[AMRadio] Audio Limiter for AM HAM TX

GRANT YOUNGMAN nq5t at tx.rr.com
Tue Aug 19 13:39:23 EDT 2014

The current gold standard in the ham community appears to be the Inovonics 222.  The most advantageous thing about it is that the gyrator-based LPF can be modified to limit AM bandwidth to something under the NRSC limit for ham use (a bezillion resistors, but not too bad).

The CRL PMC-series limiters are also very effective.  Example would be the PMC-450.

Both of those boxes seem to go in the $400-450 range give or take, although you can probably find a CRL PMC box for less. 

I currently have a CRL SMP-950.  This is a stereo AM negative peak limiter that can be strapped for mono.  It has adjustable asymmetry like most, tilt correction, etc.  There’s a simple (one resistor) mod recommended to eliminate an issue with clipping distortion and open up the audio.  For some reason, these also don’t seem to be as expensive as some others.  I have outboard filtering (4-5 KHz) to keep the bandwidth in check.  Like the 222, the AGC can be set so that a slow AGC leveler isn’t really needed in front of it — so you can drive it direct from the 528E.

The 222 can still be purchased new from BSW (maybe it’s NOS), at about double the used price.  I don’t know if the 222 is in current production or not.  The rest aren’t.

Even a CBS Volumax/Audimax would work ok — just make sure the Volumax is an AM limiter and not an FM version.  I would recommend the use of the Audimax leveler with the Volumax.  eBay prices on these seem ridiculous to me, though, given their age.  I used to use a set of these on a Globe King, and they worked fine.

If you’re worried about pushing the mod transformer with asymmetrical limiter output on positive peaks, you can just punch in the phase rotator in the 528e to essentially eliminate the asymmetry.  That’s the way I have mine set up (driving a Globe Chump), figuring there’s no need to stress old iron for a few extra positive peaks.

Grant NQ5T

On Aug 19, 2014, at 10:12 AM, pete zilliox <pete at zilliox.net> wrote:

> Forgive this query if it’s been discussed many times before....I've looked
> on ebaY (yeah I know) for a cost effective simple inexpensive audio chain
> limiter to use with my Symetrix 528E voice processor that drives my EF
> Johnson Desk KW AM transmitter.  I want to protect the very old Johnson
> high level modulation transformer from over modulation.  I'm totally over
> whelmed with way too many options on ebaY.  I am only looking for a single
> channel limiting function in a reasonably priced box.  I currently use a
> Nady active (vacuum tube pre-amp) broadcast mic to the 528E followed by a
> Dorrough  Loudness meter in parallel to a line going to the W2IHY 600ohm/Hi
> Impedance variable level control feeding the Ranger exciter.
> Recommendations for such an inline limiter, anyone?  Thanks Pete – K5PZ

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