Hi everyone,

I'm new here and I have novice electronics knowledge so please go easy on me. I own a car from the early 2000's which has a tape player and I've been trying to connect AUX to it. I don't want to ruin the factory look and install one of those flashy Chinese head units, and I've tried the fake tape which has another tape head in it, but that doesn't work well unless the heads are perfectly aligned.

So my idea was to connect the 3.5mm AUX cable directly to the amplifier chip as a lot of people do on YouTube. That work's but it doesn't sound as good as when a tape is playing. I guess there is some other chip like Dolby Digital or whatever its called that the tape head connects to first but I don't have enough knowledge to find that.

So I connected the AUX cable directly to the tape head and it sounds exactly as its 'supposed to'. However, every second there is a pulsing noise. It kind of sounds like the turn of a page in a book.

So my question is, how can I connect the AUX cable from my phone to the tape head properly so I don't get that noise? I've seen a guy on YouTube do this with an old tape head, and he put resistors in between.

Any ideas?

Thanks in advance!

 

The signal from a tape head has very much equalization in its playback preamp that boosts low frequencies a lot and cuts high frequencies a little. It is the opposite that is done during recording. Then it will sound awful if you feed AUX to a tape playback head. You need to feed AUX to a point in the circuit that follows the playback preamp, probably at the volume control.

 

The signal from a tape head has very much equalization in its playback preamp that boosts low frequencies a lot and cuts high frequencies a little. It is the opposite that is done during recording. Then it will sound awful if you feed AUX to a tape playback head. You need to feed AUX to a point in the circuit that follows the playback preamp, probably at the volume control.

There is a lot missing in your description of whatever it is you are doing. It sounds like you are connecting to an AUX input on something besides the device with the tape head, with no information at all being made about what. Then, connecting a cable to "an amplifier chip"? And where does "AUX" actually enter into the picture???
If you are getting noise pulses once per second, that is not an equalizer problem, it is a noise problem. But mostly it seems that you have left out a whole lot of information needed for anybody to provide any worthwhile assistance. So what is needed is a more complete description.

 

He has an old tape player. I think he wants to feed a line level "AUX" signal from an MP3 player or something into it so that it can use the amplifier and speakers.

He tried feeding the AUX signal into the amplifier but did not like the sound. Then he fed the AUX signal directly to the tape head but there are pulse noises that are probably caused by him not using a shielded audio cable to connect the signal so it is picking up interference.

Here is a graph showing the equalization used in a tape playback preamp that will overload and sound awful if it has an ordinary line level signal:

 

He has an old tape player. I think he wants to feed a line level "AUX" signal from an MP3 player or something into it so that it can use the amplifier and speakers.

He tried feeding the AUX signal into the amplifier but did not like the sound. Then he fed the AUX signal directly to the tape head but there are pulse noises that are probably caused by him not using a shielded audio cable to connect the signal so it is picking up interference.

Here is a graph showing the equalization used in a tape playback preamp that will overload and sound awful if it has an ordinary line level signal:

Your description is certainly believe able. "AUX", all by itself, requires a good amount of assumption that they actually meant "line level output". But that guess does make sense. It is indeed probable that among a number of challenges, connecting a line level source right across a tape head input would lead to a serious overload. The fix there will be to adequately shield the connecting wires, to properly ground the shields, and to tap into the cassette player system downstream of the tape head equalization point. Shielding is not that complex or even that hard, but locating the correct point to connect requires being able to understand the circuit. That does not sound realistic in this case.

 

Can you get access to the volume control of your stereo, you could input the signal there?

 

He has an old tape player. I think he wants to feed a line level "AUX" signal from an MP3 player or something into it so that it can use the amplifier and speakers.

He tried feeding the AUX signal into the amplifier but did not like the sound. Then he fed the AUX signal directly to the tape head but there are pulse noises that are probably caused by him not using a shielded audio cable to connect the signal so it is picking up interference.

