Fav Designs Part 3

[Paul Schreiber]

Q)I hate to bridge the gap into digital, but you mentioned the AD9825/30/31
from Analog Devices were a good DCO chip to go with as well, how would it
hold up in computer controlled situations, and what have your overall
results for this chip shown?

A) These 3 chips are all variations of a NCDO (numnerically controlled digital osc.) They will form the basis for the TItan modular synth project (coming this fall). They have 24 to 32bit phase accumulators, and some have phase control registers (for phase modulation. now there's something a Moog can't do well!!) They go from .001Hz to 100Khz in steps of 0.025Hz. Drift is what your crystal reference input drift is (usually 10ppm). Demo boards are available from AD that plug into your PC printer port. Very slick!

There is another company that is making VCA's specifically for audio, can't
remember their name, but I have some of their part numbers: 2150, 2155,
2159. Rob Urry (CEO of dbx) said that dbx had a big hand in designing these
VCA's, and also based some of the V2 and V8 VCA designs on these IC's. If
you've heard of these IC's, and have tried them, please let me know.

The company is called THAT. These parts are very high quality. In 1-5 pieces, they are around $50-$75. I have specs somewhere. The trouble is, they WON"T sell you any for less than $500 per item. Elitist snobs!!
 
Once again, what about using a uP for generating the envelope? Trade offs?

Most lof the later synths used the CEM3371/3372 chips, which offer "hardware assisted" SW EGs. Check the CEM website (www.why.net/users/paults) for details.


What about switching supplies in analog designs? Too much voltage and
current ripple?

Much to noisy. Switchers are great for power density (watts/sq in). But analog synths require little power (except to cook exponential pairs!). Most CEM chips use around 20ma. You can power 100 op amps off 1.5amps.

Paul Schreiber