Mercurial Boost (Catalinbread Vario Boost)

[MattG]

Build Rating

5.00 star(s)

There's no shortage of positive feedback around here for the Mercurial Boost - add me to the list of endorsers! I built this with the intent of generally using it as a lead boost. I've been using my Prismatic EQ for that role (and it works great for that). But I thought the Mercurial/Vario looked really interesting, so here we are! (Next on my list is to make a clone of the Boss PQ-4, similar to the Ravager.)

The build was super simple. It's a small board, with a fairly small number of parts. The only possibly non-typical part is the dual-gang A100k pot. It calls for two TL072 opamps, but I found I'm actually out of those! So I used an expensive OPA2134 for the IC1 and NE5532 for IC2. Works great!

The enclosure is a 125B Pro in "Tango Green" from @StompBoxParts. I love this color (check out my Andromeda Deluxe that also used this box).

The only thing that might need to be addressed is that my relay bypass board is just "floating" in the case, held in place only by the four wires that connect it to the effect PCB. Usually the effect PCB is bigger, and I use double-sided foam tape to attach the relay board to the stomp switch.

Edit: The eagle-eyed among you might notice a mistake: the input/output triangles are facing the wrong way. Sadly, it's not the first time I've done this!

 

[VanWhy]

Nice build! Is that a black SPST footswitch?

 

[MattG]

Yup, that is indeed a black momentary SPST from Antique Electronic Supply (aka Tubes and More aka @Amplified Parts). Kinda pricey, but I knew I was going to like this pedal, so I used premium parts: black SPST, footswitch dress washer, aluminum 1510 knobs, black LED bezel, premium enclosure.

 

[Chuck D. Bones]

Beautiful!

 

[MattG]

Edit: ignore my comments below. I think I imagined a difference that wasn’t there. There is some distortion, but after playing with this some more, I actually think it’s my amp’s input being overdriven.

As @Chuck D. Bones says, the OPA, 5532, and TL072 should all work the same in this circuit.

Minor update: I built this primarily to use as a lead boost, and it works great for that. I rarely (maybe never?) do leads with a clean sound. However, I was playing at home the other day, and using this pedal in clean mode. With the gain (i.e., overall volume) down low, I found that, even on moderately-hard strumming, there was some distortion. To my ears, it sounded like opamp distortion, particularly with the "noisy decay" (as can be heard in the Boss BD-2 or Colorsound Power Boost). I don't believe this pedal is supposed to add any dirt. Recall my original build was with OPA2134 and NE5532 opamps. My suspicion is that one or both of these opamps was hitting the power rails and causing the distortion. I finally got some TL072s, and dropped those in (as is called-for by the build docs): so far, it seems much improved!

TL;DR OPA2134 and NE5532 opamps might distort. Use TL072 as prescribed.

 

[Chuck D. Bones]

This circuit is not sensitive to opamp type or part #. OPA2134, NE5532 & TL072 should all sound the same unless you are driving this circuit to saturation. With GAIN at zero and 12dB at noon, the Mercurial Boost has unity gain. When dimed, the 12DB knob adds 12dB of boost at the center freq. The last stage adds 21dB of boost when GAIN is dimed. Turn either or both knobs too high, or overdrive this with active pickups or another pedal, and you will run out of headroom. As long as your electrolytic caps are rated for it, you can run this pedal on 18V (or high) for more headroom.

Measure Vref and verify it's at 1/2Vcc.

Did you sub any parts other than the opamps?

 

[MattG]

Thanks Dave. I went back and checked everything. Vref is indeed half vcc. I didn’t change any other parts (unless unknowingly by accident).

I wasn’t able to duplicate the issue, at least not the way I remember. I did tl072 with 5532, 2x tl072, OPA and 5532… they all sounded the same.

I could get a bit of distortion (in all configs), but I’m pretty sure it’s actually my amp’s preamp being overdriven. (My amp has two input channels, and a third linked input. The linked input is more easily overdriven. I couldn’t hear any obvious distortion when only using a single input.)

So… tl;dr, I’m imagining things.

 

[Chuck D. Bones]

I take it you don't own an oscilloscope.

 

[MattG]

I actually do own a little cheapie oscilloscope.

 

[Chuck D. Bones]

That will tell you if the pedal output is distorted or not.

 

[MattG]

Would I do that by feeding the pedal a generated signal, and then looking at the output on the scope? I’ve only used the scope once, so I’m not well versed in its use.

I also have a computer audio interface… there’s presumably software for measuring distortion…

 

[Chuck D. Bones]

Either one will work. I'd suggest looking at the output in the time domain (waveform display) and drive it with a guitar signal. With a 9V supply, the output should start clipping somewhere around 6Vp-p. The flat-tops on the waveform should be pretty easy to spot.

If you want to measure distortion, that's done in the frequency domain where we feed a sine wave in and look for harmonics coming out. Below the onset of clipping, you should not see any harmonics.