Radio

[Guido]

I replaced my factory cassette radio in my '92 LSC with a CD player but kept the factory amp and speakers. Do I use the radio's hi-output speaker connections or the line-out connections? I was told to use the hi-out wires - and I did - but with the volume at "1" it's still pretty loud and there is always some hiss, even when playing a CD. Did anyone replace their deck and maintain the rest of the factory system? Should I try the line-out connections? I was told the line-out would damage the factory amp. Is there a gain control on the amp? Any suggestions? Thanks

[Lscman]

The factory amp is intended for speaker level outputs, not line level. The problem you're having with hiss and "too loud" is the new head unit has much higher wattage (and voltage) speaker level outputs than the factory head unit. If you were to reduce input voltage to the factory amp, it would reduce both the volume and hiss proportionally. There is an inherent "hiss" noise on all head units. The signal does not overcome the hiss at low volume dial setting. The solution is to create a voltage divider at the amplifier or head unit to cut the voltage level. A series resistor in the line will do the trick. There is a resistor inside the amplifier that is somewhere between 8 and 32 ohms from speaker polarity to return. A series resistor in-line with each "polarity" wire will do the trick. They will need to be somewhere between 8 and 30 ohms to cut volume in half. A resistor bridge could also be added at the amp between polarity and return wire, across the amp input. This will drop the input voltage much more, but it must be used in conjunction with a resistor of 8 ohms or more between the bridge resistor and the radio head unit. A bridge "could" be built that preserves the 8 ohm loading your head unit is designed for, but higher resistance generally results in a better S/N ratio anyway. You should alter the resistors until you get good volume at 50% volume (per dial). This will maximize signal to noise ratio and minimize hiss. You can modify an AMP that is designed for speaker level inputs to accept line level inputs. The input loading resistors will need clipped and higher impedance resistors around 1K installed with RCA jacks. The gain may not be ideal and you may need to have some electrical circuitry to attempt it. I'd advise against it.

[Guido]

Ahhhh, OK thanks for the help. Do you know if there's a gain control in the amp that I can adjust? If so will that help? Do you think this higher watt CD player might damage the factory amp? It's rated at 17W RMS and 45W max. What's the factory deck rated at? Well, if worse comes to worse can you send me a rough sketch of what you mean with the resistors? Will a Potentiometer work? Just wondering. Thanks again!

[Lscman]

Normal fractional wattage pot will smoke, since it needs to be high wattage. However, it will not get you high. With 20 some watt RMS amp, there will be a significant amount of this power dissipated in the resistors at max volume dial settig. However, the "average" power will be substantially less and 5 watt resistors should do, depending upon the values you wind up using. Experiment with cheapo small resistors and buy power resistors when you determine what values to use. Most pots are 1watt or less. There is no gain control in a factory amp designed for factory head units. No mfr installs unnecessary componentry, especially pots that need tweaked and can fail. Install one resistor in series with polarity wire between the head unit and the amp and an optional resistor from polarity to return right at the amp for further attenuation. The series resistor can not be between the second resistor and the amp. It must be on the head unit side of the second resistor, in the polarity wire. Let me explain it in a more visual way. Imagine a 10' lamp cord with a red polarity wire and a black return wire. Remove a 2" section of the red wire at the 5' mark and solder in a resistor. At one end of the 10' length, solder another resistor from red to black. Last, connect the red and black wires on the end with the resistor to the amp and the other end 10' feet away to the head unit. As you INCREASE the resistance of the resistor at the 5' mark (in the red wire) and/or DECREASE the resistance of the resistor on the end near the amp, the volume will decrease. Less voltage will reach the amp input. The resistance value of the two resistors added together should be somewhere between 20 and 50 ohms. Definitely not less than about 16 ohms or your head unit could be damaged due to insufficient inpedance. OK now, draw out what I said on paper. It is a simple voltage/current divider. The aftermarket head unit will not damage the amp. The factory amp is probably no more than 30W RMS. Perhaps you should just bypass it to improve S/N ratio. Try the head unit only and decide if it's loud enough. That's the kiss method. Lscman, Penn State University - Electrical Engineer, Class of '81

[This message has been edited by Lscman (edited September 30, 2000).]

[Guido]

Thanks for the help, I think first I'll try bypassing the amp (KISS) then go from there. Just one more thing... where is the amp? I was told the "amp looking thing" under the rear parcel shelf is the door lock computer.
Thanks again.

[Lscman]

The amp is where you said and it looks like an amp. The door lock CPU is a 3x4" square plastic box and looks nothing like the amp. It is mounted there too, under the rear shelf that the speakers mount to.

[Sexly]

Well I just hooked up my Rockford Fosgate CD player, which does 40x4 peak, and something like 27x4 RMS, and there is no distortion through the factory amp. I am pretty happy with that, cause it gets L O U D! Anyone out there got a JBL amp they are selling?