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Headphone and Amp Impedance (2011)

nwavguy.blogspot.com

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Re: Headphone and Amp Impedance (2011)

#11

Post by marcosdumay »

michaelrpeskin wrote: post #6
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 learne…

Why would one optimize for maximum power transfer at all? Worse, why would one un-optimize the power source by adding impedance so that it works at maximum capacity? It's much better to just leave the extra capacity there.

That 1/8 rate is a property of our hearing, and so will apply to any system that creates sound. But it's a quite hard target to achieve in large bass speakers, so a lot of systems simply don't.

Re: Headphone and Amp Impedance (2011)

#12

Post by ahartmetz »

michaelrpeskin wrote: post #6
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 learne…

The radio stuff is about "wave impedance", the speaker or headphone stuff is more like a power supply situation at (in RF terms) very low frequencies. You want your power supply to have low impedance so it doesn't sag and keeps tight control over its output voltage.

There must be a mathematical description of these different situations but I can't be bothered to work it out. An important part of it is probably that an audio amplifier has feedback, an RF amplifier just emits a wave and the consumer must make itself compatible with it. Lower output impedance = more effective feedback control for an amplifier with feedback.

Re: Headphone and Amp Impedance (2011)

#13

Post by willis936 »

michaelrpeskin wrote: post #6
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 learne…

That's true for RF, but not for DC. Audio is "DC" because transmission lines are well approximated as ideal wires at frequencies below 100s of kHz.

Re: Headphone and Amp Impedance (2011)

#14

Post by frabert »

kragen wrote: post #10

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 sc…

This is unwanted advice, I know, but here is what I would do: I'd make it so that both the transmitter has a very high output impedance (so that plugging in an headphone causes its output to drop to near zero) and the receiver has very high input impedance (so that it doesn't bog the tx down).

It will cause some noise issues, because a very high impedance front end will pick up stray radio transmissions, but I think it's doable given a small enough bandwidth

Re: Headphone and Amp Impedance (2011)

#15

Post by pclmulqdq »

atoav wrote: post #8
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.

Class A and AB amps often have significant output impedance due to how they are constructed, and used to be the standard. They still are the standard for HiFi audio. The other types of amps are generally pretty low impedance.

Edit: Amp types as pointed out by another commenter.

Re: Headphone and Amp Impedance (2011)

#16

Post by geon »

kragen wrote: post #10

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 sc…

> i want to use 3.5mm jacks for super simple data communications and power

That sounds like an all around bad idea.

Re: Headphone and Amp Impedance (2011)

#17

Post by formerly_proven »

michaelrpeskin wrote: post #6
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 learne…

That's for maximizing the power transfer, which is important in radio applications because you want to maximize SNR (and because this is the condition where no power is reflected).

Power applications (like speakers or the power grid) are generally concerned with maximizing efficiency, which you get by lowering source impedance as much as possible.

Re: Headphone and Amp Impedance (2011)

#18

Post by amelius »

What I want is an electrical equivalent circuit, that models not only the ohmic losses but also the reactance and the output power. For different kinds of headphones and speakers. And then a way to tune the parameters in that model to an actual device.

Re: Headphone and Amp Impedance (2011)

#19

Post by kragen »

frabert wrote: post #14
kragen wrote: post #10

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 sc…

This is unwanted advice, I know, but here is what I would do: I'd make it so that both the transmitter has a very high output impedance (so that plugging in an headphone causes its output to drop to near zero) and the receiver has very high input impedance (so that it doesn't bog the tx down). It will cause some noise issues, because a very high impedance front end will pick up stray radio transmissions, but I think…

just in case this wasn't clear, plugging headphones or other audio equipment into these 3.5mm jacks is not the intention of this design; rather, i want to minimize the chance of destroying expensive equipment if it happens by accident

noise is pretty irrelevant on the power rail. but if i put a high constant impedance on the power rail—say, a 100Ω resistor in front of 5 volts—none of the devices powered from it can draw much power, 25mA being the limit in that case

(for the data line, sure, resistance is not at all futile, resistance is a perfectly good solution, along with some clamping diodes)

what i have in mind is two things:

1. maybe use a trrs connector and use the second sleeve for the power rail? that way the only thing that ever gets plugged into it by accident is a microphone, which i think is unlikely to burn out at 5 volts (even if normally it's not subjected to more than 2.2 volts)

2. also, limit the current with a linear current source. this design simulates as having about 10Ω impedance over the 0–100mA range and a few kilohms once you exceed the current limit: http://tinyurl.com/23qvuylw

Re: Headphone and Amp Impedance (2011)

#20

Post by smolder »

pclmulqdq wrote: post #15
atoav wrote: post #8

Earlier quoted context omitted.

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.

Class A and AB amps often have significant output impedance due to how they are constructed, and used to be the standard. They still are the standard for HiFi audio. The other types of amps are generally pretty low impedance. Edit: Amp types as pointed out by another commenter.

I wouldn't call Class A "standard", unless you're including class AB in that? Pure class A is a bit esoteric since it's wildly inefficient compared to D or even AB.
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