HVO Background
The HVO is the result of a question I asked myself -- is it possible to make a solid state pedal that sounds like a good tube amp?. 
I have been dissappointed in all of the overdrive pedals that I have tried over the years.  They all lacked dynamics and sounded like mush.  The ones that used discrete transistors instead of opamps sounded better, and I think this is because opamps use a large amount of negative feedback, which kills dynamics.  Even the pedals using discrete transistors still didn't sound as dynamic and rich as good tube amps, though.  I think part of the reason for this is that they are running at a low voltage, 9V or 18V compared to at least 250V in a tube amp.
I decided to try to design a solid state pedal that mimiced a tube amp as much as possible.  The unique properties of a tube amp are that the preamp gain stages use triode tubes and run at very high voltages.  When over-driven the grids of the triode tubes conduct and clamp the input voltage.
The I-V characteristics of transistors are very non-linear and very different than a triode tube characteristics.  After much research and thought, I eventually came up with a way to make a transistor behave much like a triode tube.  Normally transistors have characteristics that are very similar to a pentode vacuum tube.  By using some ideas that others came up with and a few of my own, I succeeded in making the transistors behave like triode tubes.
By using high voltage transistors and a proprietary internal power supply, I was able to run the transistor gain stages at 250V.  This is about the same voltage that's used in tube amps.
When I powered up the first prototype and played a few notes, I was shocked as how much it sounded like a tube amp.  All the harmonics and dynamics were there.  It sounded better than any other overdrive pedal I had tried.  There were still a few bugs to work out, but I had the answer to my question -- yes, it is possible to make a solid state pedal that sounds like a good tube amp.
It took about 6 months of tweaking to get the prototypes working perfectly.  Most of that time was spent minimizing noise and getting the most amp-like tone.
The final results have far exceeded my expectations, and I am very excited about this pedal.  I'm confident that those of you who give it a try will find that it is the best overdrive pedal you have ever heard.
Kim Hardee
Surprise Sound Lab
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