Transparent, but affordable limiter - does such a beast exist?

Cloud cx335 is worth checking out too, designed specifically to do what your asking. Built well and about £50 off ebay....

http://www.canford.co.uk/Products/93-161_CLOUD-CX335-COMPRESSOR-LIMITER

thermionic said:

Hi,

Some friends of mine have a club / venue installation. Instead of using a PA system they have have an overgrown hi-fi, using hi-fi carts on the turntables and Levinson amps etc. Rather than risk any visiting music selectors blowing the K-horns, they have a Drawmer limiter prior to the x-over; transparent, it ain't. In an ideal world they'd buy a GML 8900, but that's a lot of money, and it's way over-specced in terms of features considering they purely want to use it as a limiter. What would you fit? Is there a DIY project that's uber-transparent?

TIA

Justin

edit - you're going to suggest the PICO limiter, aren't you, right?

edit again - I now see PICO PCBs are unobtainium... Hmm...

Click to expand...

To change the subject back to the OP's actual question, limiting for sound reinforcement is a pretty mature discipline and very different from studio compressor design. Most SR power amps have clip limiters built in. Transparency for a peak limiter is all relative, but the goal is to keep peaks from hard clipping while keeping audible artifacts to a minimum.

#1 You are looking for peaks, not slower rms or average levels so you don't need fancy rectification, while full wave is better than half wave.

#2 The gain element only needs to attenuate the signal, and only needs to be active when signals are very loud, so you don't need some low noise, wide dynamic range, pristine gain element. Note: I designed one limiter back in the '80s using cheap OTAs in parallel with a simple unity gain inverting opamps feedback resistor. Below limiting the audio path was a clean as a single inverting opamp, during actual limiting the CV modulations swamped out any gain element nonlinearity the OTA contributed.

#3 Time constants, for minimal intrusion, are generally fast attack and fast release, so the gain reduction occurs quickly enough to prevent hard clipping, then releases quickly also when the overload goes away, so you don't experience pumping or envelop modulation (loud peaks ducking everything else).

It may seem contrary to use modest quality parts, but when properly configured they are only in path during peak events. In general the comparison is to hard clipping and speaker damage, not pristine audio. Amp manufacturers have invested decades in making their clip limiters as transparent as possible and IMO they do a good job, but the comparison is not to a studio comp which is different animal entirely. If anything the comparison is to another limiter. Since these are pretty much already built into SR amps, with the modern big dog SR amps including DSP with sophisticated speaker protection (LF excursion, long term average heating, "and" peak limiting), there are not many stand alone products.  The one I sold was back in the '80 before amps routinely included clip limiters.

OK you can always use parts that are better than needed so THAT VCA and rectifier will work. I would suggest an undersized cap in the RMS integrator to give faster "peak" response.  Look to see if THAT app notes describe a peak limiter.

JR

PS: There are SR forums with lots of experience dealing with such matters but you are bringing scalpels (hifi gear) to a gun fight (live SR).