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Old 13th October 2007, 02:32 AM   #1
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Default RCA: Stereo to Mono?

I am looking for a way to convert a pair of RCAs(left and right channel) to a mono channel, combining both channels, to make a center channel. I am wanting to do this at the RCA level.

I can buy parts and build the adapter if there is a schematic out there.

Any help is greatly appreciated
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Old 13th October 2007, 03:43 AM   #2
ppia600 is offline ppia600  United States
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You could do it electronically (someone here probably knows how to build one or where to buy) or just get a ground loop isolator (noise filter with two rca's on each side) and solder the +'s and -'s together on the output side and use one jack on that end. The input side will still have two electrically isolated seperate channels for input.

This type for example, but they also have square types or you could just buy some isolation transformers from radio shack and build it yourself.

Click the image to open in full size.

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Old 13th October 2007, 05:42 AM   #3
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That's going to be a tough load for the head unit. It won't hurt anything but the signal is likely to be distorted.

The left and right signals won't match (especially at high frequencies). The impedance of the transformers is relatively low. If one channel has no signal, it's output is essentially at ground. This means that the other signal line will be trying to drive a signal to ground. An audio signal won't be quite so extreme but it will be a tough load when the signals don't match.

On a pac isolator (SNI-1) the output dropped more than 10% at 1kHz when connected to a circuit to simulate the above situation.

There is a slightly different alternative. Connect the outputs of the isolator in series (left center conductor to right shield). The output signal will have an amplitude that's ~2x that of either output but you can compensate by turning down the gains.

Do not connect the brown wires to ground unless you need to and then only use the one connected to the left channel.

Electronic alternatives 'may' be a better solution but unless you use a relatively complex circuit, you would have to use an isolator anyway.
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Old 13th October 2007, 02:46 PM   #4
djQUAN is offline djQUAN  Philippines
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I have done a very simple summing circuit composed of two 3.3kohm resistors in a Y config.

the top two ends of the "Y" is the left and right signals and the output is the bottom part of the "Y".

grounds are all connected together.

as expected, there is some signal loss. but it works.
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Old 13th October 2007, 03:09 PM   #5
ppia600 is offline ppia600  United States
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Haha, I figured a post like this was inevitable, which is why I included the part in parentheses. Yeah, bridging the outputs of the transformers would also work. Either way there is going to be a loss of efficiency -with impedance and signal quality- but I don't think he will notice considering he's using it for a "center" channel.
Using op amps in a circuit to combine the two signals would probably be pretty simple especially if you were using an amp like an older ppi or orion with the din jack that has the +15 and -15v rails. OR, you could probably just find the two op amp positive and negative output signals in the amp's preamp circuit, and use two jumper wires to parallel them. L+ to R+ and L- to R-. (on the op amp(s) output terminal)




Quote:
Originally posted by Perry Babin
That's going to be a tough load for the head unit. It won't hurt anything but the signal is likely to be distorted.

The left and right signals won't match (especially at high frequencies). The impedance of the transformers is relatively low. If one channel has no signal, it's output is essentially at ground. This means that the other signal line will be trying to drive a signal to ground. An audio signal won't be quite so extreme but it will be a tough load when the signals don't match.

On a pac isolator (SNI-1) the output dropped more than 10% at 1kHz when connected to a circuit to simulate the above situation.

There is a slightly different alternative. Connect the outputs of the isolator in series (left center conductor to right shield). The output signal will have an amplitude that's ~2x that of either output but you can compensate by turning down the gains.

Do not connect the brown wires to ground unless you need to and then only use the one connected to the left channel.

Electronic alternatives 'may' be a better solution but unless you use a relatively complex circuit, you would have to use an isolator anyway.
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Old 13th October 2007, 07:34 PM   #6
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Here is what I am using it for: I have an Alpine MRV-T320 in my car currently. What I want to do is be able to flip a switch that will turn on relays to do the following: 2 to cut off the front speakers from the Alpine amp, then one to combine the 2 channels into 1 channel, then one other relay to complete the circuit between that single channels output and an 8ohm loudspeaker under my hood so i am able to play music outside of the car. It doesn't have to sound the best because the loud speaker isn't the best speaker in the world; im using a radio shack outdoor PA speaker.

