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Understanding output impedance

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Loquah:
I have an amp that normally uses resistors to tame the output in much the same way as (I think) the early SEX did. The result is a Zout of 230 ohm or so. I've created a mod with an L-Pad off the speaker taps so I now have two outputs with varying Zout.

I'm doing some testing for an upcoming review and have discovered that the high Zout of the stock output is not altering the frequency response of most of my low impedance cans, but is affecting all of the moderate impedance (80-120 ohm) ones.

Does the 1/8 rule work in both directions (i.e. is it about the distance between Zout and transducer impedance?) or is it more likely just to do with the nature of the drivers and their own impedance variations by frequency?

Paul Birkeland:
Are you looking closely at the low frequency response of the headphones?  That is where you are most likely to see lumpiness. 

Loquah:

--- Quote from: Paul Birkeland on April 26, 2021, 05:31:58 AM ---Are you looking closely at the low frequency response of the headphones?  That is where you are most likely to see lumpiness.

--- End quote ---

Hi Paul, yes I was expecting significant deviations in bass which I saw in the 80 and 120 ohm headphones, but not in 2-3 32 ohm headphones!?

For example, check out the 80 ohm Utopia measurement with the expected bass changes and then the 62 ohm AKG K712 with basically no change

Paul Birkeland:
If the impedance curve of the lower impedance headphone is super flat, you wouldn't necessarily see a significant change.

Loquah:

--- Quote from: Paul Birkeland on April 26, 2021, 04:13:22 PM ---If the impedance curve of the lower impedance headphone is super flat, you wouldn't necessarily see a significant change.

--- End quote ---

I wondered if that might be the issue. So, to confirm, the 1/8 rule does not work in reverse (i.e. output higher than transducer impedance) and damping would still be affected even if frequency response is not - right? The sound is a bit smoother from the high output socket, but it also has different (and more) components in the signal path.

By the way, I know this isn't why you guys do what you do, but I'll be thanking you publicly in my upcoming review of this amp for the help you've provided me in understanding the technical aspects of some amp design choices.

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