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I'm embarking on cloning an old analogue synthesizer, the Jen SX1000, and I notice on the schematics available several places where there is a series resistor on the non-inverting inputs of op-amps. The input impedance of the op-amps is enormous and swamps the value of the series resistor (e.g. 10k, 100k or 1Meg) so I'm baffled why they're there. The circuits are not exotic so far as I can see. Does anyone have a reason they may be there?

The first circuit simply provides a voltage that tracks with frequency to the filter. Op-amp in question is the LF353 bottom right.

Jen SX1000 filter tracking circuit

The second is a bog standard Envelope Generator, and it's the output buffer (LF353 again) on the right.

enter image description here

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    \$\begingroup\$ I can't read those schematics without wearing colored glasses or something. So I won't bother. But in general the idea for such resistors (as you describe them) is due to the bias currents present in opamps. (There's also an offset current which is a round-about way of saying how unusable their bias current spec is.) In general, you will find that the value for that extra resistor is about the same as the thevenin impedance seen at the (-) terminal. \$\endgroup\$ Dec 9, 2023 at 9:19
  • \$\begingroup\$ Yes, they're rather poor quality scans unfortunately. \$\endgroup\$
    – Ian Bland
    Dec 9, 2023 at 10:09

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  • Offset compensation

As @periblepsis said in the comments, the idea is to have each input of the opamp see the same impedance so its input bias currents cause the same offset voltage on both inputs and cancel. This is not the case here as the resistor value on the + input does not match the impedance seen by the - input.

  • Overvoltage protection

If you expect the input voltage to go outside the bounds set by the opamp's power supply, then the input protection diodes in the opamp will conduct to the power supply rail and dump input current into it. An input resistor is a way to limit current and prevent damage to the opamp. This can also happen at power-off if there's a large cap between input and ground, or during startup/shutdown.

  • Maximum allowed differential input voltage

Some opamps allow the full supply voltage across the inputs, others restrict it to a much lower value or have protection diodes between the inputs. This is often the case for low noise bipolar input opamps, where too much reverse bias on the input transistors may worsen the noise spec. This is not the case here as LF353 datasheet mentions 30V maximum differential input voltage.

  • Tame opamp misbehavior

Some opamps will reverse the output when the input goes out of allowed common mode range. This is different from the absolute maximum rating as no damage occurs, what happens is the output suddenly goes the wrong way which can make the circuit malfunction. It's called "phase reversal" and an input resistance can help against it. Also maybe it misbehaves during clipping and the resistor fixes it.

  • Deliberately slow down the opamp

It's not a very accurate filter, but the input has capacitance, so a high value resistor will change the frequency response.

I'm not sure which reason applies to your circuit. Simulation doesn't help much because opamp models usually do not contain the whole internal circuit, only what's necessary for a reasonably accurate and quick simulation under normal conditions, and some are more accurate than others. So simulating abnormal conditions using opamp models is hit or miss.

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  • \$\begingroup\$ Thanks, I think the overvoltage protection may be the best explanation here. \$\endgroup\$
    – Ian Bland
    Dec 9, 2023 at 10:08
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    \$\begingroup\$ One more: - simply copying others' designs without understanding. Sometimes people will hear advice that you should have this series resistor and include it even when it's not particularly important for the application (it doesn't usually hurt anything even if it's chosen wrong, after all). \$\endgroup\$
    – Hearth
    Dec 9, 2023 at 14:31
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    \$\begingroup\$ @hearth Nobody gets fired for leaving it in ;) \$\endgroup\$
    – bobflux
    Dec 9, 2023 at 16:16
  • \$\begingroup\$ The part of the circuit in question seems unique to this synth, it isn't on the datasheet for the tone generator IC which supplies current to the circuit (though the current mirror before it is, but with different values) so I think the designers were competent to design for themselves (they were an Italian organ company). I myself am doing a direct copy precisely to ensure it's a direct copy to get the same sound with all its flaws. The VCF tracking was "unique" in being rather bad by ideal standards and I want to preserve that. \$\endgroup\$
    – Ian Bland
    Dec 10, 2023 at 10:43
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I can see some reason for resistors in those positions as both have a capacitor of fair size connected more-or-less directly to the input. If something odd happened like the negative supply being pulled up above ground during power supply cycling the inputs could become reverse biased wrt V- and in that situation you want to limit the current. So something like 10kΩ might be prudent, so the second one makes sense to me.

1MΩ in the first position does not make any sense to me. I wonder if it's an error in the schematic. Images of the board show a 1MΩ resistor near that position but the orientation and position suggest it might not be connected as shown.

enter image description here

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  • \$\begingroup\$ Interesting observations thank you. Perhaps the 1M you mention is meant to be tee'd to ground? I don't have the real circuit board to examine as my Jen was stolen many years ago and now they sell for silly money. I'll play around with it on a breadboard. The part of the circuit in question is meant to track filter cutoff to oscillator pitch but is notoriously bad at it which gives the synth some of its audial charm and uniqueness IMO. \$\endgroup\$
    – Ian Bland
    Dec 9, 2023 at 10:19

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