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Aftermarket head, stock amp
Aftermarket head, stock amp
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Old 27th May 2004, 05:15 AM   #1
Thorshavn is offline Thorshavn  United States
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Default Aftermarket head, stock amp

I would like to attenuate the voltage reaching my car amplifier by about 50%. This is because it is a stock amp with only high inputs, and my aftermarket head unit puts out a great deal more power than the original head unit. This hasnít been a problem for me until I added a subwoofer suing the low RCA outs on my head unit, now the system isnít balanced because the stock amp is so loud.

I realize that I could wire a potentiometer and find a good level, which might be what I end up doing, but I thought I would ask the forum if anyone could guesstimate a good resistor value for me to put between the head and amp.
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Old 27th May 2004, 10:59 PM   #2
maylar is offline maylar  United States
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Whatever the impedance to the amp is?
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Old 30th May 2004, 07:14 AM   #3
tsmith1315 is offline tsmith1315  United States
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It *would* help to know the input impedance of the stock amp.
But, you can make a voltage divider that should work anyway.

Per channel, tie two 7.5 ohm resistors in series and connect the pair across the speaker outputs of the head unit. Obviously, these will dissapate a fair amount of power, so they will need to be rated for 15W or so.
Output of the voltage divider will be at the junction of the two resistors for (+), and head unit output (-) for ground. Maybe a diagram will help...

Unless the input impedance of the stock amp is very low, this should divide the voltage roughly in half. An option to reduce the voltage by two-thirds is also noted in the scan.

It may be helpful to the group to know what type of amplifier it is, as someone may have worked with it before.

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Old 31st May 2004, 12:25 PM   #4
Thorshavn is offline Thorshavn  United States
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Yes, I thought about that one, But I don't want to use big resistors, and that's just a little too much stuff to bundle up with the wiring harness for four channels. Thanks for the idea though.

I guess what I really wanted to know is if there is a standard impedance for speaker level inputs. I guess I'll just have to go to the trouble of wiring a pot inline in the car and seeing what resistance works. I imagine that the input impedance is relatively high and power resistors won't be necessary.
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Old 10th June 2004, 12:23 AM   #5
DC Dave is offline DC Dave  United States
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Depending on quality and power output of your headunit you might consider bypassing the stock amp all together.
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Old 10th June 2004, 01:32 AM   #6
xplod1236 is offline xplod1236  United States
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dont you have a volume/gain control on the amp?
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Old 10th June 2004, 03:24 AM   #7
Brian Donaldson is offline Brian Donaldson  United States
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I would carefully open the stock amp and look see if it's worth using. If it doesn't have a SMPS (switch mode power supply) It likely won't sound any better than the head directly running the speakers. I would bypass it with some jumper wires and give it a listen.
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