BAJA EVH overdrive pedal project 090218

bajaman · Post by **bajaman** » 09 Feb 2018, 09:00

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

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

Anyway, without further ado - here it is - enjoy and have fun

bajaman

bajaman · Post by **bajaman** » 09 Feb 2018, 09:15

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
bajaman

287m · Post by **287m** » 14 Feb 2018, 10:04

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

sixthfloor · Post by **sixthfloor** » 14 Feb 2018, 18:02

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.

bajaman · Post by **bajaman** » 14 Feb 2018, 20:25

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

bmxguitarsbmx · Post by **bmxguitarsbmx** » 14 Feb 2018, 22:40

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!

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.

koldman · Post by **koldman** » 15 Feb 2018, 00:31

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

287m · Post by **287m** » 15 Feb 2018, 14:18

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

i wonder, you are shaman too. How you know i want remove that dpdt switch?
ah, this circuit is hard too make onboard pots

287m · Post by **287m** » 28 Feb 2018, 13:51

Oh No. I did!

Have s some 1/8W resistor or 1/4W standing isn't crime, right?
And still can build in perf

Frank_NH · Post by **Frank_NH** » 28 Feb 2018, 13:59

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

287m · Post by **287m** » 28 Feb 2018, 14:11

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

rockhorst · Post by **rockhorst** » 13 May 2018, 13:42

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.

Nefi_fatobene · Post by **Nefi_fatobene** » 18 May 2018, 01:38

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

Hey 287m, did you test this layout?

287m · Post by **287m** » 20 May 2018, 16:38

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

Not yet Sir, out of some componen and good mood to bending leg komponen in perf

sego91 · Post by **sego91** » 06 Jan 2019, 17:12

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

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!

jhergonz · Post by **jhergonz** » 07 Jan 2019, 02:46

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

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!

how about other type of taper for the gain knob?

manelik · Post by **manelik** » 19 Apr 2019, 16:14

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)?

FeVeR2112 · Post by **FeVeR2112** » 22 Apr 2019, 05:52

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)

bajaman · Post by **bajaman** » 22 Apr 2019, 07:16

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

Zaenar · Post by **Zaenar** » 17 Jun 2020, 10:17

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 !

BAJA EVH overdrive pedal project 090218 [documentation]