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Old November 11th, 2008, 01:43 PM   #1 (permalink)
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Quick And Easy DIY SHO

Hey all -

I've heard of numerous attempts from folks building macguyverized SHO's with readily available parts (i.e. Rat Shack).

I just had to try it out myself:

1. To see if it really works
2. To see if it sounds really good

Lo and behold, both 1 and 2 are realities, and you even get the charming "crackle OK" effect as well.

Basically, you just sub a pair of 1 MEG resistors for 10 MEG units (either is adequately high enough to give a good, bright input impedance), a 22K resistor in place of the 5.1K, and a 20K/25K pot in place of the 5K unit.

Also, while the BS170 transistor is anything but unique, you won't find it at Rat Shack. But if your town has an electronics supplies store, it's probably available there. If not, you can usually find a 2N7000 at such stores. If Rat Shack is truly your only option, an IRF510 can be used, but with less gain (but the SHO has so much with the BS170 that it's not much of a factor, IMO). Just make sure you get the gate, source, and drain connected to the right spots, as not all mosfets are laid out in the same sequence.

Granted, to truly macgruberize this build, you need access to a DPDT stomp, but you could actually use a toggle instead. Since many folks leave the thing on all the time, you could even omit it - just make sure that the juice is cut when not in use, with either a dedicated power switch, or stereo jack for the input, to switch the battery off by pulling the input jack.

Then there's the enclosure. Obviously no Hammond boxes at RS. Not to worry - you could get one of their plastic boxes and shield the interior (awesome macguyverization possibilities there). I've also seen pedals built from junction boxes, PVC end caps, small baking pans flipped upside down, etc. Go hog wild, here.

How to build the circuit itself? I'll leave that up to those who are up to the challenge. It's not that challenging to build on a perfboard, in all honesty. Just two capacitors, two diodes, one transistor, four resistors, and one potentiometer is the whole circuit.

Anything else to modify? If you don't like the full spectrum bandpass effect of the SHO, you can cut down the input cap to .01uF-.047uF, and the output cap to .1uF. The stock value of 10uF for the output is overkill, IMO. YMMV.

And if you can build one of these, you can build a Super Duper, or even a Box of Rock. They both use the SHO in a modular fashion. The BOR is basically a BSIAB with mosfets in place of the jfets. I've also heard that the Box of Metal is also modular in fashion. If you're up to a slightly different challenge, you could build Joe Davisson's Obsidian. It's basically a variation of the BOR minus the boost stage and pot crackle.
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Old November 11th, 2008, 11:56 PM   #2 (permalink)
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That's cool! I had a SHO...and let it go to fund other purchases. It's really a great little box....but....

You open it up and can only think "WTF"????

There's just nothing in it (compared to other pedals). Very overpriced considering the complexity. OK....you get the kookie paint job.....
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Old November 12th, 2008, 08:54 AM   #3 (permalink)
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You open it up and can only think "WTF"???
I think that this is what tempted people to macguyverize it in the first place.

And Vex made them so that you had to open them up, because he didn't include a D.C. power jack (or LED on the early models).

What's really amazing is just how simple the SHO biasing schem is (i.e. how little circuitry that it requires to make it behave). Even the old LPB-1 has more components!

I was also surprised to find out that subsequent Vex dirt pedals use the SHO as their modular building block - the BOR is 3 SHO's cascaded into each other, with a verbatim SHO for the boost circuit. Maybe that's more bang for the buck, but I've heard folks complain about the price for the Vexter version.

The SHO is cool, IMO, but most boosts are. They all have their different strengths and applications. I just think it's cool that one can be built easily.
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Old November 13th, 2008, 03:24 PM   #4 (permalink)
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Nice work. I have been wanting to try this mainly because of the simplicity but I can't figure out schematics well enough to give it a confident try. Can you post some pics of the the board that are a little more clear than the last one your posted? Maybe one of the solder side if you have a chance.

Thanks for sharing!
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Old November 13th, 2008, 04:27 PM   #5 (permalink)
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This circuit is so derivative of Jack Orman's AMZ MOSFET Boost it is just hilarious. It is nuts that somebody has the nerve to charge $250 for something like that that was intended to be built by hobbyists for $25 or maybe what is crazy is people will pay that (no offense to present company). Actually the schematic you showed here is a worse design than the AMZ the SHO's so called "Crackle Okay" control is the result of a poor design choice that omits the blocking capacitor on the gain, which is there in the original to prevent crackle. So they are introducing crackle needlessly and calling it a marketing point. They basically butchered Orman's design to save on a 10 cent capacitor in the parts count it looks like.

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Old November 13th, 2008, 04:31 PM   #6 (permalink)
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Can you post some pics of the the board that are a little more clear than the last one your posted? Maybe one of the solder side if you have a chance.
There are so few components and perfboard is so unclear on the solder side, that I've included a layout with all of the physical connections between parts - you can then lay it out on perfboard, veroboard, or actually PTP it right to the drive pot!

