adding a feedback control to a Shin Ei Fuzz Companion

[moid]

Aha! I did wonder about that, thanks! Looks like I'll need to wait until next weekend to work on it now, had to look after my son who was ill this weekend so didn't get any time to solder anything.

[moid]

Finally an update! I managed to build your extra daughter board suggestion this evening. I used 0.01uF caps like you said in place of all three 10uF simply because they are smaller and this build is being crammed into a 125B with no space at all in it. I didn't have a 510K resistor so used a 470K instead, but everything else followed the vero layout I made. The entire creation now works (mostly) - if the Feedback knob is off, the pedal behaves like the original modified Shin Ei which is great. As the feedback knob is turned on, things get noisy/unpredictable/chaos and eventually techno random bleep pulse evilness and no guitar at all There's a fine line between music and violent noise!

Two areas could potentially do with improvement if you have any suggestions please. First as the feedback is increased the three high strings on the guitar lose the ability to produce sound (the three lower strings still work) and the effect is high E is lost first, and as you increase the feedback the B string goes, followed by the D. I am sure that you once said something to me about adding a certain capacitor value somewhere to enable high pitched frequencies to travel through more, but I cannot recall it or find where you wrote it... does that mean anything to you? Apologies if it was some other helpful soul who took pity on me!

Second - the SPDT switch doesn't appear to do very much at all - the oscillations have a constant tone on one side of the switch and a slightly more broken up pulse on the other, but there's not much difference in it - would it be worth unsoldering one of the 0.01uF capacitors that are the final board components before the signal goes to the SPDT switch and replacing it with a different value? And if so, should I go higher like your original schematic (if using a 10uF would create a cleaner sound, that would be worth experimenting with so that the switch could go from chaos oscillation to something more refined?)

Once this is finished I'll post some sound clips and do a better vero layout of the mess inside the box. Thanks again for your help

[moid]

Ooh bonus feature! If fuzz and level are set at around half way, if the scoop knob is turned to cut bass from the signal you get high pitch oscillation that can be tuned through the fuzz and level knobs... partly this is good, but partly it's a bit annoying because it never fades out... If the fuzz knob is turned up enough the pitch becomes so high that I can't hear it (doesn't mean it's not audible, I'll have to ask my son; my hearing is not good on high frequencies)... must get some sleep now, but slight changes make some very weird clanging tones, almost like it's filtering some frequencies out, this is rather fun!

[crochambeau]

First as the feedback is increased the three high strings on the guitar lose the ability to produce sound (the three lower strings still work) and the effect is high E is lost first, and as you increase the feedback the B string goes, followed by the D. I am sure that you once said something to me about adding a certain capacitor value somewhere to enable high pitched frequencies to travel through more, but I cannot recall it or find where you wrote it... does that mean anything to you?

Yeah, I might have been talking while referencing a schematic on another build? I don't recall taking that plunge on this thread..

Stupid question time: do you have an o-scope? I'd be interested in watching screen while you increase feedback. Guessing the answer is no, so....

It sounds like you'd like more HF in the feedback mode and less HF oscillation when the gain is cranked? These are interactive fronts, and diving into the circuit to fix one might tamper with the other. Everything affects everything.

My thoughts (disorganized) are:

0.01 is a fairly small coupling cap, so it will, as a general rule, allow more HF and less low frequency to pass. Depending on the impedance of the surrounding elements it might be high passing the upper strings while going into a supersonic oscillation that effects that way the circuit handles the lower registers. Experimenting with cap size might be illuminating to see if you can push the "knee" or crossover point up or down in relation to capacitor size (simply piggybacking one over the existing in parallel will make the capacitance larger).

You can stick a bypass cap on an emitter and increase AC conductance through that stage - the only emitter resistor I see is on the far right hand resistor. Increasing HF only would entail selecting smaller value caps, and the 390 ohm emitter resistor is probably not going to attenuate the other frequencies to a large degree..

To snub HF, you can grab a SMALL value (pf) cap and either ground reference that along a signal path or try the feedback approach of giving a high frequency path from the collector to base, though this entire section is really getting close to tampering with the sound to a larger degree.

