AUX input to old car cassete player - Page 1
Forum section: electronics / projects

author: darthtony
author_rank: Contributor
timestamp: March 13, 2016, 09:38:25 pm
content: Hi, i am trying to add an aux input tomy car radio, which has a cassete player. The input form the cassete head seems to go to thishttp://pdf.datasheetcatalog.com/datasheet/philips/TEA0675.pdfchip, which includes the head preamps and some other noise reduction circuitry. I lifted off the output pins and fed the line in signal directly(bypassing the TEA0675). This setup works, however the volume is significally lower(than the radio volume) and I have to turn the gain up, which introduces some noise, and the final result is not very good.(Its not awfull like those plugin cassete things, but it could be better) What can i do to improve the sound clarity?Should i feed the input signal directly to the cassete heads(Via a resistor devider?) or should I add a non inverting amplifier for some gain(How much?).

author: darthtony
author_rank: Contributor
timestamp: March 13, 2016, 10:47:01 pm
content: Quote from: wilfred on March 13, 2016, 10:26:09 pmQuote from: darthtony on March 13, 2016, 09:38:25 pmI have tried the cassete thing, but the sound quality is very bad, and i saw that it is a common and easy modificaiton so i decided ot go that way.

author: tooki
author_rank: Super Contributor
timestamp: March 13, 2016, 11:55:16 pm
content: Quote from: wilfred on March 13, 2016, 10:26:09 pmI guess you haven't actuallytriedsuch devices yourself. The sound quality varies between "decent" at best and "totally useless" for most. Aux in is a million times better.

author: calexanian
author_rank: Super Contributor
timestamp: March 14, 2016, 12:22:17 am
content: A couple of thoughts come to mind, the first of which is that it is time for a new stereo! Apart from that I might try two things. Many older stereos simply had analog volume control pots. I might try intercepting there and perhaps put a switch in to shift from the internal circuit to a jack for an aux input. Otherwise I have noticed many of the old style circuit use chip amps such as the TDA2020 or something along those lines. I might try looking for a place to input to the power amp circuit. I would think a smart phone would have enough drive on the headphone jack to operate one of those linear circuits. A follow up thought is to watch out for potential problems of shorting from smartphone and media player output jacks as they are sometimes quasi bridge tied outputs and using a non isolated charger at the same time can be problematic. I have only encountered this problem once, but it should still be mentioned. I "Survived" with one of those tape deck adapters for a number of years. They are ok. I found the frequency response of the rest of the system had more of an impact than the tape head adapter.

author: calexanian
author_rank: Super Contributor
timestamp: March 14, 2016, 12:24:52 am
content: Another thought occurred to me. Get some kind of cheap car stereo amplifier, mount it under the dashboard, and have a multi pole switch that changes the speakers from the installed stereo to the new one. I like this idea better.

author: rdl
author_rank: Super Contributor
timestamp: March 14, 2016, 12:42:33 am
content: The ones that use FM transmitters are generally poor. You can get a wired type which uses an adapter to insert the signal directly between the antenna and the antenna jack. They're a bit more expensive, but are said to work much better. I have one of the cassette adapter types (Philips brand) that works pretty well.

author: darthtony
author_rank: Contributor
timestamp: March 14, 2016, 01:10:09 am
content: Well, buying a new stereo is of course the best solution, but it really isnt that important for me, so i cant justify spending the money. I just have some spare time now, and it seems like a fun project. The volume controls are digital, and i dont like the idea of injecting the signal directly to the amp, because i want to be able to easily switch between radio and aux. Right now, i have shorted the microswitches that sense when a tape is playing, so the radio thinks a cassete is in and i can switch between them by selecting cassete/radio, so the setup functionally works as i would like it. The only problem is the low audio level, as i said i can turn the volume up and it works with acceptable quality(There is some noise probably due to the high gain, so i would like it to be a little better), however the main problem is because of the mismatch in the signal amplitudes, when i switch from cassette(aux) to radio, there is a large difference in sound volume.Quote from: wilfred on March 14, 2016, 12:20:49 amSpot on 16yrs old. Like i said if it was really important to me i would have bought a new radio, however i tried this modification and i really like the way it turned out, because i have retained all the previous functionality(Steering column buttons, fm radio) and i am using the internal audio switcher, so no ugly buttons on the dashboard.

author: jh15
author_rank: Frequent Contributor
timestamp: March 14, 2016, 06:26:47 am
content: Back in 1969 or so I tied in my battery cassette recorder to the volume center tap (with a coupling cap) on my Corvair Spyder turbo radio. (The car was turbo, not the radio). Worked fine.Then I tried some kind of way of powering from the car system, OOPS! ground issues related to Dave's how not to blow up your oscilloscope video.Fixed it, then the poor cassette recorder got hit by a rogue wave at the beach.Just get a cassette insert of good quality, I just sold a 2001 Maxima with one in it that I used for years.

author: samnmax
author_rank: Regular Contributor
timestamp: March 14, 2016, 01:55:44 pm
content: A long time ago I did the same to a Daewoo radio, cut the traces coming out of the tape preamp and inserted the audio there. I don't remember having to increase the volume, though. Maybe check further down into the circuit to see if there are some attenuating resistors or something else you can skip (I assume the next component would be the audio switching IC). I presume you also have your phone volume at near max, right?

author: darthtony
author_rank: Contributor
timestamp: March 14, 2016, 08:33:52 pm
content: The outputs of the head preamp(where i injected the signal) seem to go directly(via a 0.1uF cap) to the inputs R2 and L2 of this audio processor chiphttp://www.st.com/web/catalog/sense_power/FM1934/SC200/PF65176http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00003645.pdf

author: samnmax
author_rank: Regular Contributor
timestamp: March 15, 2016, 07:27:34 am
content: In the application circuit of the TEA0675, there are some traces that go from the output pins to various other pins of the chip. If you just lifted the output pins, this circuitry should still be there. I'm not sure, but maybe you could try lifting one pin of the output capacitors and inject the signal there to make sure nothing else is present in the circuit.