note's lifespan. But...while we're in the neigbourhood, that same high-amplitude square wave is used to provide a source of a "fuzz" signal to be blended with the sub-octave."
JohnK is a well-known DIYer in the Talk Bass community and is on many of the DIY sites as well, he had this to say about it:
http://tagboardeffects.blogspot.com/2013/04/mxr-blue-box.html JOHNK “when I added the 1 octave switch, it was really glitchy until I lowered the voltage going to it with my benchtop power supply. adding a 470 ohm resistor to the 9v supply fixed that. it also was a bit too distorted for me so I changed the 1K resistor to s 4.99K [sic] one in the the gain stage. I also omitted the 10n cap across lugs 2 & 3 on the volume control because I found the pedal to be too bright (AKA, Keeley C11 mod).”
If I recall, that 10n cap was changed to 6n8 in one of his other posts or layouts — omitted or 6n8? Whatever. Adjust to taste… but dropping the voltage may have other consequences:
https://www.diystompboxes.com/smfforum/index.php?topic=79181.0
DDPAWEL: "Tonepad layout try to make R5 =1k [R5] into 470ohm res. + 10k pot. Another way is to change R8=1M [R4 U1 FEEDBACK RESISTOR] into 100k res. + 1M pot. Or combine both of them. Be careful, I don't know how much low the values of R8 [R4] and R5 [R5] can be. MXR Blue Box have something called a Schmitt trigger around ic1b which needs a good gain."
Bold emphasis above is mine.
I hope something from the above aids you in some way.
RG Keen suggests the Anderton Rocktave as a more suitable candidate for octave-down:
Not to dissuade you from your endeavour, I am merely here to try to learn more about what makes this circuit tick — I really like my Blue Box clone.
Keen has a pretty good explanation in the DIYSB thread above of what's doing what (you'll have to translate R15/C8/Q2 etc from the GGG schematic to the Mantle's).
Here's a few things I've come across about the circuit:
Mark Hammer, in the same thread above (IIRC, I didn't mark it, but it's in my notes right below RG Keen's description of how the circuit works):
MARK HAMMER "IC1b serves two purposes. The principal purpose is to output a square wave that reliably exceeds the minimum needed to trigger the flip-flop into division, for much of the note's lifespan. But...while we're in the neigbourhood, that same high-amplitude square wave is used to provide a source of a "fuzz" signal to be blended with the sub-octave."
JohnK is a well-known DIYer in the Talk Bass community and is on many of the DIY sites as well, he had this to say about it:
http://tagboardeffects.blogspot.com/2013/04/mxr-blue-box.html JOHNK “when I added the 1 octave switch, it was really glitchy until I lowered the voltage going to it with my benchtop power supply. adding a 470 ohm resistor to the 9v supply fixed that. it also was a bit too distorted for me so I changed the 1K resistor to s 4.99K [sic] one in the the gain stage. I also omitted the 10n cap across lugs 2 & 3 on the volume control because I found the pedal to be too bright (AKA, Keeley C11 mod).”
If I recall, that 10n cap was changed to 6n8 in one of his other posts or layouts — omitted or 6n8? Whatever. Adjust to taste… but dropping the voltage may have other consequences:
https://www.diystompboxes.com/smfforum/index.php?topic=79181.0
DDPAWEL: "Tonepad layout try to make R5 =1k [R5] into 470ohm res. + 10k pot. Another way is to change R8=1M [R4 U1 FEEDBACK RESISTOR] into 100k res. + 1M pot. Or combine both of them. Be careful, I don't know how much low the values of R8 [R4] and R5 [R5] can be. MXR Blue Box have something called a Schmitt trigger around ic1b which needs a good gain."
Bold emphasis above is mine.
I hope something from the above aids you in some way.
Hi Feral,
I definitely agree with going for another octave but I'm taking it as a challenge to reduce the glitch with the current circuit configuration (limited/less components).
Thank you very much for finding other sources and forums. I'll be taking a particular look at the gain of the first stage and re-try tinkering with the feedback resistors R7/R8 (since you quoted that someone changed one of them to 100k).
Cheers
I definitely agree with going for another octave but I'm taking it as a challenge to reduce the glitch with the current circuit configuration (limited/less components).
Thank you very much for finding other sources and forums. I'll be taking a particular look at the gain of the first stage and re-try tinkering with the feedback resistors R7/R8 (since you quoted that someone changed one of them to 100k).
Cheers
Feral Feline
Well-known member
Wishing you the utmost success in this challenge!