PLAN B analog blog

The short answer is not much.


Going into a little more detail, outside of a second change to the
sine wave circuit (which was put there for easier initial set up (us)
and really doesn't effect the end user and the 15A port, the
differences are slight:

We've added an output buffer to the PWM circuit (which was handled
another way in the older versions) and a bypass cap or two.  Nothing
serious.

The new (flat) VCO also has a pathway for the (I promise) soon to be
released 15A expander.  It's another connector.  Look on this blog for
what the 15A does.  Btw - I've found the missing link which was making
the +/- octave switch behave so poorly...so this should be coming out
with the rest of the ELF's.

Most the bigger changes had already happened by the time we flattened
the unit out, namely:  the AC offset triangle output (the first
release was offset all above ground), the harmonically improved sine
wave, the use of an SMT tempco resistor, the substitution of our
analog switch - initially used only because I really thought I'd only
be making 25 of these for that semi-custom , very limited order on the
Doepfer list and I happened to have 150 of these really expensive 18
pin SPDT analog switches. So once those ran out, I replaced it with a
more affordable SPST switch which was better suited for the application. 

The additional trimmer now found on the current rev VCOs was added
stop us from having to hand-select the fixed resistor which would
yield the best overall sine shape.  When the Model 15 first came out
we used an over-speced (read:  unnecessarily expensive) FET in the
sine shaper. We found that a less expensive part offered no sonic
degradation once set up correctly, but required a bit more tuning as
the lesser expensive model had a wider gain range.  So we added a
resistor which set the amplitude where it need be.  The net effect
instead of a $3 part we could use a $.20 part with the same exact
results.  Further, we also added a new function which reduced the
amount of total harmonic distortion, so make that a $2.80 cent gain
with superior results. This did require I hand tune each sine shaper
by selecting the proper fixed resistor value to compensate for the new
FET's part to part gain variance (FETs in general are very
loosy-goosy).  About 100 VCOs after I became completely sick of doing
this I then added a trim pot to adjust this easier. Even though we
lost $.60 of our initial $2.80 gain, it was money well spent as I was
REALLY tired of going through this exercise.

That's probably more info than you wanted, but that's the full story.

Hope this helps...


- P







--- In PLAN_B_analog_blog@yahoogroups.com, Anthony Rolando
<goldenechos@...> wrote:
>
> 
> Aside from the redesigned form factor and improved sine waveform,
what are the differences between revision 1 and 2 Model 15 oscillators?
> 
> Tony
> 
> 
> 
> 
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