Film-Tech Forum ARCHIVE: summing bi-amp outputs

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Posted by Alban Birch (Member # 4121) on 01-02-2008, 02:19 PM:

Hello
I have a CP45 in Bi-amp mode and a Altec 1698 Boothmonitor.
I would like to sum the HF and LF outputs to get a Fullrangesignal on my monitor. I would like to do this on the Processor and Amp outputs since i modified the Monitor for pre- and postamplistening. How could I achieve this?
Thank you

Posted by Gordon McLeod (Member # 33) on 01-02-2008, 09:22 PM:

You will need to put a set of summing resitors to do that

Posted by Alban Birch (Member # 4121) on 01-03-2008, 07:43 AM:

what are summing resistors?

Posted by Sam D. Chavez (Member # 1841) on 01-03-2008, 11:09 AM:

These could also be called isolation resisitors.

You have two separate audio signals going to the power amp and then from the output of the amp to the speaker.

Now you want to feed both signals also to a monitor without combining or shorting the two signals together because this will lead to sending low freq. info to the tweeter and eventually blowing it up, and somewhat less importantly, sending high freq. to the woofer.

It's the size of the resistors that is important. 10K Ohms, 1/2 watt in series with the wires going to the monitor should do the trick. I knew this would be a follow up question and was surprised Gordon did not answer the question more completely, maybe he was tired.

Are you taking this signal from the output of the Amp. or the processor? The Amp would be better and more useful.

Posted by Louis Bornwasser (Member # 3063) on 01-03-2008, 01:10 PM:

Sam's way is right on. Only problem is if some form of resistor is already in the monitor input. Louis

Posted by Alban Birch (Member # 4121) on 01-03-2008, 02:04 PM:

Thanks to all of you for your replys

quote: Alban Birch
I would like to do this on the Processor and Amp outputs

The Monitor is a Altec 1698 it has a 10kOhm Isolationtransformer at the input section

Posted by Gordon McLeod (Member # 33) on 01-03-2008, 05:21 PM:

Actually I would use a smaller value on the HF input compared to the LF input
But I think it would be better to get a monitor that has this already provided for since they are relatively cheap in the first place

Posted by Sam D. Chavez (Member # 1841) on 01-03-2008, 07:58 PM:

In a perfect world, Gordon is right, but he has a monitor and wants to use it and is also trying to learn something in the process.

I tend to assume a lot of this stuff is for home style screening rooms or places where they don't have a regular service guy.

As to the resistors, the idea is to isolate the signals from each other, so even though the monitor has an input resistor, isolation from LF to HF must still be done.

Posted by Monte L Fullmer (Member # 2797) on 01-04-2008, 11:29 PM:

quote: Louis Bornwasser
some form of resistor is already in the monitor input

..and that would be what the "low", "mid", and "high" inputs on the back end of the monitor be - their version of the isolation resistor?