Apologies if that's hard to read, I'm not in a solid translate stuff to something readable head space at present, I might revisit with coffee tomorrow.

[eatyourguitar]

if you have a phase 180 switch and your buffer is TL072 based then you should have no bandlimiting and the switch lets you experiment to find one of them is actually negative feedback at most notes on guitar. this will likely not be perfect 180 out of phase so it will probably comb filter and reduce volume in the sweet spot for most notes on the guitar. if you play with spice then you already know about phase rotation in guitar pedals being completely non-uniform and dependant on frequency. it is actually more correct to say that the guitar pedal has a time delay that depends on all the filters that depends on what note you play. if you play a scale and compare the output of the pedal to the input you will notice that time is speeding up and slowing down. this is how frequency dependent phase rotation works. in phase feedback will always reinforce the fundamental or in some cases the dominant frequency band if it is on top after a scoop. the transistor buffer needs to be tweaked for correct ac coupling and no negative feedback from collector to base if you do not want it to be band limited and bass heavy during feedback.

just remember that there is the theoretical phase 0 or 180 and then there are guitar pedals and spice and damn what just happened here I hear it!

[crochambeau]

if you have a phase 180 switch and your buffer is TL072 based then you should have no bandlimiting

I haven't re-digested the thread so far, but I do not recall any operational amplifiers on the scene, so we're slamming full on into Miller Effect territory and bandwidth will probably have a very defined ceiling..

..where that IS, is another story.

[eatyourguitar]

when opamps are used, the bandwidth is whatever you want as long as it does not exceed the slew rate of the opamp. I think we can all agree that audio is well within the domain of a TL072. you are right that there is no opamp schematic in this thread but that should not stop anyone from googling TL072 9v buffer, clean blend, feedback etc... I could draw a schematic but I am already busy doing a for profit schematic right now. no time sorry. BTW I don't know why you would mention miller effect since you don't even need to AC couple anything. the shin ei has an input cap and an output cap whether or not you add a TL072. miller effect matters when you are using more than average current. not very common on small signals and small loads. TL072 will always be less problems with being muddy compared to a silicon NPN. the TL072 is basically flat and neutral with very minimal crossover distortion that I much prefer to a temperamental NPN clean boost everything needs to be tweaked for frequency response and stability. this is because you have no stability without the CB feedback cap but crappy band limiting with the CB feedback cap. getting the top end to flatten out requires a corner frequency closer to RF but that negates stability and CB feedback impedance. NPN = problems in my experience. as good or bad as a LPB1.

[crochambeau]

TL072 will always be less problems with being muddy compared to a silicon NPN.

I attribute this due to miller effect in that the loading of circuit components and the "parasitic" capacitance seen becomes more of a concern with a discrete topology, which is why I brought it up. I'm okay with being wrong, though so far I haven't seen a TL072 (which I agree, has loads enough bandwidth for audio) applied to the circuit under discussion.

[eatyourguitar]

I can agree with that

[crochambeau]

All that aside, it's entirely possible to get a discrete transistor circuit to sound like a champ, it just sometimes takes a bit more care.

Usually the detrimental stuff spoken of does not edge in that low in the bandwidth, but since we are speaking of a feedback path circuit that is not integral in the time tested design of the Shin Ei Fuzz Companion it is simply due diligence to consider that as a possibility. I think what I'm trying to say here is that: were this my build, I would most certainly explore as many value adjust experiments as I could before planting yet another active into the mix. So it might be time to grab a few alligator clips with caps & resistors and continue having fun with it.

Or chase an IC buffer, it'll allow you much higher input impedance on that stage (for better or worse, depends on taste).

[eatyourguitar]

All that aside, it's entirely possible to get a discrete transistor circuit to sound like a champ, it just sometimes takes a bit more care.

