[sdiy] The TL074 has changed!

[Mike Beauchamp]

On 2025-08-02 01:30, Jay Schwichtenberg via Synth-diy wrote:
> Mike,
> 
> Would you please share your experiences about the new chip not working 
> for you?
> 
> Thank you.
> Jay S.
> 

Hi Jay,

Sure. Here's the section of the schematic: 
https://mikebeauchamp.com/dump/tl074-prob.png

This is a little voltage to current converter used to control a LM13700 
used for resonance control in a filter. The resistor values were chosen 
by ear so that if the Resonance potentiometer is at full and additional 
CV is placed into the resonance-cv input, that CV input has a diminished 
effect as it is increased (sort of like a reverse-audio response). It 
still has an effect, but the circuit is limiting or "squishing" the 
response to avoid the self-oscillation creeping into bad-sounding 
territory. If the resonance pot is at minimum, the resonance-cv acts as 
normal.

However, with the new TL074 it does something really different. With the 
resonance potentiometer at full, adding the additional CV source works 
for a bit and then totally turns the resonance off after about 1V. As 
though the op-amp went into phase reversal, but that's not something I'd 
expect from the specs of the new TL074.


With the new op-amp I also tried modifying the circuit to match this 
excellent article I just found by Electric Druid ( 
https://electricdruid.net/design-a-eurorack-vintage-vca-with-the-lm13700/ 
) but with different scaling resistors and adjusting the additional 
current-limiting resistor to stop the self-oscillation from going into 
bad-sounding territory. Unfortunately there's no "squish' to the 
response", and even worse, another weird behavior creeped up: Once the 
maximum current was reached, additional CV voltage made the resonance 
amount reduce noticeably, so it also had a change of direction, very 
small but audible. This small change of direction also shows up on 
falstad circuit sim, so isn't a TL074H specific thing, but it's 
counterintuitive and worse than the original from a musician point of view.


Mike