Here is a graph showing the equalization used in a tape playback preamp that will overload and sound awful if it has an ordinary line level signal:

I'm sorry, I probably didn't explain myself properly.

Yes, basically I have an old radio in the car with a tape player and I want to connect my phone to it/maybe a little Bluetooth board so any sound coming out it would play through the radio as if a tape was playing.

So all I did was take an old cable with an old 3.5mm jack, cut one end off and connected the left, right, ground wires to the appropriate pins on the amplifier chip. That did the job without any pulsing noises but the sound was not the same as when you listen to the radio or play a tape, so I figured there must be some other circuit the sound goes through before it enters that chip where the sound is enhanced.

So I connected the cable directly to the two pins on the tape head without any resistors or anything in between. That produced an excellent sound, nothing gets overloaded even when I put the phone on maximum volume, the only issue is that there is a pulsing noise in the sound.

So my question is, do I have to add some resistors in to the line coming out of the phone in to the tape head, or what is causing this noise and how do I get rid of it?

I watched this guys video and he did the same thing with an old tape head, but he added 2 resistors to it:

I can make a video of whats happening and some photos of the radio if that would be of any use.

Can you get access to the volume control of your stereo, you could input the signal there?

Yes, I can get access to it, not sure if I can input it there or how it would sound. I would prefer to inject the sound as close as to where it would be coming from the tape head as possible so I could still use the equalizer and other functions of the radio.

 

I am now suspecting that it might possibly be a magnetic field getting into the tape head, since it is still connected. And if the tape player also has a radio function that may be part of the problem. If any part of the tape transport mechanism is still powered, such as the auto-stop function, that may be the cause. So checking for any power to the tape part could be the possible cause.

 

I had a look at everything again today and the board seems to be pretty simple. There are two chips on it. A TEA0676T and a HA12141.

The TEA0676T is the pre-amplifier, which from my understanding amplifies the sound picked up by the tape head, from there it goes to the HA12141 which is the Dolby Surround system or whatever it was called back in the day, that is supposed to filter out noise etc...

The datasheet for the TEA0676T is here, pin out is on page 4: https://www.nxp.com/docs/en/data-sheet/TEA0676T.pdf

I connected ground from my cable to pin 12 (for some reason the ground cable is red) and the audio channels to pins 1 and 16 which are the output pins.

I am beginning to see that I probably made a mistake because I just bypassed whatever this chip does completely which is probably why it doesn't sound like the tape head because the tape head connects directly to this chip.

I'm not sure if I should tap in to input equalizer channel A & B or input channel A1 & B1 instead.




I guess I will have to try both. Good that I bought a duplicate radio from eBay for 5 bucks to experiment on.

 

Yes indeed, having a second radio is a good choice. I suggest finding a way to anchor the large cable that you are using for the external connection and use some much thinner wire for the actual connections, because that surface mount conductors can easily get peeled off of the board and then things get ugly trying to fix it.

 

You need to feed the AUX input to a point following the Dolby circuit which might be the volume control (because the Dolby Noise Reduction circuit changes the frequency response when recording and changes it back when playing back. Whatever connects to the point now (maybe the Dolby circuit) must be disconnected with a switch when the AUX input is switched in.

 

The problem with the heads is the inputs are floating, one input is at Ref the other is to the amplifier. If you use these inputs you will need to modify the equaliser frequency response .Your best option is to input the signal after the preamp,.

 

Replying to the original post, what you have done works for me but you have to turn the Dolby completely off otherwise it sounds terrible. The picture of your wiring shows that you are working on a BMWbusiness radio cassette, p,robabbly pre 2002. I have wired my aux i/p to the chip as you have but I earthed to the near by screw on the board (missing in your picture). Also don’t plug the tape head connector back in on reassembly. To play the Aux In you will need a dummy tape to actuate the tape deck and circuit or poke a pencil in the slot LHS to operate the tape deck.