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pulldown resistor on a SHO-type circuit

Started by MarkL, February 10, 2015, 06:42:57 PM

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MarkL

Hi...I've looked at several threads in other forums on this and I'm still not sure where to go with it -- I am making a SHO-type mosfet booster, and on the original design as well as the Landgraff derivative, there's no 1M (or the like) pulldown resistor on the input.  Is the circuit designed in such a way that the 10M resistors at the gate of the Mosfet do the trick in guarding against pops?  Is there a danger, or a detriment, to including a pulldown resistor?

copachino

its supposed that it doesnt need it, but in the practice, that pedal its noisy, even with a pull down you will get the annoying pop, best choice, optical bypass
Affiliations: madbeanpedals fan and pedal porn lover....

MarkL

So the pulldown resistor at the input actually WON'T kill the pop, then, eh?  I'm not familiar with optical bypass.  What does that entail?

copachino

Quote from: MarkL on February 10, 2015, 09:51:07 PM
So the pulldown resistor at the input actually WON'T kill the pop, then, eh?  I'm not familiar with optical bypass.  What does that entail?

i tried like 10 types of layout for the SHO type, and they all pop like hell, eve the original SHO from zvex pops like hell, so optical bypass from rullywow, they are really cool
Affiliations: madbeanpedals fan and pedal porn lover....

luks999

or try a bypass relay system ;)
uk-electronic has a kit and 1776 effects has a board

garfo

I've made a SHO but used two 2.2M instead of the 10M resistors. Mine pops a bit but not too much. Not loud enough at least.

madbean

A small series resistor before the input of the circuit plus grounded on bypass wiring might help reduce that pop. With the large input impedance of the SHO, you could reduce the input cap way down without any difference in tone, too. Probably 10n or even less.

MarkL

Well, I've done some modifications that seems to have helped.  First, I've lowered the resistors around the gate of the Mosfet to 4.7M, and I've added a 47uF cap to ground on the power rail.  I don't know why, but the pop isn't nearly as problematic now.

But here's my question.  I was originally hesitant to add a 1M pulldown resistor on the input because I didn't want it to decrease the value of the 10M/4.7M resistor to ground on the Mosfet gate.  With 4.7M, I think a parallel 1M resistor lowers the resistance to about 2.2M (correct?).  But, now I'm wondering if that would be the case if the pulldown resistor goes BEFORE the input cap.  That is -- if I put a 1M pulldown resistor at the input BEFORE the input cap, does that isolate it from the 4.7M (or 10M) resistor to ground on the Mosfet gate?  Or would it still affect the overall resistance of that gate resistor?

Another question -- I noticed on a schematic of the Blue Magic overdrive that the resistors around the gate of the Mosfet in that circuit are mismatched (4.7M paired with a 2.2M-to-ground).  Does this lead the Mosfet into asymmetrical clipping?  I am guessing that if the goal of a SHO-type circuit is to do a clean boost, asymmetrical clipping when the device is dimed is perhaps not the ideal result...