Update: This project has been superseded by the Neo Series Modular Polyphonic Pickups. Same technology in a modular form. The Neos are the culmination of the Six-pack project.
Here’s the V1.1 prototype based on the V1.0 design and specification. The version is bumped to 1.1 since there are slight modifications from the original 1.0 design. If you haven’t seen the original design specifications yet, it would be good to do so now.
The smallest active magnetic pickup in the world? 😉 Here’s the Neo 1.1 prototype.
We got the PCBs from Seeed Studio’s Fusion PCB Service. Seeed Studio is an Open Hardware facilitation company based in Shenzhen, China. Companies like Seeed are rising in prominence, evidence of the emerging Open Hardware revolution spurred by the very active DIY community. We hand-solder the SMD components for all our prototypes. We get our components from Digikey, Mouser and Element14. The resistors are 0.1% thin film low noise resistors (Vishay). The Op Amps are high performance, ultra low noise OPA2209 (dual) from Texas Instruments.
Above, is the fully assembled and working Neo-2 pickup. The coils are the first batch of custom prototypes from Chipsen Electronics, a company specializing on inductors. We asked Chipsen to customize the coils for us by replacing the ferrite core with neodymium magnets. We get the magnets elsewhere and send them to Chipsen.
For this prototype, the coils are vacuum potted with paraffin wax. Vacuum potting ensures that there are no bubbles, voids or any air pockets in the coil. For production, wax will be replaced with either clear urethane or epoxy to make sure that the potted coils can withstand manufacturing.
The coils are both wound clockwise, with the two Neodymium magnetic north facing up. This deviates from the V1.0 specification with alternating clockwise and counterclockwise coils and opposing north, and south orientation. This is no longer necessary. Originally, the plan is to take advantage of the multiple coils and have the coils alternate from north-south pole orientation, clockwise winding for the 1st, 3rd and 5th coils and south-north pole orientation, counterclockwise winding for the 2nd, 4th and 6th coils. That way, the sum of the pickup outputs will cancel some more of the noise the same way humbuckers do.
Well, it does not serve our purpose. The intent has always been to have independent processing for each of the coils. Unfortunately, this scheme only works if all coils are arithmetically summed into a monophonic output. But that defeats our purpose.
Basically, we have something like the representation below, where A…F are the individual signals, and N is the noise component (assuming the PUs are perfectly matched):
- A + N
- B – N
- C + N
- D – N
- E + N
- F – N
I thought that I could extract only N and de-noise the individual signals before any further processing. I was wrong. There’s no way to do that. It is only possible to get N alone if I have another void coil that picks up just the noise. That void coil can be placed elsewhere (even outside the pickup itself). By subtracting the signal from the void coil from each of the outputs A…F, we can have something like:
- A + N – N
- B + N – N
- C + N – N
- D + N – N
- E + N – N
- F + N – N
We have noise cancellation for the individual coils.
The main board below contains the biasing electronics, allowing the Op Amps to operate on a single supply with a VR voltage reference at ½ the supply voltage. The OPA209 can operate from +4.5V to +36V, the ground is lifted and the outputs are pseudo-differential with VR as the common reference point. The output connector is a 1 mm pitch micro-miniature connector with positive lock and gold plated contacts (from Hirose).
Here’s the complete assembly with three Neo-2 on the main board.
Future Directions: Towards 1.2
There will be some more changes coming in version 1.2, which hopefully will be the final prototype before manufacturing. First, the connector will be moved towards the right as it is presented in the V1.0 design and specification. Second, instead of the OPA2209, we will be using the micro power OPA2314. This Op Amp consumes 150μA (quiescent current) with a respectable 14 nV/√Hz (measured at 1kHz) noise figure. While that is 6 times more than the ultra-low noise OPA2314 (with 2 nV/√Hz noise), it is more than adequate for our purpose. For example, we used the LME49726 Op Amp with a 15 nV/√Hz (measured at 1kHz) noise figure and using noisier 10kΩ and 100kΩ resistors in our original noise tests. Yet, the tests show that the self-noise is already quite impressive.
I learn quite a bit from the forums. In the Hexaphonic Pickup Project thread, in the music-electronics-forum, Steve Conner questioned the use of Op Amps:
This has probably been covered before, but why use differential amps? You use more components, pay a 3dB noise penalty, and the only advantage is rejection of noise picked up capacitively. A differential amp won’t reject magnetically induced noise.
What ensued after Steve’s question is a very interesting and lively discussion on noise, electrostatic noise vs. electromagnetic noise, preamplifiers, Op Amps, discrete vs. OP Amps, BJTs vs. FETs and even vacuum tubes vs. solid state. Definitely worth reading.
Op Amps Noise
David Schwab noted here that EMG pickups have the two coils connected to the op amp in a differential manner, in this case an LM4250 (disregard what the schematic says). Schematic diagram below:
What is interesting in the schematic diagram is EMG’s decision to use the LM4250. The LM4250 is a programmable Op Amp. A single external master bias current setting resistor programs the input bias current, quiescent power consumption, etc. It is ultra low power (50-100 μA), a very desirable trait, but the tradeoff is that it is quite noisy at 40 nV/√Hz. I was concerned that would amount to a lot of hiss when the gain is cranked up in subsequent stages. Yet, we also know that EMG pickups are a popular choice for shred and heavy metal guitar players, with distortion pedals and high gain amps set at 11. David comments: “Where’s the noise?”
Oh it’s there, I’m sure. I think now that it just does not matter that much. There are lots of things in the signal chain with more noise anyway. Not to mention that 50s style high impedance pickups are very noisy to begin with (even EMG still uses high impedance coils). With that in mind, I decided to switch to the modern OPA2314. It is superior to the LM4250 in terms of input-noise while still consuming very minimal current.
Single-ended vs. Differential Op Amps
Steve Conner suggested using single-ended amps, plus a Faraday shield for the pickup. It might be enough just to wire the coils so that the outer windings are grounded, then they will help to shield the inner ones.
Following EMG’s lead, I still think an Op Amp is still the better choice for the preamp. With a single-ended, single-stage, FET or BJT design, I am trading off a lot more (than common mode rejection). There’s also linearity, superior THD (total harmonic distortion), independence from temperature and more importantly superb PSRR (power supply rejection ratio). Good PSSR is important considering that I may be powering the pickup through USB. These can be mitigated to some degree, but you’ll end up with a complex discrete design. I think I’ll stick to using Op Amps.
And one more thing…
The Void Coil
I might be adding an extra void coil to the main board (coil facing down). The void pickup will have the same winding as the other coils but this time with a ferrite core instead of a neodymium magnet —it is a plain inductor. Its purpose is to pick up the magnetically induced noise. The noise will then be sent to the differential Op Amps for noise cancellation.
If we are going this route, then an Op Amp is certainly a must. So there you go, a design based on Op Amps also gives you the most flexibility.
Enclosure and EMI shielding
As for the EMI shield. I have two choices. First, Chipsen can have the coils shielded with copper tape. Second, the pickup enclosure can be shielded in the inside with conductive EMI shielding. I think shielding the enclosure should be sufficient.
David Schwab suggested using standard, off the shelf, EMG style pickup covers. We found a couple of vendors and manufacturers and we are in contact with at least one manufacturer willing to produce the enclosures for us with EMI shielding.
I’ll have a proper acknowledgment section, but for now allow me to give proper thanks to the smart folks at the Music Electronics Forum. Many thanks to Steve Conner, David Schwab, Mark Hammer, Mike Sulzer and Ralph Jude Barthine (rjb). Hey, you guys rock!