How are modern "Headphones" output circuitry designed?

[VinzC]

Hi all.

While fiddling with audio devices I've observed a couple of strange things. I have a Kenwood auto-radio with an AUX input (on the front) that is meant for plugging an iPod or some other headphones ouput. I tested a the headphones output of tablet that I connected to the AUX input of the auto-radio. Oddly enough I had to set the audio volume on the [Android] tablet AND the auto-radio to the maximum to hear something reasonably loud to my ears. Now if I wanted to listen to the FM band, I had to turn down the volume knob down to something like 8 or 10, the maximum being 35.

I hear you: «but you used an attenuated input!» Right. So I've plugged the tablet headphones output to the RCA input channel on the rear of the auto-radio... same thing. Well, not exactly: the audio volume was juuuuuuuust a slight, tiny little bit louder. I also tried with a recently bought television that has a headphones output... same thing.

I remember if you did that 20 years ago you would blow up the input circuitry of your amplifier. And most certainly you'd fry your speakers. So I tried with the headphones output of a radio bought several decades ago aaaand... surprise! it was impossible to stand the sound with the volume knob of that oldie set to more than a quarter of its full range!

It buzzes me that I nowadays need to amplify the signal coming out of a headphones output before I can shove it into the LINE IN of an auto-radio — I'd have expected to attenuate it! That problem has been acknowledged by a commercial audio reseller, who happens to work also with regional DJ's.

It boils down to my question: how are "modern" headphones designed? Since I know the Intel HDA chip (for instance) can sense the connected audio devices, I suspect there's some impedance sensing of the device that is plugged in and if a high impedance load is sensed then the allowed maximum volume is drastically reduced, you know to protect us, dummies from being deaf. Am I following the right rabbit?

Thanks in advance.

[Fungus]

It boils down to my question: how are "modern" headphones designed?

a) With much better magnets than in the 1970s.

b) With power consumption in mind (almost everything is battery powered these days)

[dmills]

Also, with much lower impedances, so they can be driven to a reasonable level with the available voltage swing from a battery without needing a boost converter.

Old school headphones were usually somewhere in the 64 - 600 Ohm region, some even higher, modern ones tend to be much lower impedance and so need far less voltage to produce a reasonable amount of acoustic power.

The 1970s stuff used power rails of anything from 12 - 30V, so swinging 2V or so peak for a line level input was no big thing, modern stuff is running less then 4V rails a lot of the time, so 2V peak (4V pk-pk) is not easy.

50mW into 32 ohms is 1.3V RMS = 1.8V peak which is pretty much the limit for a ~4V nominal rail, and 50mW is plenty into most cans.
50mW into 600 ohms is 7.75V peak which means you are looking for a 15V rail at a minimum.

Regards, Dan.

[tszaboo]

Usually the end of anything nowadays, if there is even an extra circuit, is a single chip. It is an amplifier, volume control, power supply, everything. DC blocking capacitors are a thing of past, they have dual rail. The negative rail is made by a tiny switched capacitor DC-DC. They perform awful usually. They also have sometimes a bunch of analog switches on the output, since some retard probably at apple designed two different pinout for the TRRS connector.

[VinzC]

Thanks Fungus, dmills & al. I understand my guess was wrong and impedance detection is useless. That said, just to be sure, my understanding is that line levels (as of Wikipedia) typically are 1V rms at 0dB for consumer electronics. If that is true — and I see no reason to doubt — it means that the voltage swing of the headphones/line outputs I used were much less than that, i.e. about a fourth in power (I felt the ratio was four times in acoustic power), which would suggest less than 0.5V rms at 0dB. Is that plausible? I mean does that make sense for a tablet (that wouldn't bother me that much) and a television device that is powered by the mains — or at least can use much more than the limiting 5V described earlier?

[NiHaoMike]

That seems about right for an output stage running from 1.8V.