3 ms·

Those op amp characteristics aren't really coming into play for that delay sound, it's just vanilla digital delay. Most of the "vibe" of the DD-3 is coming from

Those op amp characteristics aren't really coming into play for that delay sound, it's just vanilla digital delay. Most of the "vibe" of the DD-3 is coming from the companding and filtering scheme it uses to work around the limitations of the digital pieces. You will hear the effects of this in many hard clipping distortion circuits, though, where the amplifier gain factor will far exceed the voltage rails and be pushed into undefined clipping territory. Behaviors in this range can be an important part of the sound, e.g. the Proco Rat and the infamous LM308 op amp with its slow slew rate. Some like the TL072 exhibit a really nasty phase inversion that results in a pretty horrific (usually undesired) distortion. It's a balancing act, though; search "op amp motorboating" in any DIY stompbox forum and you'll find thread after thread of people trying to keep op amp gain stages from oscillating. I know more than a few noisier artists who enjoy when designs can be tortured into doing that, though :)

kazinator 2y ago
Motorboating feels like a strawman version of op-amp oscillation. It occurs because the tinkerers are using single supply circuits, without adequate bypassing to ensure that their virtual ground network has very low resistance. Thus the current dumped to ground by any circuit generates a voltage which appears to it and all the other circuits (ground loop). Another ingredient in motorboating is that there are multiple amplification circuits cascaded together which are coupled using capacitors (due to the single supply operation, again, which tends to discourage DC coupling). The ground loop creates a feedback path across multiple amplifiers, and the coupling capacitors create the phase shift.
- ericwood 2y ago
  I'm not sure what you mean by "strawman version of op amp oscillation"; for many it's the colloquial term for oscillation. You can accomplish it very easily with many classic distortion circuits, especially on a breadboard. Guitar pedals are almost always run off a unipolar 9V supply. Large bypass caps are the normal (usually cargo culted over as a 100u at the input). There's a lot of constraints that either remain due to the restrictive power supply requirements on top of tradition or people building off designs from the 90s that were hand assembled and took BOM golf to the next level.
  - kazinator 2y ago
    Motorboating is not a colloquial term for all oscillation. It's a very low frequency instability consisting of a train of pulses which, when played through a speaker, sound like an internal combustion engine with a short exhaust. The op-amp oscillation the article is talking about occurs in the radio frequency range. Tiny parasitic capacitances induce 180° phase shift at some very high frequency where the op-amp's open loop gain has dropped to unity. You can't hear this directly since it's not in the audio range. It brings about distortion. The amp may pass audio signal, but the oscillation prevents it from performing properly.
    - ericwood 2y ago
      In the contexts I was talking about (hobbyist DIY guitar pedal forums) motorboating is often used as a catch-all for many types of oscillation, probably because the symptoms you describe are a very common error. Sometimes you get lucky and can tune the resulting oscillator by turning the guitar's volume knob! I wasn't trying to assert anything about the article, just responding to a comment about desired op amp imperfections in guitar effects. It's fairly common practice to bandwidth limit high gain op amp stages to avoid the issues discussed in the article, amongst other things.

kazinator 2y ago

Motorboating feels like a strawman version of op-amp oscillation. It occurs because the tinkerers are using single supply circuits, without adequate bypassing to ensure that their virtual ground network has very low resistance. Thus the current dumped to ground by any circuit generates a voltage which appears to it and all the other circuits (ground loop). Another ingredient in motorboating is that there are multiple amplification circuits cascaded together which are coupled using capacitors (due to the single supply operation, again, which tends to discourage DC coupling). The ground loop creates a feedback path across multiple amplifiers, and the coupling capacitors create the phase shift.

ericwood 2y ago
I'm not sure what you mean by "strawman version of op amp oscillation"; for many it's the colloquial term for oscillation. You can accomplish it very easily with many classic distortion circuits, especially on a breadboard. Guitar pedals are almost always run off a unipolar 9V supply. Large bypass caps are the normal (usually cargo culted over as a 100u at the input). There's a lot of constraints that either remain due to the restrictive power supply requirements on top of tradition or people building off designs from the 90s that were hand assembled and took BOM golf to the next level.
- kazinator 2y ago
  Motorboating is not a colloquial term for all oscillation. It's a very low frequency instability consisting of a train of pulses which, when played through a speaker, sound like an internal combustion engine with a short exhaust. The op-amp oscillation the article is talking about occurs in the radio frequency range. Tiny parasitic capacitances induce 180° phase shift at some very high frequency where the op-amp's open loop gain has dropped to unity. You can't hear this directly since it's not in the audio range. It brings about distortion. The amp may pass audio signal, but the oscillation prevents it from performing properly.
  - ericwood 2y ago
    In the contexts I was talking about (hobbyist DIY guitar pedal forums) motorboating is often used as a catch-all for many types of oscillation, probably because the symptoms you describe are a very common error. Sometimes you get lucky and can tune the resulting oscillator by turning the guitar's volume knob! I wasn't trying to assert anything about the article, just responding to a comment about desired op amp imperfections in guitar effects. It's fairly common practice to bandwidth limit high gain op amp stages to avoid the issues discussed in the article, amongst other things.