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I'm using a 4558D JRC, and when my amplified signal gets low enough to clip, it goes to the positive supply rail rather than the negative one. Can anyone explain why that is? It makes a really horrible scratching sound when I'm trying to soft-clip my guitar signal.

I'm powering it with 9V, non-inverting; input biased at 4.5V, as I'm not using a dual-voltage power supply; and a gain of around 501 (Gain = 500k/1k+1).

Datasheet: https://www.rcscomponents.kiev.ua/datasheets/jrc45584i743ncft874nfdt34ufguygf43.pdf

Andrew Neal
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  • Maybe R7 should be 3k. Welcome to EE.SE but don't you think a schematic would be essential? There's a CircuitLab button on the editor toolbar. You can create one with that. Add a link to the datasheet for the 4558D JRC, whatever that is. – Transistor Apr 02 '20 at 17:30
  • R7? You mean the resistor that goes between the inverting input and ground? Yes, a schematic would be handy, but this is just a prototype on a breadboard so far without one drawn up, and I didn't know I could make one real quick on here. The 4558D is just a cheap Japanese op amp that has been used in vintage guitar pedals, so tone chasers like to use it, and it's what I have on hand because of that. https://www.rcscomponents.kiev.ua/datasheets/jrc45584i743ncft874nfdt34ufguygf43.pdf Here's the datasheet. Anyway, thanks for the welcome! – Andrew Neal Apr 03 '20 at 15:11

1 Answers1

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Your 4558D JRC is a sad copy of the sad old 741.

It doesn't have much operating space when powered at 9V.

The inputs and outputs must all stay about 2V from the powersupply rails. That leaves you with about 4V of input range and 4V of output range centered around 4.5V.

Call it 2.5V to 6.5V as valid input and output ranges.

Exceeding either of those limits will cause the op amp to misbehave. How it will misbehave isn't really defined.

You are also using a fairly high gain, so you will easily exceed the available gain bandwidth with just audio signals - leading to more distortion.

  • Use lower gain (maybe a couple of AC coupled stages with lower gain instead of one high gain stage.)
  • Make sure your input signal stays within the allowed boundaries.
  • Make sure your output signal will stay within the allowed boundaries.
  • Clip with diodes rather than depending on the opamp to clip when you overdrive it.
JRE
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  • Option Z: Use a better op-amp ;) – Edin Fifić Apr 02 '20 at 17:35
  • I didn't realize it was of poor quality. I've always heard it was good for guitar effects because of its warmer tone (which is most likely its slowness making it clip a little softer). I am clipping with diodes in the feedback loop, but when I increase my input level (by boosting it before it gets to the effect circuit), that is when it begins to clip all weird. I'm trying to design it so that I can drive it to distort more with higher input levels, so the op amp has to be able to clip correctly to avoid bad sounding artifacts in the sound. But looks like you're right. Thanks for the info. – Andrew Neal Apr 02 '20 at 21:07
  • @user247918 - It's not poor quality - exactly. It's just that you're using it outside of its intended range, and there are no guidelines as to how it will behave when you do. Go to the data sheet and look at the operating characteristics. They are all defined for a power supply of +/- 15 volts. Some later versions of 741s were specified down to +/- 5 volts, but even that is a greater supply range than your 9 volts. When you operate this far outside the intended conditions, there is just no guarantee what will happen. – WhatRoughBeast Apr 02 '20 at 21:17