CBS Model 450 Dynamic Presence Equaliser

[NOON]

And I worked out why I was getting confused by the switch (apart from the rats nest of wiring and links) It's ON _ON _ON, but the terminals that are ON in the middle position vary. See attached grab from datasheet. Will trace it out again and see if I can draw it more clearly.

 

[NOON]

I've updated the original schematic. Switch section is now easier to understand, started labelling sections based on discussion, corrected the PNP orientation, couple of other sundries.

 

[abbey road d enfer]

Got time to do some more research. The diodes translate as 1n456A, a pretty standard small signal Si unit. there's a couple of zeners elsewhere in the circuit, but everything else looks like these.

Actually, the simulation does not give the expected result. It may be due to several factors: limitations of the simulation algorithms, models incorrect or neglecting parasitics, value errors. Can you double-check the values of R75-78?

 

[NOON]

R75 is actually 6K8, everything else looks OK. I also added that CR29 is a 10V zener. That will likely change things, hopefully for the better.

 

[abbey road d enfer]

I've updated the original schematic. Switch section is now easier to understand, started labelling sections based on discussion, corrected the PNP orientation, couple of other sundries.

Still not entirely clear with the switch; the second section (from the top) is always passing. I would think one of the positions should interrupt.
Also the 4th section (bottom one) applies a postive voltage in the upper position (All), which turns the lamp ON, which results in no enhancement. I would think this would be the down position (Off).
Q22&23, as well as Q26&27, constitute a Shmitt trigger that forces hysteresis to teh control voltage.

 

[NOON]

The switch configuration is correct.  I don't know why they would bother soldering another pole that is just bypassed by a jumper, my initial thought was it might be a way of converting from Break-before-make to Make-before-break (or vica-versa) to reduce popping when switching modes while on the air, but the logic doesn't really support that. The wire for the Control Voltage test point connects to the other side of the switch, perhaps it's a way of not connecting two wires to one terminal because of some manufacturing process limitation?

My theory on the other switch sections is that Speech goes via some kind of dynamic filtering in the Discriminator section to set a variable threshold while All is a simple fixed threshold, set via the voltage divider and the Threshold pot. The CR12-CR13 section in the Control board seems to work this out.

When the lamp is on, it's boosting the presence frequencies.

 

[abbey road d enfer]

When the lamp is on, it's boosting the presence frequencies.

You're correct, I stupidly surmised the LED was shunting... Doh!

 

[abbey road d enfer]

OK, so I think I got it.
In presence of signal Q21 is turned on, which results in a low level at the input of the Schmitt trigger Q23&24. Its output goes low, which discharge capacitorC35 through R87. As a result the voltage at Q218 goes high, which allows the control voltage from the control path to drive the lamp. When teh discriminator detects an absence of signal, the output of Q28 goes low, which disbles the lamp drive, which avoids noise pumping in silences.

 

[NOON]

So the whole Discriminator section is effectively a digital output determined by silence or lack of? That seems to make sense. Thanks for all your help in understanding this beastie! I'll do some more tidying up on the schematic and post it in the appropriate section for future reference by anyone who needs it.

Next mission is to work out the best way of converting this thing into a compressor to make it more useful in the studio.

 

[abbey road d enfer]

Next mission is to work out the best way of converting this thing into a compressor to make it more useful in the studio.

Before doing that, I would experiment using it on snare drums or even on the kick. That may become your secret weapon.
There are so many compressors available, it seems a pity not using this piece of gear for what it is.
A nice mod would be to make the LC tanks switchable in order to tune the frequency of action, though.

 

[mjrippe]

Before doing that, I would experiment using it on snare drums or even on the kick. That may become your secret weapon.
There are so many compressors available, it seems a pity not using this piece of gear for what it is.
A nice mod would be to make the LC tanks switchable in order to tune the frequency of action, though.

+1  When I had mine I did exactly that.  Well, not the selectable frequency part but I did use it on drums.

 

[NOON]

'Final' schematic posted in the Technical Documents section.

Was it any good on drums?

To make the frequencies switchable both the Control and Input filter sections would need to be modified. This would make it possible to detect the lack of one frequency and boost another. Not sure if that would be useful or not. Maybe it could be made sweepable with a little active EQ circuit rather than switching L-C values.

Could also be possible to have it switchable between Dynamic EQ and Opto comp modes. Would just need to shunt the input Vactrol circuit rather than using it to bypass the divider, then work out mods for the sidechain.

Thought bubbles at this stage, I'll restore it as-is first and give it a good listen, then think about possible modifications. Anyone else got exciting suggestions?

 

[abbey road d enfer]

To make the frequencies switchable both the Control and Input filter sections would need to be modified. This would make it possible to detect the lack of one frequency and boost another.

Why not? but it seems too unpredictable operationally. You would need a side-chain monitor.

Maybe it could be made sweepable with a little active EQ circuit rather than switching L-C values.

That would not be very simple because the circuit would need to be balanced. In order to prove the concept, I think switchable LC's is simple enough.

Could also be possible to have it switchable between Dynamic EQ and Opto comp modes. Would just need to shunt the input Vactrol circuit rather than using it to bypass the divider,

That is a possibility, however, as I said earlier, there are so many compressors available...

 

[NOON]

Plenty of opto compressors out there, but not in my rack. I have a variety of FET and VCA comps but no opto and this one should be interesting. I can see an opto compressor getting more use than a dynamic EQ, but if it's switchable between modes then it's best of both worlds.

 

[NOON]

I've replaced all the electrolytics and tested the input and output boards successfully. Not going to restore the power supply, the mains connections are SCARY!

Will test the whole unit with bench supplies then put together some switchmode modules in the back section to bring it up to modern safety standards.

 

[NOON]

Whole unit is passing signal, CVs are changing depending on signal level. Now I need to work out a way to test the functionality of the Dynamic EQing. Not easy to send and monitor 'normal' audio signals on the test bench and no bench power supplies at the audio monitoring setup. Testing continues...

 

[abbey road d enfer]

Not going to restore the power supply, the mains connections are SCARY!

What's wrong? I would think you could rewire it in accordance with more contemporary practice...

 

[NOON]

The mains connections come in via the card edge connector. (currently no fuse there either!) The mains pins are right next to the -15V pin, about 1-2mm away. I could rout the mains around the connector, but that would mean the PSU card can't be removed.

There is a small area behind the card edge connectors where I was going to mount the switchmode PSUs, I could possibly just relocate the existing transformer into there and send the low voltage AC via the card edge connector, but new PSU's should be quieter than the old zener-regulated unit.

With the current global shutdown and the fact I just noticed the +/-15V Meanwell PSU I have to hand doesn't physically fit, I may end up doing that anyway.

 

[abbey road d enfer]

I could rout the mains around the connector, but that would mean the PSU card can't be removed.

You may use an isolated in-line connector for that.

 

[NOON]

I'll go for a rummage tomorrow and see if I've got anything suitable. Need to get into Depression-era thinking and improvisation with parts and practices rather than just ordering new stuff all the time. We've gotten lazy and spoiled.