My car's head unit does not have preamp outputs and I don't want to change it (the headunit).
To connect my external amplfier I tap into the head unit and conected the signal wire on the outputs of the preamp (a DSP made by Philips SAA7708 ,identical with SAA7706) and the ground wire to the ground of the head unit.
It works but I hear the laser head motor in the speakers. This must be because of the ground that I connected to. I tried diferent combinations of ground but can't get read of the noise.
I don't know if the power amplifier (TDA7560) clears this noise or it uses a diferent reference as ground (free of noise)
I have no ideea how to solve this problem. Please if you have any ideea.
sorry for my bad English and hope I made myself understood.
Alex
To connect my external amplfier I tap into the head unit and conected the signal wire on the outputs of the preamp (a DSP made by Philips SAA7708 ,identical with SAA7706) and the ground wire to the ground of the head unit.
It works but I hear the laser head motor in the speakers. This must be because of the ground that I connected to. I tried diferent combinations of ground but can't get read of the noise.
I don't know if the power amplifier (TDA7560) clears this noise or it uses a diferent reference as ground (free of noise)
I have no ideea how to solve this problem. Please if you have any ideea.
sorry for my bad English and hope I made myself understood.
Alex
There's a good chance the preamp and power amp use something other than ground for reference. Try using a 10uF in series with the outputs, and ground to the chassis or to the outside of the choke on the 12v power line, or even to the power amp input signal ground.
Does the signal go directly into the TDA7560? It may be buffered before arriving at the amp, in that case, you may have better results taking the signal from the amplifier input.
Tim
I have a capacitor on the input of the amplfier. why put another?
the amp has no input signal ground.
I don't know what to do.
on radio there si no background noise. only on cd because of the dam motors.
please help me solve this problem.
i am tring to find some schematics of headunits with build in outputs. none found at this time.
the amp has no input signal ground.
I don't know what to do.
on radio there si no background noise. only on cd because of the dam motors.
please help me solve this problem.
i am tring to find some schematics of headunits with build in outputs. none found at this time.
Sorry, I should have been more clear, the amplifier I was referring to is the internal TDA 7560.
I should also clarify that I'm not that good of a tech, although I can usually find a way to do what I want if I have the unit in front of me.
Looking at the TDA 7560 data sheet, the signal inputs are on pins 11, 12, 14 and 15. Signal ground is at pin 13. That's the amplifier input signal ground I was attempting to communicate.
You may have better luck taking the preamp signal and ground from those pins.
Looking at the datasheet for the DSP, there are separate signal grounds for the CD input. If they are balanced signals, that could easily pass noise with an unhappy reference. Are you taking the output directly from pins 6/9/13/15/ on the DSP? Looks like pin 12 is the analog reference... Yep, that's the actual reference for the output amplifiers built into the DSP, so that's what I'd use for ground if I was taking the output directly from them.
Another option is to tie ground directly to CD ground (if unbalanced) pin 77, but that won't take advantage of the noise filtering properties of the DSP chip.
You are absolutely correct about the series cap. If your external amp has capacitors in series with the input signal, there is certainly no need for another pair. It only serves to block DC.
Hope that may be of some help,
Tim
I should also clarify that I'm not that good of a tech, although I can usually find a way to do what I want if I have the unit in front of me.
Looking at the TDA 7560 data sheet, the signal inputs are on pins 11, 12, 14 and 15. Signal ground is at pin 13. That's the amplifier input signal ground I was attempting to communicate.
You may have better luck taking the preamp signal and ground from those pins.
Looking at the datasheet for the DSP, there are separate signal grounds for the CD input. If they are balanced signals, that could easily pass noise with an unhappy reference. Are you taking the output directly from pins 6/9/13/15/ on the DSP? Looks like pin 12 is the analog reference... Yep, that's the actual reference for the output amplifiers built into the DSP, so that's what I'd use for ground if I was taking the output directly from them.
Another option is to tie ground directly to CD ground (if unbalanced) pin 77, but that won't take advantage of the noise filtering properties of the DSP chip.
You are absolutely correct about the series cap. If your external amp has capacitors in series with the input signal, there is certainly no need for another pair. It only serves to block DC.
Hope that may be of some help,
Tim
thank you for makeing it clear.
i was thinking to connect at that pin but was afraid not to damage something. i will try first with the scope. I've managed to see the noise signal with no music so I should be able to see if it si there too or not.
looking at the datasheet and the pcb i've noticed the outputs are conected like SAA7705 not like 7706.
SAA7705 pinouts
pin 6 - rear right audio voltage output of the QDAC
pin 7 - rear right audio current output of the QDAC
pin 8 - rear left audio current output of the QDAC
pin 9 - rear left audio voltage output of the QDAC
pin 10 - ground supply for the analog part of the QDAC
pin 11 - positive supply for the analog part of the QDAC
pin 12 - decoupling for voltage reference of the analog part of the QDAC
pin 13 - front right audio voltage output of the QDAC
pin 14 - front right audio current output of the QDAC
pin 15 - front left audio current output of the QDAC
pin 16 - front left audio voltage output of the QDAC
pin 7 is conected trough a cap to pin 6, pin 8 to pin 9, like in the image attached.
pin 10 is conected trough a cap to pin 11. if there is another conection under the ic I can't see it.
pin 13 of the 7560 is conected to the normal ground. but I will try it again.
Thank you wery much for your help. I will try what you said and report later.
Alex
i was thinking to connect at that pin but was afraid not to damage something. i will try first with the scope. I've managed to see the noise signal with no music so I should be able to see if it si there too or not.
looking at the datasheet and the pcb i've noticed the outputs are conected like SAA7705 not like 7706.
SAA7705 pinouts
pin 6 - rear right audio voltage output of the QDAC
pin 7 - rear right audio current output of the QDAC
pin 8 - rear left audio current output of the QDAC
pin 9 - rear left audio voltage output of the QDAC
pin 10 - ground supply for the analog part of the QDAC
pin 11 - positive supply for the analog part of the QDAC
pin 12 - decoupling for voltage reference of the analog part of the QDAC
pin 13 - front right audio voltage output of the QDAC
pin 14 - front right audio current output of the QDAC
pin 15 - front left audio current output of the QDAC
pin 16 - front left audio voltage output of the QDAC
pin 7 is conected trough a cap to pin 6, pin 8 to pin 9, like in the image attached.
pin 10 is conected trough a cap to pin 11. if there is another conection under the ic I can't see it.
An externally hosted image should be here but it was not working when we last tested it.
pin 13 of the 7560 is conected to the normal ground. but I will try it again.
Thank you wery much for your help. I will try what you said and report later.
Alex
I tried pin 12 (DSP) but there was a little noise and then i tried pin 13 on power amplif and it worked. I think there is still a little noise but very little.
thanks again.
thanks again.
i have the same saa7708h dsp on a cd player, and the datasheet of saa7705h. where from should i tap for an input? looking at the saa7705h application the best analog input should be on pins 72, 70 and 77, but if i tap there how can i turn on that input?
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