Simple LPB1 clone with increased input impedance

[cdwillis]

I've breadboarded a couple of these a few months ago, both with the standard 430k and 43k resistors that EHX uses for biasing and the Beavis Audio 1M and 100k resistors. I'd like to increase the input impedance so it doesn't load down the signal so much. At first I tried a 10M and a 1M, but I got no signal. I reduced the resistors down to 2M2 and 220k, but still no sound. Then I removed the 220k resistor to ground and suddenly there was sound passing through, albeit sputtery gated sound.

My question is, how do I determine the maximum value of these resistors I can use to bias the transistor? I've been trying to read and someone said don't try to make the LPB1 more than it is, build something with mosfets, etc. I'm just trying to understand what it is that makes the transistor quit functioning with more resistance at both ends of the bias point.

[willienillie]

You might want to experiment with adding an input buffer stage, like a simple BJT emitter follower.

[idy]

I think the issue is BJTs are "current controled", they need actual current flowing to open up that base/emitter junction. 1M resistor=not much current. FET, MOSFET just need a "field" to bias the gate, little or no current, big resisistors OK because the gate is not "loading" the bias voltage.
If you experiment with that pair of resistors you will find the highest value that allows things to function. Appears to be between 1m and 2m.
There will be a point where there is almost enough current, and then your signal will turn the thing on at peak only, "gated sputtery" sound.
Yes the correct answer is build something else. *Any* JFET or MOSFET design will have better input impedence.
One of my first projects was BYOC tri-boost: a range master, LPB and MOSFET boost in one box (unfortunately you can only use one at a time.) But it let me (and my friends) hear the difference between these simple stages. Handy utility box.

[Eb7+9]

the culprit here is the low emitter bias resistor (330 ohms) ... even with a current gain of 200 we'd be seeing 66k AC to gnd through the base - so, no matter how much you try increasing the base bias resistors Zin equiv is dominated by what's going on in the emitter circuit ... to fix this try using Re = 3k3 and Rc = 100k (same ratio) ... and instead of relying on a pair of resistors I would set the base bias via 50k trimmer and 470k bias feed resistor adjusting trim so Vc is optimized (near half rail)  ... Zin MAX is now somewhere close to 660k||470k depending on device gain ... maybe around ~250k AC

[antonis]

What Eb7+9 said..!!

Although, I shouldn't  rely upon a 100k  Collector resistor, 'cause beta (h_FE) is severely diminished for low Collector currents, resulting into also diminished Base reflected impedance..
[(h_FE x re) + (hfe+1) X R_E]

Bootstrapping, by its own, can't help here,  'cause main issue is low Emitter resistor value, so a mix of bias resistors bootstrapping and Emitter resistor replacement by a constant current source (of a single BJT + diodes simple form) could make things much better..
(of course, the above is way far apart from a humble LPB1 booster..)

[niektb]

I came across an interesting article lately that also had a section about current sources: https://sound-au.com/articles/followers.html#s4
Would be definitely interesting to explore that route to replace the emitter resistor with something (sounds like something that you could re-use in any new pedals you might be developing)

[antonis]

I came across an interesting article lately that also had a section about current sources: https://sound-au.com/articles/followers.html#s4

Something like that but with Q2 replaced by 2 series diodes..
(one diode counting for Q3 V_BE drop and the other for setting source (actually sink) current..)

P.S.
Reference No1 in Rod Elliot article: "High Input-Impedance Amplifier Circuits - T.D. Towers (Wireless World, July 1968)" is en excellent document..
https://www.keith-snook.info/wireless-world-magazine/Wireless-World-1968/High%20Input-Impedance%20Amplifier%20Circuits.pdf