JVC KS-AX3101D high level input common/floating ground

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Hey guys,

Since I am saving up for a new head unit for my E46 I would like to connect my Subwoofer-amplifier with the high level input on the factory unit.

But there is something I don't really get...

link to the user manual it's only 2 or 3 pages (look in the pdf for ks-ax3101d 800w mono amp):
http://resources.jvc.com/Resources/00/01/50/MA481ien.pdf

My high level input user manual says:
Connect Speaker L+ to High level input pin A
Connect Speaker R + to High level input pin C
Connect ground (from the headunit chassis) to pin B

This reminds me of the old common ground systems.. But my headunit dated 1999 has floating grounds, so every speaker has it's one -..

Is it still possible to use this high level input, and do I just mix things up in my head? Or is it really better to buy a High-Low converter??

And if it is possible to use this high level input what is in this case the best place to connect my high level input ground?
1.negative battery lead
2.ground of the amplifier
3.head unit chassis

Since it is a mono amplifier couldn't it be possible to connect just the Right Speaker +, and use the Right speaker - for the ground at the high level input, or is that a bad idea?
 
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Thanks for your reply.

But I was thinking, the case of the head unit is connected to the ground of the head unit. So it's worth trying to first put the high level input ground just on the chassis of the car/on the negative battery terminal right? If it sounds right it shouldn't be a problem?

Why is it not the right way to hook the high level input ground to the negative lead of one of the speakers, would like to understand the theory..
 
The audio output is referenced to ground, the head unit's case ground. Other grounds will be 'noisy' in comparison. If you use other grounds, you are likely to get an amplified version of that ground noise in the audio.

The speaker level outputs from the head unit have 1/2 of the charging system on them. Grounding them can cause damage (expensive damage in some instances) to the internal amplifier in the head unit. The input ground for the high-level input is bypassed to ground by several components. Connecting the negative speaker output to the high level input ground will likely cause the head unit's internal amp to fail.
 
Thanks for making things clear. The only thing that I can't fully place is:

If the speaker needs a positive and a negative/ground lead (in this case a floating one) from the head unit to play music.

How is the high input on my amp able to get the full signal if it isn't the same negative/ground lead?

What happens if you connect the negative/ground lead from the speaker to the housing of the head unit? Will it play music or screw my internal amp?
 
For any speaker output pair, the positive and negative are the same signal but the negative is an inverted version of the positive.

Since the speaker output has ~6v on it, the cone would be displaced from it's resting position as DC current would constantly flow through its coil. This may eventually destroy the speaker or the internal amp.
 
Perry thanks for the answers.

Since it's a bit difficult to attach a wire to te housing of the car radio. I would like to know if it's also ok to connect it to the ground wire on the ISO connector? That would be practically the same or will you than still have the change of hearing the ground noise?
 
The inputs on this amp are not common (unless something has changed in recent years). Most take both positive and negative inputs and use electronic components to eliminate the possibility of having a ground loop.

The better quality external converters (the quality varies) use a transformer to break any possible ground loop. The output has a free-floating ground. Most also have a secondary ground wire that allows you to ground the output shields in case the amp doesn't like free-floating shields.

Page 125:
Basic Car Audio Electronics
 
Perry, I bought a external hi/lo converter from a friend. I read pag 125 but that gives me a few little questions;

Another benefit of using transformers in the converter is that it allows the shield ground on the output of the converter to float. This means that it can be grounded to any point in the vehicle and there will be no chance of creating a ground loop between the output shield ground and the head unit's ground.


Shield ground wires:
There are usually 1 or 2 extra wires on a line output converter (often brown in color). These wires are used to ground the shield of the RCA output cables to the head unit. Some amplifiers work fine without them, others don't. If the converter is installed behind the radio, I would recommend connecting the shield ground wires to the case of the radio. The next diagram shows a schematic diagram for a simple line output converter.
At first they say: It can be grounded everywhere in the vehicle (like the ground side of the rca). And later they say These wires are used to ground the shield of the rca output cables to the head unit.

I think that's a bit confusing?

On other sources on the internet they say that you can connect that extra brown wire to every ground point in the car..


Question 2 about the built in hi/lo converter of my JVC

I think I am getting further step by step. In an other thread they tried to explain me how AC power works. The thing is that I now really can't understand how the JVC unit only needs the + wires from left and right to get signal of it. You don't have a closed circuit since the ground wire is connected to the casing of the head unit and that's not the same ground as the speakers use.. even if they are btl outputs with + and - the same signal.
 
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You can connect the brown shield ground anywhere. If the LOC is near the head unit, you'd ground to the head unit case.

The speaker outputs from the head unit have an IC that's connected to ground. IT drives the signal between ground and 12v. With a series-connected capacitor, you could drive a speaker with one speaker terminal connected to ground and the other speaker terminal connected to the speaker output from the head unit.
 
You can connect the brown shield ground anywhere. If the LOC is near the head unit, you'd ground to the head unit case.

The speaker outputs from the head unit have an IC that's connected to ground. IT drives the signal between ground and 12v. With a series-connected capacitor, you could drive a speaker with one speaker terminal connected to ground and the other speaker terminal connected to the speaker output from the head unit.

Ok thanks for the reply, so technically it's the best to ground to the head unit's case but not necassary (what's the use of the ground loop transformers in here if you connect them to a ground anyway?)

I think some things are a bit mixed up in my head with the second question: in post numer 6 you said to an earlier question:

Me:
What happens if you connect the negative/ground lead from the speaker to the housing of the head unit? Will it play music or screw my internal amp?

You:
For any speaker output pair, the positive and negative are the same signal but the negative is an inverted version of the positive.

Since the speaker output has ~6v on it, the cone would be displaced from it's resting position as DC current would constantly flow through its coil. This may eventually destroy the speaker or the internal amp.

So I understood that you can't use the housings ground since they are floating ones and are completely disconnected from the housing/HU ground? Moreover, since these car amps use BTL you need both the wires for the signal right? Or do you need both wires for full power?
 
If there is no break in the DC circuit path, you MUST ground at the head unit. This is what your amp has.

With the transformers in the LOC, the transformers break the ground path.

You cannot drive a transformer with DC. Each speaker output has 6v DC on it. To drive the transformer, you'd have to have a capacitor to block the DC. Using both speaker wires allows the transformer to be driven without the cap. There is essentially no DC applied across the transformer because both speaker wires have 6v. You do not need both signal wires. It's simply easier to do use both wires.

For speaker level, you need both wires for full power (20w/channel into 4 ohms).
 
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