I can’t recall how many times we went back to the drawing board. Design, test, rinse repeat. I only wish turnaround time is shorter than it is now. After initial design, planning, and breadboarding, we start off with dead bug prototyping and do a barrage of tests to validate the design. Then, we do PCB layout using Eagle CAD (although we might be switching to KiCAD soon) and send the design to PCB manufacturing. A cycle takes around 1 to 2 weeks depending on complexity. PCB layout itself can be a demanding task, especially if space constrained, like the Modula preamplifier. What’s painful is when you are in the final stages, and there’s that yet one more test that breaks to design! It happens! And it happened again with the Modula preamp.
Nice and clean! I just got a set of sample coils straight from the manufacturer and so we got excited and assembled these guys into the Modula’s dual coil profile. With that profile (bobbin-less, 1000 turns per coil, gauge 44 at 4mm width), the coils are very difficult to wind using our home-grown pickup winding machine. Our early attempts were good enough for prototyping and one-offs, but I had to ask our favorite coil factory to do it for us for production.
So how does it sound? That’s what excites me most! I did a quick listen. The sound is very close to the XR, but with an even tighter response, having a very small aperture. Snappy, with lots of sweet harmonics, yet balanced from low to high frequencies. I think this can be a perfect general purpose pickup for a host of instruments. More on that soon!
Look what we have here! It’s the Modula Preamp. It’s a balanced design using the ADA4084 Low Noise, Rail-to-Rail Input/Output, Low Power Operational Amplifiers, with a 625μA supply current per channel and 3.9 nV/√Hz noise density. The Op Amp can go up to 36 volts (imagine the headroom)! But for practicality, 9-18v ought to be enough. Hey, this is a rail-to-rail amplifier, meaning the output can swing to the full supply range without clipping. I can’t even imagine driving a guitar amplifier with 9v peak to peak, but you can if you want to, and if your strings can generate that much oomph. Perhaps the bass guitar can? Yet, if your battery is running low, it can still work down to 3v!
After at least 5 revisions, this is so far, the best small preamp we can come up with. This guy went through a long incubation period (tests, tests and more tests!), until we finally settled on a design that I really like. I’ll share some test results soon.
The design is modular too! This preamp is reusable. You have balanced inputs —headers at the top for +in, -in and ground, and single-ended output, plus supply —headers at the bottom for out, ground and supply.
The preamp assembly connects to the S-shaped Modula adaptor board and which can accommodate mono or stereo (using two Modula preamps). Stereo Modulas allow for split pickup configuration (See A New Breed of Pickups).
I needed to implement real-time, multichannel pitch detection in software using a small ARM Cortex-M4 microcontroller (MCU). My all-time favorite is the STM32F4 family from STMicroelectronics. It has DSP and single precision FPU instructions and can reach up to 225 DMIPS/608 CoreMark at up to 180 MHz operating frequency. Not too bad, actually, especially for this class of MCUs, but it can easily get overwhelmed with complex DSP code we normally take for granted in a desktop or laptop machine with multi-cores running in the GHz range.
I’ve been working on this for quite some time now and I am quite pleased with the results. I now have a fast, accurate, low-latency, phase-correct and efficient multichannel pitch detection. I thought I’d like to share. In case you are wondering, no, it is not for note to MIDI conversion, although that is obviously one application.
Look what we have here! Concentric pots, anyone? We’re developing the next generation Resonant Filter and this might just be the key. Before, we had to use two separate pots to control frequency and resonance, which required more space! With the Modula project, we’ll be using concentric pots, along with a redesigned State Variable Filter E.Q., thanks to recent developments in low-power, low noise, Op Amp technology. A full-frequency response pickup (Nu, XR, Modula) plus a resonant state variable filter can simulate all types of pickups by mimicking their electrical characteristics (Resonant frequency and Q). See Helmuth Lemme’s table: Resonant frequencies of some well-know pickups for various parallel capacitors. With a state variable filter, we can even go beyond the sound of the traditional pickups, for example using very high Q settings. Synthesists know this very well!