Notice how lonely everything looks on my perfboard (i.e. room for growth) - I think I'll fill it up with at least the Super Duper circuitry.

BTW - I've also seen the pair of 1N4148's replaced with a single 9.1 volt Zener from the G to S. I believe that this is how Vex is currently doing it, as well. He's also dropped the pair of 10 meg resistors to 1 meg in the BOR and similar pedals.
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Old November 13th, 2008, 04:44 PM   #7 (permalink)
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Actually the schematic you showed here is a worse design than the AMZ the SHO's so called "Crackle Okay" control is the result of a poor design choice that omits the blocking capacitor on the gain, which is there in the original to prevent crackle. So they are introducing crackle needlessly and calling it a marketing point. They basically butchered Orman's design to save on a 10 cent capacitor in the parts count it looks like.
This is not completely accurate. The reason that the crackle occurs is not that there is DC on the pot - it's because all of the rebiasing occurs at that point. The crackling sound is the transistor actually rebiasing, i.e. switching on and off.

Jack's design is a true class A, well regulated, super clean boost. The SHO is about as "caveman" as it gets, but there is an audible difference - the super crude biasing scheme actually causes the mosfet to generate more even order harmonics than you'd ever get with a really stable design - just like a very crude tube preamp stage.

I don't disagree with you at all that Vex is banking on the crackle being some kind of novelty, as well as overcharging for the equivalent of a D grade (at best) science fair project. Users have pointed out that the crackle isn't an issue really, since they aren't turning the knob on the fly. If the crackle is really a nuisance, you could actually use a rotary switch with different value resistors.

EVERYONE has tried to reverse engineer the SHO to get rid of the crackle, without success. When you lose the crackle, you stabilize the biasing, and end up with the Orman design, or something similar.

But this crude little boost is not without it's merits. The very fact that you can make a mosfet actually bias up and produce any audible signal with only $2 worth of components is actually pretty intriguing, as well. Vex obviously saw $$$$$$ when he realized that it would work.

We can certainly debate the flaws or merits of the SHO circuit if you wish, but that wasn't my original intention. If someone is really intrigued with building boosts, I suggest they build all variations, including Jack's super clean mosfet design.
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Old November 13th, 2008, 07:05 PM   #8 (permalink)
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Ha you are right, looking at Jack's schematic again to see what is missing there is no bias resistor on the SHO schematic therefore there is nothing to be decoupled from the gain pot with a cap, and that makes the pot itself the bias resistor instead of a gain pot. I see how it would saturate the transistor as you bias it way into non linear. It is real strange to have the only control on the pedal be a bias adjustment. That sure is a quirky design.
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Old November 13th, 2008, 07:12 PM   #9 (permalink)
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Reminds me of the Dallas Rangemaster (in that design it really was DC through the volume control making it crackle) but the maker figured you would set it and forget it too.

I just might build one myself tonight just to see if it sounds any good it looks like a 30 minute job.

By the way Jack uses the 9.1v zener across the source and grid. The schematic as drawn does not seem to have any overvoltage protection for the MOSFET.
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Old November 13th, 2008, 10:00 PM   #10 (permalink)
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I just might build one myself tonight just to see if it sounds any good it looks like a 30 minute job.
Give it a shot - I did, and it's been fun, so far. Really cool with a tube amp just on the edge of giving up the goods.
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Old November 14th, 2008, 04:57 AM   #11 (permalink)
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OK I built it tonight

OK as promised I built the Z Vex "S. H. O." pedal clone using the above layout (thanks) on perf board. It worked fine the first time. Extremely easy build slapped it together in no time.

Here it is on the bench while I was playing it through my 5 watt TDA 2002 practice amp I keep in my lab:



Here it is mounted in a Hammond ABS case that has a built in 9 volt battery compartment with a battery door on the back that slides off. This is a nice slim case that fits comfortably in a shirt pocket. It also has mounting points that fit the holes in the perfboard. The perf is the standard Radio Shack wafer that snaps into 2 pieces.



As you can see I am using a board mounted 5K trimpot in place of the the silly Crackle knob. I have the bias set so I can change the color of the sound by turning up the volume on the guitar. Definitely in saturation mode with compression and swelling of the notes but still sounds very clean, getting the 2nd and 4th harmonics very emphasized.

I have to wait until daylight to really test it but it does add a very sweet harmonic tone however there is a whole lot of boost at that point. The bias adjustment makes it much louder but I want to set it at a fixed bias and control the gain separately oh well more for another day. I cannot do much because I have to play whisper quiet it is the middle of the night. So far so good.



