Grounding problem: stereo speaker output to amp input

[dumpystig]

I'm in the process of adapting a tablet to install it permanently in my car. I want to connect the tablet speaker outputs to a separate small stereo amp which will power two small independent speakers solely for the sat nav guidance. The tablet headphone jack is being used for connection to the headunit aux-in for music playback.

Here is my problem; the tablet speaker grounds are separately connected within the tablet, they are not commoned. All of the small amp circuits I have considered have a stereo jack input with commoned L / R grounds. So what would be required to have a common ground input to the external amp whereby the tablet amp will not be damaged?

Thanks.

 

[MikeMl]

Are you sure that the tablet stereo speakers are not connected to bridged outputs, where both ends of the speakers are driven out of phase. I have encountered this with (for example) Garmin sat nav units. I had to use an audio coupling transformer between the bridged output an the ground-referenced input of an aircraft audio system. I first tried just using only one side of the bridged output referenced to chassis ground, and had a horrible ground-loop.

 

[dumpystig]

Thanks Mike but TBH I don't have a clue about that, and I wouldn't know how to test it. The only thing I have done is measure resistance across both speaker output grounds. Although I've been into the guts of the tablet there's absolutely no way of tracing about 98% of the circuitry. I've removed the power and volume buttons, soldered thin leads in their place and brought the leads out the back to be connected to dash mounted buttons. Also done the same for both speakers, mic, audio jack and battery (to facilitate charging; the tablet charging circuit is shot).

 

[alec_t]

Perhaps instead use a splitter on the headphone jack? Wouldn't that have a common L/R ground?

 

[tcmtech]

The simplest way to make a common ground output from a floating ground amp output is to just use the + output line going through a capacitive coupler. By doing so you eliminate the floating ground effect for that side of the amps output giving it reference to the common ground.

For a preamp level output a 1 - 5 uf capacitor on each speakers + line should be more than enough to give you a proper common ground referenced audio signal.

Depending on the amps original output levels you may still have to attenuate it down a bit through a pair of resistors.

 

[dumpystig]

I think I get the gist of what you suggest. I think...

I've just connected a speaker across both L + R negative outputs and it worked; so would I be correct in assuming this indicates the tablet amp is a bridged output?

 

[dumpystig]

Just found this:

https://electronics.stackexchange.com/questions/12187/stereo-line-level-converter-odd-grounding

The first answer is excellent, balanced out - to - unbalanced in (bottom left Diag.) seems to be what I have and what I'm after. Anyone else agree with me?

 

[tcmtech]

Yes that would indicate you have bridged amps driving each speaker.

If It was me I would put a light dummy load resistor of maybe 50 - 100 ohms in place of the original speakers then use the capacitive coupler to get your two outputs that are referenced to the common ground.

 

[dumpystig]

Great, I may be getting somewhere at last. At the risk of seeming lazy, could you give me a bit more info on the cap. coupler, maybe a diag. even...?

 

[audioguru]

A speaker connected across the L + R negative outputs plays the difference between L and R if the outputs are bridged. Then mono is nulled and ambiance and extreme stereo (left and right ping-pong) is enhanced.

If the outputs are not bridged then the L + R negative outputs are grounded.

 

[dumpystig]

Well I've done plenty of reading but I could still do with some help with this, especially regarding the capacitive coupling.

 

[audioguru]

Each of the two wires of the Tablet's speakers are probably driven by its own amplifier. Then the output is called "bridged". Most car amplifiers are bridged.
Then the voltage swing to the speaker is almost doubled which makes the speaker current almost doubled which makes the power in the speaker almost 4 times as much as a simple amplifier with one wire of the speaker grounded.

Each output wire of a bridged amplifier has a DC voltage which must be blocked by an input coupling capacitor on your circuit.

You can take either output signal wire and feed it to the input coupling capacitor of your little amplifier.

 

[KeepItSimpleStupid]

You can see a bridged amplifier configuration here: https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers

What you get out of a bridged amp is two signals 180 degrees out of phase. When added together by the speaker, you effectively have a power rail 2x what you would with a single amplifier. Power = (V^2)/R, so it's a big gain. Very popular in automotive amplifiers.

On any output there is sometimes a very small amount of DC which may to be removed. In high end systems, the removal occurs at the power amp. e.g. a frequency response of 0.5 Hz to 40 Khz +- x db.

Just taking one of the outputs to ground gives you 1/2 the usual voltage. Capacitive or transformer coupling can remove the DC. The whatever you feed this into can also be an AC coupled amplifier. i.e. It won't amplify the DC.

 

[dumpystig]

Thanks for the input. I've already got more of an understanding of the bridged amp output, but it's the actual interconnection between this and the secondary amp input that I can't figure out. I know I need to combine both Left + and - to make one output and Right + and - to make the other output. But it's how to do it, what components are needed, and how/what to connect to the ground input on the secondary amp.

 

[KeepItSimpleStupid]

NO! All you need is the L+ and ground and the R+ and ground for your input. No connection to the L- and R-. Now it could be too loud or too soft, but that's another problem.

 

[dumpystig]

OK, here's what I have.

The tablets internal speakers (absolutely crap, the worst sound imaginable) have been removed and the connections to them have been extended and brought outside the tablet.

Connecting DVM Ohms across speaker wires, tablet powered off -

L- and R- 23K
L+ and R+ 23K
L- and L+ 23K
R- and R+ 23K
L- or L+ or R- or R+ to tablet ground 11.5K

So it's definitely a bridged amp, yes? Taking the above 4 speaker outputs I don't have a separate ground connection. Now as you suggested above I understand that I use L+ and R+ and obtain the ground from the tablet ground, thereby having the required L and R outputs with (tablet) ground connection as inputs to my secondary amp with L, R, ground inputs? If that's the case then it makes plenty of sense; I seem to have been assuming this was a much more involved solution...

 

[KeepItSimpleStupid]

Yep, but here still may be a more involved solution.

The simplest, "more involved solution", might be a LOG pot connected across the speaker terminal and take your output from the wiper.
It also might mean there isn't enough current to drive the input.

 

[dumpystig]

OK, I'm going to give your first suggestion (i.e. my last post) a go. Make those connections and see what results I get. I already have my secondary amp and speakers: I've robbed the internals from a Dell Soundbar, 12VDC supply @ 1A, 2x 5W, ideal for in-car plus it sounds great hooked up to a PC so should be fine for the sat nav app on the tablet. If any attenuation is needed I'm sure that can be sorted.

Off to do some wiring, back soon. Fingers X'd. And thanks for your help.

 

[dumpystig]

Excellent! That works great, although it now needs some attenuation. I'll go back to google now but if anyone has an idea where to start I'm all ears.

 

[tcmtech]

As I mentioned earlier it would be a good idea to put a light load across the + and - outputs of each channel. Power amplifiers tend to like having a light load opposed to no load at all. anything from 50 - 100 ohms would be a good place to start.

As far as attenuation a 5 - 10K potentiometer across the output side of the capacitor would give you a simple but effective volume control.