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Old 29th September 2012, 05:33 AM   #11
n0nspaz is offline n0nspaz  United States
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i had the idea of switching in C14 to make up for the passive EQ loss (when EQ is engaged). maybe i should just put a state variable in there. if anyone knows of a small footprint EQ that will give me HPF, LPF, wide mid scoop 'acoustic' guitar sound, and modest mid boost (Clapton or whatever)...this is what i am after. what you see is the closest i have come.

challenges:
1. circuit inverts signal polarity
2. EQ is done by brute force modelling and none of this is tested yet in the real world

cheers
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File Type: png S-tastic strat wiring with Fetzer Revisited-14.png (98.6 KB, 76 views)
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Old 5th October 2012, 11:34 PM   #12
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Default More mods based on your requirements.

Hi nOnspaz,

I attached a schematic of suggested mods to your original preamp design. I recommend that you keep the original JFET front end but to achieve low-Z drive (into 10kOhm) and stay in-phase with your passive signal chain, I would suggest using an inverting opamp stage as described in the schematic.

I like your ideas about cut-and-boost and diode-clipping circuits but I would suggest using an active circuit for those applications. There are some pretty cool tone circuits that are passive (e.g. the original Donahue Tele passive) but you get many more options when you go active.

Please let me know your thoughts. JP
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File Type: pdf nOnspaz_lowZ_driver.pdf (73.2 KB, 34 views)
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Old 6th October 2012, 04:46 PM   #13
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Please note that on the last schematic post (nOnspaz_lowZ_driver.pdf), R2 is an 11.3K Ohm resistor and not an 11.3 Ohm resistor. In addition, check to make sure J1 is biased correctly by measuring the DC voltage at the Drain of J1; it should be between 1/2 and 2/3 V1 (9 volt battery).

JP
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Old 7th October 2012, 05:54 AM   #14
CBS240 is offline CBS240  United States
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Hi

The 4117 J-fets I use in my projects have Idss from 40-60uA. So a class A bias of 20uA is reasonable, could even be more like 15uA. Expect Vgs @ 20uA to be around -0.2V so 11K for the source resistor should be fairly close. Because of the variance in J-fets, this resistor value may need some small adjustments.

Add too many caps and filters to such a high impedance circuit, in the real world, and you might end up with a radio reciever.
J-fets are well suited for direct coupling to BJT's. Why capacitor couple the J-fet to the BJT? In this similar circuit I just made up, the drain resistor drops 4V resulting in 4.6Vce on the 5089. The 5089 transistors I have, has a current gain of about 500. J202 Idss is about 2 or 3mA. 3mA/500=6uA, so the drain resistor would be 4V/(20uA + 6uA)= 153.8K or 150K. For 15uA Id, 4V/(15uA + 6uA)= 190K.
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Last edited by CBS240; 7th October 2012 at 05:57 AM.
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Old 8th October 2012, 04:05 PM   #15
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Hi CBS240,

I agree, I like your design for it's elegance (without the coupling cap), very cool. The only reason I would suggest the more complex coupling cap and extra biasing resistor(s) configuration would be due to the slight variance in biasing parameters for the average JFET. Unless you predetermine what those biasing levels would be, you might have a symmetry issue with the output biasing point of the emitter follower stage. Probably not an issue for this application though. Simpler is almost always better. Thanks, JP
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Old 12th October 2012, 11:19 PM   #16
n0nspaz is offline n0nspaz  United States
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the challenge is keeping the footprint as small as possible since the idea is to build it into the body of a strat.

since i use a fixed bridge, not tremolo, i plan to mount the 9V battery in the rear bridge compartment and the circuitry in the pot/switch cavity, somehow.

that's why i was thinking of using the Orman AMZ tone control (big muff modified tone control). but then i wanted a mid boost as well as scoop and i went off the deep end...now considering active EQ lol

fact is, small footprint is big here.

is it possible: Fetzer preamp + EQ + simple diode clip + Zout <1k
with:
- noninverted signal at output (in any switched position)
- using transistors, not opamps if possible (just because i want to learn more about them)

thank you guys
cheers
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Old 12th October 2012, 11:41 PM   #17
n0nspaz is offline n0nspaz  United States
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so JP, if i measure the actual Idss and Vp (pinch-off) of the exact transistor i am using, which i am doing using the circuit at the bottom of the Fetzer Valve Revisited page, are there caps or resistors that can be eliminated from the circuit?

i am new to transistor circuits (but am famil with opamps) so i apologize if some of this really basic.

other curiosities:
1. How do the 4117 & 4118 draw such low current but provide so much gain? what is the catch? it does have a high Zout. but does it sound nasty or something?

2. i am thinking of switching to an NTE457 for lower Zout, which will allow me a smaller series volume pot before the EQ (less noise?). do you guys think this is wise?

3. is there a noninverting buffer with Zout of <1k using a transistor?
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Old 14th October 2012, 08:56 PM   #18
n0nspaz is offline n0nspaz  United States
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here is my latest attempt

1. i moved the GreenBean Buffer biasing resistors to the 'power supply'. is it ok to create a Vb like this and bias the 2 emitter followers like so, or do they need dedicated resistor dividers for each tranny? also- how do i know these values are what i need. do i calculate current flow for ~1mA per tranny? i am new to transistors.

2. is this the best place for the volume pot?

3. i am still seeing loading effects on the passive EQ circuit (The New AMZ Tone Control) I am trying to isolate each section with caps, etc. but i must be missing something.

thanks
hope this helps others too.
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Old 14th October 2012, 09:41 PM   #19
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R15 looks a high value to me.
The guitar cable driver should be fairly low impedance to counteract the capacitance of the lead or you will lose high frequencies
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Old 14th October 2012, 11:45 PM   #20
n0nspaz is offline n0nspaz  United States
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where is R15. i don't see it.

the biasing looks ok in the sim using Vb. but i don't know, this is my first time using transistors
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