BAJA EVH overdrive pedal project 090218  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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bajaman
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Post by bajaman »

Here it is at long last - the Peavey EVH 5150 preamp section in a nice friendly 9v overdrive pedal format.
You can build it with or without the tone stack and recovery stage or make it switchable - the choice is all yours :wink:
This circuit is my attempt to mimic the frequency and phase response characteristics of the original Peavey EVH 5150 head preamp section.
I also paid close attention to the clipping behaviour which occurs asymmetrically in the 5th gain stage first and then in the 4th gain stage.
There are six gain stages in this amplifier with a lot of attenuation and select filtering between each stage to get the desired overall frequency response.
I found it easier to get the desired double asymmetric clipping by using a red led and a pnp transistor in this pedal and considerable gain juggling was required to get the same response to clip as the actual amplifier does :wink:
Anyway, without further ado - here it is - enjoy and have fun :thumbsup
bajaman
Baja EVH overdrive pedal with switchable tone stack 9v 090218.GIF
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bajaman
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Post by bajaman »

Here are the plots for the frequency and phase measured at the output just before the tone stack - the red line is the peavey 5150 schematic simulation and the green line is the baja pedal simulation.
As you can see they are very closely matched between 100Hz and 100kHz.
cheers
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EVH 5150 .gif
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287m
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Post by 287m »

Baja, what function of last 1M resistor? if use tonestack, so parallel with level, right?
Can safely remove?

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sixthfloor
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Post by sixthfloor »

As far as I understand this resistor is a pull-down resistor. You can remove it, but you might experience popping sounds when turning the pedal on.

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bajaman
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Post by bajaman »

as sixthfloor states - it lessens any popping when tone stack is switched on / off.
If you are using an unswitchable tone stack you could leave it out :wink:
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bmxguitarsbmx
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Post by bmxguitarsbmx »

Theoretically, the two clipping sections will dump current into the high impedance bias supply and cause bias shifts with long integration times. Could be a problem, could sound awesome! 8)

These high-impedance Cap filtered Resistive divider bias supplies will drift when expected to absorb asymmetrical wave forms. The large value Capacitor doesn't lower the impedance, it only lengthens the time constant to return to equilibrium. Not to mention that DC shifts will cascade through each stage to the next, even if they are not DC coupled.

A 10uF or so cap on the output of the op would let you ground the rectifier/clipping transistor.

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Post by koldman »

Would these circuits work much differently on a dual rail supply? +/-15v

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287m
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Post by 287m »

sixthfloor wrote:As far as I understand this resistor is a pull-down resistor. You can remove it, but you might experience popping sounds when turning the pedal on.
yeah, i think that too.
my small brain always think the pulldown is the first or the last to ground. hahaha
bajaman wrote:as sixthfloor states - it lessens any popping when tone stack is switched on / off.
If you are using an unswitchable tone stack you could leave it out :wink:
i wonder, you are shaman too. How you know i want remove that dpdt switch?
ah, this circuit is hard too make onboard pots :scratch:

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287m
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Post by 287m »

Oh No. I did! :lol:
Have s some 1/8W resistor or 1/4W standing isn't crime, right?
And still can build in perf :hmmm:
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BAJA-EVH.zip
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baja-evh-value.png

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Frank_NH
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Post by Frank_NH »

Have some 1/8W resistor or 1/4W standing isn't crime, right?
Image

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287m
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Post by 287m »

Ah Frank
dont compare with old goldie.
we just want see beauty of component, not solder side when open the box :o

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Post by rockhorst »

I would suggest an AC coupling cap after IC2b and tying the red clipping LED to ground instead of Vbias. I haven't tried this specific design, but I tried something similar recently and the LED to Vref made the bias network unstable, resulting in a sort of fuzz like effect to the note decay.

Impressive work by the way. Interesting how all the opamps tie together and the clipping is done by just two components at the end. Diferent approach from most pedals.

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Post by Nefi_fatobene »

287m wrote:Oh No. I did! :lol:
Have s some 1/8W resistor or 1/4W standing isn't crime, right?
And still can build in perf :hmmm:
Hey 287m, did you test this layout?

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287m
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Post by 287m »

Nefi_fatobene wrote:
287m wrote:Oh No. I did! :lol:
Have s some 1/8W resistor or 1/4W standing isn't crime, right?
And still can build in perf :hmmm:
Hey 287m, did you test this layout?
Not yet Sir, out of some componen and good mood to bending leg komponen in perf :(

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Post by sego91 »

I've Just built this beast and I really like the tone I can get out of it! :D

However I'm having a couple of issues with the Tone stack and the gain Knob, seems like they are working only on the first quarter of turn...

Any idea of how can i fix this thing? Thanks! :D

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Post by jhergonz »

sego91 wrote:I've Just built this beast and I really like the tone I can get out of it! :D

However I'm having a couple of issues with the Tone stack and the gain Knob, seems like they are working only on the first quarter of turn...

Any idea of how can i fix this thing? Thanks! :D
how about other type of taper for the gain knob?

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Post by manelik »

rockhorst wrote:I would suggest an AC coupling cap after IC2b and tying the red clipping LED to ground instead of Vbias. I haven't tried this specific design, but I tried something similar recently and the LED to Vref made the bias network unstable, resulting in a sort of fuzz like effect to the note decay.
This is interesting because I made a MB throttle box clone recently and got "a sort of fuzz like effect to the note decay", and found the boost/eq section was the culprit. Do you think a similar fix could be done (before IC2a in the schematic)?
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Post by FeVeR2112 »

The BC557 is being used as a diode (emitter not used). In 287m 's layout, he used a 3mm LED as the diode. What would be another (or better) diode to use in this build?

Also, wouldn't a transistor be better instead of the single-OpAmp (IC4)

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bajaman
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Post by bajaman »

FeVeR2112 wrote:The BC557 is being used as a diode (emitter not used). In 287m 's layout, he used a 3mm LED as the diode. What would be another (or better) diode to use in this build?

Also, wouldn't a transistor be better instead of the single-OpAmp (IC4)
Actually it is the collector that is not used ;-)
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Post by Zaenar »

Hey Bajaman !

It's a great job you did and I have a little question about it (ok, maybe a dumb one)

I was thinking if it is possible to "daisy-chain" this OD/Preamp pedal with your presence and resonance schematic
(viewtopic.php?f=13&t=29369)

Something tell me it's not that simple but maybe it is !

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