The "Crank" - a not so perfectly clean booster

[Mark Hammer]

My feeble attempt at a booster.  An op-amp based double clipper design that will look all too familiar because it's a very cookbook type design.  The neat part is the way it uses a dual-ganged pot and the two different legs of a non-inverting op-amp.  The hard part was futzing around with different values of resistors and caps in the gain-setting parts and thinking through it until I got it right.

Provides an overall gain of about 42 and just a hint of dirt at max gain.   Very gradual and smooth change in gain and character across the control's range.  Keeps sparkle and bite of single coils but rolls off around the 6-7khz zone so not something you'd call "silky", though it certainly isn't ragged. I a/b'd it with Jack's MosFet booster and it has a bit more oomph though there is no denying Jack's is cleaner.

The pleasing thing is how it keeps roughly the same bandwidth (including bass) over a reasonably wide range of gain settings.  Could probably benefit from a treble cut control for tone preset, but certainly fine as a 2-knobber.  Foolishly, I perfed it but didn't use sockets, so I can't say whether op-amps besides an NE5532 might sound better or worse.

There is a fairly thorough walk-through in the posted doc.

http://img.photobucket.com/albums/v474/mhammer/TheCrank.gif

(I entered it at hammer.ampage.org also but somehow T-boy still needs to fix the permission rights for all graphics files added after this past spring.  So chekc the URL above instead.)

[Ben N]

That looks great, like the boost I used to try to get from a turned-down Blues Driver, but less noisy and tailored to the purpose.  Kudos!  

May I ask, did you consciously choose to go symmetric in the clippers on both stages?  Do you think assymetry would work in a circuit like this?

Ben

[Mark Hammer]

Thanks, Ben.  I didn't think asymmetric would add anything, either tonally or in terms of headroom.  What I wanted was to have some sense of roughly how much clipping would occur in stage 2, based on the known status of stage 1.  Asymmetric just complicates calculations.  I suppose one could try it, but there is plenty of dynamic range, even at full tilt.  I'm still mulling over sticking a small resistor in series with the diodes in stage 2 for a little more range and gentler clip.

[tungngruv]

Hey Mark, after seeing all of your projects, creations and technical support, don't you think it's time to write your stompbox book? :wink:

[MartyMart]

That looks great,  would there be any benefit from having two 10k pots instead of the "ganged" reverse wired pot ?
Could you perhaps have a bit more variation/interaction between the two gain stages that way ??

Thanks for sharing and another very well explained circuit  

Marty.

[Mark Hammer]

I pondered using a pair of pots, but thought the dual ganged thing was more elegant and lent itself more to a small enclosure.  Besides, the effect of manipulating either of the gain controls is likely to be subtle tonally, and not nearly as noticeable as moving both at once.  I guess you just have to learn to think of it as being like one stage even when it's two.  

BTW, it was a thought-out shot in the dark, so if anyone DOES have suggestions for improvements on the basic idea, please offer them.  For instance, I'm wondering if the output pot value is ideally chosen at 10k.

[Peter Snowberg]

Very nice Mark.

Here's what I hope is the smart part.

I used a dual ganged 10K pot to simultaneously adjust the gain of both stages...

I have had similar hopes with dual ganged topologies and the results have sounded great to me.

Here is a similar concept from the CMOS inverter fuzz direction, but with an asymmetric twist thrown in which I think I'm the first to use.

NOTE: The chip is unbuffered! MC14000UB

[Johan]

cool..

why the cap/resistor between the two stages?..the frequency cut is so low, you could just have a short between pin 1 and 5 and let the first stage bias the second, cuting the partcount a little..( I just love to cut out components )..perhaps also connect the 10k pot to Vb instead of through that cap, making it NO BASS CUT ( one component less)perhaps being so bold as to do the same in the next stage ( even one more..).. then you could also put that 220k ground (Vb) reference resistor on pin 3 between pin two and three (..yes, you would loose a tiny bit of gain but also all thermal noice from that resistor would cansel itself out..)

...I do need a new booster, as I just gave mine away...I tink I'll build this one...

Johan