Couple of questions regarding chorus, trems, and LFOs

[PenPen]

Ok, I'm working on a design that is a tremolo and chorus pedal in one, with a few added twists. I've been saving up a couple of questions, and I finally decided to just ask.

1) Small Clone, is it correct that it uses a CD4047 for the clock? I thought I had filed away that it used an MN3101 chip for the clock driver. Am I mistaken, and it does in fact use the CD4047? If so, theN what application would one use the MN3101 to drive the MN3007? A flanger, a la Polychorus?

2) I noticed several LFO designs use the LM358 for the opamp. Is there any particular reason why this chip is chosen? Does it have superior rail-to-rail properties over, say, a TL072?

3) There are several ways to do a tremolo, but one I haven't seen is using a jfet like the j201 with the drain/source as the path for the signal, and attaching the LFO to the gate to attenuate the signal. Any reason why this isn't done?

4) Assuming the tremolo method of using a jfet with the source to ground, gate to LFO, drain to emitter of a transistor, to vary the gain level of the bipolar by changing the amount of resistance to ground, as in the EA trem. If one wanted to, instead of vary the volume/gain, vary a tone control, wouldn't it be as simple as moving the drain to a cap that is attached to the collector of the bipolar instead of the emitter of the bipolar, so it would vary resistance of the high frequency roll off?

5) What should I avoid doing in my design to side-step clock tick? I'm mostly concerned with LFO ripple in the power supply line, the design I'm working on will have three LFOs and I'm concerned with clock tick in the signal path, or even having each LFO interfere with the others. Any tips on things I should watch out for?

I think that is it, I much appreciate any info and give thanks in advance.

[Mark Hammer]

1) The MN3101 is a custom chip intended top provide two complementary clock outputs in a small package with the fewest external components, smallest footprint, and low current requirements.  Is it the "best" solution or method to accomplish the task of clocking a BBD?  No, but it works remarkably well under certain circumstances.  One of those circumstances is that you try not too hard to clock the BBD really fast.  Is it somehow cheaper than other approaches?  Not necessarily.  A 4047 is likely cheaper than an MN3101, but I suppose that depends on who you buy from and what you factor into the cost.  A great many time-based effects use chassis smaller than what E-H used for many years, so while E-H incurred no "space penalty" from using larger chips, other companies might have to ask themselves if they could fit the entire circuit into the smaller chassis they has chosen.

2) Every 2-opamp LFO produces both a square-wave and a triangle wave.  the triangle wave is essentially the smoothed out version of the square wave.  At the point where the square wave swings from one extreme to the other, it draws a lot of current.  Enough, sometimes, that momentarily sucking that current away from other things creates an audible "tick" at the rate of sweep speed.  There are a variety of things one can do to eliminate that threat of audible tick, and one of them is to use op-amps in the LFO whose current draw is modest, such that they don't suck too much current away from other things that it might be heard.  The LM358 is perhaps not creator's gift to audio, but it uses less current to accomplish the LFO task than some other op-amps might, thus helping out the objective of quiet sweep.  The TL022 is also often used for the same purpose for the same reasons.

5) We've already provided some hints about providing tick-free LFOs: use low current op-amps (assuming the LFO is op-amp based).  Another strategy is to use well-filtered supply and reference-votage lines.  You will often see the reference voltage provided via an equal-resistance divider circuit cutting the supply voltage by half, with a cap between Vref and ground to provide a sort of "emergency reservoir" of current to help out with those sudden voltage swings.  If the cap is a little over-sized (i.e., higher value, like 22uf instead of 10uf), that can help out, and so can providing individual Vrefs (with their own subcircuit) for each LFO.

[PenPen]

Thanks Mark. In my SPICE models I'm not seeing any ripple on the 12V supply line after I put in a 400uF filter cap, but models can be deceiving and not work the same as in real life. From what I've been pricing out, using the CD4047 appears to be a cheaper solution than using the MN310x chips, so I suppose I will just continue with that.

Thanks also for the opamp info. I appreciate it.

Can anyone comment on questions 3 and 4?

[slacker]

You can use a jfet like you suggested with the signal going from drain to source with the gate driven by an LFO. Ages ago I built Tim Escobedo's wobbletron and if you remove C1 you get a very nice tremolo that uses a jfet as you've described. RG's variable stuttering pedal also uses a jfet like that.
I think the only reason not to use them is that it's quite easy to make the jfet distort.

[PenPen]

Thanks slacker. I suspected it would be fine, from what I could see the Tremulus Lune used a variable resistor this way to get the effect, so I figured a jfet could be used the same way. I just wasn't sure if there was some pitfall to doing that. I want the speed of a jfet for the effect instead of an optoresistor, so I was trying to see if this was a good idea or bad idea. I just read how to input a guitar wav file as input in another thread, so now I can actually realistically model what I've got so far, and I'll test that method out also. Thanks for the response.