I was going to say this but I didn't want to sound like I was disagreeing with you after a long rant. I also don't want to make anymore bold claims in this thread without actual schematics, LTspice screenshots etc.. to back up what I am saying. there are formulas that can be used to design the single stage NPN amplifier you want that meats your requirements of a mostly flat frequency response and very little phase angle rotation at all guitar related audio frequencies. the benefit to that would be that it is class A and has no crossover distortion. if you have no base shunt, your miller effect is basically zero since the base with an input resistor will always be passing such small currents.

http://www.physics.mcmaster.ca/electron ... lifier.png

add a 10k resistor after the input capacitor. change the input cap to 100nf polyester film or don't change it. at 10uf it actually gets really flat because the impedance is low all the time. the maximum current this circuit will consume instantaneously is 1.77ma when the voltage across the 4K7 resistor is 8.3v. figured this using a CE voltage drop of one diode voltage drop (0.7). this happens when the base is saturated. you can make crude estimate of maximum operating current amplifying a sine as much as possible without major clipping by dividing 1.7ma by 2. then from that 0.8ma you can work back from the beta (guessing 300). 0.8ma/300 = 2.6uA or 5.2uA peak for the base current. the CB resistor is about the same for current. 10uA thats 0.01mA is not enough for miller effect to matter much. I would be more concerned about driving a long cable into a low impedance load from a common emitter amplifier that basically works as a voltage amplifier and a current amplifier simultaneously on the same signal.

[crochambeau]

It's hard to share a beer on opposite ends of the continent!

[moid]

Thanks to both of you for the rather impressive discussion of my rather unimpressive pedal! I'm sorry I don't follow a lot of it, but hopefully I will understand more of the debate in the future. In regards to one question, sadly I don't own an oscilloscope, but I've always felt that I could pass off the mad scientist look much more successfully if I did indeed own one.

Before I can go any further with asking for help I have some bad news; my son who was playing with the pedal in its partially assembled state while I was at work somehow dropped it, open side down and it is now mangled... it generates shrieking oscillations but won't pass any guitar sound at all I'm not even sure where to begin on this, but I will spend some time with an audio probe trying to find what part (s) are unhappy and hopefully fixing them. Once I get it back to the state it was in before its accident, I will return and ask more strange queries. Thanks for your help.

[moid]

Yay, after lots of bending parts back into place the pedal has come back to life In a fit of enthusiasm for circuit bending (and noise) in general I decided that the SPDT switch was just too subtle in terms of what happened when I switched it, so I decided to muck around with it and add a new cable from the Collector of Q2 to one of the outer lugs on the SPDT (not removing the previous connections, because, hey, why not?) and placing in the middle of this new cable a 1000pF capacitor. I did try a selection of values and most either cut all the sound when the switch went in this direction (not good) or cut nothing I could hear, but values around 1000pF (I tried 440pF and 2.2nf) seemed to break stuff up more in a pleasingly distressed fashion, while leaving the option to flick back to a sort of more normal fuzz when the switch was in the opposite direction.

So I boxed that lot, my son and I spent a good hour making some wondrous noises with it (and it is seriously good on bass too, on the right settings it sounds like a synth wah without the annoying attack note the synth wah always generates) and so later this evening I decided to record some samples and plugged it into other pedals, whereupon the feedback knob no longer does anything, except at maximum when it cuts all noise out and passes clean guitar instead... I checked this wasn't a power issues by using different plugs / power sources for the other pedals, so I don't think it's a ground issue (or maybe it is?)... once more any thoughts on how I can get this to now play nicely with its new pedal friends? Thankyou and please don't feel obliged to reply - the pedal is still awesome on its own, but part of me wants to run weird sounds into it to see how it messes them up

I can re draw the vero board layout if that will help (to show all the things I've done to it)

[eatyourguitar]

building it on vero, and dropping it, and I don't know how much experience you have, I would say maybe it would be good to build another one but this time clean the copper with scotchbrite before you do anything and then finish soldering in less than 24 hours to avoid oxide build up. I would even clean with alcohol again before soldering. watch the these videos. only difference is that you use scotchbrite instead of the rubber eraser in the video.

http://www.amazon.com/Scotch-Brite-Heav ... 003FYJ83S/