This is a car audio project. I want to build a little box that has 3 pairs of RCA inputs and one pair of RCA outputs. It will be hidden so it doesn't need to be pretty, I just want to figure out how to do the circuitry. The inputs should be selected as follows:

Input A - active by default

Input B - selectable with a switch mounted somewhere in the car

Input C - hardwired to the car's navigation unit low-level output. It is activated by the signal that would normally be used for muting the factory stereo while the voice command plays, and will serve the same function here.

I know how to do this with a bunch of relays, but it won't be a clean solution. I'm also worried about the speakers firing when the relays make contact. I've scoured the Internet for a 12V RCA input switcher and it doesn't appear to exist, so... how do I make this device?

Dan

 

You're wanting to switch line-level audio, right?

 

No, low level. Cars do not use RCAs for line level.

 

Here is a basic sketch of the concept.

 

I would suggest you have a look at analog switches. There heaps of different types, some can be controlled by switches others by micros.

 

I would suggest you have a look at analog switches. There heaps of different types, some can be controlled by switches others by micros.

I'm not coming up with anything that runs on 12VDC.

 

Line-level audio is 10k impedance; low power, low voltage.

A 4000-series CMOS 4066 IC is a quad switch. They have an impedance of ~150 Ohms.

You could use opamps on the output side as a voltage follower/buffer to keep the impedance low.

You'll need to use a cap in series on the input, and a couple of 20k resistors to "float" the inputs at Vcc/2.

I'll see about posting a schematic later.

 

Line-level audio is 10k impedance; low power, low voltage.

A 4000-series CMOS 4066 IC is a quad switch. They have an impedance of ~150 Ohms.

You could use opamps on the output side as a voltage follower/buffer to keep the impedance low.

You'll need to use a cap in series on the input, and a couple of 20k resistors to "float" the inputs at Vcc/2.

I'll see about posting a schematic later.

That would be sweet. Just to be clear, by "line level" I mean an amplified signal and by "low level" I mean a preamp signal. I am trying to switch between non-amplified signals (like out of a phone or an MP3 player) to expand the single RCA input on my head unit.

 

No, low level. Cars do not use RCAs for line level.

Actualy, line-level audio means cca 1.5VAC signal or less, which is pretty low level. You may have tought of power lines which are kind of a a different league.

 

Different league? How do you know I'm not rocking 800kV 3-phase under my hood?

 

Because you are still alive.......

I think

 

Tell me about the signal from your navigation unit to mute the stereo.

Is it normally 12.7v, and 0v to enable sound? Or the other way around? Or something completely different?

 

The factory wiring diagram doesn't say, but I'll figure it out as soon as I tear it apart. Let's assume for now that it's a +12V signal. Since you're helping me out I'll dive in there and test it as soon as I can find the time.

 

A 4066 bilateral switch will work, unsure if any noise would be added, you'd want to have a very clean/regulated power supply.

In the application you are using, are the RCA jack shields at automotive ground, or are they floating? This isn't something to assume, measure with an ohmmeter when power is off.

 

RCA jack shields are at chassis ground, and all my RCAs will be grounded at the source end to reduce induced noise.

 

What about your MP3 player?

 

What about your MP3 player?

Can you clarify your question? I assume the MP3 player (aka cell phone) chassis uses a negatively grounded RCA. Also it charges from the car's 12V source.

 

By the way, to give you an idea of my knowledge level, the first two links on op-amps in your blog post are over my head. I am going to try to digest the op-amp basics document because I want to learn about this stuff.

 

OK then, see the attached.

The TL07x can be a couple of TL071's, or a TL072 dual opamp. They're used as a voltage follower/buffer. You could use a TL074 if you wanted to, and use one of the channels for the Vcc2 signal instead of a 741 opamp, but you'd have to do something with the unused channel.

You could use a TL082, but they are much noisier than the TL07x opamps.

The 741 opamp and it's R10/R11/C15 input network establish a reference voltage that will be Vcc/2.

x-In-A is normally connected to the xouts, where x=R or L.
When S1 is closed, x-In-B is connected to the xouts.
If at any time NavInput goes high, the C input is connected to the xouts.
The 10k resistors and the 100nF (0.1uF) caps slow the switching speed to help avoid "pops" when transitioning between different signals. You may have to "tweak" these values a bit (increasing the resistance) if the popping is excessive, or decreasing the resistance if the switchover is too slow.

Note that the 4093 quad Schmitt-trigger NAND gate needs a 100nF cap across it's Vdd/GND pins; they are not shown in this schematic.

 

Wow, that's awesome, thanks Wookie! This circuit is a little more complicated than I expected. I'm planning my audio install for late this month, and building this will probably come afterward. I might have questions, so don't be surprised if I revive this thread.

BTW, I did a Google search for some of the component part numbers and I came upon a forum where they were discussing the tonality and noise floor of different op-amps in audio circuits. I'm installing a high end, low noise system so acoustic transparency is very important. If necessary I would trade noise and coloration for a pop while switching. My original plan was to use DPDT relays, but I don't know if it's a good idea to run the signal path through a relay. Considering thatoneguy's point that the car's charging supply isn't perfectly regulated, the coil could cause induced whine in the signal. (Edit: that is, unless I could make the signal go through the relays while in the normally closed position)

Oh well, experimentation is half the fun, right? When I finish your circuit I'll report back and let you know how it works out!