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Originally Posted by UKenGB
Doug, I see what you're saying, but in that case any amp would be able to handle speaker level inputs and that cannot be true otherwise there'd be no need for high level (speaker) -> line out converters.
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I don't think you quite followed me
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Originally Posted by UKenGB
Let's forget the 5v bias of this BMW system,
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You can't - it is the critical factor in what you are trying to do
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Originally Posted by UKenGB
it is certainly not true that a line level signal can drive speakers and although far from being an electronics expert, it is my experience that indeed you should not send the speaker level ouput of one amp straight into the line level input of another.
I'm not being trying to be argumentative about this, I just want to be able to understand, partly as I want to feed the output of my radio module into my Nakamichi amps (line level input only) and partly just because I always need to understand with what I am dealing as I am then better able to figure out any variations I may need and/or fix any problem that arise subsequently.
Some facts would help me here. A traditional line level signal would be about 2v, with some system up to about 4v (IIRC Sony started doing this with their ICE), but what would typically be the equivalent at the speaker?
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Let me answer your last couple questions first and then you can read the long long long answer
1. Consumer Line Level Inputs traditionally meant 100mV - 1V, but this has been blurred recently as you noted.
2. Read the diatribe and I'll answer you at the end...
Okey Dokey - let's start with some basics
And remember, you asked to
learn, not just be given an answer!
First: Power = Amperage * Voltage, also represented as P = I * V
Second: Voltage = Amperage * Resistance, or V = I * R
Therefore: Power = Amperage^2 * Resistance
The critical point of this if resistance goes down, then amperage goes
up to maintain the same power. Why does this matter? Because electronic components are rated to handle a given amount of power, and typically a given maximum amperage. If you exceed this amperage (or power), bad things happen.
Now, let's talk amplifiers. Amplifier is a general term used to describe any device that makes a signal greater. Therefore, in general terms, all headunits have an amplifier in them. In each case, that amplifier was designed to produce a given amount of Power when presented with a given Resistance.
Think of the power amplifier you'd like to add to your system - it probably produces X watts into 4ohm, 2*X watts into 2 ohms, and likely 4*X watts bridged into 4 ohms. That ohm rating is determined by the speakers you hook up to the system, but the extra Power doesn't just magically appear - it requires additional Amperage to run through all of the components of the power amplifier. Therefore, even though the amplifier
can produce X watts into 4 ohms (and the associated amperage, I), the designers must use internal components that can handle 4*X watts, and consequently 4*I amps. Still with me?
Ok, so back to your head unit. There are really only 2 types of load a head unit will ever be presented with: speakers (as in your case) or the input stage of an amplifier. Speakers, as we've already discussed, tend to be in the 4ohm range (8 ohm for home audio). The input stage of an amplifier, however, is closer to 1,000,000 ohms. Why is this the case? Recall the above equation: V = I*R. If the designers are trying to produce a 10V input
to an amplifier input stage, then we can quickly see that I = 10V / 1,000,000 Ohms, or a very very very small amount of current. Conversely, if the same radio were being designed to drive speakers the components would need to be able to handle I = 10V / 4 = 2.5 amps. This enormous difference in build requirements is why a head unit that is capable of driving speakers to 10V (and therefore handling 2.5 amps) can also drive the input stage of an amplifier with no issue, but the reverse is not true.
Note that in ALL cases, the amperage flowing through the head unit amplifier stage, and thus through the lines to the downstream component (whether amplifier or speaker) is determine
by the downstream component. Hook up a speaker and get high amperage; hook up an amplifier and get low amperage.
As if that weren't complicated enough, there is the issue that BMW chose to use balanced inputs. In a "regular" RCA signal, the tip of the RCA jack carries a signal and the ring of the RCA jack is a reference ground (meaning: no signal). In balanced inputs, however,
both the ring and the jacket carry signal, but the signs are opposite. So, when someone says the BMW radios produce a 5V balanced signal, they actually mean that the tip carries a 5V signal and the ring carries a -5V signal. Since amplifiers take the difference between the tip and ring, this means that a 5V balanced signal is actually a 5V - (-5V) = 10V signal!
Since we know your head unit is capable of driving speakers directly, we also know that it is capable of driving an amplifier input stage. What we need to ensure, therefore, is that the amplifier input stage is capable of accepting 5V balanced inputs.
This is why Line Output Converters exist - they are little more than a reducing transformer than takes the higher voltage signal and steps it down to a lower voltage signal that your amplifier input stage can handle.
I hope this all makes sense, but if not, ask away and I'll try to clarify...
btw, the answer to question #2 is that the question really doesn't make sense. Because line level outputs are designed to drive a huge resistance, you can't really say what the voltage would be if they drove a speaker. If you make the simplifying assumption that we won't exceed the maximum amperage design spec of the head unit, then using V = I*R, where I is basically 0, we find that V is also basically 0.
d-