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Modding MXR Zakk Wylde Overdrive

Continuing coverage of Tube Screamer like pedals – I’ll cover a sort of a Boss Super Overdrive clone. I’ll be covering MXR’s Zakk Wylde overdrive (ZW44).

This post continues building on previous posts: breadboarding the pedal, how it shapes the tone and I even did an alternative version. I have already covered SD-1 too.

I’ll cover quickly the schematic, and then do a quick analysis of it – in comparison to the TS808 & SD-1 circuit. I’ll crack open the pedal and see what’s hidden inside. I’ll pay with the pedal and we’ll see how that goes. If you don’t want to read, just skip to the video.

Btw, I threw in SPICE circuit in there if you want to play with it yourself.

Schematic

Let’s have a look at the schematic:

Schematic of MXR ZW44 pedal
MXR Zakk Wylde overdrive (click for full image)

I marked all components to match what I have on TS808 circuit in one of the earlier posts (I did a similar thing for SD-1). Designators are offset by 300, so R301 on the schematic here corresponds to R1 on TS808 schematic (or R401 on SD-1 schematic), C311 corresponds to C11 and so on.

SD-1 and ZW44 are very very similar. Again, some of the components have different values, but really a handful only. Outside input buffer, I pointed out in the video, just a difference in 3 capacitors.

Analysis

OK, let’s get into details here. I’ve annotated the schematic as in some earlier posts:

Schematic of ZW44 pedal with annotated tone shaping elements
Annotated Schematic of MXR ZW44 (click for full image)

You’ll notice that there’s a lot of highlighted elements in red indicating that they might be affecting the sound. I also added a little warning sign onto the ones that are different from TS808. There’s a yellow warning sign with green matching sign when there’s difference between ZW44 and TS808, but no difference between ZW44 and SD-1. Single warning sign means – different from both TS808 and SD-1.

For example, 720Hz high pass filter in clipping section affects the sound but has exactly the same cut-off frequency as TS808/SD-1. On the other hand, ZW44 and SD-1 have much more gain than TS808 in the clipping section.

Let’s zoom in a bit.

Clipping Section

Let’s look into the clipping section.

Annotated schematic of just clipping section
Clipping section of ZW44

In the post about what does it mean to be a Tube Screamer, I covered this section in more detail. Looking at this, the only difference from SD-1 circuit is added C304 capacitor. For the most of the rotation of the gain pot, that should not affect the sound though.

One thing to note though is the taper of the (over)drive/gain pot. SD-1 has linear pot, TS808 has log pot but ZW44 has D taper originally. D taper is sort of a log pot but slowly rising resistance part is longer than “normal” A taper log pot. Should not affect sound, but rather “controllability” of the gain.

Tone Section

Tone section differences look like this:

Annotated tone section schematic of ZW44
Tone section of ZW44 (click for full image)

The tone sections look nearly identical between SD-1 and ZW44. The only differences are in C305 & C306 values.

Again, seemingly big differences in cut-off frequencies of filters due to component values, but again, there is a very big proportion of tone pot rotation that gives nearly identical sound on both pedals.

Tone Section Frequency Response

What I said earlier, let’s look into the frequency response of the tone section and compare SD-1 and ZW44.

OK, when you move the slider to the right you reveal SD-1 tone section response, when you move to the left you reveal ZW44 tone section response. If you look closely, huge part of the sweep overlaps (about 60% of the pot rotation).

This is why the pedals when compared, sound quite similar for many knob positions.

SPICE

Here’s the SPICE file if you wish to play with it yourself:

Hidden Secret

There are few videos out there showing how to mod this pedal. This is my take anyway.

There’s a hidden switch inside the pedal. What the switch does is flips between GT-OD and ZW44 modes. Googling some, looks like MXR Classic Overdrive is the same as GT-OD.

Photo showing where the hidden switch is on the PCB of ZW44
Switch on ZW44/GT-OD

I don’t know if that switch is still there on the new GT-OD pedals. My pedal has date 02-08-2006 printed on the PCB, and CUSTOM OD, ZW-44 and CSP-021 (with some numbers following them).

Photo of ZW44 PCB showing ZW44, CUSTOM OD and CSP-021 markings
ZW44 PCB

In the video, you can see how I desoldered the switch, replaced it with a toggle switch and screwed it onto the enclosure. Here’s the final result:

Photo showing modified ZW44 with the switch brought outside.
Modded ZW44 with the switch on the outside

I don’t recommend doing this mod unless you know what you’re doing since you could destroy your pedal. I destroyed the original switch in the process. You’ve been warned.

Trying it Out

Finally, let’s try this out:

Sounds good, and I think I did surprisingly neat job of it 😁. Let’s move on to designing our own version of Tube Screamer – yay!!!

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