Sorry to come back to you so late - I was busy with lots of other things like bringing V220 / V281 on its way in the last days.
The information from Thomann, that G109 has an autosensing power supply is definetely wrong.
I should know !
All our headphone amps from the Lake People G 1xx series and the F 3xx series and the Violectric amps own standard transformers because of the sound.
These transformers may be set to 230 OR 115 V AC supply. European models are set to 230 V ex works.
When a unit is set to 115 V AC supply (ex works) this can easly been recognized by a sticker on the back panel reading "115 V AC".
This sticker covers the standard silk screening on the back cover reading "230 V 115 V" which indicates that the unit may be set to 115 V AC (covers the range from 85 to 125 V AC).
There are also auto sensing devices in the Lake People and Violectric production range.
Here the back is marked "90 - 250 V AC" to indicate an autoranging switch-mode supply. See drawings from the F 4xx and F 6xx series on the web.
Why it still sounds so good is easy to explain:
Your G109-S (set to 230 V AC but connected to 110/120 V AC supply) now runs with only half of the intended internal supply voltage.
This will not bother the overall quality of the amps nor the sound under normal conditions - but it will half the max output voltage swing which is noticeable when high impedance headphones are connected. The output power now is only 50% which may violate the performance when extremely hungry low impedance headphones are connected.
Also the max input voltage is reduced to +15 dB instead of +21 dB what is relevant only in a professional environment.
Further hum may appear as parts of the internal power conditioning are not working due to the halfed internal voltage.
If you would have the G109-P model, chances are high to not hear anything because the relay which is responsible for the delayed switch-on will not properly work (or only by chance).
Again, the operation of a European 230 V model in a 110/120 V environment is absolutely safe (!!) and will not bother the sound (or only little) under "normal" circumstances.
However, the full performance can only be achieved when set to the correct AC mains voltage.
If in doubt, contact www.Violectric-USA.com for more details.
Fried Reim (CEO of Lake People / Violectric)