Can it get better than this? Oh, I’m sure it will! I’m pretty sure we’ll see continuous improvements in Op Amp performance over the years. It is 2017 and we’ve come a long way since the early days of the Fairchild µA702 op amp, created in 1964 by Dave Talbert and designer Robert Widlar, and then a few years later: the μA741, also from Fairchild —the most popular op-amp of all time. I fondly recall the smell of solder (oh my… lead!) and a drawer full of LM741 DIP ICs as I was learning about these magnificent Op Amps with the Art of Electronics book at my side.
Check out our Op Amp Shootout page for more Op Amps. If you need some background information, I invite you to read the series of articles by Douglas Self (EE Times) Op amps in small-signal audio design (part1, part2, part3, part4).
So… I’m always on the lookout for low-power, low-noise Op Amps. Last time, we had a look at OPA1692. With a 4.2-nV/√Hz noise density and only 650 µA of supply current per channel, it is hard to beat. Alas, it is not available in singles (only duals and quads), which makes it a non-contender for the Modula pickups.
So, the search continues and as I moved outside my typical search area, I discovered a splendid Op Amp from Analog Devices —the ADA4084:
- Rail-to-rail input/output
- Low power: 0.625 mA typical per amplifier at ±15 V
- Gain bandwidth product: 15.9 MHz at AV = 100 typical
- Unity-gain crossover: 9.9 MHz typical
- −3 dB closed-loop bandwidth: 13.9 MHz typical at ±15 V
- Low offset voltage: 100 µV maximum (SOIC)
- Unity-gain stable
- High slew rate: 4.6 V/µs typical
- Low noise: 3.9 nV/√Hz typical at 1 kHz
- Long-term offset voltage drift (10,000 hours): 3 µV typical
- Temperature hysteresis: 4 µV typical
The ADA4084-1 (single), ADA4084-2 (dual), and ADA4084-4 (quad) are single-supply, 10 MHz bandwidth amplifiers featuring rail-to-rail inputs and outputs. They are guaranteed to operate from +3 V to +30 V (or ±1.5 V to ±15 V).
These amplifiers are well suited for single-supply applications requiring both ac and precision dc performance. The combination of wide bandwidth, low noise, and precision makes the ADA4084-1/ADA4084-2/ADA4084-4 useful in a wide variety of applications, including filters and instrumentation.
Wow! 625μA and 3.9 nV/√Hz!!! Is this it? We’ll see… I have a few samples coming and I’ll share our test results.
Aside: At 1mA, you get the ADA4807 3.3-nV/√Hz noise density with vanishingly low distortion (−141 dBc/−144 dBc at 1 kHz). A fair tradeoff, I would say.
This LTD SC-208 began life as an entry level 8-string for those interested in dipping their toe in the “deep end”. The stock setup included passive humbuckers, a 3-way selector, master volume and tone control. By converting to a Flex Set the switching options expanded from 3 to 5, the frequency response was greatly extended, and the guitar shed some weight along the way.
What do you get when you cross the Nu and the XR? The Modula. Our new baby: A full-range, semi-modular pickup that can be used for just about any guitar or bass. Yeah, finally the bass! So far, the results of our tests have been very positive. We applied what we learned over the years from the development of the Nu and the XR and the result is no less than outstanding!
Presenting the Modula
- Modular. Three module types can accommodate all guitars and basses.
- Aesthetically pleasing, sleek, modern design.
- Allows arbitrary placement, either straight or angled.
- Traditional mono or (custom) stereo split configuration.
- Lo-Z full frequency response. The blank canvas.
- Tweakable passive tone-blocks emulates all known pickups.
- Fully balanced, low-noise preamplifier. High headroom Rail-to-Rail output (5v to 18v).
- Dual-coil humbucking, dual rails design for each module. Quad coils for a single coil (Strat-style) pickup. Octa (8) coils for a full-width (PAF-style) pickup.
I’ll post more information as this project evolves. I value feedback. The Nu and the XR projects started from online forums, fueled by feedback from people all over the world. I invite you to come and join our facebook forum. Be involved in the discussions that will shape Modula’s future design and implementation.
Here are two pictures of an early prototype for a six string guitar. There are two modules for each 3-string group, reminiscent of the Fender precision bass pickup. Click to zoom in.