Univibe concerns

[RedHouse]

... I've built and seen many built with 2N5088 exclusively that worked fine. ...

Same in my experience, I've bult dozens with 2N5088's exclusively and had no problems either.

[Brossman]

Brad, I just took a look at your website, and I have to say I'm seriously considering scrapping my project altogether...

I must admit...the VB7 looks rather enticing!

[jasperoosthoek]

Who was having oscillation problems in the front end?

I had those problems with the Neovibe board ordered from GGG. I replaced the transistors with lower gain 2n3904s (190). But the latest batch of 2n3904s I bought have a gain of 290. It might be that I soldered a wrong resistor in there, I would have to check that. But the board layout should not be the problem as I didn't design it myself.

[RedHouse]

Who was having oscillation problems in the front end?

I had those problems with the Neovibe board ordered from GGG. I replaced the transistors with lower gain 2n3904s (190). But the latest batch of 2n3904s I bought have a gain of 290. It might be that I soldered a wrong resistor in there, I would have to check that. But the board layout should not be the problem as I didn't design it myself.

Ah, I see.

[R.G.]

I had those problems with the Neovibe board ordered from GGG. I replaced the transistors with lower gain 2n3904s (190). But the latest batch of 2n3904s I bought have a gain of 290. It might be that I soldered a wrong resistor in there, I would have to check that. But the board layout should not be the problem as I didn't design it myself.

It is always tempting to say that whatever caused something to work again must have been what caused the problem to start with. Tempting, but open to error.

Oscillation in something that normally amplifies is what is called gain-phase oscillation or Nyquist oscillation, after the guy who did some of the early work on negative feedback. There are two things which are necessary for a negative-feedback amplifier to oscillate. These are (1) a gain through the amplifier and then back through the feedback loop attenuation that is greater than one and (2) a phase shift in the amplifier plus feedback network which causes the 180 degree phase inversion of the amplifier to get turned around to 360 degrees so that the feedback reinforces the input, not opposes it. If you have those two things, it oscillates. Eliminate either one, and it quits oscillating.

Accordingly, if something oscillates, you can make it quit by either (1) lowering the forward gain of the amplifier (2) increasing the attenuation of the feedback path so it feeds back less to the input or (3) messing with the phase shift properties of the amplifier to get the phase shift down or (4) messing with the phase shift properties of the feedback network to get the phase shift down. And note that "gain" here is voltage gain unless you're way out in the tall grass messing with current mode amplifiers and feedback.

In the Univibe, the voltage gain of Q3 is unity, because of that unbypassed emitter resistance. So the voltage gain that matters for stability is all in Q1 and Q2. For this to be unstable, either Q1 or Q2 has too high a voltage gain, or they both do, or one or both has too much phase shift. Then there's always the possibility of wrong/misplaced components, soldering, etc. But the voltage gain of a bipolar is NOT equal to the hfe. That's the current gain, and is highly variable. The voltage gain depends on the value of the resistances connected to the transistor AND the current gain. High current gain *lets* you think about high voltage gain. Proper design *lets* you use high current gain either for high and variable voltage gain or lower but more dependable voltage gain depending on the circuit.

It was recognized back in the 1950s that reliable design of transistor amplifiers used local feedback to stabilize the gain of a transistor over variations in hfe, temperature, leakage, and so on. In a well designed bipolar transistor circuit, you can stick in just about any transistor of the right polarity with enough gain and the circuit will work properly.

[Mich P]

Here a thread from last year talkin' about Q1, Q2 and Q3
http://www.diystompboxes.com/smfforum/index.php?topic=69918.msg607080#msg607080
Mich P.

[RedHouse]

Here a thread from last year talkin' about Q1, Q2 and Q3
http://www.diystompboxes.com/smfforum/index.php?topic=69918.msg607080#msg607080
Mich P.

The distortion issue is different than the oscillation issue.

