Pocket Operators (PO) are a range of synthesisers by Teenage Engineering, designed to be small, portable and battery powered. Modern battery technology has far exceeded the AA form factor that the PO supports, but to utilise this an external power supply is required. Enter PO-PSU.
The objective is to take the 5V supply from a USB and use it to power one or more PO. By using USB, a variety of different power sources can be used, specifically power banks to keep the PO system portable. A secondary objective is to not modify the PO in case it has other owners in the future!
This project is a derivative of a board I saw elsewhere online. That project was not open source so I decided to make my own version and open source it.
This project is not in any way affiliated with Teenage Engineering, but I am using their nuts and bolt design without permission because it looks fucking cool.
Output current (max)
The Pocket Operators were measured to find their current draw, which is around 30 mA RMS.
I'm signing this project off as finished as I don't intend on doing another board revision, the second revision accomplishes the design goals. The remaining work I intend on doing is writing up about points of interest, and ensuring the BOM and CAD files are up to date.
Here are a couple of close-ups of the second revision:
I used exposed copper on the bottom to replicate the pocket operator visuals.
Two versions of the hardware exist: one with burst mode enabled and the other using traditional pulse skipping. R4 enables pulse skipping, R5 enables burst mode (stuffing both leads to a damaged USB port).
Using the first round of builds (minus the USB jack and with a couple of mod wires attached) the project has now been tested on some dummy loads.
Oscilloscope on output
Bench supply powering the board
100 Ω resistors mimicking a PO (V=IR, 3=0.03*R, R=100Ω)
With no load, there is 140 mV ripple current in the output.
Emulating one PO
100 Ω (30 mA)
Bench supply current
(2.98^2/100)/(5*0.02) = 88.8 %
Emulating three POs
3 x 100 Ω in parallel (90 mA)
Bench supply current
((2.91^2)/(100/3))/(5*0.058)= 87.6 %
This shows that PO-PSU can generate enough current for three POs without dropping the voltage significantly. The efficiency numbers seem good, although they are very approximate as I didn't measure the value of the resistors so they could be ±10%. The device is currently configured in burst-mode which offers better efficiency at low output currents, but it may be worth testing the device in pulse-skipping mode to see if there is a large loss in efficiency. Pulse-skipping mode is stated in the datasheet to offer better output ripple so would be nice to reduce the noise transmitted to the PO.
Just an update on the inspiration: the link to the product I copied can be found here. I copied more than I remembered, all the way down to the name! However I stand by the purpose of this project, to open source a design (and also to make a design that doesn't require modifying the PO).
Further design analysis looks like the voltage converter is an LM317 or something similar. If this is the case (big if) then there are numerous possible advantages:
Using a switch-mode regulator can offer improved efficiency.
Using an LDO means that the small voltage drop from 5V to 3V wouldn't cause any problems, which may be using a linear regulator.
The part (LTC3405) I am using instead does have several negative aspects though:
Only 300mA output current as opposed to over an amp. This doesn't matter significantly for two reasons: USB is spec'd to supply 500mA so we're not losing much here, and also the PO only draws 30 mA so this would be enough to supply many (where other issues like noise would become a bigger issue).
More expensive. There is no way around this, the LTC3405 is almost $4, and $1 on the output inductor. To me this is a benefit though for efficiency and improved functionality. I like to have good accessories for my musical instruments.
Specifically, the LTC3405 has a mode designed for operating at very low current draw which is attractive as there may not always be a PO connected. Further, the designed as it is at the moment disables the LDO if there is no jack present in the DC connector. Both of these prevent wasting energy.
The first prototype run of boards has arrived from OSH Park! Looking very pretty.
The parts arrived from Digikey last week so all I need is some time to build it up. Testing the through-hole components in their places reveal the DC jack to fit very loosely as KiCAD doesn't support slotted plated holes. As well as this, the micro USB doesn't fit the front holes and will have to have the pins bent into place. I suppose this is why we do prototypes!