DIY Headphone Amp Limiter

[m00dawg]

So SmashTV planted the see of building my own limiter for a custom headphone amp (stage monitor / IEM amp) and I was wondering if anyone has tackled something similar?

 

I've got a design working, largely based off of Chu Moy's (designer of the CMoy Audio Amp) headphone amp and limiter designs. The idea is clear. Where it gets hazy is where to add the limiter. I read up on the various mentions of a limiter here:

http://headwize.com/?page_id=707
http://headwize.com/?page_id=800

Where I'm a bit confused is Figure 1 on the Limiter page (800) seems to have the limiter after the volume but before the amp. Figure A9 on the pocket amp page (707) has it after the amp. I noticed both the value and method of connecting the trim-pot are also different, I assume because the latter case is attenuating an already amplified signal meant for headphones?

The bit I'm wondering about is - wouldn't it be best to limit before amplification or volume control? That way the audio source never goes past a max no matter what the headphone volume is set to, so a dropped mic will always hit the limiter at high or low volume. It also would not be affected by differences in headphones - though it would need to be calibrated for the actual "normal" levels from my mixer (which is +4dBu, so I'm guessing 1.7V or so is what I want to center around), such that having an adjustable knob or trimpot would be wise. But I wouldn't need to calibrate it for differing kinds of headphones since the volume would be independent of the limiting?

Another curiosity - I will be feeding the headphone amp a mono input so was going to split the signal to both the left and right. I am going to use a TLE2426 (rail splitter, virtual ground) so shouldn't have too much DC offset, but anything else I might be wary of? The headphones I would likely use are the common Westone UM1's probably and their impedance is 24 Ohm, which means I'll end up at 12 Ohm if I just tie them together and do nothing else if I understand impedance correctly.

Finally, since I'm using a balanced (XLR) input, I assume using a two channel op-amp, with one channel for the XLR balance; and the other for the actual headphone amplification won't be an issue? Unfortunately, if I want an LED indicator (to indicate when the limiter is active) it seems I will need another op-amp channel from the designs I've seen.

 

Thoughts?

Edited February 16, 2013 by m00dawg