[sdiy] The TL074 has changed!

[Jay Schwichtenberg]

Mike,

Thanks for the details!

Jay S.

On 8/3/2025 8:43 PM, Mike Beauchamp wrote:
>
>
> 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
>
>
>
>
>
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