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Audio Input Decoupling Question

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indecided

Member
Hello guys,

I'm trying to patch an auxiliary input into a car stereo. This modification taps into the tape deck input.
Some adventurous souls had already done this and scoped out the board to find the suitable input pins as per the picture below.

pcb.PNG circuit.PNG
He has put in a little decoupling circuit (I assume) , with an electrolytic cap to the output and a resistor to ground from the output.
I was always under the impression that a decoupling cap should be non-polarized (as not to block AC) or perhaps i have my concepts wrong.

Could someone kindly explain the above circuit as well as comment on the component choice? I'm particularly concerned about the values as to prevent creating too high of a high pass filter (say 20 hz would be a good HPF value - don't need the infrasonics).

In case anybody's wondering, The radio in question is a Becker Audio 10 (6019) Radio from a 15 year old w203 C-Class.

Thanks!
 

Externet

Well-Known Member
Seems from the picture there is no need to add any 'decoupling' circuit. Pads paths have 10μF capacitors for blocking DC, and a resistor placing some load. You can get away with polarized capacitors as existing in the picture.

What needs is tricking the circuit to fake presence of tape loaded in the (removed?) tape mechanism in order to accept the auxiliar input when 'tape' is selected. That black connector should be hacked properly. Otherwise, an endless cassette would need to be inserted.
 

audioguru

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Most Helpful Member
A car does not have a negative supply in addition to its positive supply, instead it has only about +12V and 0V. The audio circuits are biased at half the supply voltage (biased at about +6V) then the audio can swing up to almost +12V and down to almost 0V which is fine for the polarized capacitor that has its minus end at 0V and its positive end at about +6V.

If the signal source impedance is low like the output of an opamp then the value of the capacitor is calculated by 1 divided by (2 x pi x f x R) where f is the -3dB cutoff frequency and R is the input impedance that the capacitor is feeding.
We do not know the input impedance of each pad, it might be the 3900 ohms that is shown. Then for a 20Hz cutoff frequency the capacitor value will be 2μF but 2.2μF will be fine. The article uses 22μF for earthquake infrasonics or the input impedance of each pad is 390 ohms.
 

audioguru

Well-Known Member
Most Helpful Member
The tape head input has equalization that will mess up your line level audio and you need attenuation. If the pads are where the output of the tape deck preamp connects to then the pads might need the tape deck unplugged from them to work as aux inputs.
 

indecided

Member
Thanks for the replies everyone! Good to see you again audioguru. It has been a while since i've been on the boards.

I've hooked it up as per recommendations and unfortunately the outcome was rather disappointing. It would appear that there is TOO MUCH attenuation in the system. With the volume cranked up, i don't get much output.

There is also a rather noticable HPF effect, leaving the lowend/mids very empty.

It would appear that it's back to the FM modulator for now until I can figure out how to disconnect the tape deck alltogether. Unfortunately it is not just discrete circuitry and not some outputs i can pull low/high to be able to strip out the tape deck portion.

Barring which, I would have to cannibalize the entire tape deck to get things working, which given that this tape deck is the only surviving cassette player in the family, and there might be a old recorded tape that I might need to play back.. someday, i'm hesitant to tear up the tape deck for now.

Any ideas on resistance/values? I've noticed that the other variants of the stereo have an attenuation resistor which the guides say to bridge over to cut out the attenuation.
 

audioguru

Well-Known Member
Most Helpful Member
We do not know the output impedance or output level of your signal source and we do not know the input impedance or sensitivity of the amplifier portion of the car radio.
A high impedance feeding a low impedance results in attenuation. A coupling capacitor with a value too low for the impedance it feeds results in low end cut.
 

dr pepper

Well-Known Member
Most Helpful Member
As mentioned the tape head input uses a 'nab' circuit and isnt good for line in, great scott on youtube has done this properly and posted videos, the way to do it is to search for the datasheets on the ic's in the set and find out what they do, in the mentioned vis one was found to be an input selector and it wasnt hard to find which i/p was the processed line in from the tape deck.
Theres been enough explanations on the dc blocking cap so I'll not bang on about that.
 
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