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Topic: Dolby reverse scan pcb supply voltage

Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 01-14-2002 09:58 PM

I am finding some discrepency in the supply voltage fomr different manufacturers. I ahve a reverse scan unit fomr a Christie projector and for the analog convertor it measures plus and minus 7.5 volts. On a C.E. unit it measures plus and minus 10 volts and on a Kelmar unit it measures plus and minus 12 volts. There are a pair of zeners there that are I believe 5.1 volts with a 180 ohm limiting resistor. Dropping 12 volts to 5.1 through a 180 ohm seems like a bit too much drop, while the 7.5 volt supply seems more realistic to me. So which one is correct?
Mark @ GTS

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 01-15-2002 04:03 AM

It doesn't really matter what the supply rails are, within certain limits, as long as they are symmetrical. The TL072 & TL074 chips on the preamp board can happily run on +/- 15V, but the audio signal output level is so small that +/- 7.5 volts is more than adequate. (The supply rails determine the maximum signal level that can be handled before clipping, although with some opamps, higher supply rails result in a better slew rate).

Can't remember seeing preamps with 5 volt rails, I mean why would you bother dropping from 12v down to 5v? Unless maybe the power supply is dirty and needs further regulation. There should also be some smoothing caps in parallel with the zeners if that is the case. A 180 ohm resistor dropping 12v down to 5.1v would be passing about 38mA and dissipating about 1/4 of a watt, which would be enough to make a 1/2 watt resistor get warm.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 01-15-2002 07:41 AM

I had thought that one chip got the supplied voltage and the other the zener/resistor regulated V. But close examination of the traces seems to indicate both chips on the lowered voltage. So I can't imagine why Kelmar is feeding the card 12V, at least on the units I have. One of mine went to 13.4V for unknown reason (anyone know much about Lambda OEM switching supplies?) and that may have been the proximate cause of one of those 180 Ohm resistors burning (well, darkening anyway). Unit never failed though. I think I may just throw some chip regulators into the line from the P/S and cut the V to something reasonable.

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 01-15-2002 09:36 PM

Actually after doing some checking, some readers including Kelmar and Kinoton use the Cat655 preamp board made by Dolby which does have 5 volt rails with the resistor/zener divider arrangement. Presumably it was done this way to allow the use of less well-regulated power supplies and also maybe to try to minimise the injection of interference picked up by the supply cables or from the use of switchmode power supplies. The problem with using the zener regulators is you need decent sized, good quality filter caps to keep the power supply impedance as low as possible. IMHO it is better to use an appropriate power supply in the first place.

The CE readers use boards which are not manufactured by Dolby and use a decent linear power supply, so don't need the zener regulator.

Steve, if you are going to add linear regulators you might as well rip out the zeners and run the thing at +/-12 or 15 volts. Also I'd change the TL072 to an NE5532 or LM833 while I was at it.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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Steve Kraus
Film God

Posts: 4094
From: Chicago, IL, USA
Registered: May 2000

posted 01-17-2002 07:04 PM

Ray Derrick wrote:
>Steve, if you are going to add linear regulators you might as well
>rip out the zeners and run the thing at +/-12 or 15 volts.
>Also I'd change the TL072 to an NE5532 or LM833 while I was at it.

Interesting idea; thanks. But before I do anything that invasive I think I'll just stick some regulators ahead of it on the one with the overachieving power supply that needs taming. 13.4V isn't really enough headroom for regulating down to 12 but now that I know the chips are really getting 5.1 maybe I'll try a 7809 & 7909.

Any idea why they are using 12V to ultimately create 5V? Perhaps a function of what P/S they wanted to use? But all the more strange if, as you say, the chips would be quite happy at 12.

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 01-18-2002 03:57 AM

If your power supply is outputting +/-13.4V then you are correct, 9V volt regulators would be the right choice, although you would use the 78L09 and 79L09. These are the little fellas in the TO92 pack which handle 100mA, more than enough for the Cat655 which only draws about 20mA or so.

As to why a reader manufacturer would chose 12V supply rails for their power supply, if it is a linear regulated supply, it is probably because 12V regulators are more common and therefore generally cheaper than 9V types.

BTW the TL072 & TL074 have a maximum differential supply rating of 36V so you can even run them at +/-18V if you need about 10.5vrms of output signal. Dolby obviously chose 5V because it is more than enough for the tiny signal levels coming out of their preamp board.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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Bill Purdy
Expert Film Handler

Posts: 139
From: Seattle, WA
Registered: Oct 1999

posted 01-18-2002 11:10 AM

I guess it's time a manufacturer put in a word. The "why" of the 12 volts is that Dolby has 12 to 15 volts available on the back of their Cinema Processors and is usually enough that it can be regulated to 12 volts. The trouble is that using their bi-polar DC means a lot more cable runs plus a gadget to isolate and filter and regulate the voltage, so I don't know of anyone who does this. But, Dolby said "+/- 12 Volts" and that is what we did in order to keep them from grumbling. However, when you realize that the audio voltages involved are very small, you could operate the rails at 2 or 3 volts and have plenty of head room. What really matters is as mentioned above that the plus and the minus are balanced. In the case of our latest supply (the LS-40) the voltage is what was convenient based on what was available from the transformer we chose which in turn was based on what we needed for the current regulated LED part of the supply. As long as we can supply some balanced voltages well above the signal level in the pre-amp it doesn't make any difference.

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Bill Purdy
Component Engineering

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System Notices
Forum Watchdog / Soup Nazi

Posts: 215

Registered: Apr 2004

posted 06-10-2004 02:31 AM

It has been 873 days since the last post.

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Giorgio Volpi
Film Handler

Posts: 51
From: Caracas - Venezuela
Registered: Mar 2002

posted 06-10-2004 02:31 AM

Hi,
I need to know about Cat 655 connections.
There are 9 connections divided into 3 (near to J1 indication) and 6 (near to J2 inication).
My board is identified by A1PD4731 Rev. E S/N 500078.
Tanks.
Best wishes,
Giorgio.

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