ENS-76 VCO Option 2 question

Bob Zimmer bzimmer at voicenet.com
Tue Mar 25 04:28:59 CET 1997

At 01:31 PM 3/24/97 PST, you wrote:
>     That's nuts, Bob! Should work. Ya got me.
>     I've used the 082 in this way plenty o' times, no problemo. There is, 
>     somewhere, a rational explanation for this. Maybe the 071 is damaged 
>     inside, weakened perhaps? So that it works in some apps and not 
>     others? I can't remember the headphone-driving capabilities of these 
>     JFET's off the top of my head... But as a I-V converter like in this 
>     3080 circuit, any of these op-amps should work the same (except for 
>     bandwidth). The external compensation devices like the 301 and the 748 
>     may have some extra hi-freq oscillations, but not the 071.
>     Be sure ya let us know what it turns out to be (so we can put it in 
>     our op-amp X-files).
>     - Gene
>     gstopp at fibermux.com
>     p.s. I'll be out of the country on a business trip for a week, so 
>     ASM-1 queries should be postponed till April.
That's what I thought also.  According to the PCC, the 556 was selected
because of the need for slew rate, not bias current (not that I pretend to
understand bias current!).

I have found that a later circuit (VCO with Through-Zero FM Capability) uses
the same triangle-to-sine converter.  There is a difference though!

The 39K resistor between the inverting input of the 556 and ground is
replaced with just a ground connection.  After finding this schematic, I
tried the VCO Option 2 without the second resistor and now it works great!
As expected the higher slew rate of the TL071 gives a more rounded sine wave
than the 741 did!

I'm just beginning to understand op-amps and have no clue of what problem
that resistor caused!  Now if only I can get rid of the glitch in the middle
of the sawtooth waveform and do a couple of level adjustments.  I'm trying
to setup all the outputs for +-10V peak-to-peak.

  >=== Bob Zimmer -- Philadelphia PA ===<
  >=== bzimmer at voicenet.com          ===<

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