Amplifier Load Box Project

Well, this is the start of my Headphone Amplifier load box project, Mouser just delivered all the parts.

Will have an 8 load values

  • 12 Ohms Aeon Flow Open (13 Ohms)
  • 25 ohms Ananda ( 25 Ohms)
  • 30 Ohms. Rad-0 (29 Ohms)
  • 40 Ohms Close to Arya( 41 Ohms) & HEDDphone (42 Ohms)
  • 75 Ohm Close to ELEX (80 Ohms) & Utopia ( 80 Ohms)
  • 120 Ohms. ( used in SPL Phonitor x mesurements)
  • 200 Ohms LCD -4
  • 300 Ohms Vertie & HD-6xx


  • 4 wire XLR
  • 6.5 mm Stereo Headphone Jack
  • 3.5 mm Stereo Headphone Jack
  • two BNC outs for test equipment

Needed to 2 x 1 pole 10 selection pots to do selection for left and right load channels

The case i am going to use


Very cool.

This will be a fun thread to follow. :+1:t4:


Looks like an awesome project, looking forward to your updates!


Nice, but box is fugly, my sg. is Hammond 1455N1602.

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?

Nice project.
I got something similar for general use in electronics projects.

Will you switch by replugging or by rotary switch?

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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.

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Toggle bank sounds like a good (and expandable) setup.
Circuit would be like so: