2-stage Magnavibe

[strungout]

Heyp.

So, I breadboarded the Magnavibe and liked it, but... I wanted the pitch change to be wider. I added a second stage. At one stage I had a bit of fuzz if I hit the strings hard (kind of pleasant fuzz, not gated). A second stage created more fuzz, still pleasant. But I want it clean!

If this circuit had opamps, I would know what to do to lower the gain, but with transistors, I'm kinda lost...

Here's what I have on my BB:

First, I need to understand the topology so I can reduce the fuzz. From what I could gather from searches, this seems to be a common emitter with shunt feedback, to set biasing on the base. Is this right? I've lowered the feedback resistor to 680k. The base is at 4.4V (should it even be at that voltage?) It helped some. Thing is, I'm using 2N5088s, they're lower gain than the MPSA18. Maybe I'd have a vibro-fuzz if I had used the latter ones...

Second, I read that people used a 10n instead of the 100n coming out of the collector because they didn't have enough vibrato on the high strings. I have the opposite problem, it's the low E string where I have a more subtle vibrato. Not as much depth to it. I raised my 100n to 1u (I figured if 10n helps with the high end, a higher cap value would help with the low end). I also added a 47k in parallel, which I read helps with range. The effect is less subtle, but not quite as strong as on the high strings.

Third, I settled on 200R to 11M dark LDRs and used a 5mm white LED with 15k mdc (I used a calculator that said at 15k millicandela at 90 degrees is about 27 lumens, I think. Doesn't seem like a lot...) and used shrink tubing to roll a dual LDR vactrol. The LDRs are at a slight (around 10 degrees) angle inside the tube because the leads are poking out the sides. It works well. Should work even better when the thing is boxed and it's in complete darkness.

So, here I am with a great sounding vibrato... If I don't slam the string too hard. Which means that if I put anything that boosts the signal in front of it, it will fuzz out.

Any ideas?

EDIT: Oops... I reuploaded the schematic.

EDIT 2: Here's a schematic with the stock values:

[bluebunny]

Here's what I have on my BB:

No pictures.   

[bool]

The base should sit at cca. Vcc/4 + 0.6V = 2.85V for max. headroom.

Imho get the bias right first.

[strungout]

Marc: Fixed the scematic not showing up.

bool: Ok, I adjusted the bias. I have both transistors at 2.78V. Should be close enough? I'm gonna get some trimpots later today to get to 2.85V. Right now, the fuzz is pretty much the same. Q1 has the 4M7 res and Q2 has a 3M3 + 680k in series.

I should've posted my voltages:

Power: 9.37
Ground: 0

Q1
E: 2.58
B: 2.78
C: 6.78

Q2
E: 2.51
B: 2.78
C: 6.83

Q3
E: 0
B: 0.57
C: 1.93 - 2.77

[bool]

Not sure what are two 47k resistors in || to 1u caps doing there? These aren't really supposed to be there.

If you are still getting fuzz, perhaps your input is too hot; so try to pad it down. Easy way is to replace the 2k2 resistor at the input with say a 220K trimmer etc.

Other than that; no suggestions. Except that the 1u caps in both stages seem a bit too much. Try to swap the 1u for a 220n at both stages..

[strungout]

bool: That did it, thanks! I considered my pickups, but I thought, they don't do that to other pedals, so I dropped it. But I was wrong. I have a set of JB/Jazz from Seymour Duncan, hotrodded pickups. I had to raise that 2k2 input resistor to 510k before the fuzz got tamed. Maybe I'll put it on a switch, so it I can revert to the 2k2 for pickups with lower output.

I removed the 47k in parallel to the 1u. I mixed up the info (sometimes I can't remember stuff I read just a minute ago...) with other info and thought it was helping with string range, but I guess I heard what the other changes I'd made did. Or I just heard what I wanted to hear. It was actually used to provide a better mix of vibrato and tremolo, when modded for it.

Those 1uF though, they do make the vibrato on the lower string more pronounced. I raised them slowly for the stock 100n (220n, 330n, 470n, etc) and was satisfied at 1uF. I just rechecked, at your suggestion, and it confirmed it. The 220n made the vibrato more subtle.

I'm satisfied with my Magnavibe atm. Will mess around some more just for kicks.


One last thing: Am I right in thinking this configuration is a common-emitter with shunt feedback? I just want to know so I can research this a little more. Learn a few things for the next time.

[duck_arse]

those are both concertina phase splitters, i'd guess with collector bias. you want both collector and emitter resistors same value [ .... ] so they both develop the same voltage swing (across them). the bias resistor let's the transistors gain decide what to do at the base.

there is a dissection of this stage somewheres abouts this very forum ........

[Mark Hammer]

As I have done in various places recently, I will put in a plug for a stereo (one-in/two-out) Magnavibe, using independent unsynced LFOs.  You can easily get dual-ganged 20k/25k pots, such that rate and depth can be varied simultaneously in both LFOs using just two knobs.  The important part is that the two LFOs are not synced, or counterswept.  Individual differences in the components, because of part tolerances, will assure that, even if the two LFOs do line up at some point, they don't stay that way for very long.  Normally, the pitch-wobble is what we pay attention to.  When the pitch is wobbling gently, in stereo, with no apparent pattern, it provides a very immersive kind of shimmer to the sound.  Most of us would not likely want to leave vibrato on for very long at a time, but in stereo it starts to feel like a nice light chorus that you can leave on for a while.  Recommended.

[strungout]

Stephen: Thanks, I will look into that! I can do the 'how it sounds approach', but the I need to learn more about the 'why'. Makes for better modding decisions.

Mark: Oh, forgot about that! I came by those while I was searching. I wanted to make another one tailored specifically for my 5-string bass. I guess I can use a splitter to get stereo. Problem will be matching the depth... Or I'll just breadboard another circuit. Hmm, should be interesting either way.


I'm out of parts now. And money. Built a Small Clone, Echo Base and Tremulus Lune, and I have a Phase 90 and two Magnavibes to build. I'm binge building... or something.

[Rob Strand]

wanted to make another one tailored specifically for my 5-string bass.

Good idea.   I've been thinking I should a simple 2-stager for bass for about a year now.   It would only take about 1/2 hr to do a VST plug-in to tweak the parameters but I've got a heap of other stuff to do.