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Old November 14th, 2008, 08:53 AM   #12 (permalink)
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it does add a very sweet harmonic tone however there is a whole lot of boost at that point.
Vex only scraped the surface with regards to minimal boost with just the extra harmonics - even the Box of Rock runs the three cascaded mosfets fairly hard...

...I've heard the Super Duper with the bias on both channels set pretty low (not much above unity), and it's pretty cool.

I think that the best approach would be to run 5 or more SHO stages with very soft bias, different coupling caps, and a modified BOR/BSIAB tone stack that would allow for flat mids at the pot's midpoint.

Joe Davisson's Obsidian uses three P channel BS250's that are supposed to have much less hiss, if you don't mind a positive ground circuit. He also removes the drive control from the bias point. So I guess you could then call the control "crackle not okay." It would also be interesting to take just one stage of the Obsidian and A/B it against a SHO.
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Old November 14th, 2008, 02:44 PM   #13 (permalink)
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Such a big difference between 20/25k and 5k pots. Is it ok? And the 22k resistor as well.
Are these values optimized for the 2n7000?
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Old November 14th, 2008, 04:46 PM   #14 (permalink)
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Such a big difference between 20/25k and 5k pots. Is it ok? And the 22k resistor as well.
Are these values optimized for the 2n7000?
It's all a matter of establishing a bias that will make the transistor happy. The difference may seem big, but it really isn't. All that the pot is doing is reducing the overall value - if you have the fixed 5.1K drain resistor and a source pot set to 4K, it's similar to having a 22K drain resistor with a source pot set to ~17.5K (these are crude estimations, since the relationship isn't linear).

As for being optimized for the 2N7000, no they aren't. But neither are they for the BS170, either. The SHO is a fairly rigged circuit that just happens to sound good for guitar.


As long as you have ~4.5-5.5VDC at the drain (at idle!), the drain and source resistance values are somewhat flexible. I've even heard of someone using a 1K source pot, but that sounds a little twitchy, to me.

...But you could probably use a 10K drain resistor with a 10K source pot, etc...
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Old July 29th, 2009, 06:42 PM   #15 (permalink)
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My first DIY project - only Crackle, No OKAY!

Im wondering if you guys can help me troubleshoot my SHO build. This was my first DIY project and I don't have the $$ to abandon it for something else that may work. Plus I could probably learn something by toughing it out! What would you guys do if all you got out of it was a BUZZZZZZZ???
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Old July 31st, 2009, 01:13 AM   #16 (permalink)
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What would you guys do if all you got out of it was a BUZZZZZZZ???
Troubleshoot it. Rebuild it. Maybe build another one, if building on perf. The circuit is so simple that it's cheapest and fastest to just build another one. The trick is to not make the same mistake(s) in subsequent builds.

As far as the protection diodes go, you can drop the "top" one, and use an LED for the "bottom" one. Here's a project called the Multi Face, and when a mosFET is used in Q1, the LED protects it from static and voltage spikes.
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Old October 12th, 2009, 07:28 PM   #17 (permalink)
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Thought I'd resurrect this thread...

I just finished my SHO clone from .011's helpful schematic, and in just over an hour. It sounds every bit as good as my original SHO. I did go with the original component values posted, and not the Radio Shack subs because I was able to get them at a local electronics supplier. Now I just need to pop it into an enclosure.

Anyone wanting to see what all the "Clean Boost" fuss is about for cheap/easy should give this little guy a try.

Hey .011--I'm wanting to try the 2N1 next, would you need to use a 5k pot for the "Master" control as well?
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Old October 12th, 2009, 07:44 PM   #18 (permalink)
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I'm wanting to try the 2N1 next, would you need to use a 5k pot for the "Master" control as well?
No - the master volume is a 100K unit that takes the place of the 100K resistor from output to ground on the second SHO. It's wired up like a basic level control.

If you are only using the separate SHO's at different boost levels (and not together), you simply keep the master full up, which is exactly the same as the 100K fixed resistor. It's only if you cascade the two together that you cut the master back, so that you don't blow out the next thing in the signal chain.
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Old October 12th, 2009, 10:52 PM   #19 (permalink)
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Thanks .011

I'll be cascading the two so, yeah, I'm going to need the ol' master vol. I was a little nervous just testing out my first build today--knowing the massive amount of boost available, and imagining a puff of smoke or something coming from my amp when I turned it on, but there was nothing to worry about I guess.

I'll report back when I've got the 2N1 sorted out. Thanks.
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Old October 20th, 2009, 10:21 AM   #20 (permalink)
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THANKS 11 !

here's my SHO,took longer than it should have due to a faulty jack.............urghhh!
I too used the original values,except for a 10k reverse log pot. everything works as it should and sounds great! I wish there was a way to attenuate the volume so you could get those overdrive tones at a realistic volume,cause it is LOUD before that starts to happen.
As a clean boost,this thing works like a charm.
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