From what I've read so far, and from my experience, I've gotten the following facts (pls correct me if they're wrong):
- Most aftermarket head units output speaker-level signals at a max of 10V
- Some aftermarket head units also have line-level (pre-amp) outputs with a max of 1V (usually on RCA plugs)
- The stock Alpine head unit in a 98 M3 with the HK option outputs something in-between: a maximum of 4-5V
- When adding aftermarket head units, most people connect the speaker-level outputs of the aftermarket head unit to the stock inputs of the HK amp (which expect 4-5V max), and it works fine

My question: Does this mean that It's not a good idea to turn the volume of an aftermarket head unit above 50% (assuming speaker-level outputs are connected directly to the inputs of the stock amp)? Can the stock HK amp take up to 10V of input signal, or will that damage it? Is it a good idea to put some custom resistors to attenuate the 10V to 5V (using the kind of line-level-adapter circuit described here: http://www.termpro.com/articles/faders.html except with different resistors so that the output is 5V)?

Thanks in advance.

Is it a good idea to put some custom resistors to attenuate the 10V to 5V (using the kind of line-level-adapter circuit described here: http://www.termpro.com/articles/faders.html except with different resistors so that the output is 5V)?



You asked a very valid question, and indeed it would be worthwhile to attenuate the output of a new HU to something closer to what the stock HU delivered. If you attenuate even 3 dB, that should be adequate.

Your link tells it very well, but I should add that speaker level outputs (as well as the stock HU) are balanced and not grounded. Neither signal lead is at ground. So you will need 3 resistors, although I tend to do it with 4 for convenience. This is what I do:

Referring to your link, wire 4 resistors in series. I would use (4) 100 ohm resistors. At the ends of the string connect your speaker leads from your HU. At the mid point of the string, you can ground this (as shown in the link), but I would try it floating (unconnected) first to minimize noise. You get your new attentuated outputs at the junction of the outer resistors and inner resistors. Basically the opposite ends of the outer resistors---look at the link and it makes sense.

Anyway, I wish I could draw a nice pic for you, but I have no scanner at home. Good Luck.

BTW, this is what you would do if you wanted to connect the speaker leads of a HU directly to the inputs of a power amp that has differential (balanced) inputs. Some amps, notably JL and Zapco, accept balanced inputs.

Matthew

Haha, nevermind, it turns out the HU i'm planning to get has 5V pre-outs! :) sweet

it's a pioneer DEH-P9800BT, the only reputable-brand HU with bluetooth cell phone integration on the market (from which you can make calls as well as receive calls).

i really appreciate the helpful info though. i was just about to buy the resistors and start soldering, and i kinda randomly went on Pioneer's web site just to look at the specs once again, and i saw that they advertise "High-volt (5V) pre-outs" for that model.

on a different note, does anyone have any experience with or opinions about this model? it's very new (came out this month)... but i've been waiting for it for a while.. they announced it last year, along with the specs and everything

Haha, nevermind, it turns out the HU i'm planning to get has 5V pre-outs! :) sweet



But they are not balanced and therefore not intended to directly interface to the factory amplifiers. You will have to run all new signal cables also, as the factory twisted pairs are not adequately shielded for single-ended preamp signals. Ultimately, you may find it easier to install new amplifiers as well.

But good luck.

Matthew

Alpine has a BT interface (although not as good) and 5v isn't too bad for preamp output voltage. BUT if you are trying to integrate into the factory amp you'll end up just using a different harness rather than your RCA outputs.

Hmm.. then what about the speaker-level outputs: are they balanced on aftermarket head units? I'm not very clear on what it means for a circuit to be balanced, but from what I understand, it means that the "ground" (aka "negative") audio signal is NOT wired to the chassis ground. And the cable has the 2 wires twisted around each other, possibly with a shield around them that's not connected to anything (or is connected to chassis ground?).

So you're saying that the factory HK amp takes in a balanced 5V input? If the speaker outputs of the aftermarket HU are balanced, then I guess I'll go with the solution we talked about - with the resistors. If not, then it seems like I don't have many options here... so I'll just connect it to the unbalanced 5V RCA outputs of the Pioneer CD player and hope that it doesn't pick up too much noise...

What do you recommend? I don't want to spend money on a new amp or speakers. I like the clarity of the sound with the stock HK system. Haven't tried it with an aftermarket HU, but if the quality of the sound doesn't go DOWN when I do, I'm fine with it.

Hmm.. then what about the speaker-level outputs: are they balanced on aftermarket head units?

The speaker outputs on all modern HUs is a balanced signal. What that means is that each lead carries a signal, and neither is ground. The signals are the inverse of each other, hence "balanced". This topology is essentially immune to most types of noise.

It would be easiest to use the speaker outs from your new HU, as you can retain the stock wiring to the amplifiers, and really only need to add the attentuation resistors. Or, run without resistors if you feel the HU will never be played excessively loud.

Matthew

Cool, thanks for the help. I think I'll run it with the resistors.
Just out of curiosity, how did you come up with the 100 ohm number? Is there a formula that would give me the output voltage?
To make sure I'm getting this right: is this the circuit that you're talking about? (the non-grounded version... for the grounded one, I'd split the 200-ohm resistor in the middle into 2 100-ohm resistors and ground the point between them... but wouldn't that unbalance the whole thing?)

IN+
-------+
|
100 ohm
|
+--------- OUT+
|
200 ohm
|
+--------- OUT-
|
100 ohm
|
-------+
IN-