Installing an Auxiliary input on a Stereo (updated!)

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Goal

Almost everyone has an old boombox, tape player or old car stereo lying around. As CD and tapes became obsolete, this equipment (even the high quality pieces) has been collecting dust. The salvation for this devices is an auxiliary or “phono” input, where a RCA-to-Stereo cable can be connected. For equipment that doesn’t have this capability, an auxiliary input can be easily implemented.

This project requires basic electronics knowledge. If you don’t have practice with electronics, ask a friend with experience to help you.

Required Materials

1. Old Stereo, car cassette deck, or car cd player.
2. iPod, mp3 player or whatever source you use to listen music.
3. Stereo cable with 3.5 mm plug
4. Double Gang potentiometer (on some cases you can scrap it from the same stereo).
5. Two ceramic capacitors, start with 0.1 uF
6. Soldering Iron, screwdrivers, etc.

Procedure

My example device is this Sony boombox. I like it because sound quality is decent, construction quality is greater than average (It was made in Japan) and can operate with batteries, which is a very nice feature for camping or pool parties.

First unplug the device and take the cover off.

This is how the main board looks.

On devices designed to operate with 110/220 VAC you will see a separate circuit board that steps down and rectify the voltage coming from the wall. Ir’s very easy to spot due the big transformer. We don’t need to touch it.

Ok, let’s take a quick look to the theory. The signal generated by either the Radio Tuner or Tape player is sent through a volume and tone control. Then the signal reaches a Power Amplifier that (as the name suggest) amplifies, “boosts” or “magnifies” the signal enough to make a speaker vibrate and produce sound.

We will inject the signal coming from our mp3 player directly on the amplifier input, and disable the other parts of the system. We will also include a new volume control.

On the original Engallamientos’ post I have maintained the Radio and other original functions – the mp3 player just overpowered them when connected. This scheme worked well on some systems, but in other cases the impedance mismatch burned the mp3 player. I prefer to avoid you any risk and opt for the safe path.

Finding the amplifier

We need to locate the amplifier chip. It’s easy to spot because it’s (normally) under a metal heat sink. Each chip is labelled with a code. Click on any picture to enlarge.

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Getting the amplifier Datasheet

Google the code printed on the chip to make sure about its identity. We are looking for the “amplifier” or “power amplifier”, not ”pre-amplifier”.

Don’t forget to put word “datasheet” after the name. A datasheet is a document that describes all the functions and usage of an electronic component.

Once you located the datasheet, take note of each pin number and its function. Some old datasheets are harder to understand, like the following  (TA7282)

Newer datasheets are far cleaner and self-explanatory for the beginner. The following example belongs to a modern integrated circuit.

Determining the necessary pins

Using the information from your datasheet, locate the required pins on the amplifier circuit board. The following pins are required for our project:

• [NON INVERTING INPUT A] (corresponds to the left channel input)
• [NON INVERTING INPUT B] (corresponds to the right channel input)
• [GROUND]

At the same time please be careful with the following pins, do not connect anything to them or your music player will be damaged.

•  [V+]
• [V-]
• [OUTPUT A]
• [OUTPUT B]

Coupling the new Signal

Now take your stereo cable with 3.5 mm plug and peel the end exposing the wires. You will find tree smaller wires, they correspond to the left channel, right channel, and ground.

 

They will be soldered on their corresponding amplifier pins. But wait! don’t forget the coupling capacitors.

Don’t forget the coupling capacitors!

Solder a small value ceramic disc capacitor (like 0.1 uF or 0.22 uF) between the signal cable coming from the music player and the amplifier input. Repeat the same on the other channel.

If this sounds confusing don’t worry, on the following picture the step is explained graphically.

After soldering the respective cable on its amplifier input pin, notice that the pin is connected to the other components on the circuit board via printed tracks. We need to scrap both the right and the left channel tracks.

Do not scratch the track going to the Ground Pin. Without it the unit will not work.

 

You can scratch the track using an x-acto knife or box cutter

The volume Control

The volume control it’s very easy to implement, you only need a dual gang potentiometer. Note that the following drawings are the same circuit. The first drawing shows how the right channel is wired to the “top” potentiometer, and the second drawing shows the left channel and the “bottom” potentiometer.

Testing and finishing

This is how the whole circuit looks. Hell yeah!

If everything works as desired (I’m sure it will do!) install the new potentiometer and any cosmetic features.

13/08/2012 errata

I (incorrectly) stated that connecting the input signal to the tape player head was a good alternate method. Is not. Sound volume will be hard to control, requires an isolation transformer, cassette deck switches must be bypassed, and overall results may be frustrating.

Below is a picture of a car stereo where I employed the tape deck method.

The switch on the top is to “simulate” the presence of a cassette on the deck. An air fan was also installed.

As I said, using an isolation transformer (sold in stores as “noise suppressor”) will be necessary.

Spare you the annoyances, the main method is better.

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