Tap into head unit preamp. NEED HELP PLEASE! - diyAudio
Go Back   Home > Forums > General Interest > Car Audio

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th June 2004, 11:17 PM   #1
alexv is offline alexv  Romania
diyAudio Member
 
Join Date: Jan 2004
Location: Bucharest
Default Tap into head unit preamp. NEED HELP PLEASE!

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
  Reply With Quote
Old 21st June 2004, 03:12 AM   #2
diyAudio Member
 
tsmith1315's Avatar
 
Join Date: Jul 2003
Location: Doerun, GA
Quote:
I don't know if the power amplifier (TDA7560) clears this noise or it uses a diferent reference as ground (free of noise)
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
  Reply With Quote
Old 21st June 2004, 11:37 PM   #3
alexv is offline alexv  Romania
diyAudio Member
 
Join Date: Jan 2004
Location: Bucharest
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.
  Reply With Quote
Old 22nd June 2004, 06:23 AM   #4
diyAudio Member
 
tsmith1315's Avatar
 
Join Date: Jul 2003
Location: Doerun, GA
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
  Reply With Quote
Old 22nd June 2004, 08:43 AM   #5
alexv is offline alexv  Romania
diyAudio Member
 
Join Date: Jan 2004
Location: Bucharest
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.

Click the image to open in full size.


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
  Reply With Quote
Old 22nd June 2004, 03:08 PM   #6
alexv is offline alexv  Romania
diyAudio Member
 
Join Date: Jan 2004
Location: Bucharest
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.
  Reply With Quote
Old 8th December 2004, 07:59 PM   #7
ciocos is offline ciocos  Romania
diyAudio Member
 
Join Date: Nov 2004
Location: ploiesti
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?
  Reply With Quote
Old 14th December 2004, 06:03 PM   #8
ciocos is offline ciocos  Romania
diyAudio Member
 
Join Date: Nov 2004
Location: ploiesti
pls anyone, heeeeeelp, give me an advice, anything....
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Head unit greg928gts Car Audio 9 10th June 2009 06:26 PM
head unit basenjifan Swap Meet 0 24th September 2007 06:58 PM
Looking for a head unit with eq (and XO) xplod1236 Car Audio 41 20th April 2006 11:01 AM
Head Unit VoLCoM Car Audio 4 31st October 2005 02:51 PM


New To Site? Need Help?

All times are GMT. The time now is 06:00 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2