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Tuesday 17 January 2012

Zvex - Super Hard On (SHO)

[Note - This post also contains info on the Landgraff Clean Boost]

I didn't realise I hadn't put up a post for the Zvex Super Hard On - it's one of my favourite little circuits. It's a great booster and also a really good building block when you're designing circuits...

Here's the description from the Zvex website:
"This is the perfect preamp pedal. The "Crackle Okay" volume knob is a negative-feedback control styled after classic 60's recording console inputs. (They crackled when adjusted too.)
Most vintage guitars suffer from steadily deteriorating magnets in their pickups, since permanent magnets aren't really forever. The Super Hard-On's input impedance is so high (>5 Meg) that it refuses any current flow from your pickup... maintaining the most magnetic field around each string, so you can hear exactly what your pickup sounded like the day it came off the winder.
The ouput level can exceed 8 volts peak, and when it finally distorts, the wave is shaped like triode overload, not fuzz."

And the standard demo video of the Super Hard On in action:


Now there are several schematics available for the Zvex Super Hard On as it's circuit has changed slightly over time. The schematic I use is this one (It's the MKII):


Here's a nice little vero project file from IvIark (http://tagboardeffects.blogspot.com):

[As you can see the zener diode is positioned slightly differently here - it doesn't make a difference to the tone!]

Estragon from the Freestompboxes forum helped to clean up the understanding of the Super Hard On circuit a little with this great post:

"The SHO is a deceivingly simple circuit. There is a lot going on when the source/bias pot is adjusted.
First, as the bias pot is adjusted from max (5k) down to min (0 ohms), the DC drain voltage changes roughly from 7.2V down to 4.0V. I have measured this in the breadboard and verified it in the sim (using a BS170 Spice model by Zetex).

Second, the gain varies from 0 dB to almost 40 dB as the bias pot is adjusted. Notice how the variation of the DC drain voltage tends to keep the dynamic range nearly centered. At min gain, drain and source resistors are 5k and 5k, respectively, thus the output can vary from about 4.5V up to 9V, and the 7.2V bias voltage is nearly centered. At max gain the output can vary from about 0V up to 9V, so the 4.0V bias voltage is again close to the theoretical center of the dynamic range.

Third, the input impedance of the SHO is not 5 Mohm as the popular belief dictates. No, it varies big time with the bias pot. At min gain, the end of the upper 10Meg resistor is tied to an inverted copy of the input voltage, not ground, so overall input impedance actually becomes 3.3Mohm when you do the proper math to find out the equivalent thevenin resistor (this has been verified in the sim as well). As the bias pot is reduced and gain increases, something very convenient happens: the input impedance starts going down, down to 140 kohms at max gain. This certainly helps taming the high frequency resonance of any guitar pickup (around 2-5kHz), thus taming highs and helping to the musicality of the clipping. Again, this is because the upper 10Meg resistor is effectively tied to a voltage source that is an amplified version of the input voltage, and not a simple ground point."

There are also other projects available based around the Super Hard On topology. Soulsonic's "Crackle Not Ok" (A Super Hard On without the crackling gain pot":


And Rick Holt's (a.k.a FrequencyCentral) Super Heated Super Hard On (Which changes the circuit from standard Mosfet booster into a Mu-amp mini-booster. *Warning - this thing is seriously loud!*):


Oh, and one more thing - here's the schematic for the Landgraff Clean Boost (Drawn by Clay Jones):


Yep - it's a Zvex Super Hard On [MKI]

17 comments:

  1. Hmm, What about the sho pedal using Two diodes for clipping? Never seen a diagram showing this.

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    1. I tried different diodes in symmetrical hard clipping configuration. Ended up using 1N5818 schottky and a 3.3nF input capacitor (could be smaller value for a treble boost flavour) Makes a great fat bluesy overdrive that goes well after fuzz-face and before octaver. Maybe I will add a tone control at the output like the AMZ Presence control... But Yes it sounds great with clipping diodes :)

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    2. Wicked cool concept! can you point me towards any schemes or articles explaining how to set up those different diode clipping mods?

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    3. Sure,
      Looking at IvIark layout just change the 100nF cap for a 3.3nF and add two diodes (each in opposite polarity) from the output to ground. I'd suggest putting sockets if you want to experiment with different input cap values and diodes.
      Here is article that can help:
      https://www.electronics-tutorials.ws/diode/diode-clipping-circuits.html

      Have fun!

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    4. thakns dude!

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  4. What about modifying this pedal to be more bass guitar friendly? Obviously Change input/output caps, but anything else I'm missing?

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  5. Hi, I don't know what is your rig and what kind of bass sound you're after but IMHO it is already "bass guitar friendly". Plenty of low end on the stock version. This circuit is probably one of the easiest to build and tweak. As you mentionned, a socket to try different input caps is a good way to start. The output gain is very high so it is also possible to add a passive tone control/filtering and still have a very strong signal. Good luck in your Quest!

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  6. Yes it is actually pretty bass friendly,I guess I just want a little bit more. I'll try the cap changes And I think the tone edition as well. Thanks.

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  7. I built this. It works except I’m getting an oscillation. I believe it’s from the zener diode. Is there anything I can do to fix that?

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    1. Hi, oscillation or noise? Single transistor design with so much gain can be a little noisy... but not on a problematic level. Basic trouble shooting : Check for ground loops, check your transistor (swap it if you don't have a tester at hand) and be sure to have a clean power supply. Have you tried removing the zener according to your hypothesis? Hope it helps!

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  8. What's the use of the 2x 1N914 on the landgraff version?
    Voltage stabilisation? Regulation?

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  9. Great question. Would love it if someone has insight in those 2 diodes and their function.

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    1. ESD protection for the MOSFET. the newer versions with a single zener might be better, but it's the same function in the circuit. one good part about the single diode is fewer components to build out.

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    2. came to post the same. They are labeled "protection diodes" in the landgraff schematic. The two on the input are protecting from noise from power/ground lines. the one on the power circuit is doing a similar thing

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