low-level output on stock system

[Brian-bmw]

Originally Posted by DouglasABaker

So there are no systems that have just a head unit without the additional radio box in the trunk?

That is interesting only because I helped someone (via phone) with an install who said he only had a head unit w/ no box in the back...

d-

At a high level, there are probably 6 major permutations of the audio system:

1) In-dash radio with no amplifier (6 speaker)
2) In-dash radio with standard HiFi amp (10 speaker)
3) In-dash radio with Top HiFi DSP amp (12 or 14 speaker)
4) Navigation with trunk radio and no amplifier (6 speaker)
5) Navigation with trunk radio and standard HiFi amp (10 speaker)
6) Navigation with trunk radio and Top HiFi DSP amp (12 or 14 speaker)

The first three perrmutations have the radio in the dash. It is the component with the tape or CD player. The last three permutations have the board monitor screen up front and the separate radio module in the trunk.

My hunch is that the install you helped with must have been system #1. That would have the standard radio with MID in the dash and no separate external amplifier. In that setup, the speakers are powered directly by the radio module. It is analogous to permutation #4, but in that case the radio module is in the trunk with no external amp, and the speakers feed directly to the trunk radio.

The other point to note about the above is that the same radio module part functions for scenarios #1, #2, #3. And, the same trunk radio supports scenarios #4, #5, and #6. The radio scans the I-Bus for which amp (if any is installed) and then decides out it should change its output.

[Brian-bmw]

Originally Posted by UKenGB

...the radio module could drive a third party amp without having to fool it into believing the OEM amp is connected.

Yes, the radio can drive a third party amp perfectly fine. Many folks have done it. However, you may need a line level converter. I am not sure of that, because I have not studied those posts as much.

You do not have DSP, so the radio's volume, fader, balance, tone, bass, etc. will all function like any ordinary radio. The only question to deal with is whether you need a line converter and whether the balanced output is an issue. I want to say that neither has been a big challenge for anyone who has done an aftermarket amp installation. In fact, I think most folks fed the radio outputs directly into the amps with no line converters, but I would search around some more for that info.

[UKenGB]

Ah, but does it really scan the I-Bus, or is it simply designed to automatically supply the correct output for either high or low impedence (i.e. amp or speakers)?

This is relevent as in my situation (no amp) the radio powers the speakers directly, but if it is connected to an amp instead, does the level drop to what could be called line level (ok, with a 5V bias) or will it still be trying to drive speakers, which will surely fry an amp with line level only inputs?

[DouglasABaker]

Originally Posted by Brian-bmw

At a high level, there are probably 6 major permutations of the audio system:...

That is terrific info - I greatly appreciate it

d-

[DouglasABaker]

Originally Posted by UKenGB

Ah, but does it really scan the I-Bus, or is it simply designed to automatically supply the correct output for either high or low impedence (i.e. amp or speakers)?

This is relevent as in my situation (no amp) the radio powers the speakers directly, but if it is connected to an amp instead, does the level drop to what could be called line level (ok, with a 5V bias) or will it still be trying to drive speakers, which will surely fry an amp with line level only inputs?

Yes, it really scans the IBus. When attached to a DSP amp the radio puts out a constant volume signal. You must reset the entire audio system to change it to variable outputs.

Electrical outputs are designed to handle a maximum amperage / minimum impedance. For the most part, systems can easily handle lower amp/higher impedance circuits. In this case, a system designed for speakers (high amp/low impedance) will easily drive an amp (low amp/high impendance). There is no concern of the system accidentally sending a high amperage signal to the amplifier because the amplifier determines what it receives. In other words, because the amp is high impedance, it will receive a low amperage signal.

The long and short of it is that you need only concern yourself with whether the amplifier can handle a 10V input signal (balanced 5V signal is 2x5V = 10V total peak). If it can, then don't worry about it.

Doug

[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.

Let's forget the 5v bias of this BMW system, 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?

[DouglasABaker]

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.

I don't think you quite followed me

Originally Posted by UKenGB

Let's forget the 5v bias of this BMW system,

You can't - it is the critical factor in what you are trying to do

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?

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-

[superior]

Wow, excellent explanation of the situation (and general audio/electrical physics too). I just had a minor question as an aside. I may purchase an amp capable of 10V line-level inputs (got some Kickers in mind) to power a new subwoofer. After I have located the signal wires from the radio headunit going into the factory amplifier, what is the best way to tap them so that the signal is split to both amps? The balanced 5V seems like it would need a special wire harness or something to maintain that +/- charge separation.

Thanks in advance for the help!

[DouglasABaker]

Originally Posted by superior

Wow, excellent explanation of the situation (and general audio/electrical physics too). I just had a minor question as an aside. I may purchase an amp capable of 10V line-level inputs (got some Kickers in mind) to power a new subwoofer. After I have located the signal wires from the radio headunit going into the factory amplifier, what is the best way to tap them so that the signal is split to both amps? The balanced 5V seems like it would need a special wire harness or something to maintain that +/- charge separation.

Thanks in advance for the help!

I try Glad you like it since it took a little while to write up!

Balanced signals don't require any special wire or harness - it is still a 2 wire RCA, it is just that both wires carry a signal. Regular RCA's maintain separation just fine.

Anyway, to answer your question:
Some amplifiers can accept 10V inputs into their RCA jacks, others accept them into speaker terminals. Assuming the amp you are looking at accepts them into the RCA jacks, then I would simply take the speaker wires from your head unit and wire them into RCA pairs. In other words, I would take Front Left + to the tip of one RCA and Front Left - to the corresponding Ring. Do the same with FR, RL, and RR. Once you have male connectors on each speaker wire you can then get a Y cable to with 1 female end (to mate to your newly wired male end) and 2 male ends. Run those to the 2 amps.

Alternatively, simply wire up RCA's and run them directly to your 4 channel amp and then use the passthrough RCA jacks on the amp to run to a subwoofer amp.

Doug

[DouglasABaker]

Originally Posted by superior

Wow, excellent explanation of the situation (and general audio/electrical physics too). I just had a minor question as an aside. I may purchase an amp capable of 10V line-level inputs (got some Kickers in mind) to power a new subwoofer. After I have located the signal wires from the radio headunit going into the factory amplifier, what is the best way to tap them so that the signal is split to both amps? The balanced 5V seems like it would need a special wire harness or something to maintain that +/- charge separation.

Thanks in advance for the help!

Just realized I may have misunderstood your question. If you were asking how do you take 4 channels of audio and run it into an amplifier with only 2 channels of input then the answer is a bit different.

Typically you'll find that subwoofers are driven off of a front and a rear channel - so I would take the FL and the RR RCA's and run it to one amp, and the FR and RL RCA's and run them to the other amp. This way you are splitting the load between both subs regardless of fader setting.

If this still didn't answer your question then I need you to ask if differently

d-