Adding preamp outputs to factory radio

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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?
 
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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.
 
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...
 
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.
 
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.
 
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.
 
What's the simplest way to attenuate speaker outs from a stock stereo to be safe inputs to an amplifier?

A series resistor on the RCA center conductor. The value should be about one to four times the amplifier's input resistance.

A voltage divider would be a better method, and may require capacitor coupling if the stock stereo is BTL.

Good line level adapters use audio transformers followed by voltage dividers.

A voltage divider followed by a differential amp would probably be about the best solution to be worth it's cost or effort.

Tim
 
I've done both pre-amp input and output from a stock Toyota radio.

The input was done by removing the AM Stereo decoder chip, and substituting an op-amp in negative gain on an adapter board. (It was also reverse-phase, but I figured that was relatively unimportant.)

Output was realtively easy. I followed the signal path past the pre-amp to the tone/volume/balance/fader, and where it led back to the board was where the input coupling capacitor was located. I simply tapped into the 4 outputs after the fader control and ran them to RCA jacks using some 63 ohm resistors and cat5 network wire. (I found that the pre-amp would sometimes oscillate without the resistors, especially when changing inputs.) I'd guess signal level at that point was pretty close to 500mV, as the amplifiers treated it similar to an inexpensive aftermarket deck, in terms of gain settings.
 
that's what i plan on doing to my ford radio. i was just gonna tap off right before the power amp chip. i'll be sure to add some output resistors too.

as for the input, i used an adapter for the cd changer that allows me up to 3 inputs plus the changer itself.
 
Yes, I do!

I have about a dozen boards coming in next week designed specifically for this very purpose. I have been doing custom line drivers for OEM radios for about 5 years, from simple two channel to others with aux inputs. The boards themselves are comprised of all SMD components, and utilize a BUF rail splitter (cleanest sound) for the OPA driver. I also include a half-second soft mute turn on circuit for those that need it. Each board is two channel, and must be installed by Envision Electronics...unless you REALLY know what you're doing and don't want a warranty. They are $110 each, installed...a bargain considering you retain factory cosmetics and seriously decrease your chances for getting it ripped out of the dash one night.

I will post more details and pics once the boards come in...

my site: www.envisionelectronics.com
 
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