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ECM 19th July 2004 02:36 PM

Adding preamp outputs to factory radio
hello all, the title says it all. i'm installing a system in my truck, but i want to retain the factory radio. instead of using a hi-lo level converter, i want to add preamp outputs. can i just tap off the input to the internal power chip? will this give me preamp level signals?

audioPT 19th July 2004 06:16 PM

Yes, you can.
Use a 1uF Cap.

This is NOT the best way, but can do the job.

ECM 19th July 2004 06:27 PM

thanks for the reply. what would be a better way? by tapping of the inputs to the power chip, you bypass the internal amplifier stage. wouldn't that lower the noise floor?

sangram 20th July 2004 09:33 AM

No, that's fine. Except you should have a line driver and disconnect the input stage of the internal amp completely, by removing the coupling cap. This is to:

1. Prevent loading of the preamp stage by adding a second load to it.

2. Bring the signal up to a level sufficient to drive amps and reduce chance of interference.

ECM 20th July 2004 02:07 PM

ok, so let me get this straight. i remove the coupling caps to disconnect the power chip. then i use some caps and connect them to some shielded wire and make a preamp out. bump the signal level up some by using a line driver. is this the way to do it?

how much signal does the factory unit output, typically? can i remove the power chip completely?

thanks for all the inputs...

richie00boy 20th July 2004 06:37 PM

I really wouldn't bother with all this messing about. Just attenuate the speaker outputs at the amp end. If you did it properly you could even get a nice low-impedance hi-volt balanced line. Wooo just like on the top dollar units...

The internal amp in the headunit will sound a lot better driving a high-impedance amp because it will be running in class-a at the currents required to drive the amp.

ECM 20th July 2004 11:06 PM

i want the cleanest signal possible since i have an eq and a couple of crossovers down chain. i want to keep the noise floor as low as possible. by bypassing the power chip, that is one less gain stage.

richie00boy 21st July 2004 10:01 AM

If noise floor is your priority and you don't mind building a small box of tricks, you could very easily knock up a balanced line receiver. This would be driven directly from the headunit speaker outputs (which are differential) and then attenuate and convert to unbalanced to drive your crossover/eq etc.

This solution would remove any ground loop issues and also offer by far the best rejection to RF and other noise pickup.

ECM 21st July 2004 02:01 PM

how would i build that? are there any schematics online?

richie00boy 22nd July 2004 05:04 PM

A standard balanced or differential receiver would do. You would just need to either attenuate the signal prior to going in the receiver, or set the receiver itself to attenuate.

The cheapest and simplest way would be to use a differential amplifier as found in any op-amp text book. There are variations of this using upto 3 op-amps that improve the common-mode (noise) rejection.

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