Headphone Limiter Simple

[Jbliss]

Looking for a simple Headphone Limiter for some 60ohm headphone. Something simple using Zener's. How do I work out what zener's and resistors to use to work with the impedance. I would prefer if it was a pretty hard limiter.

Thanks

[linux-works]

empiracally, I would say.  probably not the answer you are looking for.

back to back zeners would limit and you would want a series R before that, of course.

but there are other ways to do limiters.  AGC is one.  some volume controls can have a soft clip style (LDR is one that I'm told can have a knee and will soft-clip as you get close to a threshold).

there's also audio compressors, of course.

the zener trick is the harshest one, though.  you sure you really like that method?

[Jbliss]

Will the zener method change the sound of the headphones when there not compressing?

[rstofer]

The zeners aren't ever going to compress, they are going to limit in a very harsh manner.  But they shouldn't effect anything as long as the voltage is below their threshold.  I have never measured the voltage on headphones so I have no idea what value zeners will be required.

Compander circuits are generally used to restrain volume and often need some kind of non-linear element like an incandescent lamp.  Here is one using a MOSFET for the gain control:

http://www.electroschematics.com/9400/audio-compressor-agc/


Google for compander or automatic gain control to find other circuits.

[linux-works]

for radio communications, we did use back to back diodes (not zeners) for ham radio.  a burst of static could be really loud and it made sense to hard-limit that.  we didn't care about sound quality; but we wanted ear-blast protection

if this is for that kind of use-case, diodes work well.  regular signal diodes and a series R; wire up a pot for the R and vary it until you get the clipping behavior you like at the threshold you like.

if its for music or non-radio style sound, I'd prefer a compressor that kicks in once it hits the threshold.  go to a pro audio/music store and see what they have.

years ago, there was a small semi-affordable box called RNC (really nice compressor).  it was back when USENET still mattered and was on rec.audio.pro, iirc.  maybe it (or something like it) is still around.

and don't forget, computers are now small and fast and fanless; maybe even a rasp pi doing audio in/out filtering could be fast enough to be a nice soft limiter in PURE software.

[Jbliss]

Thanks everyone. I would like to try the back to diode way just to see how harsh it is and as a bit of fun / learning. What equation / how would i choose the values of the diodes and series resistor. Has any one got a simple schematic with values on it.


Thanks Josh 

[linux-works]

diodes will clip at .6v, approx.

those will be in parallel to your phones.  your phones may be 8ohms or as high as 600ohms.  it varies based on model and design.

your source voltage and Z is also an unknown.

like I said, you can ask for math all you want,  but in the end, buying a pot, varying it while listening IS going to get you the fastest and most real answer

once you get what you want, remove the pot, measure the value and put a fixed R in there.  carbon, not metal film, as carbon fails as an open and metal fails as a short, which you won't want.

[rstofer]

As mentioned just above, we need to know the impedance of the headphones (high, like 2k ohms or low, like 8 ohms) and the output impedance of the source.  Again, high or low...

Here is a document that shows both setups and the high impedance headphone circuit is trivial.  Unfortunately, the low impedance setup needs a couple of matching transformers.

http://www.wa0itp.com/audiolimiter.pdf

This stuff is all over Google...  Search for 'diode headphone limiter' among other things.

[linux-works]

http://www.diyaudio.com/forums/class-d/210466-ldr-limiter-ta2020-ta2024-spirited-playback-without-clipping.html

that may be a helpful idea.  maybe.