Adding external input jack (stereo to mono)

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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.

2. 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

 

[Eeyore]

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?

How are you going to connect to that point ?

I'm guessing it's a pretty similar level to what is coming
from the cassette head pre-amp circuitry.

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

2. 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?

Yes.

Graham

 

[Bob Masta]

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.

<snip>

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

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