v8voodoo

Don't ask why, it's a long story, but imagine the car this is for has no head unit to connect into........

If I get a lead which plugs into my iPod (3mm headphone type jack) and terminates in two RCA outputs, can I connect my iPod directly to an amp in my car with the desired effect of my iPod playing through the amp into the speakers?

My main "I wonder" is regarding the switching on of the amp that's normally done via the wire from a standard head unit and the loudness of the iPod.

Anyone done it and does it sound ok?

cheers,
Nige.

ru'

Should be fine; you'll need to connect the remote switch wire to the positive feed to turn the amp on - best do this through a switch so you can turn it off and save your battery when you're not driving.

graham72

dont think this will work, the voltage that goes through to the amp via rca's is a lot lower than headphone voltage thats why they are called pre outs, they go before the amp in a normal head unit whereas your ipod would already have amped the signal to go to headphones! seek pro help before trying in case you damage ipod or amp or speakers!!!

ru'

In fact, with 4V pre-outs these days I'd say that the iPod may actually produce less voltage from the headphone output than a HU pre-out.


It'd be safe to try; just make sure the iPod volume is right down when connecting, then turn it up when it's all switched on. I've fed PA systems straight from my iPod headphone out (into an input expecting 0dB, which is roughly 0.775V) and the iPod volume needs to be quite high to get a decent signal. You could call this 'pro advice' by the way...

wannaB4

Try finding the cheapest thing you can that has a docking connector on the cut it off and fit RCA plugs to give you a line out. Or there maybe something like this already.

v8voodoo

This is getting pretty interesting..!!
The last thing I fancy doing tho is b@llsing my ipod or amp up, then I'd have to sing to myself..lol
Keep the ideas flowing guys..! I'd like to get it done before next weekend...

Cheers,
Nige.

ru'

As in your original post, a 3.5mm stereo jack plug to 2 x Phono lead will be fine - using the headphone output at least allows you to control the volume (as far as I know, the line out on an iPod via the dock connector is fixed volume).


Still not convinced? Lets do some maths...

1) I'm worried the iPod will produce too much voltage and fry my amp inputs:

I'm not sure about the impedance of standard headphones, however the after-market ones I've seen are 16 Ohms. Let's say they're 32 Ohms to be 'safe', as this would mean the iPod needs to produce more volts.

So, again I'm not sure about the sensitivity of the standard headphones, but again the after market ones I've seen seem to produce about between 112dB and 123dB with an input power of 1mW (1mW = one thousandth of a Watt).

Okay, let's assume the worst that the standard headphones only produce 105dB at 1mW. Note that when you double the power, you only gain 3dB. We could assume that iPod SPL won't exceed around 130dB as this would result in hearing damage pretty quickly (if not instantly). So let's see how much power we need to get to 130dB:

105dB = 1mW
108dB = 2mW
111dB = 4mW
114dB = 8mW
117dB = 16mW
120dB = 32mW
123dB = 64mW
126dB = 128mW
129dB = 256mW
132dB = 512mW

Okay, let's assume the iPod won't produce more than about 512mW.
So, with our assumed 32Ohms load and a power of 512mW we can work out the maximum voltage from the iPod:

Power = (Volts x Volts)/Impedance
512mW = (Volts x Volts)/32
(Volts x Volts) = 32 x 512mW
(Volts x Volts) roughly equals 16 (as 512mW = 0.512W)

This means the absolute maximum voltage is 4V in our example. This is not high enough to cause damage to a line input through overvoltage, even if the amp isn't built for 4V pre-outs. It could cause the signal to distort, hence suggesting the volume of the iPod is turned up slowly from zero, but it won't damage the amp. Okay, distorted signals can damage connected loudspeakers (especially tweeters), but this is easily heard.

So, if you've followed me so far the voltage isn't a problem.

2) I'm worried the power from the iPod may damage my amp inputs:

Okay, so we know the estimated absolute maximum voltage. It's likely the line inputs on the amp will have an impedance of around 10kOhm (ten thousand Ohms) or more. We can use the same equation as above to work out the power going from the iPod to the amp inputs:

Power = (Volts x Volts)/Impedance
Power = (4 x 4)/10000
Power = 0.0016 Watts

That's 1.6mW, i.e. pretty much nothing at all!

So, hopefully we're happy that the power won't damage the amp!



Okay, I've simplified some things (no, honestly!) and I've made some assumptions (but always looking pretty much worst-case) but hopefuly I've added some weight to my previous statement.

Add this to the fact that I've done this many times (okay, not using car stereo amps) and hopefully you'll believe what I say.

wannaB4

Nice maths but how about using something like a sub level controller, which has rca in and out and would work just like a volume control.

wannaB4

one of these..

http://www.maplin.co.uk/images/Full/-10740i0.jpg

and one of these..

http://www.soundoffaudio.com/showpho.../name/n-2079-1

add an amp and job done!

ru'

Or just use the iPod volume control and the headphone output and save some money/clutter...