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-   -   summing rca's... (http://www.diyaudio.com/forums/car-audio/151659-summing-rcas.html)

jkwylde 15th September 2009 01:58 AM

summing rca's...
 
Got a ???... I am attempting to use a stereo amplifier in mono mode, it is internally inverting, but requires the summing outside the amplifier - I can't just Y a stereo pair of outs can I??? The source is an alpine 7904, through a nakamichi sp200 x-over, into a hifonics Zeus series VI.

My last crossover summed for me, but I wanted to use the 2 Nakamichi x-overs to match my mid and hi amps.

what must I do to sum the line level?
Is there a small circuit I could build?

Perry Babin 15th September 2009 03:30 AM

Insert a resistor in each RCA output and connect the other ends of the resistors together. Use that (and the shield) to drive the next component with a mixed signal. The resistor value isn't critical. If the RCA output is driving a sub amp and nothing else, 100 ohm resistors would be a good choice. If you were splitting the RCA output to go to both high and low frequency amps, you'd want to use a higher value to help prevent introducing distortion (which is more likely to occur with lower value resistors).

To get a summed output, you'd have to use op-amps which would make it much more difficult.

jkwylde 15th September 2009 06:21 AM

Thanks...another???
 
You solved the question for me, and as an uneducated bobo i ask two more questions :

with a preamp signal of 1.6 volts, through the resistors, what type of voltage will I see, and....



The risk of distortion is the devil - - what is a super safe value on the resistors? A good midway point to keep distortion absolute minimum without degrading the signal....?

Sorry for being so inquisitive...

Quote:

Originally Posted by Perry Babin (Post 1923985)
Insert a resistor in each RCA output and connect the other ends of the resistors together. Use that (and the shield) to drive the next component with a mixed signal. The resistor value isn't critical. If the RCA output is driving a sub amp and nothing else, 100 ohm resistors would be a good choice. If you were splitting the RCA output to go to both high and low frequency amps, you'd want to use a higher value to help prevent introducing distortion (which is more likely to occur with lower value resistors).

To get a summed output, you'd have to use op-amps which would make it much more difficult.


Perry Babin 15th September 2009 09:38 AM

The resulting voltage will depend on the difference in the signal between the individual channels (signals should be identical at the lower end of the spectrum) and the input impedance of the next device (likely 10k ohms). For your application, I wouldn't expect to see any significant loss.

Since most op-amps can drive 600 ohm loads and the RCA output is likely driven by an op-amp, 600 ohms should be safe.

tsmith1315 15th September 2009 04:04 PM

Quote:

Originally Posted by Perry Babin (Post 1924138)
(signals should be identical at the lower end of the spectrum)

And for that reason, you can actually just skip the summing node and bridge the amp using the R & L outputs as they are.


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