Dolby Cat757 ?

[Magnus Eriksson]

What is it and what does it do?
(found it in an old sound rack)
/M.E. IMG_3593.jpg

[Steve Guttag]

I believe it was used with the DMA8 Plus, which had balanced analog inputs. So, if you had an existing unbalanced wiring from your analog source (as may have been wired to the original DMA-8), it would jump your ground/return to all of the "-" terminals of the DMA-8 Plus.

[Jim Cassedy]

Steve (as usual ) is correct. I had to hunt one of these down when I needed one last year,
but you managed to find a spare one sitting in a rack. Some guys have all the luck! lol

For the schematically inclined, here's the wiring diagram of the little bugger:
Dolby_CAT757.jpg

But, wait. . . there's more:
DolbyDMA8pMatrix.jpg

[Paul Finn]

To clarify things, the Cat.757 was NOT intended for application on the multi-channel analog input side of the DMA8 Plus. The adapter was intended to correctly connect the multi-channel balanced analog OUTPUT of the DMA8 Plus to the Dolby CP650 and earlier Dolby processors, all of which had six channel unbalanced external analog inputs. As Mr. Guttag pointed out, it provided the correct/required negative(-) balanced to unbalanced channel connections between the DMA8 Plus and the following cinema processor.

Also thanks to Jim Cassedy for the additional information.

Paul Finn

[Steve Guttag]

I did say analog inputs but clearly, it is for the analog outputs since the DMA8 Plus would feed a host processor that lacked AES3 digital audio from various sources, including DCinema, when it finally showed up. I believe the adapter would work on both analog inputs AND outputs.

[Paul Finn]

Yes, Steve, I neglected to mention that the adapter should also work on the DMA8 Plus multi-channel analog input and save rewiring if the upstream source (DA20, DTS6D, etc.) required it.

Paul Finn