Adding external input jack (stereo to mono)

Aug 13, 2007 2 Replies
C

I bought a replica of an old cathedral radio (Crosley CR32) for my office. It is a radio/cassette player. I would like to add an input jack so that I can connect CD, MP3, Internet radio or whatever source to play big band tunes through the antique-looking radio. I tried one of those cassette adapters, but it made too much noise as the motor turned the fake tape transport. It also seems like such a kludgy work- around when an input jack is so much simpler. I looked up the IC's on the circuit boards and located the Amp (TBA820M) and downloaded data sheets. It's a simple MONO amp. Inserting a cassette automatically switches the audio input, so I would like to use that with the external input as well (remembering not to insert a cassette when using external source). I have two issues:


  1. Is it OK to feed the input of a CD/MP3 Player headphone jack into the Amp IC? I'm guessing it's a pretty similar level to what is coming from the cassette head pre-amp circuitry.


  1. How to PROPERLY combine the two STEREO channels coming from the CD headphone jack into a MONO input to the Amp IC? I found a diagram on the Net that shows a 10K Ohm resistor in series with each channel then connected together for the input. Will this work for me?



Thanks! Crusher300



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E

How are you going to connect to that point ?

It may be somewhat higher in voltage. You can always attenuate it.

Yes.

Graham

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B

Here's a tip that might help: Presumably the unit has a normal potentiometer-type volume control. (As opposed to the digital attenuator found on some modern equipment.) If so, that is almost certain to be a good place to connect in. The wiper goes to the amp input, the fully clockwise contact comes from the existing signal source. So if you want to add a jack that still retains use of the volume control, you put it in the CW contact lead. If you want to control volume only with the external device, you put the jack in the wiper lead.

In the "olden days" this sort of surgery was trivial, since the pots were on the front panel and connected to the circuitry by nice long leads... easy to find and connect to. Modern equipment often has the circuit board mounted right behind the front panel, with the pot directly mounted to the board. It's a bit tougher to work with, but still do-able.

In general, with the pot method you don't need to know much (or anything) about the preceding stages or the following amp.

Best regards,

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

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