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Headphone and Amp Impedance (2011)

nwavguy.blogspot.com

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Re: Headphone and Amp Impedance (2011)

#31

Post by kazinator »

screcth wrote: post #28

Why don't sources try to measure the frequency response of the driver + cable + headphone circuit and apply a digital prefiltering step to compensate these distortions?

Because it has to be done individually for each headphone. It would require specialized equipment, like a super accurate reference microphone capable of accurately listening to a headphone speaker, in the same way as your ear.

The best thing is for the speaker manufacturer to assume zero ohm output impedance from every amplifier, and for them to do their work to get their speaker to have a close to flat response. (Or whatever: some markets prefer a huge bass.) Then it has that designed response in any such amplifier.

Re: Headphone and Amp Impedance (2011)

#32

Post by instagib »

lelanthran wrote: post #2

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

Smith charts

https://en.m.wikipedia.org/wiki/Smith_chart

You can put an amp 50ohm output into a 75 ohm cable or connectors but there will be negatives. The further away from output impedance the more negatives. Loss, distortion, and worse roll off, etc. Cables are fun too. for much higher frequencies, the less of the actual copper is used. The signal uses the surface of the wiring. For lower frequencies it uses the entire strand/solid copper wire.

RF mostly uses 50ohm but not always. Cable video (tv) is usually 75ohm.

Re: Headphone and Amp Impedance (2011)

#33

Post by kazinator »

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.

Class AB amplifiers (basically class B but with an overlap region when switching between the output devices) have very low output impedance when they feature negative feedback. Which is the standard for hi-fi audio.

Re: Headphone and Amp Impedance (2011)

#35

Post by kazinator »

instagib wrote: post #32
lelanthran wrote: post #2

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

Smith charts https://en.m.wikipedia.org/wiki/Smith_chart You can put an amp 50ohm output into a 75 ohm cable or connectors but there will be negatives. The further away from output impedance the more negatives. Loss, distortion, and worse roll off, etc. Cables are fun too. for much higher frequencies, the less of the actual copper is used. The signal uses the surface of the wiring. For lower frequencies it uses the e…

The 75 ohms is meaningful when there is no reflection (like when we imagine sending a signal into an infinite piece of the cable).

If you are using a short piece of the cable to convey a voltage signal between two pieces of equipment (like the output of one amplifier into the input of another), it has no meaning. The voltage on the other end rises almost instantly and is reflected back.

Re: Headphone and Amp Impedance (2011)

#36

Post by kazinator »

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…

Unlike what other commenters are saying, this Zout == Zin is applicable to audio.

For a fixed Zout much higher than zero, if we want to suck the most power out of the amplifier, we should use a matching Zin indeed. E.g. if an amplifier has 16 ohms output impedance, then we will draw the most power out of it with a 16 ohm load.

Loads higher than 16 ohms will result in less current flowing.

Loads lower than 16 ohms will cause power waste in the source impedance. For instance a zero ohm load will mean that all the power is dissipated in the 16 ohm source impedance: the zero ohm load draws current, but no power because it I^2R is zero.

So from there, if we increase the impedance gradually, we obtain more and more power until we get to 16 ohms, and after that less and less power.

In amplifiers we care about efficiency, not with operating at the theoretical point where absolute maximum power is drawn. With a near zero ohm output, we ensure that all the voltage is dropped by the load rather than wasted in the source.

When we have a near zero ohm output impedance, Zin == Zout still applies! E.g. theoretically, the maximum power transfer from a 0.1 ohm amplifier would take place if we connect it to a 0.1 ohm load. And that reflects the trend in low-output-impedance audio amplifiers: the lower the speaker impedance you plug in, the power power you get: 16 ohms, 8 ohms, 4 ohms, ... You just can't go anywhere near 0.1, due to the practical limitations in the amplifier: ability to deliver current without frying itself.

But, so yes, Zin == Zout is relevant, but in the majority of the audio amplifiers built, which have very low output impedance, that theoretical point occurs at low value of Zout/Zin which usually out of reach of the absolute current delivery capability of the amplifier.

Re: Headphone and Amp Impedance (2011)

#37

Post by Kirby64 »

screcth wrote: post #28

Why don't sources try to measure the frequency response of the driver + cable + headphone circuit and apply a digital prefiltering step to compensate these distortions?

You don’t generally want to compensate, as impedance that isn’t flat doesn’t necessarily mean that the output SPF isn’t flat.

Re: Headphone and Amp Impedance (2011)

#38

Post by semi-extrinsic »

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…

Why not just go with RJ11 (6P4C) instead? The PCB mount connectors are cheap, cables are easily available even from most local electronics stores, up to 10m lengths.

Re: Headphone and Amp Impedance (2011)

#39

Post by shadowpho »

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…

What’s wrong with type c connectors? They are dirt cheap, psu/cable is already done for you, thoroughly tested, and easy to solder(6pin version)

Re: Headphone and Amp Impedance (2011)

#40

Post by boomskats »

For context, the NwAvGuy O2 amp[0] design is pretty famous now. The dude even lays it down right there[1] in the comments of the linked article. And then soon after that iirc he just disappears?

[0]: https://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html [1]: https://nwavguy.blogspot.com/2011/02/headphone-amp-impedance...

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