Difference between Dolby optical preamp cards

[David Ferguson]

I'm aware that there have been many different optical preamp cards in the various Dolby processors and units over the years. From a quick count, I found the following:

• cat 137A in the OPU1
• cat 108 in the CP50/CP200
• cat 240 in the CP55
• cat 240A in the CP65
• cat 514 in the CP45
• cat 661 in the CP500

What is the difference between all these cards? I'm assuming they are all at least physically incompatible with each other (expect perhaps 240 and 240A?), but what are the other differences? Are there differences in quality, ease of use, reduction of components, ease of dolby to build, newer components?

[Sam Chavez]

Must be a slow news day.

You forgot the CP100. Cat. 75?

Don't think the 137A is actually a preamp. More like a bridging card.

There were various refinements to the 108 preamp. To allow more HF response in the slit loss correction circuit. There was an important fix on the 108C having to do with unintended crosstalk in backplane layout that affected projector 2 hf response.

240 to 240A was the elimination of the input transformer. The A version worked better theoretically but nothing your would ever hear. And despite all the very cool circuitry didn't provide true isolation.

514 was a very low cost version so minimal parts count. There was other linking circuitry to the 45, but nothing important.

Don't recall anything noteworthy on the 661.

[Pete Naples]

Cat137, OPU1 I only every saw one of these. A very long time ago and I really can't remember anything about it.
Cat108, there were various revisions over the years. As projector optics got better. I seem to remember they were quite noisy.
Cat240/240A are cross-compatible IIRC, the fundamental difference is one was transformer balanced input and the other transistor. I cannot remember which was which.
Cat514 was an odd looking thing, previous optical pre-amps all carried gain and slit-loss setting pots. On the CP45, those were on the MB. So if a Cat514 failed, no re-calibration was necessary. Also, if in 99% of installations, a single projector was in use, and the Cat514 failed, the show could be continued by removing the Cat514, turning it 180º and putting it back. thereby using the other 2 channels.
Cat661 was IIRC based on the Cat240A, if you look at a Cat661 PCB you'll see the silk screen for the level and HF pots which dont exist.

Somewhere I have course notes from last century, where this was all discussed, when I remember where I filed them, you are welcome to a peruse.

[Steve Guttag]

The real movement on the various optical preamps is continual miniaturization of things. The CAT 75 and CAT108 were discrete components, the CAT 240/240A moved to mostly IC (lots of CD4000 series and TL07x series. As others have noted the CAT240 had transformers on the board (the CAT108s had their transformers on the motherboards). The response of the CAT240A was better than the CAT240 but not as well isolated but the tradeoff was good. I'm sure omitting the transformers lowered the cost of manufacturer quite a bit. The CAT108 did evolve over the years with the CAT108C getting a ground-plane fill so it was a bit quieter than the predecessors. I think the CAT 108-4 got the crosstalk fix mentioned earlier. I'd have to check a schematic to see...it's on the output of the board.

[David Ferguson]

Thanks Sam, Pete and Steve - all really great information to have! I'm very grateful you're around to answer my weird questions.

Must be a slow news day.

You forgot the CP100. Cat. 75?

I was doing some reading of the manuals and was curious to why all the CPs had different cards! I do find it interesting how some cards like the optical pre-amp changed with nearly ever different CP, but others like cat64, remained the same. And yeah I did forget the CP100, nice to know about that one too.

Cat137, OPU1 I only every saw one of these. A very long time ago and I really can't remember anything about it.
...
Also, if in 99% of installations, a single projector was in use, and the Cat514 failed, the show could be continued by removing the Cat514, turning it 180º and putting it back. thereby using the other 2 channels.

I'd actually never heard of the OPU1 until I saw a picture with one in it. I'm assuming this was only really used before the CP100 came out? And that's a cool trick!

the CAT 240/240A moved to mostly IC
...
I'd have to check a schematic to see...it's on the output of the board.

That explains the wildly different pictures I've seen of the 240 and 240A! And now I'm curious as to what schematics are available - did Dolby ever publish one for the 240A?

[Steve Guttag]

Actually, Dolby was quite slow to change the Optical Preamp. The CP50 came out in what 1975 or 1976 as the economy processor to the CP100. With the exception of the CAT112/113 (Switch card), most every card in the CP50 was reused in the CP200. The CP200 used the MPU-1 preamp that the CP100 used. Sure the cards evolved as they were improved/adapted to changes but it wasn't earth shattering.

Also remember, when the likes of the CP50 came out, it was thought of as a box that could be spliced into an existing 4-track magnetic theatre...hence no volume control for "Non-Sync/Mag", The shortcomings of the CP50 in the "modern" '80s theatre became ever more apparent (really poor automation interface, limited inputs, and was becomes kludged more and more to get it to work where it was needed). The CP55 sort of improved all of that in one fell swoop. And, why not, while one is at it, redo a 10-year old design with the collection of knowledge plus circuit improvements over discrete semiconductors. The VCAs on the CAT 117 cards were great for a time when the alternative was a stacked daven pot but they tracked poorly from channel-to-channel as one drifted away from 7.0 (where things were optimized). Again, everything went towards integrated circuits where one is going to get an inherently more stable/consistent result instead of metal strapping pairs of discrete transistors to keep their temps the same.

The CAT64 did evolve into the CAT64B in about 1984 (the last runs of the CP50 had them). Primarily the head EQ and input/output were omitted/fixed (can't use them in a CP100 or E2) but you picked up stable calibration and a little less noise.

The CP65 was a refined CP55 that ushered in the Dolby Digital era (created a 5.1 signal path). The CAT240A only really changed by using active balanced inputs...the rest was the same with maybe a tweak in the slit loss area (I recall the CAT240A setting up better with narrow .7mil (red line) slits better than the CAT240. CAT64B and CAT242 remained...it got a beefed up power supply regulator (CAT249A), cound handle REAL Dolby SR and the CAT 441 provided for a stereo surround. Oddly, the CAT441, being newer, used a newer and better VCA than the CAT242 so the Rs and SW were cleaner/quieter than the other 4 channels. There was a studio version of the CP65 where the CAT242 was kludged to use the better VCAs on the other 4 channels.

Really, until the CP500, all of Dolby's analog processors were evolutions, in terms of signal path, from their predecessors. How they were controlled changed between the CP100, CP50, CP200, CP55...the CP65 was controlled just like the CP55. And then you get to the CP500..their first digital processor where things entered and left analog but were processed digitally. Naturally, that is going to have a different topology and we would be over a decade beyond the ideas of the CP55. The CP650 shared nothing with its predecessors as its entire signal path (Except at the extreme input/output stages) was digital. And while the CP500 did incorporate Dolby Digital internally...it did so by using, essentially the same cards from the DA20 with some extra for B-Chain facilities. The CP650 was fully an integrated design where Dolby digital was a daughterboard versus a collection of cards. That is just electronic evolution.