Film-Tech Forum ARCHIVE: Modification for changing CP45 crossover frequencies

This is topic Modification for changing CP45 crossover frequencies in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Demetris Thoupis (Member # 833) on 10-12-2010, 10:07 AM:

Besides Dolbys' field bulettin for changing the crossover frequencies of a CP45 to 1K and 1.2K does anyone have any table with resistor values to match other frequencies as well? Dolby does not have any documentation regarding any other value.
Demetris

Posted by Robert Minichino (Member # 3495) on 10-12-2010, 12:30 PM:

The resistor value is R=7234316/f, where f is the frequency in Hz, and R is the resistance in ohms. Pick the closest value resistor.

The "LO" setting adds another resistor value to "HI" value, so if you want to pick a second, lower, frequency, find the resistor value for that frequency, subtract the "HI" value resistor, and that's the second resistor you want.

So if you want to set the center channel to have two selectable crossover points, one at 600 Hz and the other at 900 Hz, you'd start by calculating the high resistors:

Rhi = 7234316/900 = 8038 ohms (8.04k)

Now, calculate the low resistance:

Rlo = 7234316/600 = 12057 ohms (12.06k)

Subtract the two, to find the low resistors;

12.06k - 8.04k = 4.02k

So you would then install 12.06k resistors in R614, R616, R618, and R621, and 4.02k resistors in R613, R615, R617, and R619.

If you only one want frequency selection, just calculate the resistance, substitute the first four resistors per channel and leave the switch on "HI."

Posted by Demetris Thoupis (Member # 833) on 10-12-2010, 03:48 PM:

Excellent information. Thank you for the info.
Demetris