[sdiy] Discrete OTA

[David G Dixon]

I don't know, Andy... I just call 'em like I see 'em!  I've never put a DC
blocking cap in a filter circuit in my entire life, nor have I ever seen the
need for one.  I only built one filter with OTAs (the earlier version of the
Dr. Octature cascaded four-pole) and it works very well without DC blocking
caps.

> Hi David,
> 
> Can you please elaborate as to why the 33u is needed after 
> each sallen key? As you can see from the schematic:
> http://machines.hyperreal.org/manufacturers/Korg/MS-synths/sch
> ematics/KLM-307.GIF
> , the design actually doesn't use dc blocking caps after each 
> stage of the sallen key, they only have them after the last 
> stage. I would have thought there was enough DC blocking from 
> the negative feedback to not need them at all, the final 
> 2SC945 x 2 transistors already distort a lot anyway, so a 
> tiny amount of DC input here isn't going to matter much, and 
> the amp adds dc anyway which needs blocking before the final output.
> 
> So the only thing I can think of is they are protecting the 
> 4558 from DC, since there is a 10u / 33u DC blocker just 
> prior to each one of those. So is this a case of protecting 
> 4558s rather than a need for an OTA + dc adding buffer design 
> needing AC coupling? The IR3109 roland designs are basically 
> an OTA + dc adding buffer (in this case a p-mos
> buffer) and if you check the schematic of the Juno-6 or 
> similar there is a single 10u DC blocking cap at the input, 
> and output, but nowhere else in the filter core or resonance 
> feedback path, and the filter will work just fine if they 
> were removed and operate on DC signals.
> 
> --Andy