It sounds like a mess but i know it works. I have setup this before in my car, but had the speaker bridged across the amps 2 channels which at high volumes or any peak power immediately blew the speaker.

If i am able to combine the channels onto 1, then i can use just one single channel which will be perfect what im using it for.

Is the summing op amp circuit also called a 'mixer'? I have looked at a few on the internet and have seen a few schematics of them but im not sure what the proper one to use would be
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Old 13th October 2007, 08:53 PM   #7
ppia600 is offline ppia600  United States
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Oh, why didn't you say that in the first place, I thought you were running a center channel on your dash, haha.



If you're going to use relays, don't even worry about combining the signal before the amp. Just find the two "bridge" terminals on the amp and connect the speaker under the hood to those. Now, cut one of the leads right after the amp (doesn't matter which one) and insert a spdt automotive relay. Use terminals 87 and 30 for connecting the speaker wire back together. Wire lead 85 to ground and lead 86 to the output of a rocker or toggle switch. Now connect an accessory power source (like the red accesory lead that feeds your radio) to the input of the rocker or toggle switch. Now when you flip the switch the under hood speaker will be connected and automatically bridged with a mono signal that combines both channels.

Now, here is the part that will disconnect your two stereo speakers at the same time:

You will need two more spdt automotive relays for this part. Take one relay and cut one of the speaker leads on the left and insert the relay in between using the 87a and 30 pins. Also take the other relay and do the same thing with the right speaker. Be sure to use the 87a and 30 pins as well. Ground both of the 85 pins. Connect both of the 86 pins together and run them to the output side of the switch you already have. All three relays will now kick on when you flip the one switch. The center channel relay will be connecting the underhood speaker, and the other two left and right relays will be disconnecting the two stereo speakers at the same time.

Just be sure to have the volume almost all of the way down to avoid any popping sounds or voltage spiking on the speaker leads. There are lots of guys doing this where I live. I've actually done it quite a few times at work. It seemed kind of funny the first time somebody asked us to do it, but now its not a big deal.

Have fun
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Old 14th October 2007, 03:27 AM   #8
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Quote:
Originally posted by ppia600
Oh, why didn't you say that in the first place, I thought you were running a center channel on your dash, haha.

If you're going to use relays, don't even worry about combining the signal before the amp. Just find the two "bridge" terminals on the amp and connect the speaker under the hood to those. Now, cut one of the leads right after the amp (doesn't matter which one) and insert a spdt automotive relay. Use terminals 87 and 30 for connecting the speaker wire back together. Wire lead 85 to ground and lead 86 to the output of a rocker or toggle switch. Now connect an accessory power source (like the red accesory lead that feeds your radio) to the input of the rocker or toggle switch. Now when you flip the switch the under hood speaker will be connected and automatically bridged with a mono signal that combines both channels.
I had it setup this way but using the bridge directly on the speaker im using will blow it. The reason why I want to combine the channels before the amp is because when i flip the switch, i want to combine the 2 channels into one, send that to the right channel then run the PA speaker off of the right channel of the amp, that way the wattage is limited. It will put out 80watts/ch. at 4ohms instead of the 220 bridged.
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Old 14th October 2007, 10:32 AM   #9
Glowbug is offline Glowbug  United States
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If you're needing a center channel for 5.1 content, why not get a real 5.1 processor and do it that way?
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Old 14th October 2007, 04:29 PM   #10
ppia600 is offline ppia600  United States
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Quote:
Originally posted by silentblackhat


I had it setup this way but using the bridge directly on the speaker im using will blow it. The reason why I want to combine the channels before the amp is because when i flip the switch, i want to combine the 2 channels into one, send that to the right channel then run the PA speaker off of the right channel of the amp, that way the wattage is limited. It will put out 80watts/ch. at 4ohms instead of the 220 bridged.
It will only blow if you don't pay attention to how loud you play it. You can always just use a capacitor inline with one of the leads to prevent it from getting bass too. Thats another reason to turn the volume down before hitting the switch.

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