DIY Stompboxes => Building your own stompbox => Topic started by: RickL on January 18, 2021, 04:42:53 PM

Title: Distortion plus DOD 250 op amp
Post by: RickL on January 18, 2021, 04:42:53 PM
I finally got around to building these, with a small mod to select Si or Ge diodes. They'll be combined in a single box. I don't actually have any 741 op amps and figured (correctly) that pretty much any single op amp would work. I'm currently using LF351s and they sound like, well, distortions.

Has anyone done any systematic testing of the differences in sound with different op amps? It'll be easy enough to swap 741s in if I ever bother to get some more but I thought I'd ask if anyone had any preferences.
Title: Re: Distortion plus DOD 250 op amp
Post by: antonis on January 18, 2021, 04:52:43 PM
It will be evolved into a looooooong story.. :icon_cool:
Title: Re: Distortion plus DOD 250 op amp
Post by: iainpunk on January 18, 2021, 04:55:01 PM
the difference is quite small, mainly in the higher frequency's. the slew rate is what is important here. LF351 has 16v/uS
if you are building both in a single box, i'd recommend the LM358, it has a slow slew rate of only 0.3v/uS
the lower the slew rate, the 'softer' the distortion, although it only makes a marginal difference.

cheers, Iain
Title: Re: Distortion plus DOD 250 op amp
Post by: Mark Hammer on January 19, 2021, 12:33:35 PM
Hi Rick,

My view is that circuits like the DOD250, that use a diode pair (or trio) to ground on the output, may well be clipping twice.  Headroom limitations, dictated by supply voltage and gain used, can result in clipping within the op-amp, followed by diode-clipping of its output.  When diodes are used in the feedback loop, signal amplitude is generally constrained to be well within the headroom limits of the op-amp, such that one hears only diode clipping.

But what follows somewhat from this reasoning (assuming I'm correct)  is that increases in supply voltage - conceivably via charge pumps - can take the headroom limits out of the equation, assuming the gain is not set ridiculously high.  And the related inference is that chip-to-chip differences in a 250-like circuit will be most detectable when the supply voltage is kept low.  You might even want to tinker with descending below +9V, perhaps via diodes in series with the supply.  For all we know, forcing a stable supply down to 8V or 7.5V might result in more audible tonal differences than changing clipping-diode type.  Worth experimenting with.

And FWIW, Ottawans are generally behaving themselves, with our case-count being a smaller share of the provincial case-count than our population size would predict.  Stay well.