Too late at night to test this stuff, so I thought I'd come here and ask for you guys to tell me the holes in what I'm thinking (it might just be all together wrong) I've installed an amp in my car (cheapo amp, not worth talking about) and I want it to accept two signals (one from my iPod and one from the CD player). Now, the original plan was to put the signals on a switch, then just flip the switch to pick iPod or CD player (of course, I'd need a 4PDT ON/ON switch) but then I was thinking: could I set it up so that both signals can go to the amp and just turn one signal source off to choose between them? This would be preferable b/c there would be no switching and I wouldn't need to find a spot to stick a switch. So, the signal from my iPod is unamped into an 1/8" jack I made today. The CD player's signal is lightly amped (I say that b/c it's something like an internal 50 Watt amp.. something just to push the speakers). Obviously I don't want the signal from one travelling the line to the other, so I thought "Maybe if I put a diode on each signal line to keep the signal flowing one way down the line, that would work." I would have just tested it tomorrow and seen what happens with diodes on the unamped iPod signal (to see if it messes it up any), but wanted to see what you guys thought. Anyone know if the voltage required to turn on the diode would F with the quality of the signal? Think my iPod could satuate the diode and cause the signal to clip? If nothing else, I'll go back to the switch idea, just thought this way would be a little better Thanks for the responses in advance
The Diode would rectify the signal. A decent audio signal will be an AC signal centered around 0V. You would destroy the signal if you used a diode. I would just use a switch to be safe.
Easier than a switch, hang a relay between the two sources & the amp. Let the off state be the CD, power the relay by plugging in the iPod & have the on state make it the source ?
explain what you mean. A socket with a switch? I want to get away from physically flipping a switch. The relay sounds promising if I can find one that will work with the low current of the iPod. I'm not sure if they make relays that output to the same pin. If not, I'd have to tie two output pins together and then I'd be afraid that current would flow the wrong way up my iPod line which I don't want. I've not used many relays in my time, usually just what was provided me (nothing for a personal project) so I don't know all the different types.
you can get 3.5mm (1/8") stereo sockets with bypass connectors; when a plug is inserted the two signal connectors are connected to the signal parts of the plug, when the plug is removed the signal connectors are connected to the bypass (ie. your CD player)
that would be fine and dandy if I were making a box, but it's my car and I've run wires... it's very difficult to run wires from the dash to under the back seat and I've already had to do it 3 times (speakers (and CD player signal), power, iPod signal). And I have my male 1/8" jack sticking out from my E-brake. I'd have to rewire the entire thing, so I think the relay would be easier at this time
Of course, you could just use an op-amp adder circuit to add the signals... see a circuit diagram here ('cos I'm too lazy to draw one) - note that the feedback and input resistors are the same value (10k in this version), so the amp will have unity gain (i.e. non-amplifying). You can ignore the OS pins, and you can ditch the trimming caps from the power lines, assuming you have some fairly close to the circuit. Obviously, you would need two of these to add two stereo signals. I would definately suggest not using a 741 op-amp, tho
Well, depending on whether you are an audiophile or not, just soldering the wires together would work. I would not suggest the relay from and audiophile perspective, as most relays have a great deal of interference they put out when the relay is *on* (due to the electro-magnet). However a solid state relay doesn't have this particular issue, but then it has the issue of having a high-impedance which may reduce the quality a bit. The op-amp solution by Pandalet is one of the more elegant solutions. However, wiring the plugs into a 'reverse' y-splitter would work just fine two. The extra bit of copper isn't going to hurt the signal quality at all (at least not in the 20-20k range), and doesn't require you to do any switching. It also offers the cleanest signal path.
but wouldn't that possibly cause current to flow INTO my iPod from the CD player? don't I not want that?
Yes, and no. If your IPod is off, while the other unit is on, it really shouldn't be an issue, since the IPod will be shown as an open circuit then. The output amplifier would not be activated. In addition, even if it was on, you are talking levels of approximately 2v P-P, at about 100-200ma. This is because those outputs are set to drive headphones or line-outs, and as a result, have a very high impedance (about 10k Ohms worth). Also, the amplifiers will have already been built to handle the back-emf from the headphones. So all in all, it'll work just fine, using some of the engineering that needs to go into these products. If you are *really* concerned, or a purist, then go with the switch or Pandalet's suggestion. It all depends on how much work you want to put into the solution. Some people go for 'elegent', others just go.
I think I'm gonna try that op amp adder. I ordered some op amps that will easily run on car voltage and it said "very low noise, high speed" in the description, so hopefully it'll work out well (OPA725 if anyone cares). I'll build it and test it in a lab on campus to make sure it works like I want it to (to make sure I don't mess up) and they only came in SMT, so I'm gonna get some SMT-DIP converters so I can make it look nice on a breadboard (maybe I'll go to campus and make myself a nicely traced copper clad board). Thanks for all the input guys I knew I could count on you
