How to amplify car speakers without stereo?

Question

I have a 1980 VW Dasher with its original speakers but no radio. It has an unusual (erroneous?) electrical configuration where most accessories (e.g. lights, cabin fan, etc) still run even with the ignition switched off and key removed (I've heard that's just how old VWs work, but never confirmed that). This results in the problem that installing any aftermarket radio or amplifier will run the battery down if I connect it in the conventional way. So 2 related issues here:

1) I don't really want a radio. I'd actually be just fine with some sort of simple, low-power amplifier that can drive the speakers, which I could plug a smartphone or something into (like with a TRS connector or whatever) to play music and whatnot. Has anyone had any success doing something like that? What kinds of products should I look for? Right now I don't even know what such a thing would be called. "Amplifier" encompasses too many different products.

2) I need to put my own toggle switch into this system somehow, so I can switch the radio/amp off when I'm not using it, and avoid running down the car's battery. How should I approach wiring this? I'd like to avoid breaking out any books on Electrical Engineering right now, and hopefully just accomplish the mission with the bare minimum of knowledge (I'm assuming the people who install car stereos don't necessarily know everything about voltage and current, so hopefully I don't need to either). In the absence of specific suggestions, links to more general guidance about doing this sort of thing would also be helpful.

Answer 1

What you need is a simple switch. Here's an example of one that you can order from DigiKey.

Your radio will use two wires for powering itself, the ground (GND) and V+. The ground wire should be grounded, that is connected to negative (-) terminal of the battery. The V+ wire should be connected to one connector of the switch. Battery positive terminal (+) should be connected to the other connector of the switch. Switch will simply short-circuit positive battery terminal to the radio when turned on.

This is the simplest solution that I can think of. Well, there is a simpler one, using just bare wires and connecting them when needed, but I guess that would be somewhat weird solution.

Answer 2

You can also get source sensing amplifiers, which will further make this simpler. If they detect signal from your smartphone, they'll turn on, if not, it'll turn off after some amount of delay.

The next question is whether or not you can (or would want to) hook your smartphone or other source directly to an amplifier. It might work, it might not. There certainly have been a number of developments as far as equalization and other features that they used to lack, but it'll all come down to the output voltage of your smartphone, and the acceptable voltage expectation of the amplifier. It won't hurt it to try it out; but it just might be too quiet or distorted (I'm guessing it'll be quiet if it doesn't work).

Answer 3

Many amplifiers have low-level inputs using RCA style connections. You can use one of those with a RCA to 1/8" stereo jack for your phone. Switching it could be accomplished through a switch like you mentioned or hopefully there's an ignition-only wire that can be used on the amplifier. They typically have an ignition input just used to switch the power.

Answer 4

While you could buy an amplifier, you won't save money that way vs. buying a simple car radio. $100 gets you a unit with a built-in amplifier (17W per channel) and an interface for connecting your phone. It also offers a convenient way to change the volume (instead of having to unlock the phone and messing with a touchscreen).

Amplifiers are generally intended for audio geeks and are more expensive. They also take up more space.

To avoid running the battery down, just switch off the radio. No external switches necessary.