input resistors in boost circuits.

[TimWaldvogel]

i was readin up some stuff on booster pedals and got a question in my head.

if any of you guys have listened to the zvex super hard on clean boost pedal, you will hear how it adds clarity to your signal a bit in the high end. zvexs site says its largely because of the 5 meg resistor to ground rather than your average 1 meg to ground you see 80% of the time.

is it that simple? can i change the clarity of a lpb1 a bit by swapping out that resistor value it the beginning? would this prove problematic?

[R.G.]

if any of you guys have listened to the zvex super hard on clean boost pedal, you will hear how it adds clarity to your signal a bit in the high end. zvexs site says its largely because of the 5 meg resistor to ground rather than your average 1 meg to ground you see 80% of the time.

is it that simple? can i change the clarity of a lpb1 a bit by swapping out that resistor value it the beginning? would this prove problematic?

Sadly, no, it's not that simple. The SHO is a special case, along with other pedals that use MOSFET input resistors. The input resistance of a MOSFET is equal to the resistance of a sheet of very pure glass 25 volts thick. It's titanic - maybe a million megohms. So MOSFETs can be biased by very large resistances.

The LPB is not a MOSFET input as I remember - I think it's a bipolar transistor. Bipolars have input resistances of just the base itself in the 10 ohm to 1k ohm range depending on a whole lot of stuff. A lot of design work with bipolars is devoted to trying to get that input resistance higher by circuit tricks.

The only reason that increased input resistance makes guitar "clearer" is that it loads the guitar pickup less. Even that gets to diminishing returns somewhere around 1M. Is 5M "clearer"? Maybe, on some pickups which produce sounds high enough for the 5M to make clearer, and with low enough number of turns to reach that high in frequency. Not all pickups do that.

So, no. It's not that simple; although using a higher input impedance booster may make your guitar brighter, you can't just whip a 5M resistor in there and have it be magically better.

[TimWaldvogel]

well in that case is there is magical mods people HAVE found when doing a diy lpb1 circuit? or even a microamp circuit. op amps seem much simpler to me, however there seems to be a magic that people find when using transistors at times.

my other question would be, is there a mosfet radio shack available substitute for the bs170 that is in the SHO that might turn up somewhat similiar results?

i am kickin my self in the rear cause i skipped out on buying a bunch of transistors to start playing with last time  ordered from smallbear. i have a bunch of NPN transistors thats why i am asking about the LPB1 SO MUCH. it sounds great but is ripe for little improvements.

[aron]

Build the beginner project. It sounds GREAT. To me (and my opinion), better than the micro amp and some others you mentioned. How do I know? I have used all you listed.

[GibsonGM]

+1, the beginner project is very much related to an LPB-1 (I like the LPB-2 a lot; it gives you some grit to your signal but isn't overwhelming). 
I don't think the Shack carries much for MOSFETs, gotta suck it up and make another Bear order ;o)   Grab some JFETs, too!

An opamp is going to have a MUCH higher input impedance than a BJT, so will be 'sparklier', but some think it's more sterile.   The LPB-2 seems pretty well-designed to my ear....you could play with it some (change the 390R emitter resistor to 1K for better clarity, you could switch in/out other cap values - halves most likely - at the input/output to cut bass and create a treble boost...).