Modification of AM-5040 amplifier, installation of RCA connectors for use on newer devices

Welcome back to the Klinkon Electronics workshop. Today we will see how to modify an AM-5040 amplifier from the RCF to be used via two RCA connectors, overcoming the compatibility problems of the DIN connector.

The AM-5040 is a 3-input amplifier (2 microphones and 1 aux line on DIN connector), with treble and bass output control and boasting an output power of 40 watts RMS, on the back are outputs for 4, 8, and 16 ohm speakers and 62ohm@50V and 250ohm@100V, the power supply also accepts 230V AC and 12V DC. There is also a protection fuse and a DIN connector Tape In/Out.

Today's goal is to make this 1970s amplifier compatible with the latest technology by equipping it with a little circuit that can take the AUX input and convert it to a mono signal for use via RCA connector with modern devices such as smartphones, laptops, or audio equipment like CD players or stereos.

Let's start by analysing the AUX connector, currently our aim is to adapt the stereo signal coming out of the classic 3.5mm jacks (about 1Vpp at 1kHz frequency) to the working voltage of the amplifier, which as we can see from the diagram (detail in the next image) is about 160mV, by isolating the GND and the signal line

Thanks to the suggestions of Fabio Bosco, an active member of the group telegram in our laboratory, in this signal line we can introduce a stereo signal by converting it to Mono and then adapting it to the logic level of the amplifier circuit, using a few simple components such as two 3.24k resistors, a 10k potentiometer, a 0.22uF ceramic capacitor and a small millefori in which to solder the components

As we can see the circuit is very simple, R1 and R2 combine the stereo signal coming from the device, and adapt it in terms of signal to the voltage accepted by our amplifier. The potentiometer allows the device to be fine-tuned using the oscilloscope to obtain the desired attenuation for the device in use, while the capacitor allows us to filter out noise while keeping the audio quality as clean as possible, and then send the signal to the amplifier via the 'purple' wire originating from the AUX channel mentioned earlier.

By inserting a 1kHz signal into our circuit, we can use a 2-channel oscilloscope to verify the behaviour of the circuit, measuring the attenuation with respect to the input and fine-tuning the system according to the type of device connected. In our case, we used a mobile phone whose output voltage value from the 3.5mm jack was 0.8V, trying to obtain an attenuation at maximum volume of 5 times.

By connecting the RCA connectors to the ends of the circuit and attaching them to the faceplate after drilling through the metal plate, we have completed our modification, thus giving new life to our amplifier.