Any questions of safety are actually quite simple - but they can still be confusing.
The power going to the headphones is coming from the output of a very powerful amplifier designed to power speakers.
With the jumpers OUT, there is a 220 Ohm resistor connected between the output of the amplifier and the headphone.
With the jumpers IN, the output of the amp is connected directly to the headphones.
First off, with any headphones whose manufacturer recommends connecting them directly to a speaker amplifier, JUMPERS IN is the best option, and is perfectly safe.
With the JUMPERS IN you are connecting the headphones directly to the amplifier - as recommended.
(Of course, you do still need to exercise some care.... it would be possible to burn out any headphone if you turn the amp up too far - but your ears will probably be bleeding long before then.)
With other headphones, how the jumpers act will depend on the impedance of the headphones.
For example, if you have 300 Ohm headphones, the 220 Ohm resistor reduces the voltage reaching the headphones by about 40%.
These headphones require a relatively high drive voltage, so they'll work well on the A-100, and they'll only be a little bit louder if you install the jumpers (something like 2 dB louder).
HOWEVER, the lower the impedance of your headphones, the more effect the resistor will have.
For example, with 62 Ohm headphones...... the resistor in the A-100 will have a much more significant effect.
With 62 Ohm headphones, and with the A-100 playing at the same loudness, the headphones will receive
16 TIMES as much power with the JUMPERS IN as with the JUMPERS OUT.
This isn't specifically dangerous, but it means that you will have to set the volume on the A-100 MUCH lower with the jumpers in to reach similar loudness.
It also means that, if your 60 Ohm headphones were playing loudly without the jumper, by putting in the jumper you might risk burning them out - or hurting your ears.
It also means that, depending on the level of your signal source, and the efficiency of your headphones, the noise floor of the A-100 might be audible, and you might simply have trouble adjusting the volume control carefully enough.
The main danger is that, WITH THE JUMPERS IN, if you set the A-100 to play at a comfortable level with high impedance headphones, and you then plug in low impedance ones without adjusting anything, they will be VERY LOUD.
In your case, the A-100 will work great with your k340's with the JUMPERS IN (and probably won't work very well with the jumpers out).
It will also work great with your K712's with the jumper OUT.
However, with the jumpers IN, you might notice some noise with the K712's, and you may barely be able to move the volume control before they get really loud.
But the big DANGER is that, if you have the JUMPERS IN, and you're playing the A-100 with the k340's, then you plug in the k712's without remembering to turn the A-100 all the way down first, you may fry the K712's.
Hello KeithEmo, I think this product is fantastic, how safe is it with the akg k712 pro? (62 ohms), taking into account that the jumpers would be connected. I have a Akg k340 electrostatic-dynamic, with 400 ohms, according to the same AKG, were recommend connected directly to a speaker amplifier, I am very curious how they can sound on the A-100.
I'm sorry for my bad English.
Greetings.