- 3,134 Posts. Joined 2/2007
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I know this thread is six years old—positively Jurassic—but I do have the definitive answer to the question posed.
I had wondered about the need for impedance matching 600 ohm headphones to 4/8/16 ohm amplifier outputs, a topic of some debate, and that led me here.
This is correct.
This is mostly right, except for your assumption about 8 ohms source impedance, which diverted you from the answer and suggested unusually high impedance headphones.
The T-1 was intended to be used with 4 ohm amp outputs—more common for the time (early 1970s) than the 8 ohms currently used—so the ratio is correct. To recap, it goes by the square root of the ratio between the input/output impedance. So sqrt(156 / 10) = sqrt(15.6) = 3.95, which is essential 4 ohms. This is why you concluded it must be a 1,200 ohm impedance headphone, because as you've doubled the input, so must the output similarly double.
The Koss Catalog circa early 1970s, the piece of the picture you were missing, explains it all:
T-1 Monitoring Adapter
For monitoring tapes from high impedance sources. Adapter contains matching transformers to match 600 to 10,000 ohm outputs down to 4 ohms. 2 output jack for phones, 2 pin-type jacks for inputs.
I hope that answers the question.
And, yes, I own both the catalog and the T-1, although I haven't yet used the T-1. I'm not the original owner of either, in case that matters.