Newly uncovered Univibe quirks

[R.G.]

While I was messing with the 'vibe, I uncovered some things which seem to change things for the better in some cases.

1.The 330pF compensation cap, C4 on the Neovibe layout is from Q2 collector to ground. This is odd compared to more modern feedback amp practice. I think it may have been selected that way to ensure that it swamps out the parasitic capacitances on Q2 collector and Q3 base. However, it does give a resonant peak out at about 900kHz to 5Mhz depending on what transistors, gain, etc are in there. On a whim I removed it and replaced it with a 30pF from Q2 collector to Q2 base. The result was more phase margin, and no peak. Now the gain rolls off nicely at about 2MHz, with some variation with the gains of the transistors, primarily Q2. It's a major step to better stability on that input feedback amp setup. Note that it is possible that some combination of transistors will not work well with this, so this must be classed as likely to work, but experimental.
2. Buffer, who needs a buffer? I figured out the input impedance at the input to Q1 independent of the 22K/47K input resistors. Turns out it's respectably high. Want to end tone sucking? Replace the 47K with 1M. This seems to work fine with my two "victim" boards as well as simulation. No loading, and it also makes the "unity gain mod" of questionable value since it gets to unity or above all by itself now that the input network is not loading down the signal source and dividing the input down. Again, experimental, but try it out.
3. I'm pretty sure I had another notch here somewhere. The 'vibe has the makings of a two-notch phaser, but the second notch never raises its head (lowers its feet??  ) in simulation. I had always put this down to the funny spread-the-phase distribution of the phase capacitors, which may have been to get a better vibrato sound. When I posed myself the question of what could be making this not show up, I looked at the low end response of the circuit, and noticed that the low end has several rolloff time constants overlaid. Hmm. What's cutting bass? Well, for one thing, the 1uF caps in series with the LDRs. These are nice, low impedance connections when the LDRs are up at a megohm, but what about when the LDR goes down to 500 ohms? I popped in 10uF for caps C6, C8, C11, and C14. The second, lower notch peeked out!

But it still wasn't very prominent, only about 12db of notch depth, which correlates to how audible phaser notches are. You need to get -20db to get real jet-plane-y.

So I thought "those bootstrap caps seem to be big enough, but why not just try it?". I subbed in 10uF for C7, C10, C13 and C16. Now the lower notch dropped much lower. Here's an oddity, though. C16 being left at 1uF with the others being 10uF was even better. I gotta do more thinking about that one. Messing with the other 1uF caps didn't seem to do much.

4. Dinking with the phase caps is a popular pastime amongst 'vibe afficionados I know; I got interesting results from making the phase caps be 22nF, 47nF, 1.5nF and 470pF(unchanged, that is). Worth playing with in concert with the other cap changes.

Interesting stuff. You may want to play with these. Of course these are mega-serious, trademarked, hyper-protected Keen Mods (tm) and can only be found in special Keen Modded (r) (tm) pedals.     

[John Lyons]

Go R.G. go!

[jessetrbo]

Just in time!  I'm starting to populate my newly acquired NeoVibe board...  I hope to take some pictures and post a build report ("hope" being the operative word).

Thanks R.G.

(Jesse Trbovich)

[R.G.]

A bit more.
As has been discussed here before, the LFO must have a high impedance. That applies to both the darlington transistor current gain stage (Q11/Q12) and to its bias resistor, the 2.2M. A quickie mod to fix is up is to use a packaged darlington, more transistor stages, etc.; again, been discussed here.

A simple mod is to put in an N-channel JFET for Q11. Suitable JFETs have very low Vgsoff. In this case, the following seem to work fine:
J201
PF5102
2N4117
PF5103
2N5484
2N4293
J210
2n4118
2n5953
J305
and in that order, because that's the order of increasing Vgsoff. Since N-channel JFETs bias with their sources at a higher voltage than their gates, you can run into the LFO output being size limited by banging against V+ if the source is very high, hence the J201's somewhat remarkable Vgsoff of -0.1 to -2.0 value.

However, if you will reverse the values of R39 and R41, this makes up for it a lot, and gives, as a generality, a bigger output from the LFO. With a JFET for Q11, you can then raise the value of R40 from 2.2M up to 4.7M or even 10M, and the LFO will now start faster and more reliably, as well as having a bit more frequency range. This change in bias voltage will make even the bigger Vgsoff JFETs work OK, and probably others I didn't look up as well.

Note that every single one of these JFETs have a pinout that is different from Q11. If you do this mod, you simply must look up the datasheet and pinout for the device you actually use, and get the leads in the correct holes in the PCB.

I experimented with the values of C22 and C23, but raising them above 10uF didn't seem to change things much.

[bluesdevil]

Great observations. Really glad you're revisiting this circuit!!

[Jaicen_solo]

While I was messing with the 'vibe, I uncovered some things which seem to change things for the better in some cases.
2. Buffer, who needs a buffer? I figured out the input impedance at the input to Q1 independent of the 22K/47K input resistors. Turns out it's respectably high. Want to end tone sucking? Replace the 47K with 1M. This seems to work fine with my two "victim" boards as well as simulation. No loading, and it also makes the "unity gain mod" of questionable value since it gets to unity or above all by itself now that the input network is not loading down the signal source and dividing the input down. Again, experimental, but try it out.

