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?
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?
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?
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?
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.
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.
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?
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?
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.
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.
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!
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!
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.
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.
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.
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.
I came upon this thread by accident.
I am building a sub amp for my car and realised that I need to roll off the bass in the rear speakers but doing that with a passive crossover is going to be tough due to the impedance peak at resonance in the driver.
This means I need to add an external amp and crossover to achieve that . So I bought a TDA1554Q for the rear speakers and a TDA1560Q for the subwoofer. That produces enough volume for my use , I don't like it too loud. The sub is an 8 inch driver in a very low profile sealed flat box. Acoustically it measures very well in the car. Good from 10 Hz to 120 Hz before it starts rolling off due to the location. The box rolls off below about 66Hz in free air.
I pulled out my sub crossover from a system I built some 15 or more years ago. It used a differential input using a TLO74 and works very well with the HU and sub amp grounded at different points. Previously it was connected to the bridged output of the source and so did not use a ground. Iwill have to connect it to the new HU with RCA outputs and see what happens. RCA Ground will go to one of the diff inputs and I guess will need a cap to chassis ground on the power amp to keep out RF ?
I haven't tried it yet as the original chip power amp ( uPC 1230H2) seems to be doing some strange things on one of it's bridged outputs ( as seen on the scope ).
I am building a sub amp for my car and realised that I need to roll off the bass in the rear speakers but doing that with a passive crossover is going to be tough due to the impedance peak at resonance in the driver.
This means I need to add an external amp and crossover to achieve that . So I bought a TDA1554Q for the rear speakers and a TDA1560Q for the subwoofer. That produces enough volume for my use , I don't like it too loud. The sub is an 8 inch driver in a very low profile sealed flat box. Acoustically it measures very well in the car. Good from 10 Hz to 120 Hz before it starts rolling off due to the location. The box rolls off below about 66Hz in free air.
I pulled out my sub crossover from a system I built some 15 or more years ago. It used a differential input using a TLO74 and works very well with the HU and sub amp grounded at different points. Previously it was connected to the bridged output of the source and so did not use a ground. Iwill have to connect it to the new HU with RCA outputs and see what happens. RCA Ground will go to one of the diff inputs and I guess will need a cap to chassis ground on the power amp to keep out RF ?
I haven't tried it yet as the original chip power amp ( uPC 1230H2) seems to be doing some strange things on one of it's bridged outputs ( as seen on the scope ).
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