Mixer PSU - Two
Power In
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Power In 〰️
A little hand solder job on a USB-C receptacle. VBUS and GND are tapped off with small wires, while CC1 & CC2 are tied down with 6k8 resistors.
So I after concluding the last post, I now know what specs this new power supply has to deliver. Everything upstream of these outputs is tabula rasa and ready to be imprinted on. The original mixer input was an odd “10V AC 500mA” requiring a unique power brick and barrel jack; so how would I seek to improve this?
I really don’t want to have to use a device specific external power supply. Something where the user has to keep track of the power brick and know that it is for this device and this device alone. I must have a dozen small power supplies for various pieces of audio equipment; 2.1mm barrel jack 6V DC centre negative for example, is far too specific for that power supply to see any other use. In my mind it creates e-waste and is incredibly clunky as a solution.
This should bring you physical pain. On top of the size and shape of adapter, one must consider its polarity and voltage. Even if two power supplies are identical in those specs, plugging a higher power-draw device into a lower-rated charger is going to cause issues. Its a minefield for the user experience.
So what generic power standards are there? Ignoring the ugly barrel jack, line AC and USB seem the only two left. Bringing AC line straight into the unit itself isn’t awful - kettle or figure-eight leads are both ubiquitous and there is an appeal in having no external power supply. The downside is now there is 230VAC inside the metal box that a user continually touches, stressful from a safety standpoint (what if it gets wet?). USB power supplies and their cables are common as muck and USB-C is fast becoming somewhat of a real universal standard. However not all USB ports are built equal, with power delivery specs ranging from 2.5W to 240W for an identical looking connector. In addition, the connectors themselves are very consumer grade and can be quite flimsy sometimes. This is not a great mix as pro-audio equipment is often used heavy-handedly.
So USB-C Then
It does seem like a bit of a waste to bring 230VAC at up to 13A into the mixer. The bigger cables, higher voltages, etc. and for what? This device certainly won’t make use of the kilowatts of power available to it. Also I just think it would be a bit more fun and fitting for this to all be USB powered - and thats good enough reason for me.
Last post I looked at the power rails inside the old PSU and came up with some specs for this new design. The total power requirement was about 7.5W across the three rails. Thankfully the most basic USB-C power delivery spec (“Profile 1”) delivers 15W; 5V at 3A. Any power draw above this requires a bit of negotiation from device to host, a back and forth requesting and (hopefully!) being provided with higher voltages and currents. The allure of Profile 1 is its convenience and ubiquity; it supposedly only needs two pulldown resistors to request the full 3A, and pretty much all USB-C power supplies should be able to provide it.
Before I get ahead of myself lets make sure.
I get a USB-C receptacle and tack the required pull-down resistors onto pins CC1 & CC2. This should indicate to an upstream host that the device is going to want up to 3A at the standard 5V. And thats it. I tack two more wires onto VBUS and GND and hook it up to an electronic load.
USB-C receptacle with a truly minimum circuit built around it, connected to an electronic load. Here the load is off, and shows the idle voltage on VBUS of 5V and change.
Happily, when plugged into the first USB-C power supply I could find, the electronic load quickly flashes up just over 5V. Slightly higher than 5V under no load is typical; the USB spec knows that there will be voltage sag under load and permits up to +10% voltage tolerance for this reason. I start small - sinking 100mA and slowly increasing to 1.5A and beyond. The mock power supply cruises through these steps, exceeding the maximum current one would expect from the old “high-power” and “battery-charging” specs.
The (almost) 15W of the (somewhat) promised land of USB-C Power Delivery Profile 1
The aim of 5V @ 3A was handily met, with the supply kindly providing 3.1A when it was asked. Another 100mA beyond that was a bridge too far however; the supply would go into an overcurrent protect mode and withdraw the sweet 5V nectar. Yes the electronic load reads roughly 4.3V indicating a voltage sag of close to a volt under load. I’ll attribute this to the resistance of the tiny wires and long cables I was using without hesitation - it’s very believable that these could reach 300mΩ total resistance. Yes I should have done a four wire measurement with a separate multimeter to make sure.
Thats the input decided on, to go alongside the output rails needed. Next up will be the circuit design itself; no doubt using some switching regulators to handle the transmutation of a lowly 5V to higher and negative voltages.
