# Mantle Fuzz / MXR Blue Box - Reduce Glitch



## lukegalea (Aug 9, 2021)

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?


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## Feral Feline (Aug 11, 2021)

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:





						mxr blue box: theory or operation?
					

mxr blue box: theory or operation?



					www.diystompboxes.com
				



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 em*pha*sis above is mine.


I hope something from the above aids you in some way.


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## lukegalea (Aug 12, 2021)

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 🙂


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## Feral Feline (Aug 12, 2021)

Wishing you the utmost success in this challenge!


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