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-   -   Limiter circuit for Headphone amplifier wanted (http://www.diyaudio.com/forums/headphone-systems/208319-limiter-circuit-headphone-amplifier-wanted.html)

diy didi 7th March 2012 08:47 AM

Limiter circuit for Headphone amplifier wanted
 
Hi all.
Does anyone have a design for a limiter for my headphone amp. I built my amp using single supply 12v.
Regards.

Chris Daly 7th March 2012 09:03 AM

Quote:

Originally Posted by diy didi (Post 2936591)
Hi all.
Does anyone have a design for a limiter for my headphone amp. I built my amp using single supply 12v.
Regards.

Hi
Are you needing compression to reduce peaks , or just volume reduction ? If volume reduction then a simple L pad. If compression the usual method is to introduce limiting
that also preserves audio presentation. i like the use of very small voltage globes as they do not mess around with sound at all, but provide small amounts of limiting, the lower the voltage globe the better .

Here is a link discussing limiters and headphones:
HeadWize - Project: Designing A Limiter For Headphone Amplifiers (A HeadWize Design Paper)

Cheers / Chris

diy didi 7th March 2012 09:17 AM

Hi Chris.
It is for when something goes wrong on stage and or I forget to turn my volume down at plug-in. Needs to limit loud peaks and have an adjustable
threshold control.

Chris Daly 7th March 2012 10:05 AM

Quote:

Originally Posted by diy didi (Post 2936612)
Hi Chris.
It is for when something goes wrong on stage and or I forget to turn my volume down at plug-in. Needs to limit loud peaks and have an adjustable
threshold control.

Hi diy didi
A possible circuit using a nat Semi LM324 suiting single supply, it is a 4 channel op amp so you could build facility for two stereo headphones and use and the LED limiter in Figure 4 of the Headwize article across each channel and each headphone set. The gain is R2/R1 so keep resistors the same ohms if unity gain is required

Also or just, In fact try this FIRST use a pre limiter using back to back Red LED's subtitute zeners for 3 mm Red Leds in Figure 5 (a) as it will not interfere until the audio level is sufficient to conduct the LED region voltage, of 1.7v could be a wise choice. You would only need this on the L and R once on the input.

For even lower voltage protection you could use transistors connected as diodes, but voltage may just be too low then and may limit too early.

Cheers / Chris

diy didi 8th March 2012 06:51 AM

Chris, wont any form of diode type clipping cause severe distortion when reaching threshold level of diode?

Chris Daly 8th March 2012 07:00 AM

Quote:

Originally Posted by diy didi (Post 2937889)
Chris, wont any form of diode type clipping cause severe distortion when reaching threshold level of diode?

Yes, but the idea is that level is too high for listening anyway, so therefore clips to protect on fault conditions, rather than clips to annoy during normal listening. ;)

So the method would be to choose the type of diode or LED observing conduction voltage to pass normal audio, but rapidly protect when faults occur.

Cheers / Chris

m00dawg 17th February 2013 03:50 PM

1 Attachment(s)
Sorry to resurrect and old thread, but I'm trying to build something similar into a headphone amp to function as an in-ear stage monitor solution (IEM). Where I am getting stuck is where the limiter should go. The docs on Headwize have it after the volume pot or even after the op-amp depending on which article I'm looking at.

But if I want a hard limiter to protect me from things like feedback or dropped mics, shouldn't the limiter go before the volume pot? Then I can tune it to the expected nominal levels from my mixer so that the limiter kicks in if that source is too loud (regardless of my headphone volume).

Another potential issue is tying the output to L and R as the input will be mono. If I understand impedance correctly, that means I'll halve the impedance (to about 12 ohms for the in ears I plan on using). I'm using a TLE2426 so as long as I don't exceed it's power rating, I should be good there. Anything else I should be mindful of when splitting the output?

JMFahey 17th February 2013 04:47 PM

You must first define what you want it to do.

1) do you want to protect your earphones from destruction and you from deafness?
Meaning it will never act except on accidents?
Then limit *after* the volume pot, or at the power amp itself.

2) do you want to set a limit level which will be reached often, and then set said level to some user adjustable comfortable volume?
Then limit before the volume control, as you show in this schematic, but in that case I warn you that you need to add some resistor (say, 1K to 4K7) between the first Op Amp output and those clipping diodes.

Personally I'd go for option #1, but of course it's your choice.

m00dawg 17th February 2013 06:12 PM

Doh you're right I am missing the resistor 4k7 resistor. That was by mistake, although it's function is a bit unclear to me. In circuit simulations, it seems needed - just not sure why? Is it for impedance matching or?

Indeed the purpose is to protect against ear damaging noises and deafness. With regards to #1, that basically means there is a maximum volume attainable before limiting, yes? That means I have to calibrate the limiter to both my headphones, sensible dBu, and my listening preferences? I guess that's probably more wise - I was just worried if I set the limiting too low and needed more volume, say, in an actual show, but have no way to fix it (I was going to make the limit adjustments internal so they don't get accidentally bumped or otherwise changed)? I suppose that is where some sort of headphone measurement system would be helpful. I've got an o-scope but that's about it.

In Headwize's schematic for a post amp limiter, I noticed the trimpot output is hooked up in a feedback sort of things instead of from the input to ground which I found odd. In searching around, I didn't find a good explanation of why that is the case, though I assume it's because we're tuning an already amplified signal and connecting a pot normally might cause too much current to flow to ground?

My thought with #2 was to calibrate it so that normal audio is right before limiting. So anything that wasn't desired is limited and, with a hard limiter, also rather obvious. In that way, it's still generally safe against mic drops, but, to your point, doesn't prevent me from turning up the volume past safe limits for normal audio. At least I assume that was your concern?

I suppose I could technically do both - set an absolute protection limiter after the amp and a configurable limiter before? I could even make the first limiter a soft limiter too (though that would likely demand some sort of indicator so I know when I'm hitting the soft clip).

diy didi 28th October 2013 05:43 PM

moo how is this circuit working for ya??


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