Revised Coxing Amplifier design

Background

The original Coxing amplifier design was based around the National Semiconductor LM383 chip, and this has been in production from at least 1986. However, this chip was withdrawn in October 1998, with the drive to new low power high efficiency amplifiers (Class D) for use in PC's and small mobile devices. The bad news is that these are generally not powerful enough for a coxbox application, and are not suitable for home construction as they require specialised components.

Scouring catalogues, after experimentation with different IC's the replacement I have found is based on the TDA1020 chip. The Circuit diagram below is lifted directly from the Phillips application datasheets, and on my testing seems to work quite well.

Application notes :

A few thoughts from the circuit that I built up using the above design - proper diagrams & photos to follow when I can lay my hands on a digital camera!

• Input directly from the condensing microphone to pin 8 seems to be fine. Make sure you power the mic as in the old design, Using Stages A and the first part of  B to power the microphone input. 
• Ignore the stand by switch above, wire directly through.
• Volume control goes between pins 7 and 6, using DC blocking caps. I used a 47k switched log pot which did a good job. See Stage C from the old design to give you an idea of how this could be done. 
• C8 is 1000uF = 1mF
• The above design has no power supply decoupling - you'll need at least 1000uF across the power supply rails
• In testing, the design worked down to a supply voltage for 4V - although I'd recommend running it from 8x 1.2v NiCd's, giving a totoal of 9.6V to give a decent level of power output.
• You will have to heatsink the tab on the chip - a DIY design should be fine ...

Concerns : Making it a bit louder - the above works fine, but you're not clipping the signal at full volume - it may be worth putting a low gain preamp in, but this goes against the KISS principle ! You could re-use the whole preamp design from the previous design, but this may be a bit of overkill.

You'll see some photo's in due course, but the external bits of the coxbox will remain the same so everything is compatible across the two designs.

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