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Headphone and Amp Impedance (2011)

nwavguy.blogspot.com

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Re: Headphone and Amp Impedance (2011)

#21

Post by icegreentea2 »

geon wrote: post #16
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.

It's not the best idea, but it's not the worst idea either. The original Stripe reader used the 3.5mm jack for example. It's even been used in some power applications (https://www.amazon.ca/UY-CHAN-Original-Soldering-Replacement...)

Re: Headphone and Amp Impedance (2011)

#22

Post by kragen »

geon wrote: post #16
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.

it's like a ratburger: it's not good, but hey, it sure is cheap

Re: Headphone and Amp Impedance (2011)

#23

Post by ahdsr »

geon wrote: post #16
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.

ZSA uses this for their keyboards. The moonlander I'm typing this on uses a headphone jack for all of the above [1]

[1] https://www.zsa.io/moonlander/

Re: Headphone and Amp Impedance (2011)

#24

Post by kazinator »

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.

I also never heard of this 50-ohm-1996 thing. Any headphone output worth its salt in the last 70 years is going to have near zero ohm output impedance.

Some headphone amplifiers use cheap integrated circuits that can only drive outputs no lower than around 150 ohms, like the JRC4556 (a device that NavGuy has written quite a bit about, incidentally).

I suspect that's where that idea might come from? If you have a JRC4556 that doesn't go lower than 150 ohms, and the headphones are 32 ohms, you can pad in some series resistance to make up the difference.

Re: Headphone and Amp Impedance (2011)

#25

Post by em3rgent0rdr »

amelius wrote: post #18

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.

SPICE model ideally.

Re: Headphone and Amp Impedance (2011)

#26

Post by screcth »

ahartmetz wrote: post #12
michaelrpeskin wrote: post #6

Earlier quoted context omitted.

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 a…

Analysis becomes much simpler when frequencies are low enough. Wavelength is large compared to the dimensions of the circuit so you can ignore the effects of wave propagation.

Re: Headphone and Amp Impedance (2011)

#27

Post by frabert »

kragen wrote: post #19
frabert wrote: post #14

Earlier quoted context omitted.

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 d…

You could do what USB does and limit the current until the device signals it wants full power!

Re: Headphone and Amp Impedance (2011)

#29

Post by em3rgent0rdr »

geon wrote: post #16
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.

Apple's iPod Shuffle 2G did charging and USB syncing over its 3.5mm TRRS socket.

And for people concerned about accidentally shorting a power line when inserting the plug, hypothetically could use a socket with built in switches so power is only sent once the plug is fully inserted into the socket.

Re: Headphone and Amp Impedance (2011)

#30

Post by kazinator »

This business of damping or lack thereof, is really just a frequency response change.

The concern with the resonance of the speaker is overblown.

When you boost a frequency range electronically with an equalizer, that is also resonance (electronic rather than mechanical).

Instead of your speaker ringing for a couple of cycles, the circuit is doing it.

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