I was just hoping for a little clarification as to which resistor R.G. is referring to? I'm building the current GGG board, and i'm wanting to implement this mod. Which resistor number needs swapping out for a 1M??

[Jaicen_solo]

Looking at the schem, looks like I need to tweak R3, is that correct? Not meaning to bump the thread, but I can't seem to edit my previous post 

[R.G.]

Looking at the schem, looks like I need to tweak R3, is that correct? Not meaning to bump the thread, but I can't seem to edit my previous post 

Sorry, took me a while to get here. With reference to this schematic:
http://geofex.com/FX_images/vibeschm.pdf
The resistors to be messed with are R1, R2, and R3. In the stock Univibe, R1=R2 = 22K and R3=47K. I don't know why this was that way, possibly because the 'vibe was designed in an era when designers thought more than one guitarist/singer would share.

In any case, you can leave either R1 or R2 out, and only connect to the one you're using for a one-input stock vibe clone.

To do the high impedance thing, change R3 to 1M, and if you like, change the input resistor you're using (either the R1 or R2 position) to anything from a wire jumper to leaving it at 22K. Leaving some resistor in there could conceivably be some protection against input spikes that would over time make the input transistors go noisy, but that's kind of a foggy possibility anyway. I'd whip in a 1K for R1 or R2 and a 1M for R3 and call it good. And I have...  .

[Jaicen_solo]

Thanks R.G. that clears up a lot. Currently R1 and R2 are unpopulated so that makes it easier than the unity gain mod, no desoldering.
Actually, I can't locate any 200K trimmers. If I did this mod, swapped out the 22K's for 1K, would it be ok to use a 250K trimmer?

[R.G.]

Thanks R.G. that clears up a lot. Currently R1 and R2 are unpopulated so that makes it easier than the unity gain mod, no desoldering.
Actually, I can't locate any 200K trimmers. If I did this mod, swapped out the 22K's for 1K, would it be ok to use a 250K trimmer?

I'm a little slow this morning. What position are you using the 200K/250K trimmer?

[Jaicen_solo]

Oh, sorry! 

I was referring to the 200K trimmer in place of the two 100K mixing resistors. 200K seems like a tough value to locate, although I have a 250K. Would a lower value work any better?
Sorry for the stupid questions, but i've been looking forward to building this for many years!

[R.G.]

No biggie. I personally like to replace the two fixed 100K resistors with 68K or 75K resistors and use a 50K pot for that, pulling up the ends of the two resistors which would otherwise be common, and connecting each one to an outer lug of the trimmer, and the wiper of the trimmer to the point where the two resistors would have commoned.

This lets you have resistor values between say, 75K/125K and anything through 100K/100K to 125K/75 by turning the pot. The exact balance point is unlikely to be exactly 100.00000K on both resistors, but it also is highly unlikely to be unbalanced by more than 50K one way or the other. Using a 200K (or 250K, which was what you asked, and which also works OK) lets you go all the way to zero on one side or the other by turning the pot. The balance point is in the middle there, but chances are it's very, very close to the middle, and using a 200K pot makes the adjustment much touchier, since the mechanical turning range of good values is so small. Using a smaller pot and some fixed resistors makes the mechanical range of the pot turning cover a much smaller range of resistor ratios on the mix, so the mechanical range of "good" values is spread out over more of the pot rotation. It gets easier to adjust it finely, less touchy to adjust perfectly.

[Jaicen_solo]

Thanks R.G. that clears things up a bit for me. I'll try it with the 250K first and see if I can dial it in.
One last question though. I'm building a Jimi In A Box, and I was hoping to use the power directly off the Neovibe board.
Given that the GGG board has space for the two types of regulator, can I just tap the power at the output pin, regulate to 9v and use that to power my FF and Octavias?

Actually, maybe I should make a new thread, but I wonder how well those two circuits would perform under 15v power?

[R.G.]

Thanks R.G. that clears things up a bit for me. I'll try it with the 250K first and see if I can dial it in.

That's fine - just a tricker adjustment.

One last question though. I'm building a Jimi In A Box, and I was hoping to use the power directly off the Neovibe board.
Given that the GGG board has space for the two types of regulator, can I just tap the power at the output pin, regulate to 9v and use that to power my FF and Octavias?

That can work. As long as the power supply giving power to the Neovibe can supply enough current, then regulating to 9V either from  the raw DC at the input of the 15V regulator or from the output of the 15V regulator and sharing the Neovibe ground should work fine.

Actually, maybe I should make a new thread, but I wonder how well those two circuits would perform under 15v power?

I don't know. I've never tried that. I suspect the FF would need rebiasing, and maybe the octavia as well.

[axg20202]

I just stumbled across this thread. Good work RG! The neovibe remains one of the most challenging but rewarding builds I have completed to date (...oh hang on, the Meatball was a complete bugger to build...:-) ) so it's great to see that tweaks to make this great pedal even better are still coming through.

If I may, a quick (stupid) question on the input resistor swap: given that many boxes out there, including my own, already have the unity gain mod, does this mod need to be undone to play nicely with this new input resistor mod?

[Jaicen_solo]

I used a 4M7 to ground in place of the 47K, and a 1K rather than a 22K on my build, which I finished today.
As far as I can tell, it raises the level of the pedal to at least unity, and perhaps slightly more. It's difficult to be sure, because of the colouration that the pedal adds, but certainly I don't need a booster. My guess is that this would only improve on the existing mods that you have already.