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Old 2nd November 2008, 10:28 AM   #1
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Default Car amp design, differential input, ground

Hi!

I have some question regarding car amplifier design.
I understand that, on a vehicle, HU ground and AMP ground (heavy gauge wire attached to chassis ground) have, of course, different voltages; I think mainly because of the small gauge wire which the HU uses and the big current of HU integrated amplifier.

Right now I am using a self-made car amplifier for a rear speaker built around a TDA1557 chipamp. This chipamp has a proper pin for the signal ground.

The RCA shield coming from the HU is attached only to signal ground. If I try to tie together signal ground pin and power ground pin the amp emits strange loud noises and does not function at all.

So this means that sgnd and pgnd are not connecte internally. So this is a differential amp?
But then why TDA1556 chipamp is said to be differential (looking datasheet) while TDA1557 is not?
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Old 2nd November 2008, 10:33 PM   #2
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The 1557 has a common reference for both channels (for an unbalanced input signal). It does not have differential inputs. The amplifier amplifies the difference between the input signal and the reference voltage.

The 1556 has differential inputs. It can accept a balanced signal as well as an unbalanced signal. The amplifier will amplify the difference between the two inputs for the channel.

The head unit and amplifier have different ground potentials because the ground conductor has resistance and there is current flowing through the ground conductor.

Why do you want to tie the signal ground to the chassis ground?
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Old 2nd November 2008, 11:34 PM   #3
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You may couple signal ground to power/chasis ground at high frequencies through a 1nF to 10nF capacitor, and at low frequencyes through a medium value resistor, say 1K to 10K.

This usually prevents RF pickup problems and signal ground is kept at a safe potential when nothing is connected to the input.

Another common practice is to use one or two pairs of clamping diodes (just 1N4005) to prevent signal ground from floating more than +/-0.5V or +/-1V from power/chassis ground. This is to prevent damage to the input circuit.

The great voltage differences found between different ground locations in cars are not only due to resistance and DC currents, but also due to wiring loop inductance and AC currents from the alternator and other systems. Always place +12V and ground wires together to avoid the latter.
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Old 3rd November 2008, 07:42 AM   #4
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Thank you for your replies.
I know this issues about ground has been asked many times, but I want to go a bit depper as I found next to nothing about car amp ground design without using an isolated SMPS.

So basically you are saying that the amplifier is not true differential because the signal ground could only be different by the actual power ground by a low voltage (let's say 0,5V?). While true differential, I guess, could float between power gnd and vcc, right?
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Old 3rd November 2008, 01:12 PM   #5
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This depends a lot on how the input stage and the PSU are designed.

Some PSU use optocouplers and allow the output ground to float even +/-100V with respect to input ground. Other PSU couple the signals directly and only allow the output to float a few volts. In other circuits like yours there is no ground isolation.

Same happens for inputs, some balanced ones with op-amps may handle over +/-25V of ground swing while others (mostly built into ICs) are intended to handle only +/-0.5V or +/-1V, which is usually enough.
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Old 3rd November 2008, 06:45 PM   #6
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Thanks.

I tried to tie together sgnd and pgnd because I thought it would only force the ground potential to be the same and the current through the rca shield wouldn't have been much (and in fact it is not much, some mA!), but it happens that it mess up something:
right now the amp powers a speaker with a lowpass built in the HU set to 80Hz, when I connect the grounds, the filter doesn't work anymore and the speaker reproduces also the mids... sounds strange, doesn't it?!

Some more questions:

1) Wouldn't having different signal ground and power ground cause a degrade of sound quality?

2) What would happen if I connect sgnd through a capacitor instead of DC coupling?

3) Let's say I want to make a discrete amplifier which behave as my IC, e.g. with signal ground and power ground (usually schematics meant for home hifi don't have different grounds). Does it simply mean that I should have all the ground of the preamplifier/input section named "signal ground" and the ground of power stage names "power ground"? Doing so the signal ground (RCA shield) carries all the current required for the preamp section, if, as you say, I mustn't link together the two ground on the amp side...

4) In the case I want to use a truly differential amp, I have to couple both signal and signal ground through small capacitors. I read somewhere that these capacitor must be perfectly matched to keep good cmrr, true?

Thanks in advance, friends!
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Old 9th November 2008, 09:37 AM   #7
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Nobody is going to answer these questions anymore?
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Old 9th November 2008, 03:55 PM   #8
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1. Not necessarily.

2. Try it.

3. Home equipment has a transformer that floats the secondary ground (much like many car amplifiers). If you're going to build a car amp, you would probably use an isolated secondary (assuming that you would use a switching power supply) to prevent forming ground loops. The secondary center tap of the transformer would be the audio ground.

4. Ideally, you want the caps to match but in most cases, using capacitors from the same batch is good enough. The resistors used in the differential input circuit are more critical than the capacitors.
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Old 10th November 2008, 02:02 PM   #9
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Thank you, but as regard question 3) I mean WITHOUT an isolated PSU. I mean a discrete circuit which behaves as the IC...
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Old 10th November 2008, 04:46 PM   #10
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If your discrete amp has differential inputs, you don't need two grounds at the amplifier.

When the signal is produced at the head unit, the output signal is referenced to the shield ground. The amp has to use that same reference (the RCA cable shield) when it takes in the signal.

On the differential input amp, it can use one of the independent inputs for each channel as the reference.

On the 1557, the input references are tied to a common point and that common point is used as the reference for both channels. If you connect the reference (the shield -- which is connected to the head unit ground) to the chassis ground at the amplifier, you will get noise because the shield ground potential is different than the ground potential at the amplifier. The reference will be a combination of the ground potential at the head unit and the ground potential at the amp.
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