Choruses, Flangers, BBD & LFOs - Did I get it right?

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ubersam
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Post by ubersam »

I have a dead DOD FX75 that I am trying to revive. As I have no idea behind BBD based flangers and choruses, I decided to find some info on the subject. I think that I understand some of the technology but I would like some verification or correction from the more knowledgeable people here. This is what I understand so far:
  1. The chorusing/flanging effect is a result of mixing the original signal with the delayed signal, where the delay time is cycled (modulated or swept) from minimum to maximum delay time (Speed/Rate). The amplitude of the delayed signal is also cycled as well, although, I am not sure if it is "from same level as original" to "louder than original" or "softer than original" (Width/Depth).
  2. A flanger has shorter delay times (ex. 0ms-10ms), and a chorus has longer delay times (ex. 20ms to 30ms).
  3. In a MN3101 / MN3007 BBD delay combination, the delay time range is determined by the small capacitor between pins 7 & 5 of the MN3101. A smaller capacitor (ex. 27pF) would result in a shorter delay time range and a larger capacitor (ex. 47pF) would result in a longer delay time range.
  4. The speed (or the rate) at which the delay time "sweeps" (changes or transistions) from minimum to maximum delay time is faster for the chorus and slower for the flanger, and is determined by the LFO.
  5. The LFO also controls the modulation or sweep of the delayed signal's amplitude.
  6. Using the CE-2 schematic as an example, changing C19 from 0.1uF to a larger value, like a 10uF for example, will slow down the "sweep" of the LFO. Changing C19 to a smaller value, like a 0.01uF, will speed up the "sweep".
So, did I get it right or am I missing something(s)?

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Post by Howlinfox »

Hey, Im trying to bring a DOD FX75(b) back too, maybe we can help each other, Where did you learn what you did about yours?
Steve

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Post by ubersam »

Howlinfox wrote:Hey, Im trying to bring a DOD FX75(b) back too, maybe we can help each other, Where did you learn what you did about yours?
Steve
That sounds good to me. I learned all that just from reading stuff I found on the net. Posts by Mark Hammer @ d i y s t o m p b o x e s forum, articles by R.G. Keen, some other articles (not posts) at h a r m o n y c e n t r a l, and reading some schematics of choruses and flangers. I am pretty confident that I have a workable basic understanding, I just need some verification, or correction in case I got something completely wrong.

I spent sometime last night tracing the circuit and so far my FX75 seems to follow the FX75B schematic. I haven't gotten to the LFO side yet. That is another section that I really need to understand.

So, what is wrong with your FX75B? Mine lost all flanging and sounded like a delay without any repeats, and without the original signal. It was like, you hit the strings, then a few milliseconds later you hear the guitar. There was no discernible modulation of the delay, so i figured that the LFO is not doing its job. Since my FX75 is about 20yrs old, I though that some of the electrolytic might have dried out or otherwise failed. If you look at the schematic of the FX75B, you'll a 15uF NP cap in the feedback loop of the LFO opamp. On a hunch, I took that out and replaced it with a 10uF cap, positive side towards the output of the opamp. That brought the modulation of the delay back. There is still no flanging, it is more like a vibrato now. I know that at least the modulation is back so I'm pretty sure I am on the right track. From my readings, you'll get a vibrato like effect if the original signal is not mixed back in with the delayed signal. Now I just have to figure out how the original signal is supposed to be mixed back in and why it isn't being mixed back in. I'll tinker with it some more tonight. I'll let you know if there is any progress.

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Post by Dirk_Hendrik »

Unable to tell you. Your schematic contains errors. Prime example for concluding so is the fact that the input opamp, U1A, has positive feedback, according to this schematic. Furthermore I'm unable to tell, frim this drawing, how the dry and wet are mixed together at the summing opamp, U2B, since the wet signal is fed to the same junction where that opamps is biased at 1/2 vcc trough those 2 22k resistors of which one is, corrcetly, decoupled with a 10uF.

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Post by analogguru »

Do a search for the FX75-B schematic -there everything seems to be drawn correct.

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Post by ubersam »

analogguru wrote:Do a search for the FX75-B schematic -there everything seems to be drawn correct.

analogguru
Ahh... The one I linked to above proved to contain errors, as Dirk pointed out. I noticed the errors when I started tracing my pcb. For instance, there is no input buffer in the schem but there is one on the actual board. So the FX75-B Schematic that you suggest does look like the correct one. That is the one I have been following as a guide for my non-B version FX75, there are only some minor differences.

Using the correct schem as a guide, it looks like the original signal is fed into the inverting input of U5A through a 47K resistor. When I measured that resistor (R8), it was about 20% less than 47K. I'll have to pull that out and measure it again, maybe just replace it.

But going back to the points I outlined in my first post, do I have the correct understanding or is there something else I am missing.

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