Extra Medium v2

Details

I’ve been talking to a friend of mine for a while about a pedal that he’s wanted. He’s a guitar player in a gigging band and has a pretty sophisticated modern live rig that he likes the sound of, but sometimes wishes that he could be a little louder and more prominent on demand, like during a solo, independently from whatever else he has going on. I said that I could do that.

The only boost I’d ever made before was the Mild Stallion, which is in the style of a Rangemaster and definitely not “transparent” (to barf out an overused pedal marketing term). It’s got a significant high-pass filter at the front that cuts out a lot of the low end and its single transistor is biased such that it’s basically clipping all the time (at least according to Electrosmash’s write-up of the original). That can be cool, but my friend wasn’t looking for something with a lot of character, just something louder. I decided to start from scratch with a basic op-amp gain section, which seemed straightforward.*

Op-amps often come in pairs, which meant that I’d have a spare to do something interesting with the mids. Guitars make sound across a wide range of frequencies, but are fundamentally mid-heavy. If you want a guitar to stand out more in the context of a full rock band, boosting mids—where it won’t compete as much with the bass or cymbals or whatever—can be more useful than making it louder overall. My issue was that I didn’t really know how to do that. I initially thought that I could use my knowledge of high-pass and low-pass filters (i.e., my knowledge of where to find the online calculators that do the math for them) to amplify something up and knock off both ends, but my tests could all be described as “bad sounding.” So like all great pedal builders, my thoughts turned to theft.

• ~4000 BCE: There was something called Electronotes. I have no idea what this is. Possibly a zine that you could get from a Radio Shack of ill repute? I may have actually found it, but it would require a librarian, a DBA, and a shaman [walking into a bar] to make any sense of it.
• 1995: Craig Anderton published the aptly-named “Frequency Booster” circuit, based on something in Electronotes. Jack Orman reproduced it here.
• 2010: Jack Orman made his own version of the Frequency Booster, and modified it to run off of a 9V power supply.
• 2022: Some folks on diystompboxes.com had the same question I did about Jack Orman’s design: how do you get this thing to go flat? It’s always boosting at least a little bit, even when the knob is all the way down. They had some ideas, and wrote about it here.

While I was doing all this research, I kind of figured out how to use LTspice: the anachronistic yet deviously powerful circuit simulation software. I’m not saying that I mastered it—it’s the kind of thing that someone should have taken back to Babbage’s and asked if it was supposed to come with a keyboard overlay—but I had a personal breakthrough with it and got to the stage of grief where I accepted that I’d have to use F9 for “undo.”

Being able to brute-force my way through many iterations of the circuit quickly, along with constant help from aotmr and dylan159, allowed me to arrive at something that clearly shares a lineage with the Frequency Booster, but is also not the Frequency Booster, and is different in ways that addressed all of my perceived drawbacks from the previous designs.

* And then my buddy said “What if I wanted to run it in the effects loop of my amp?” It’s probably not a universal truth, but I think a lot of effects loops run at line level. The whole point of a loop is that you can insert effects between the preamp and poweramp of a guitar amplifier, and unless the amp is really tricky about it, the output of the poweramp is likely to be line level, or at least something way hotter than a typical instrument level signal. The boost pedal does multiplication, so if it’s expecting a 0.2V input signal and multiplies by 10, the output would be 2V, which is fine with a 9V power supply. If it suddenly gets a 1V input and multiplies by 10, it would be at 10V, but you can’t get 10V out of a 9V power supply, and it would clip. That’s oversimplified, but since I was going for clean boost, increasing the input level meant that I’d need more headroom to be able to accommodate the gain without clipping. So for the first time, I adapted the circuit to use a charge pump and run at 18V internally (dual supply +9/-9 for an 18V potential). That change probably made it better in all circumstances (even if it’s not in an effects loop) and was an interesting exercise for me.

Sound

It makes little sense to me to include a sound sample of a device that is designed to barely impart its own sound, so I will instead use this space for numbers. The following is based on the simulation results and not real-world measurement, but should be close.

The clean boost has about 16dB of boost available. That’s ~6x voltage gain if my calculations are correct. I could have given it even more gain and left it up to the user to keep it out of clipping (or not) for a given input, but for this use case, this seemed like plenty. I didn’t want the range of the control to be too fiddly, and also wanted it to be able to go all the way down to unity gain and operate as a buffer. If I were to keep messing with this in the future, I might try out some rail-to-rail op-amps for even more headroom.

The mid boost follows the clean boost in the circuit, and can add an additional 13dB of gain (~4.5x), centered at about 870Hz. That’s not far off from where a Tube Screamer boosts mids, although the Extra Medium v2 has a higher Q and impacts a narrower band of frequencies overall. When the knob is turned all the way down, the mid boost is flat (i.e., it can’t become a mid-cut).

When both knobs are all the way down, it does basically nothing except act as a buffer. When both knobs are all the way up, it will push 16dB over the majority of the audible spectrum and close to 30dB at the peak of mid bump. At that setting, it’s likely that there would be some internal clipping, although it might be hard to hear it over whatever it’s doing to the amp.

---