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Old 30th July 2012, 03:19 AM   #1
n0nspaz is offline n0nspaz  United States
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Default emitter follower for onboard guitar preamp

looking for a BJT buffer to follow the Fetzer Valve Revisited possibly like this emitter follower

i understand some basics of transistors but i could use some help.

Does Q1 need a bias resistor from gate to V+ ? (the Fetzer design does not use one)

Q1 Zout seems to be 14k according to Multisim. is the volume pot in the best place, impedance wise?

the pot wiper position seems to change the voltage at the output of Q1 (loading). i could eliminate C4 if i knew & can count on the voltage at Q2 to be steady. otherwise i have to bias it, is this correct?

any way to make this simple 2-stage amp more elegant?
any other suggestions are welcome! thanks.
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Old 30th July 2012, 03:32 AM   #2
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forgot attachment.
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Old 1st August 2012, 10:40 PM   #3
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i am still getting strange interactions between the 2 stages although they simulate fine separately. and now i see a half-wave rectified type of output on the scope.

i have tried removing R6 pot & C9 coupling cap (and rebiasing), no help there.

Reducing emitter resistor R15 to 10k makes things even worse, creates a strong LPF at about 1kHz.

I am new to transistors and am just trying to add a low Z buffer to the Fetzer stage. I am lost..

any advice is really appreciated!!
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Old 2nd August 2012, 01:58 AM   #4
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Post some DC voltages.
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Old 3rd August 2012, 02:42 AM   #5
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In the simulation posted in reply #3 there is no coupling cap from the FET to the volume control. That could disrupt DC conditions causing clipping.
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Old 3rd August 2012, 05:17 AM   #6
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thank you Loud, i have added coupling caps and voltage probe points.

when i reduce R15 to say 10k, i get a full waveform on the scope. but the Bode plotter then shows a huge frequency dip -10dB centered around 4500Hz, i can't find where this filter action is happpening. when i looked at either transistor stage alone, they seemed to be ok, not totally sure about this tho.

Note that I chose the 4117 FET based on desired specs for Idss and Vp specs according to the Fetzer Revisited article. I am not sure if I should take more gain in this stage, however (or how to do it). I would like to design for a volume range from pristine clean tone with maximum headroom, to a slight overdrive, if possible. I want a Zout of 600-1k after the emitter follower for plugging into a console/audio interface. I am open to changing anything you see. Thanks so much for taking a look.
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Old 3rd August 2012, 05:57 AM   #7
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this shows what happens when i change the pot to a rheostat type of style, and change emitter resistor R15 to 10k (approx. 1/10th of the R21 || R22 which i think is more textbook)

is the pot even in the best location? considering i want very low Zout, i thought this was the best place for it instead of at end of circuit. if i could have something like a ~5k pot on the very output, then that would also get my output impedance within spec, since the Zout of the pot would be 0-1250 ohms max. even this is higher than i had hoped for though.
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Old 3rd August 2012, 06:25 AM   #8
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Default GOT IT

now THIS is what i am talking about!

the Multisim impedance meter (with the recommended 50ohm input resis) was causing that huge drop in level in the high freq's. makes sense, a 50ohm load just doesn't sound right at all.

i also temporarily got rid of the volume pot and biasing resistors for the EF and everything seems to be fitting into place now. very encouraging.


now, how do get the volume range i would like (clean/max headroom to slightly overdriven. slightly overdriven (driving a console input) is what, maybe 2-3V pk-pk? i probably need a better guitar circuit model (see inset) to start with. mine is a guess.

and then, to find the best place for the volume pot. i am also considering adding a passive 2-pot tone circuit between the 2 transistor stages.

thanks, hope this thread helps someone else too.
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Old 28th September 2012, 10:19 PM   #9
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Default Enhancements for the "emitter follower for onboard guitar preamp".

Hi n0nspaz,

I attached a jpg schematic of your design with some suggestions that will fix some of the biasing issues you mentioned and improve the gain of Q1. Please let me know if this was helpful and/or if you have any questions.

Good Luck, JP
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Old 29th September 2012, 05:19 AM   #10
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Quote:
Originally Posted by JP Hugsley View Post
Hi n0nspaz,

I attached a jpg schematic of your design with some suggestions that will fix some of the biasing issues you mentioned and improve the gain of Q1. Please let me know if this was helpful and/or if you have any questions.

Good Luck, JP

Thanks so much JP. can you help with my goals?

- passive & active use of the Strat via push/pull pot, active out is in phase with passive
- final Zout of the circuit <1k - to interface with a console/audio interface
- EQ with mid boost & cut, LPF & HPF -this is another circuit and another topic, not sure active or passive
- switchable diode clipper

i am thinking 6-12dB of gain upfront should be plenty.

Fetzer Valve Revisited at the bottom of the page has a Rd and Rs FET resistor calculator that I was blindly following. Will your R4 reduce my input headroom?

trying to insert a passive EQ circuit after the vol pot now.

although the 2N4117 uses VERY low battery power i am thinking of using an NTE457 (Vp 1.26, Idss 1.09mA) instead. this will give me a Zout (R4 in your schematic) of 5.3k according to the Fetzer calc. thus my vol pot can be 10k or 25k, reducing noise and allowing a passive EQ using reasonable Zin, to follow it. still drawing this up.

love to hear your thoughts! thanks again!
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