Onward
Hello! We’ve been busy. It’s been a while, but we are inching slowly, but surely, towards production. Let me present the Six Pack production prototype (v1.4). If you have been following this project, you will notice that this is very similar to the Six Pack version 1.1 prototype apart from a few design changes.
Features:
- Six low impedance coils (1300 turns, 350Ω D.C. Impedance, flat frequency response, 20Hz-20kHz).
- Modern, Low power, Low noise OPA2314 differential Op Amps.
- Single supply (1.8 V to 5.5 V).
- Stainless steel height adjustment screws and springs.
- Premium gold-plated, 2 mm pitch header connector.
- Precision 1% thin-film, low TCR, low noise, Yageo and Vishay resistors.
- Panasonic low noise film capacitors.
- Fender Stratocaster Profile (14 mm total height excluding connector).
- EMG style pickup enclosure.
Like before, the design is modular. The Hexaphonic Pickup actually contains three smaller Neo-2 active pickups sitting on a main board.
Logistics and Development
A lot has happened since v1.1. There is actually two more variants —v1.2 and v1.3, as stepping-stones before this latest v1.4 production prototype. Time and attention were given to optimising the design for production, sourcing and searching for suitability and availability of parts and dealing with manufacturers of these parts. It’s an iterative process. For each iteration of the design, we prototype using one of the nice PCB prototyping services, purchase the required parts, request for customised coil samples (or wind the coil ourself), assemble the prototype in-house and finally, test the prototype with the previous prototypes as baseline.
The difficult (expensive and time-consuming!) part is coil customisation. We had to go through several revisions until we finally settled on this version you see now. The turnaround for each iteration was slower than anticipated, but it’s getting better as we iron out the issues related to logistics.
Op Amp
From v1.2, as hinted in the Six Pack version 1.1 article, we started using the micro power OPA2314. This Op Amp consumes 150μA (quiescent current) with a respectable 14 nV/√Hz (measured at 1kHz) noise figure. It is more than adequate for our purpose —there are a number of things in the signal chain with more noise. Compared to the Fender Stratocaster single-coil, the Gibson Les Paul humbucker, and the EMG active pickup, our hex-pickup has by far the lowest noise. The quietest guitar I have here has an EMG 85 and 81 PUs. We’ve successfully surpassed the signal to noise ratio (SNR) of these active humbuckers even at heavy-metal gains. The hum emanating from the EMG is noticeable at heavy-metal gains. The EMG hiss is also noticeable (tolerable and a lot less than the hum actually, but still noticeable). I’ll have more on the noise measurements in a future instalment.
PCB Base
You might notice that each coil sits on its own PCB base that is surface mounted using a technique called castellation. The image at the right shows the individual pickup before assembly. You can see the magnet at the left, the PCB base in the middle and the coil at the right. Notice too that we no longer use the v1.1 thru-hole bobbins. I prefer SMT components over thru-hole. They make production a lot easier.
Discrete Design
I actually tried to use single-ended discrete transistors instead of Op Amps in v1.3. I had a nice BJT design with very low noise and very low power. Much to my amazement, the result was impressive! Still, I stuck to using Op Amps because in the end, the design required more parts (not less!) than the design using Op Amps. Also, and again as I noted before, linearity, superior THD (total harmonic distortion), independence from temperature and more importantly superb PSRR (power supply rejection ratio) are also important factors. Op Amps are definitely superior in these regards.
Reducing Crosstalk
Nice try. The idea sounded good in theory, but produced unsatisfactory results and was ultimately dropped. Here’s what happened…
There were attempts to further reduce cross-talk. We tried all sorts of magnet and coil configurations and magnetic isolation. See images below. First, we tried smaller coils. The smallest bobbin we could find are surface mount devices, which is good for manufacturing. The bobbin has an outside diameter of 6mm and a core diameter of 2.8mm. The idea is that with such a small bobbin, we can place magnetic isolation rings to further reduce crosstalk.
|
|
|
|
|
|
The most significant drawback is the narrow focus of the pickup and its effect on string bending. The signal is very strong as long as the strings are aligned with the magnet poles. String bending brings about uneven response and noticeable signal drops. Lateral string movement also generates a weaker signal compared to vertical string movements, unlike the original pickup design with a wider focus and balanced response. Furthermore, the rings somehow soaks the overall power with marginal improvement in crosstalk reduction.
Moving On… The Neo Series
This article concludes the Six-pack project. I think the design is in a very good shape now. Everything is in place now and so we can proceed to production. Will we be selling these pickups? You bet! There will still be design changes. Most significantly, there will also be a change in direction as we focus more on the Neo series. The Six Pack pickup will therefore morph into Neo6. I will introduce the Neo series very soon. The series will include Neo1, Neo2, Neo6, Neo7 and Neo8. Like before, the final designs will be Open Hardware. We will make everything available: the designs, schematics and all.