CBS TUBE SCHEMATIC

Desesperatly searching MEMORY UNIT or MEMORY UNIT SCHEMATIC
for a CBS AUDIMAX 2.

It will save the machine!!!

Best,

Oliver :shock:

Hi Xela,

You can find the full schematic at the US patent office online. Search for:

U.S. Patent No. 3,260,957 (A. Kaiser and E. Torick, "Compensated Platform
Gain Control Apparatus," July 12, 1966)

In all honesty, I can't see why you'd want the memory unit if you're working in the studio, it will always be very slow.


Cheers,
Justin

I agree, I had an Audimax, and as it stood it wasn't fast enough for practical recording use. The mod sounds good, DIY to the rescue.

At the old forum, PRR was kind enough to offer a history lesson on how the Audimax came into being. It was intended to be followed by a fast peak clipper limiter (i.e. the Volumax), and was never designed as a conventional "limiter" in it's own right.

The concept was that it would either amplify or attenuate if the level dropped below or increased above a certain threshold, idea being that the radio-station would maintain uniform loudness even if the DJ was clumsy with the faders. I remember PRR's description was something along the lines of a "dumb DJ" in that it wasn't fast enough to qualify as a limiter, and attenuated the level at about the same speed an observant DJ could grab the fader.

Unless you run a vintage-style radio-station I can't see why you'd want to keep the memory unit, I can't envisage how it could be used in the studio other than to add some "valve colour" to the signal.

Justin

I' m also rebuilding a Langevin Prograr 119.
There is an automatic gain control stage and after a limiter stage.

Those machines were built for broadcast... not for recording.

I 'm a bit curious to see what was the memory unit...

Many,many thanks for helping me.

Olivier / Plus XXX/ Paris/ France

> attenuated the level at about the same speed an observant DJ could grab the fader.

Turn-down of large over-level was semi-slow, a fraction of a second. You had to follow it with a Limiter. However when the pair was set up well the limiter would just catch sudden loud parts, a steady loud passage would cause the AudiMax to reduce gain so the Limiter was just topping the occasional peak.

At small overload it was even slower, drifting gain to nominal level.

What was special (and not useful in most music studio work) is that it would turn-up low levels. If the DJ didn't pot-up a weak record, and the VU was hovering around -10VU, the AudiMax would increase gain, but slowly, several seconds to bring-up 10dB to reach 0VU. "Sneak it up."

Any idiot could design that. But in real life, if the program goes "silent" (DJ hits the wrong switch, network gets knocked-out, movie has dramatic silence), a dumb AGC would turn-up the silence until the noise was loud. And in most such program silences, then next thing is the DJ wakes-up or the network connection is fixed, and normal levels go through maxed-out AGC gain and bash the Limiter and deafen the listener until the AGC drifts down to normal gain.

What the AudiMax does different is that if it has signal, and then it has silence, it "holds" the previous gain level for many seconds. It does not try to pump-up every quiet passage. But if the low level persists for more than about 10 seconds, it will "sneak up" the gain, but very slowly, and going back to "Hold" mode when it detects significant signal.

So in a movie with dramatic silences in conversation, it will hold a fairly constant gain throughout the conversation, not pumping noise into the silences.

But if the DJ is very careless, or the movie or network feed is very weak, after a while it will pump-up gain and deliver a near-normal level.

It isn't as good as any human with a brain. But it does cover most broadcast accidents and stupid mistakes, and it is really a very simple contraption. And some days, there are no spare brains around a broadcast studio panic.

BTW: the orginal research was for a war-tank intercom. The inside of a tank is VERY LOUD. The intercom must not pump-up ambient noise: it would deafen the users. It must not pass a SHOUT at normal-talking gain: that too would be deafening. Yet it must bring-up a quiet talker above ambient noise. There is a narrow range of levels that are useful: above ambient noise but below quick deafness, and it aims to put any significant signal in that range. The intercom system time and level constants must be different from the broadcast model, and the in/out interface is totally different, but it is the same basic AGC system.

I've found the SS versions of the Audimax useful with individual tracks in a very dense mix; punk rock or thrasher type stuff, and on the fastest release setting. Helps give maximum presence for a vocal or bass, for example.

Pretty colored output also.

Haven't used the tube versions, but I'd bet it's a similar effect. It is with the Gates Solidstatesman AGC.

Can't see myself tracking through either one, but sometimes handy for mixing.

Nothing to compare between the solid state and the tube one.
It is like a 1176 and a Manley Vari Mu...

Sure, but I was comparing what they do to units with other purposes (limiters), more than how they sound outside of the gain aspects.

Ive got a pair of these that I use for mastering and occasional tracking. Joe figured out what you need to do to give them attack and release controls, as the simple mod thats floating around the web doesnt work properly. There are other time constants that must be eliminated and a few other changes that need to be made, but once they are the Audimax IIs are great comps, especially considering how cheap they can be on ebay. Ill see if Joe can put up this info soon, but dont hold yr breath as hes just a little tied up with a 'new JLM product' right now. :wink:


M@

The mod shown further up the page works ok but the RC from the input level to the 6386 makes a second time constant which would have been fine in the original use but not when faster setting are needed in recording. The mod below fixed the problem and also added remote threshold. The threshold and attack and release are in a external box that plugs into the old memory plug on the back panel. Release can be done as shown or with a fixed 3.3Meg resistor and switched capacitors from 1uF to smaller values instead which is what we did in Matt's Mastering ones.

Original Audimax II circuit.
http://www.jlmaudio.com/Audimax II.pdf

Modified Audimax II circuit.
http://www.jlmaudio.com/Audimax II JLM Mod.pdf

Control box to go with above modified circuit.
http://www.jlmaudio.com/Audimax II JLM Mod control box.pdf

All above circuits are now sitting in the schematic section on the JLM Audio web site

WOW! Thanx Joe!

:thumb: :sam:

:grin:
Bloody oath thanks! Ive brought the parts home to try some of the new mods and will be instaling them over the next few days. Ill be sure to report the results. These things are already fantastic but theres always room for improvment.
:wink:

:thumb:

M@