Headphone and Amp Impedance (2011)

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Re: Headphone and Amp Impedance (2011)

#5

Post by squarefoot » Sat, Feb 17, 2024, 7:41 PM UTC

I'm very skeptic of that 1/8th rule. Usually output impedance should be as low as possible, so that the resistance added by the cable becomes negligible in comparison, and its intrinsic capacitance won't turn it into a low pass filter. Unless I'm missing something, at least in audio, amplifiers with lower output impedance are always better than those with higher output impedance.

Re: Headphone and Amp Impedance (2011)

#6

Post by michaelrpeskin » Sat, Feb 17, 2024, 7:41 PM UTC

lelanthran wrote: post #2

Interesting. Does the 1/8th rule apply to matching amplifiers with speakers as well, say for car audio?

Digging deep into my memory here, but I always remember that when Zout==Zin that's what you want for maximum power transfer.

My guess is that the 1/8 trick is for the typical characteristics of small-speaker headset. Big speakers may have different capacitance and inductance characteristics.

I don't know though. That's the first time I heard that approximation, and it doesn't seem to align with anything that I learned in electronics.

Re: Headphone and Amp Impedance (2011)

#7

Post by Kirby64 » Sat, Feb 17, 2024, 7:45 PM UTC

Honestly I don’t know what reasonable headphone output today has these high output impedances. They’re all class D amplifiers with quite low impedance directly to the power supply. This 1/8th rule is kind of pointless… you just specify output voltage drive at various loads and measure frequency response. Most do this very well. No need to faff about with damping of responses.

Re: Headphone and Amp Impedance (2011)

#8

Post by atoav » Sat, Feb 17, 2024, 7:49 PM UTC

Kirby64 wrote: post #7

Honestly I don’t know what reasonable headphone output today has these high output impedances. They’re all class D amplifiers with quite low impedance directly to the power supply. This 1/8th rule is kind of pointless… you just specify output voltage drive at various loads and measure frequency response. Most do this very well. No need to faff about with damping of responses.

Jup, but when the post was written good class D headphone amps weren't as readily available as it is today, if I recall correctly.

Re: Headphone and Amp Impedance (2011)

#9

Post by notfed » Sat, Feb 17, 2024, 7:55 PM UTC

squarefoot wrote: post #5

I'm very skeptic of that 1/8th rule. Usually output impedance should be as low as possible, so that the resistance added by the cable becomes negligible in comparison, and its intrinsic capacitance won't turn it into a low pass filter. Unless I'm missing something, at least in audio, amplifiers with lower output impedance are always better than those with higher output impedance.

> most headphones work best when the output impedance is less than 1/8th the headphone impedance

This doesn't seem to disagree with what you're saying?

Re: Headphone and Amp Impedance (2011)

#10

Post by kragen » Sat, Feb 17, 2024, 7:59 PM UTC

this is great, i've been reading a lot about headphone impedance this week, but from the perspective of 'i want to use 3.5mm jacks for super simple data communications and power for small devices, so how do i keep from burning out headphones if someone accidentally plugs them in'

(also it turns out that these jacks briefly short-circuit as you plug and unplug them)

because 'super simple' excludes power negotiation schemes like usb-c, whatever power i deliver to small devices is also available if someone plugs in a resistor, and earspeakers are pretty similar to resistors at dc, and normally driven at only about a milliwatt

the vast majority of current headphones (and headphone-driving amplifiers) out there today conform to the android specs (though probably approximately 1% of the headphones of interest to nwavguy)

jack: https://source.android.com/docs/core/interaction/accessories...

plug: https://source.android.com/docs/core/interaction/accessories...

relevant appnotes on driving them include david guo's https://www.analog.com/media/en/technical-documentation/appl... and the anonymous https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx...