This is the CBS Section of
The Broadcast Archive

Maintained by:
Barry Mishkind - The Eclectic Engineer

CBS Labs

At about the same time Rock and Roll began, radio stations began to make an effort to raise modulation levels. Prior to this "arms race," most stations ran approximately 30% average modulation. There was usually an engineer on site to watch and "ride gain." If a peak went too high, it would often knock the station off the air.

Among the most famous processors of the time were the "Max Brothers" ... the Audimax compressor and the Volumax limiter.

With these boxes, stations dramatically raised their loudness. So much so, that many of the modulation transformers of the 30s and 40s transmitters would burn out under the stress. (A common site in the 60s and 70s: a mod transformer sitting on 2X4s, to keep the case from ground because one of the windings shorted to case.)

Almost immediately, the "Modulation Wars" escalated. Modifications to "speed up" the attack and release times of the Max Brothers led to the development of a very "pumpy" sound, which many people to this day recall as an integral part of Rock Radio. The Max Brothers ruled until the advent of multiband processing in 1971. But the love affair with the Audimax and Volumax continues. 

Audio Processors

Click on picture for a larger image
  Audimax - Stereo
  Audimax 4450
  Volumax - Stereo




Among the many modification popular with the Audimax, Burt Weiner shares his favorites:

I always thought that the Audimax was a pretty nifty device and still has its place. I will agree that a properly designed "multiband" system has more room for customizing and less of the bass poking holes in things.

But to pass along a couple of my favorite modifications for the Audimax I offer the following:

A little remembered fact is that the Audimax has a tube in it that must be replaced. The tube is an 8056 Nuvistor. When this tube goes flat the AGC action becomes poor at best. I've replaced the Nuvistor in mine with a general purpose N-Channel FET from Radio Shack. The difficult part of doing this is figuring out which pins of the Nuvistor socket are which and then getting the correct FET pins into the little holes. The "source" is the cathode, The "Gate" is the grid and the "Drain" is the plate. As they say, "This can be done by a four year old". The reason for that saying is that you will need the eyesight of a four year old! Once the FET has been installed the unit will easily re-calibrate without any further circuit changes. It wont know that you replaced the Nuvistor with a FET.

The Gated Gain Stage platform: I replaced both R-45 and R-45A (paralleled resistors) with a pot. As I recall I used a 10K pot. This allows you to set the sensitivity of the GGS, which is essentially the gating threshold. I found that this will allow you to drive it with less audio to get the desired action and greatly reduce the distortion although the vario-losser circuit which is a couple of biased diodes is not the easiest on distortion figures.

The infamous 10 meg recovery resistor has been changed to a POT. I used a 15 Meg pot with a 1 Meg series resistor. This allows me to change the recovery time and the series 1 Meg sets a limit on how fast you can set it to recover before really sounding bad.

It goes without saying, so I'll say it, that all capacitors should be replaced, particularly the original black electrolytics.

The pots can generally be cleaned and continue to work for a long time. it is necessary to remove the pots from their respective boards. You will see that these are three-section pots. Not all sections have three pins. However, the center pin of what would be the three pins in a section has an opening that goes up into the pot. You can spritz your favorite pot cleaner (not Brasso) up into the pot through that hole. I use WD-40 and have had pots working for over ten years and when I need to adjust them they are still quiet.

My favorite place to put an Audimax is on the back haul side of an ISDN, or whatever your favorite remote transmission scheme is, to control the levels of your typical "all over the place" send buss. Being a gated AGC device they still have a lot to offer. The Audimax makes a nifty addition to any Ham Repeater.