I’m not sure what you are trying to accomplish here. Ideally, the amp output impedance should be less than 1/8 the headphone impedance. I built a box that provided a 3 ohm output impedance that worked well for all my headphones. Output was taken from the speaker terminals of the amp.
A load box like that is to allow you to measure amplifier performance under a known, static, load - not to increase the effective output-impedance when using headphones.
I see - you’re emulating the impedance of specific headphones for measuring, not trying to match the output impedance of an amp to the impedance of a headphone for listening. Makes sense, but what about the frequency-dependent impedance variations that most headphones exhibit?
Plan on using Rotary switch, I thought about building a toggle bank like on old PDP-11 just for fun.
@Torq thank you for answering, work is a little crazy today, getting ready to fly out to see customer in Chicago.
@jnorris For this type of testing, you normally look at fixed-resistance but remember I have a pretty wide swing in resistance that would cover most state you see. Now I could put 475 Ohm resistor for the peak of HE-6xx for the issue you raise, also put in 600 Ohm resistor.
