small clone build report and mods (and questions)

[Chico]

I just built a small  clone using Francisco's layout.

It fired right up, but when comparing it to an Analog Man Chorus, mine sounded much more subtle.  If any of you get a chance to check out Analog Mike's pedals, please do.  They look and sound fantastic.

At any rate, my first assumption was that the delay signal was too weak.  It turns out that the dry signal was the culprit.  I twiddled with the resistors and got this ironed out.  That helped, but it still did not have the depth I was hoping for.  So next I turned to the modulation section.  I socketed the 150pf cap on the clock driver and began to experiment, watching the clock signal an a scope.  

Stick with me on this one as I cannot explain the theory (yet), only what I observed.  I wanted to vary the base frequency of the clock.  I thought my delay time was maybe too short, hence a more subtle chorus.  Instead of a rotary switch and several cap values, I took an alternative route.  I replaced the 39k resistor between the LFO and the clock with a 1K in series with a 100K potentiometer.  Presto, I can now vary my clock from subtle to wildly fast, with the same capacitor.  This gave me the most dramatic improvement in the sound as I could now twiddle with the depth, rate and clock freq to get sweet chorus sounds.   I would really like to understand how the LFO interacts with the diode, capacitor and resistor that form the timing circuit for the clock driver.  Can anyone explain how all these components interact?

That got me to my next mod, and my question.  With the ability to vary the clock so drastically, and to get really short delay times, I wanted to try to turn the clone into a chorus/flanger.  From an earlier noted advice from Mark Hammer (thanks for your help and great tips, Mark) , I thought I would give it a try to see how it sounded.  Taking Mark's suggestions in mind, and based upon the parts I had lying around, first, I added a switch to change the 2.2uf cap in the lfo to 4.7 to get a slower, more flanger approved lfo.  I then turned my new clock speed to get a faster clock signal/shorter delay.

I tapped off pad D (after the delay section and filter, just after the 1uf cap.  I added a 3k resistor in series with a 100k pot, added a series cap (.03uf - it was chosen because it was closest to my reach).  Feedback was brought back to either pin 2 or 3 (switchable) of the first op amp.

At low feedback settings, it was like a warming effect.  The sound got a little thicker.  However, it went from VERY subtle thickening to flanger oscillation howl with no flanger swoosh in between.  I tried every combination of depth, rate, and clock speed that I could.  As I turned the feedback pot, I hit a "spot", and the thing kicked into howl.  Anyone know how to correct this?

Any tips?  I think that the LFO is really not suited for use as a flanger, but I was hoping to get something passable.  Any tricks for modding that LFO?  is that where my problem is?

Can I add a pair of diodes in the feedback path of the first op amp to limit runaway oscillations?

Also, btw, I found that the chorus sounded really cool when I twisted my new frequency control by hand.  I may try to figure out how to hook an opto-resistor to that control.

Thanks for your help.

[Ed G.]

As to the diff. b/w your homebrew and the analog man clone chorus:
There was some discussion on this a while back that perhaps the diff. b/w the schematic and the 'real' small clones was that the .0027 cap in the schematic was actually a .0039.

[Chico]

That could be the case.  I am going to socket that cap and play around with values.  

Through experimentation, I found that twiddling with the delay clock helped to add more depth to the chorus sound.

My first version used a 100pf cap and the chorus sound was subtle.

Once I worked in the clock frequency  control potentiometer, I had much better control of the clock, so I replaced the 100pf cap with a 250pf cap.  This let me get significantly longer delay times, which sounded to my ears, like more lushness to the chorused sound.

Thanks for the tip.

[Mark Hammer]

My impromptu regen control was a real shot in the dark.  It seems when the lights went back on there were some bodies lying scattered on the ground! :lol:

There are two things to consider here.  One is that it may be better to tap the regen signal from the output of the post-BBD filter, rather than the mixing stage.

The other is that many flangers use a diode soft-clipper in the op-amp stage where the regen comes back to to keep the thing from running away with itself.  Since the Small Clone does not have a separate op-amp based input stage and regen-mixing stage (see the schem for BOSS BF-2 for an illustration of this), I was reluctant to suggest a diode clipper for signal-control purposes since in this design even the "clean" signal would have to pass through it.  On the other hand increasing the harmonic content of either wet or dry may well lend some animation to the sound.

Given that the input stage has very modest gain (just under x6 by my calculations), a back-to-back diode pair in parallel with the 33k feedback resistor would not likely yield all that much grit to the clean signal coming out of there, but would keep the regen signal in check.  Worth a try and certainly easy enough to tack onto the copper side of the board.  Worse comes to worse, stick a 1-4k7 resistor in series with the diode pair to make it a softer clip.

Finally, check out the Anderton Pedal Flanger that appeared in Guitar Player years back.  It also uses a 4047 for a clock driver, although it pushes a Reticon chip.  You may find some ideas for adapting your SC with an insert jack to produce a remote controlled pedal-flanger.  I'm not positive, but I think that one was scanned and posted on my site.

[Chico]

Mark:

Thanks again for your help.  I was able to track down that Craig Anderton flanger article on your site.  Interestingly, the potentiometer mod that I mentioned is right there in the article.  Craig must have been sending me some vibes last night.  At any rate, it works like a charm.  The rising edge of the clock signal is rounded off when the clock speed is set too high however, but I think that the clock is pretty much out of usable range by the time this happens.

Interestingly, in that particular Craig Anderton flanger design,  the clock does not appear to be modulated by an LFO.

I will play around with finding different places to tap off the bbd line for my regen.  The previous spot you suggested is so darned convenient however, because FP has provided a pad (pad D) for his stereo mod, which makes it an ideal location to split the signal off.

I think that I will try adding the diodes in the feedback path of the op amp as well.

[Muldoer]

Couldnt you add a Feedback control like on the Electric Mistress? Theres a 100k trimpot in series with a 10k pot for control. I imagine that you max your 10k pot and adjust the other one until the oscillation just goes away. Slap me silly if I'm wrong.