Electrical Characteristics


A couple of questions about the OWL Pedal.

I saw in the schematic that there is no input protection for the codec. Since the amplifier is powered with 5V, is it possible to damage the codec with input signals greater than 3.6Vpp (±0.3V is the acceptable margin for the WM8731)?

And about the power supply, it seems that the USB_VBUS is connected to 5V through a jumper. What happen if I connect both the USB and 9VDC power supplies? Would the USB source be damaged?
This is just for convenience, I can remove the jumper when using it on the pedalboard.

That’s right: external power and USB should not both be connected at the same time. Note that you’re likely to get better audio quality with a decent external PSU.

The WM8731 inputs on the pedal may see signals up to almost 5v, but only momentarily since the inputs are AC coupled. With full amplification on the PGA the input impedance is at least 10k Ohm, and the current will be less than 2uA, which won’t cause damage.

On the OWL Modular which has DC coupled inputs there are diodes to protect against over voltage.

Very hot signals will cause distortion though:
‘The ADC Full Scale input is 1.0V rms… Any voltage greater than full scale will possibly overload the ADC and cause distortion.’

Then I suppose while the signal comes out clean the codec will be safe.

I ask about the power supply because I intend to control the pedal with a Raspberry Pi and Pure Data. Sending modulations to the parameters on the OWL gives a lot of versatility. I actually made a whole synthesizer with the OWL, a Pi and TouchOSC.
For this kind of setups would be nice to have an automatic power selector, something like Arduinos have.

That sounds like a great project. Have you got any video or photos?

It should be possible to add on a small circuit similar to the arduino one that could plug in to JP1 (which connects USB_VBUS to +5v) and a cable soldered to VIN.

Alternatively you could fit a diode across the jumper to prevent current going backwards. This will lead to a small voltage drop on the +5v rail and therefore reduced headroom on the op amps, but this shouldn’t be much of an issue: the op amp is rail to rail and goes to VDD-0.3v.
You will need a Schottky diode with less than 0.5v forward voltage drop (minimum voltage for the 3.3v regulator to work is 4.55v). And minimum forward current of 250mA. 1N5817 has 0.45V Vf so should work under most circumstances.
BAT48 small signal diode looks like a good choice: 350mA continuous forward current, but forward voltage drop goes up to 0.75V at 200mA which is not so good.


I’ve been busy lately, but yes I want to record something with it. Maybe not video but at least some audio clips. I’ll upload the patch when I finish it, it’s a mess right now. I suppose I can upload the Pure Data patch and TouchOSC layout.

The diode solution seems ok. I just saw that Arduinos have a MOSFET driven by a comparator. This gives almost zero voltage drop. Probably it can be done with only one MOSFET. I may try it.

Thanks for your help!