Well it worked in the simulations!?

Time for a slight change of topic, leaving project Hermes for a while and detailing some of the fun of developing ‘simpler’ circuits.

I have just finished a long over-due update of a simple, single supply audio mixer, primarily aimed at the Amiga, details are here: http://www.ianstedman.co.uk/Amiga/designs/Audio_Mixer_MK2/audio_mixer_mk2.html, this is a simple design, based on a classic summing amplifier. For the original prototype, designed back in 2012, I simulated the circuit using a SPICE simulator, all worked well, tracked the PCB, ordered a PCB and built it. It did not work!

The output of the op-amps was a DC level, it would not respond to any input stimulus. I isolated the other input, in case this was causing an offset, no joy, isolated the inverting output stage, no joy. Left the workshop tired and confused before I did something crazy.

The design uses an LM358 operational amplifier, nothing special, it’s a voltage feedback device that can run off a single supply. It was the single supply that was part of the problem. I created a 1/2Vcc reference, buffered by another LM358, for the other devices to use, this worked.

A search of the Texas Instruments website, found an application note, AN-116, http://www.ti.com/lit/an/snoa662b/snoa662b.pdf, in there was a critical note, missing from the datasheet,

The LM358 will operate satisfactorily in balanced supply operation so long as a load is maintained from output to the negative supply

, this is achieved by adding a 10K resistor from the output to the negative supply, which is GND for a single supply. Ironically, if you use a non-inverting amplifier, the Vq resistor is not required, but to have a summing amplifier (to mix audio signals) requires an inverting amplifier.

So based on simulations which worked well, I spent £40 getting two PCBs fabricated, which did not work! An expensive lesson learnt. It also show the importance of a prototype to prove the theory.

It took about two years to go from the prototype to a final PCB, which is now available to purchase (marketing plug), http://www.ianstedman.co.uk/Sales/sales.html.

The final design works well, even if I do say so myself. The frequency response has improved slightly, it covers the full 20Hz to 20KHz range, more than I can hear, it cures the harshness of the left/right Amiga audio output and looks quite nice.

There is one final tweak to the design, which involves a few component changes. The ‘Better Paula’ circuit implementation, is slightly wrong. It requires a different mix ratio, which thankfully is just a few resistor changes. I will test this shortly and update the MP3 samples on my website.

This audio mixer was a simple design, which all told, with design, re-design (grr), PCB tracking, component selection and website updates to document it all has taken around 30 hours to develop. Surprising but not un-expected.

That’s all for now. Back to digital electronics.


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