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Old 2nd June 2008, 07:12 PM   #1
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Default In-car mixer for first-time DIY audio-er

So I've done some DIY electronics stuff before, but usually stay closer to software. I've built a CMOY (but that hardly counts) and I've done some stepper motor stuff, but never a serious audio project. I'm interested in building an in-car mixer.

Basically I have a line in jack to my car, which is currently just hooked up to either an XM receiver or my MP3 player through an RCA connection (with a GLI to keep the buzzing out of the XM's signal). But now I'm looking at getting a GPS receiver, which also has a line out. I'd like to be able to have it hooked up along with the XM/MP3 player. So at first I was thinking just a two-input mixer, nothing crazy. But the MP3 player can have trouble with the car speakers, so maybe I'd add a post-mix amp, too. And then, well, why not add a toggle to choose between a few inputs. So the final idea is:

Two-channel stereo mixer. One channel is always from GPS. The other toggles between three inputs. Amplify it post-mixing. All powered through a cigarette adapter (ugh). I've got 15A combined power through those, so I'm thinking that should be enough to power the mixer, the XM radio and the GPS.

I'm new to audio stuff, so any input/advice/pointing me in the right direction would be greatly appreciated. Thanks!
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Old 2nd June 2008, 10:07 PM   #2
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You can use a couple of op amps to do the trick.

The inverted input of the op amp will be at virtual ground, so you just parallel the input signals with a series resistor per input.

Gain is adjustable, so if you need a little extra output from one signal, that can be covered with resistor values.

Rather than switching inputs out of circuit, you can use you switch to mute the signal from the unwanted device.

15A is a lot to draw through the lighter constantly. If you're going to this effort, grabbing an accessory lead behind the dash will be a simple effort.

If someone else doesn't follow up, I will after my 6 yr old isn't helping me type.
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Old 4th June 2008, 11:43 PM   #3
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A quick follow-up…

What I’m talking about is using op amps (IC’s in small DIP’s) to make an inverting voltage follower. They have two inputs, one inverts the signal 180 deg, one keeps phase intact.

The inverting input can be used as a summing node. Because it presents a very low input impedance to the source, current from an incoming signal will flow only towards the input, and not back into another device that may be connected to the same input. As a result, signals into this input are mixed.

The output from the op amp can be inverted back to 0 deg by another inverting follower if you’re one who’s concerned with absolute phase.

Gain can be adjusted by a resistor or two in the circuit, and modern op amps can pass output signals that swing pretty much to rail voltage.

This would be a fairly simple and inexpensive circuit, but would need to be built well enough to hold up to vibration.

Perry probably has one or something essentially the same (maybe to sum R+L to mono?) on his site:

www.bcae1.com? I think that's right.

Be careful if you go there, you'll be on it for hours. It's very comprehensive.

Or do a search for “voltage follower”

You mentioned another point:

Quote:
But the MP3 player can have trouble with the car speakers, so maybe I'd add a post-mix amp, too.
If it’s going through the line-in jack, the amp inside the head unit should be powering the speakers. An external amp wouldn’t solve that problem. What kind of trouble do you have?

And do all 3 units have mini-phono type plugs (like headphones), or something else?
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Old 5th June 2008, 10:31 PM   #4
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Thanks for the help! This all looks like the way to go.

Regarding the mp3 player - basically, I have to max the volume on both the car speakers and the mp3 player itself to get anything near acceptable volume.

I have 3 mini-phono stereo jacks, and want one additional input in case someone in the passenger seat wants to use their mp3 player. So it looks like it should be pretty straightforward.

Thanks again!
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Old 5th June 2008, 11:02 PM   #5
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Quote:
Regarding the mp3 player - basically, I have to max the volume on both the car speakers and the mp3 player itself to get anything near acceptable volume.
So that's from a headphone output. If there's no line output (which wouldn't go through the mp3 volume control), it'd be wise to give the mp3 a dedicated input with a gain of 2x input voltage or so.

The reason I asked about mini-jacks is they are available with switches built-in. It may be reasonable to have each jack switch itself into mute "mode" when the plug is removed and do away with the toggle altogether. The drawback is that anything plugged in will be mixed into the output.

If you take this overall route, be aware there will be some power supply considerations. They can be handled a couple of ways, and aren't really anything to worry about at this stage IMHO.
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Old 6th June 2008, 01:03 AM   #6
dangus is offline dangus  Canada
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You could first try a passive mixer... just use something like 10k to 47k in series with the two inputs and then to the volume potentiometer. This would be a lot simpler and have much less to go wrong.

Google passive mixer schematic and you'll find ideas like this:
Passive Line Mixer
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Old 7th June 2008, 03:34 PM   #7
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dangus:
Quote:
This would be a lot simpler and have much less to go wrong.
True, but it wouldn't address the mp3 volume issue.

It sounded like DeepOmega wanted to build something challenging. There's not much to go wrong with simple op amp circuits, and this may spark an interest in further projects for our new member. I could be wrong.
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Old 8th June 2008, 06:58 AM   #8
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Quote:
It sounded like DeepOmega wanted to build something challenging. There's not much to go wrong with simple op amp circuits, and this may spark an interest in further projects for our new member. I could be wrong.
Pretty much. I do better work when I'm doing harder work, so hey. I'll start getting my hands dirty planning this. Rest assured if I have issues I can't work out, I'll post here! Thanks all!
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Old 8th June 2008, 04:25 PM   #9
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To avoid noise from ground loops and so you don't need ground loop isolators, you should use noise cancelling input circuits for each of the sources. Something like the attached circuit will work.

When prototyping the mixer, you should build the first one on a breadboard (the type with spring-loaded contacts). This will allow you to test the circuit and make quick revisions.

Have you decided whether you'll use a split supply (positive and negative voltage) or one with a virtual ground (reference voltage at 1/2 of the supply voltage)?
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