[sdiy] usage of 3080 in micromoog

[Dave Manley]

I almost buy the simplicity argument, but to argue a bit:  the hiZ is 
provided by a FET stage not by the 3080, and the 3080 requires the gain 
setting resistor, a voltage divider to bias the output, and a darlington 
pair.  So I'm not sure it's that much simpler.  Is there some other 
characteristic of the 3080 they are exploiting that an opamp wouldn't 
have provided?

Also was the motivation for the ladder simplicity or that at the time 
there really weren't a lot of choices for making a voltage dependent 
resistance?

Benjamin Riggs wrote:
> The output of the ladder needs a balanced hiZ input with a gain ~ 100-1000.
> to do that with an opamp, you need to load the top ladder rung with the
> balanced resistor network (of 4 resistors) with an opamp to set gain (5
> components), or without loading the ladder use a more complicated
> instrumentation configuration (3 op-amps, 7 resistors), or a discrete
> circuit using a bunch of transistors and resistors) or use a OTA with one
> resistor setting gain and 1 resistor doing the I-V conversion at the output
> (1 OTA and 2 resistors). There are probably more ways of doing it, but with
> so few parts?
> 
> Moog liked elegance, the best/simplist solution with the least number of
> parts. That after-all was the motivation of the ladder filter in the
> firstplace.
> 
> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Dave Manley
> Sent: Friday, 6 October 2006 8:52 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] usage of 3080 in micromoog
> 
> There are a few places in the micromoog where a 3080 is used with the 
> bias current fixed.  For example look at the output of the ladder 
> filter.  Why was a 3080 used here instead of an opamp?
> 
> -Dave
> 
> 
>