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May 08, 2021, 06:27:31 AM

Author Topic: Understanding output impedance  (Read 150 times)

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Offline Loquah

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Understanding output impedance
« on: April 25, 2021, 08:09:16 PM »
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?
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Online Paul Birkeland

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Re: Understanding output impedance
« Reply #1 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. 
Paul "PB" Birkeland

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Offline Loquah

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Re: Understanding output impedance
« Reply #2 on: April 26, 2021, 03:50:31 PM »
Are you looking closely at the low frequency response of the headphones?  That is where you are most likely to see lumpiness.

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
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Online Paul Birkeland

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Re: Understanding output impedance
« Reply #3 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.
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Offline Loquah

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Re: Understanding output impedance
« Reply #4 on: April 26, 2021, 06:04:09 PM »
If the impedance curve of the lower impedance headphone is super flat, you wouldn't necessarily see a significant change.

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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Online Paul Birkeland

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Re: Understanding output impedance
« Reply #5 on: April 26, 2021, 06:18:22 PM »
You can do the calculations if you like.  Suppose you have 1V on the amp side of the series resistor going to the headphones.  Find the highest impedance in the impedance curve and the lowest impedance in the curve, then you can fill those in as the bottom leg in the voltage divider and calculate the voltage appearing at the headphones, then take 20*log(Vmax/Vmin) to find how much deviation you should have.

The amp you were looking at I think had a voltage divider at the output also, so it may be helpful to start there and work all the way to the headphone driver.

Paul "PB" Birkeland

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Offline Loquah

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Re: Understanding output impedance
« Reply #6 on: April 26, 2021, 06:43:54 PM »
You can do the calculations if you like.  Suppose you have 1V on the amp side of the series resistor going to the headphones.  Find the highest impedance in the impedance curve and the lowest impedance in the curve, then you can fill those in as the bottom leg in the voltage divider and calculate the voltage appearing at the headphones, then take 20*log(Vmax/Vmin) to find how much deviation you should have.

The amp you were looking at I think had a voltage divider at the output also, so it may be helpful to start there and work all the way to the headphone driver.

Thanks Paul. You're correct about the amp having both a permanent resistor and also a variable voltage divider. I've applied a really nifty mod (thanks to the input from you and PJ) that bypasses the whole convoluted output circuit and it sounds much more transparent to my ears so I'm just filling in some knowledge gaps now. I was taking measurements to show why the high output impedance was a problem and was surprised at what I saw on a couple of the low impedance cans like the Meze 99 Neo and Ollo S4X & S4R. At first I thought Ollo had some clever impedance matching tech built-in to account for different mixing desks given their focus as a mastering headphone, but then the 99 Neos and K712 kind of ruined that theory.

At this point I think I'm comfortable just discussing the inconsistent results people will get from high output impedances with a couple of measurements and leave it at that. Thank you very much for your input though!!
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