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Thread: Fading with a 2 Channel Amp?

  1. #1
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    Fading with a 2 Channel Amp?

    I'm working on a mac mini install which only outputs stereo sound. So I figure I'll simply use splitters for the front and back speakers and a 2 channel amp. But since I have kids I desperately want to be able to fade the sound to the backseat of my van so I don't have to hear the same kid music every single day.

    I know very little about car audio so I'm not sure if this is either possible and/or a good idea. But it seems to me that it would be relatively easy to get parts from radio shack that would contain four RCA female adapters connected to two potentiometers or two switches that would sit between the amp and the front L/R speakers. Then simply flip the switches or dial down the volume in just the front speakers. No voltage, no sound. Viola! 2 channel fade for less than $10.

    Or... is that a stupid idea?

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    It seems that you have 2 ideas in mind- fading the speakers(ie:changing the volume level) or switching different pairs of speakers on and off using a switch.

    I think that all of this would be easier if you used a 4 channel amp- and will also give you better sound. Most of the time, motorcyle amps are cheap and small and work very well.

    Using switches would be the easiest way- but the speakers would only be on or off, no middle ground.

    The (more expensive) way to fade the sound would be to use either a remotly mounted gain control knob for the amp, or to use impedance matching volume controls- niether is very cheap.

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    I'm leaning towards trying the switches at this point. Seems simpler to me. My goal is to turn off the sound, not make it quieter. So on/off should do exactly what I want. It'd be nice if i could find some type of dual switch that flipped them both on and off at the same time. But that's not the end of the world.

    I guess my biggest concern is whether this would harm anything i.e. the speakers/amp etc? I don't know why it would, which is why I thought I'd ask. Then secondly I'm concerned that it'll cause any distortion/buzz in the sound quality.

    Can you think of any reasons why this may happen?

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    On a 2 channel amp, you are increasing the risk of hurting the audio- you might not, but it is a possibility(remember, the resistance the amp 'sees' changes when you add or remove speakers, so having multiple speakers on the same outputs will make the sound louder or quieter every time the switch is flipped- this is part of why I recomended a 4 channel amp to try to separate the different speakers. Assuming that you can fit it into your budget of course...

    There are a couple of different ways to control the audio to the speakers, there are also other ways than what I will mention here, so don't feel limited to my suggestions.

    First, I think you will need to understand the different signals entering and leaving the amp- the rca inputs are known as low level, or are a very low voltage. The speaker outputs are known as high level- because the voltage is much higher

    it is always easier to control the low level signal than the high level signal- because the high level will directly affect the loudness of the speaker, whereas, with the low level signal, you can compensate for anything with the amp gains, and because it is a lower voltage signal, there is less clarity lost in the added stuff.

    The absolute, easiest way to do this would be to use 1 double pole, double throw switch on the low level side.
    that switch would have 2 spots, to allow it to control both the right and left audio channels.

    The best way to control the high level side would be to use a switch that controls a relay that controls which speakers work. The switch might not work by itself because it might not be rated high enough to allow the audio to properly pass through.

    Wow, sorry about the book...

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    Aha! Now I understand what you meant about the 4 channel. Thanks for the clarification. I was wondering about whether the volume would go up. Makes perfect sense now. So you're saying the volume would go up based on the resistance difference between the front and back speakers. So if the front and back speakers were all the same resistance, then twice as much wattage would go to the back speakers when I flip the switch. Is that the idea?

    I see what you mean about the low level side of the amp. But I don't think I can do that with the mac mini without an external sound card. Because it only outputs a stereo signal. If I flipped the switch on the low level side it would just cut the signal off completely to the amp. Wouldn't it?

    This is a cheap enough solution that I think I'll just give it a try. If it annoys me too much I'll try something else.

    double pole, double throw. Sweet, that sounds perfect.

    Thanks for everything. I'll post some pics when I finally get this ***** installed.

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    Quick electrical question... The DPDT switches at radioshack.com have various volt and amp ratings on them. I assume I want 12VDC, it's the amps that I'm unsure of. If I pull my speakers out and put a multimeter on them, do I use that ohm value and 12 volts to calculate the amps? V=IR?

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    Quote Originally Posted by macminivan View Post
    Aha! Now I understand what you meant about the 4 channel. Thanks for the clarification. I was wondering about whether the volume would go up. Makes perfect sense now. So you're saying the volume would go up based on the resistance difference between the front and back speakers. So if the front and back speakers were all the same resistance, then twice as much wattage would go to the back speakers when I flip the switch. Is that the idea?

    I see what you mean about the low level side of the amp. But I don't think I can do that with the mac mini without an external sound card. Because it only outputs a stereo signal. If I flipped the switch on the low level side it would just cut the signal off completely to the amp. Wouldn't it?
    you really don't need a sound card unless you feel that the sound quality is too poor for your listening tastes

    this is kind of where it gets tricky-- if you were using a 4 channel amp, you would split the stereo signal from the mac-- so that you now have 4 low level cables(both positive and negative wires)-- you would send 2 of them to the switch, and then on to the amp to work the front speakers, the other ones would go to the amp to work the rear speakers.

    using one 2 channel amp is where the differnt impedences come into play-- the best way to switch speakers on and off here would be to use relays on the high level side, because there is no way to separate the front and rear speakers before the amp

    Quote Originally Posted by macminivan View Post
    Quick electrical question... The DPDT switches at radioshack.com have various volt and amp ratings on them. I assume I want 12VDC, it's the amps that I'm unsure of. If I pull my speakers out and put a multimeter on them, do I use that ohm value and 12 volts to calculate the amps? V=IR?
    most low level signals are between 1-8 volts, and while i have not measured them, i would guess that they are not over 1 amp current draw, so almost any DPDT switch will work(i like the toggle switches that snap into place, instead of the slide switches that might get bumped halfway).


    just wanted to add, if you wanted, you could hook up a aux input on the other side of the switch, so that you could still listen to a mp3 player up front, while the other speakers are playing the computer audio, you would only need to add a little bit of cabling and a small headphone jack.

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    Well, you've sold me! A four channel amp it is. Thanks for all the advice.

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