How to add a depth or mix pot to The Worm (EHX).

[j_flanders]

Like most people, I find the effects in this pedal a little too subtle, totally un-ehx-like.
I read pure-tube's posts over here about the origins of this pedal (wahbrato) and it originally had more knobs, but M.M. asked for 3 knobs and so the mix/depth was set at a fixed ratio.

How would I go about adding a 'depth' or 'mix' pot to this circuit to have more pronounced effects, especially on the vibrato and phaser setting?
The only schematic I've been able to find is this hand drawn schematic.


Usually I can somewhat find my way in a circuit for doing a simple(?) mod like this, but with this one I literally have no clue whatsoever and thus no idea on how to even get started modding it with a depth or mix pot, so any help or pointers would be much appreciated.

[willienillie]

-

[anotherjim]

I can see a dry path to the mode A switch via C4 and a feedback/Q path via C14 to mode B switch. They might be key to adding depth and resonance control.

[j_flanders]

I can see a dry path to the mode A switch via C4 and a feedback/Q path via C14 to mode B switch. They might be key to adding depth and resonance control.

For vibrato it (the dry) doesn't seem to connect to anything, which I guess makes sense as vibrato is wet only. I guess that rules out a mix pot for vibrato.
It already has a 'range' control which seems to control how hard the light can shine on the ldr and thereby controls how much the resistance can fluctuate?
Maybe the way this circuit is setup is as deep and seasick as it can go for vibrato?

[StephenGiles]

https://www.dropbox.com/s/imdfpaj13u0uin4/EH%20Worm%20Factory.jpg?dl=0

Any help?

[j_flanders]

Any help?

Thanks for the factory schematic (though I don't think we're allowed to post them here)
Nevertheless, it's good to have and check the hand drawn (copied) version against it.
It only contained one missing value and one incorrect value.
The layout was identical.
Corrected (in red) version is below:

[StephenGiles]

Any help?

Thanks for the factory schematic (though I don't think we're allowed to post them here)
Nevertheless, it's good to have and check the hand drawn (copied) version against it.
It only contained one missing value and one incorrect value.
The layout was identical.
Corrected (in red) version is below:

I thought links were OK, I will stand corrected if needs be!

[vigilante397]

Per forum rules, you can post any schematics, factory or otherwise, as long as they aren't Teese or Klon schematics.

[j_flanders]

Per forum rules, you can post any schematics, factory or otherwise, as long as they aren't Teese or Klon schematics.

I thought links were OK, I will stand corrected if needs be!

Sorry, false alarm apparently. I thought EHX schematics (or those of Ton specifically) were off limits as well.
Suggestions for more viby seasickness are still very welcome.

[j_flanders]

I've simplified the schematic and only kept the vibrato (vibe?) part. Maybe that makes it a little easier to suggest a more depth mod.

[Mark Hammer]

The current-limiting resistors for the vactrols (after the Range control) are 1k.  I suspect the LFO is making the LEDs inside about as bright as they are permitted to be before frying.  I might be wrong about that, but I have yet to see lower current-limiting resistances in other circuits.

I guess the question to ask is whether you find the acceptability of sweep width different for the various modes, or whether i's just "not enough" for all 4 modes.

[j_flanders]

I guess the question to ask is whether you find the acceptability of sweep width different for the various modes, or whether i's just "not enough" for all 4 modes.

I'm not much of a Wah connoisseur or user but I don't have the impression it's not pronounced enough. I'm rarely on that setting though, but when it's on there's no denying you're using a wah...

The range on the tremolo setting is kinda weird: it's either no tremolo but with a huge bass boost, overloading the pedal and everything up in the chain, or it's a very intense tremolo but no bass and very spikey highs and lots of white noise, and something in between where's it's kinda ok. I already have an EHX pulsar on my board so I didn't buy the pedal for the tremolo setting.

The range, depth, intensity, swirl on the phaser and vibe?/vibrato setting are lacking imo. It's those two settings (and the manual mode, which is a kind of an awkward EQ) which I bought the pedal for. A rough guess would be that they need an extra 20 or 30% of extra depth to make the effect stand out more.

I don't have a phaser to compare with and the vibrato I was using before I got this pedal is my EHX deluxe memory man. (fully wet, modulation at max, delay at minimum).
I realise it's bbd based and because of the relatively huge delay you can set it at it can get over the top seasick, but I have no need for that crazyness. I just wanted a more pronounced, swirly phaser and wobblier vibrato. But it's a pita to switch between delay and vibrato on the DMM because I have to turn all knobs to an entirely different setting.

Currently, especially with reverb on (ehx holy grail), the phaser and vibrato on the Worm are almost so subtle you don't notice they're on until you turn them off.

[j_flanders]

Still not giving up on this.
This time I drew the phaser / vibrato switching.

Am I right assuming:
* vibrato = phaser without the dry signal
* if so, does that mean r27 is not the feedback resistor (and this phaser has no feedback)?

Based on that what would the mixing resistors be? r26, r27?

