Alesis Andromeda A6

While searching for noise samples to compare my breadboarded noise against, I had a look at the Andromeda A6 circuit diagram. It is fun to study what the big guys think are good designs and I've picked up a few ideas already. I will probably post more as I go anlong.

My first findings

Separate center reference voltage

The A6 seems to use a reference voltage, PMID/PDMID in place of ground whenever ground would be expected on any of the inputs of an opamp. My guess is that PMID/PDMID means Power Middle/Power Digital Middle or something like that, as ground in these places would normally be the center of the wave.

The PxMID originates with a 5V voltage regulator. I've calculated PxMID to be 2.05V. The A6 uses +/- 12V internally, with an offset of +2V this means that any positive amplitude is reduced to +10V minus any opamp limitations.

The PMID line is littered with bypass caps. I assume that by using a separate, well regulated reference voltage, one may get rid of some noise from other components, and also that its hard to get 0V from a regulator, thats why it's 2.05V (?).

Noise

The A6 has most of its analog circuitry inside a bunch of ASICs. The noise section however, is discrete. Only one noise source is used for all voices and it supplies white, pink and red noise.

The noise source seems fairly standard, it is based around a zener diode (which is the same technique as using a transistor without connecting the collector). The pink noise filter seems to be a true pink noise approximation (3dB/oct) as it uses a multiple filter sections in the same fashion as Elliot Sound Products. It is not entirely similar as it has the filter sections connected to between the positive input and ground, whereas the Elliot version have them in the feedback loop, but I assume this is only because the A6 one is a non-inverting filter and the Elliot an inverting filter. All noise outputs are connected to an inverting amp which also adds the PMID reference and acts as a low pass filter with cutoff at 493Hz.

Analog pots

All analog pots (that are read digitally) have 10nF caps across them - I assume this is to stabilize values or prevent that noise from one pot falsely triggers another one. 8 5kOhm pots are multiplexed through one CD4051 mux.

FX bus

The FX send has both positive and negative sends that are inverted versions of each other. Not sure if this means that it uses a balanced bus but it could be likely. Further investigation necessary :-D

Master volume

Master volume is a physical pot connected directly in the audio path, no VCA is used. No master volume saving is possible which makes sense.