Audio Mixer

Having quite a few radios, mostly with very poor speakers and in the case of the IC718 a very poor volume control, I thought an external amplifier and decent speaker would serve me well.

I will add circuits here later but they are basic...

This progressed into a full blown audio mixer project which was initially lashed up and had some issues. The original had a TDA 7052 amplifier which was pretty decent but was quite noisy and as a bridged amplifier there is no 'ground' side of the speaker so any connector needs to be isolated from a metal case - not as easy as it sounds. This was changed to a TDA 2003 amplifier which is very simple to make; gives 7 Watts if needed so plenty of headroom and the sound quality is far, far superior with less noise - a no brainer!

I'd also used 741 op-amps directly soldered onto a scraggy piece of strip board so I wanted to smarten this up and use sockets in case I want to upgrade the op-amp in the future.

I'd previously used a plastic case and need to swap that to a metal case to prevent RFI.

The new modules were built and battery tested with an audio source and oscilloscope which I have found to be extremely useful to check level and waveforms.

The new boards (old ones further down) after testing individually and mounting, note the liquid paper on the ICs with their ID written on (I do this when I receive new chips) ....

The innards on show, all shielded cable for the inputs grounded as they enter the case and a big ferrite on the DC input (power poles on the end connected to a good PSU - must fit a fuse!). Update: Clip on ferrites added to to all input cables and ESPECIALLY the speaker and headphone leads - my antenna is pretty close to the operating position and RF via the speaker leads was a killer, also added ferrite cores inside the box for the speaker and headphone leads.

In use, note the use of laminated paper as the front panel - it works really well!

All the rigs are connected up, the scanner that used to scare me to death when it broke squelch is now mellow and I can easily control the volume. The mixer is immune to mobile phone EMI and HF too.

Here are the previous incarnations...

It was based on this circuit 3 which is very simple to implement, I used cheap 741 op-amps and a simple TDA 7052 PA circuit

As you can see I have not labelled it yet and it fits quite neatly on the IC 718. The mixer is the central board and the amplifier is the board on the right. The boards are temporary at the moment, and there is plenty of space to make changes.

The additions were:

• added a power switch and LED

• gave it a higher powered PA (TDA 2003) - YES, YES, YES - this is so worth it!!!!

I use the mixer a lot, it is very handy to manage the output of all my rigs and keep the noise down for the rest of the family!