[j_flanders]

I'm just going to continue posting here.
So, I've also did the simplified schematics for tremolo and Wah:




With those in mind I went back to the simplified Vibe(rato) and Phaser schematic and removed all the feedback/forward/mix paths and parts for the Tremolo and Wah settings to have an even simplier and hopefully easier to understand phaser circuit:




After reading (and trying to understand at least a little) R.G.'s article on the univibe: http://www.geofex.com/article_folders/univibe/uvfrindx.htm
and looking at the mxr univibe: http://4.bp.blogspot.com/-5J3L9R1_Chk/T_WXTQl5NqI/AAAAAAAABo0/K70MXGGHHVE/s1600/fv_sch_vint.gif
and Mayer's Voodoo vibe: http://diy.suregork.com/wp-content/uploads/2011/04/audio-schematic-corrected.jpg

for now I wonder:
- what the 'phase-caps' are in the Worm? Is it those three 10nF caps (C6, C8, C10)?
- if so, why are they the same value, while R.G's article states they need to be different and a certain value (0.015uF, 0.22uF, and 0.0047uF)
- why was it so important that the coupling(?) caps (C5, C7,C9) are exactly 200nF instead of a standard 220nF? (The worm uses 2 parallel 100nF caps instead of a single 220nF cap. (see factory schematic below)

[rockola]

- why was it so important that the coupling(?) caps (C5, C7,C9) are exactly 200nF instead of a standard 220nF? (The worm uses 2 parallel 100nF caps instead of a single 220nF cap. (see factory schematic below)

Presumably EHX was sitting on a truckload of 100nF caps...

Are you working on the original Worm (big box), or the reissue? I have a reissue and am following your exploits with much interest.

[j_flanders]

Presumably EHX was sitting on a truckload of 100nF caps...

Yeah, probably that.

Are you working on the original Worm (big box), or the reissue? I have a reissue and am following your exploits with much interest.

I have both, but for these tests I'm working with the big box. So far, the components and their values match the factory schematic posted above.
If this thread comes up with useful mods I'll apply them to both. (big box is through-hole and XO smt)

[j_flanders]

I've been tinkering with the phase caps C6,C8,C10. I've tried values ranging from 250pF to 1uF and just about any combination possible for the three.
It does provide some tonal variation but it's either more woofier and swooshier wobbling the lower frequencies or trebbly swishier, wobbling the higher frequencies.
None of the tried values sounded better than the stock 10nF values.
Staggering cap values (15nF, 220nF, 470pF for example) didn't give a more deeper vibrato either.

I then tried some 'circuit bending' and used the various feedback or mixing paths between stages but it either looses the effect (tremolo becomes dry signal, vibrato becomes phaser) or it turns into a form of Wah, but sounding more "Yah Yah" or "You Win" instead of "Wah Wah". Fun for a couple of minutes but not worthy of a permanent mod to the circuit.
Another dead end.

So, I turned my attention to the LF0 section.
I tried to simulate this part of the crcuit in Falstad (free online circuit simulator).
Here's a link in case someone wants to play with the LFO section: https://tinyurl.com/yfh32fan
Here's a screenshot of what the output looks like:

[j_flanders]

I've been listening to some MXR phase 45 demo's and I'm slowly getting the idea that The Worm is equally subtle because of the number of phase stages (and the lack of feedback).
I've also been listening to some Phase 90, 100, Bad Stone and Small Stone demo's and I'm realising that I actually have no need for such a strong phasing effect.
So, as far as as the phaser goes, maybe the worm will grow on me and might, as far as subtlety goes, even be better when paired with dirt or fuzz.
I would still very much like a more inspiring vibrato but I guess it'll never sound as lush and deep as a bbd-based vibrato.

Nevertheless, I invested so many hours fiddling with this circuit that I cannot let this thread go before I understand the circuit a little better.

So, I did some reading again:

R.G.'s article: http://www.geofex.com/article_folders/phasers/phase.html
this random tutorial: https://www.electronics-tutorials.ws/amplifier/phase-splitter.html (although slightly off-topic, this was the eye-opener for me)
and this MXR Phase 90 breakdown: https://www.electrosmash.com/mxr-phase90

Hopefully someone here can confirm my conclusions or tell me where I'm wrong.

1)The Worm is a 3 phase stages phaser with 1 notch. (similar to MXR phase 45 which does it with 2 phase stages)

2)

At the frequency where the capacitor's impedance is equal to the resistor, the phase shift is exactly 90 degrees, which is equal to a time delay of 1/4 of the period of that frequency.
So we have a frequency dependent time delay, more shift at some places than others.

From that I gather:
-90° is the maximum phase shift and for the deepest possible notch (180° out of phase) you need at least two phase stages.
-there are other frequencies where there's less phase shift (60° for example), so you could use three stages (instead of multiples of 2) and also end up with a freq which is 180° out of phase
-my guess is that The worm relies on 3x60° degrees phase shift

3) As far as the phaser setting goes, I could replace the stock circuit (3 x 60° = 1 notch) :



with this (2 x 90° = 1 notch):



and it would sound very similar, apart from the fact that the notch would be at another frequency (or move from/to another frequency in case you modulate it)

4) the phasing effect is strongest (as I found out trying all those different cap combinations) if you use the same RC pair in each phase stage. Otherwise, at the end, not a single frequency will be exactly 180° out of phase. Which would explain (as Mark Hammer has said in other threads) why phasers have deep notches and in (uni)vibes, with different, staggering caps, they're more shallow.

I'd like some feedback.

[puretube]

WOW!

[j_flanders]

Most users mention the lack of headroom with the old big box Worm.
Putting the entire circuit in a simulator I noticed that the biasing of those transistors is 'skewed' which is causing the lack of headroom.
In the newer XO version they solved that biasing problem like this:
(It also shows where some slightly different values were used compared to the original version, but they're not related to the biasing problem. Everything underlined in green is the same as in the original.)
https://i.postimg.cc/x0kscrkn/XO-EH-Worm-Factory-simplified.jpg



Here are some hi-res pictures where I traced the newer XO version:
https://i.postimg.cc/NYLPnHG5/FRONT-WITH-BACK-TRACES.jpg

https://i.postimg.cc/x0CKdL4m/back-light-flipped.jpg