When you have the blocking distortion problem all you (normally) need to do in increase the 22k input resistor to like 28k or so that usually fixes that even with Fairchild 2N5088's.

[jasperoosthoek]

It is always tempting to say that whatever caused something to work again must have been what caused the problem to start with. Tempting, but open to error.

Very true. Of course a wrong component or faulty layout can cause oscillation too which is then solved by changing the transistors. 

I have a tried and tested layout (by GGG) and have rechecked all components of the input buffer on the board. They all have the correct value according to the GGG PDF, even the 330p capacitor. At least, they were all new components and the color codes and descriptions match. Also, Mich P reported the same problem on page one of this thread. He also fixed it with changing the transistors. This means that at least two people have reported this now.

There is a discrepancy though: The resistor at the base of Q3 has to be a 3.3k according to the information on your site. However in the GGG layout it is a 3.9k. Maybe not a spectacular difference but this does increase the feedback slightly. Was it was put there on purpose to have more similar gain from both sides of the phase splitter?

Oscillation in something that normally amplifies is what is called gain-phase oscillation or Nyquist oscillation, after the guy who did some of the early work on negative feedback.

I am aware of the Nyquist theory. But, of course, the theorem is based on a perfect differential amplifier with some voltage gain A which is completely independent on frequency voltage and whatnot.

Accordingly, if something oscillates, you can make it quit by either (1) lowering the forward gain of the amplifier (2) increasing the attenuation of the feedback path so it feeds back less to the input or (3) messing with the phase shift properties of the amplifier to get the phase shift down or (4) messing with the phase shift properties of the feedback network to get the phase shift down. And note that "gain" here is voltage gain unless you're way out in the tall grass messing with current mode amplifiers and feedback.

So I lowered the "gain". 

In the Univibe, the voltage gain of Q3 is unity, because of that unbypassed emitter resistance. So the voltage gain that matters for stability is all in Q1 and Q2. For this to be unstable, either Q1 or Q2 has too high a voltage gain, or they both do, or one or both has too much phase shift. Then there's always the possibility of wrong/misplaced components, soldering, etc. But the voltage gain of a bipolar is NOT equal to the hfe. That's the current gain, and is highly variable. The voltage gain depends on the value of the resistances connected to the transistor AND the current gain. High current gain *lets* you think about high voltage gain. Proper design *lets* you use high current gain either for high and variable voltage gain or lower but more dependable voltage gain depending on the circuit.

I admit I used the term gain too freely. I know what hFE means, I've spend some hours explaining this stuff to undergraduate students . I'm not an expert but at least I have some idea what we're talking about .

It was recognized back in the 1950s that reliable design of transistor amplifiers used local feedback to stabilize the gain of a transistor over variations in hfe, temperature, leakage, and so on. In a well designed bipolar transistor circuit, you can stick in just about any transistor of the right polarity with enough gain and the circuit will work properly.

Also true, but as you say this assumes that the circuit is designed well and that local feedback is used. But just take a look at the emitter of Q2. What's that capacitor doing there? I smell a complete lack of local feedback there. Clearly a part of the circuit where the voltage gain of the combination of Q1 and Q2 is directly related to the bare transistor characteristics (hFE is some sort)  . Of course the combination of the 47k and 330p should stop feedback but I can imagine that if Q2 is completely open that the 'resistance' of collector/emitter pair will be much smaller. Definitely the transistor is loaded capacitively for positive input swings.

I think that you or probably anyone else here would have never designed that buffer like that.

I spent some time today performing circuit simulation of the input buffer. I could not change the gain freely in my spice program. I was not able to get the circuit to oscillate, even with darlington transistors so in 'theory' it should be stable. So that's one point for the Univibe.

But at least two people have reported oscillation of the input stage, both with the same solution. I think that maybe stray capacities play a role. Just things that are harder to predict. My unit also sounded great except for distortion/clipping at relatively high input levels. That's how I found out. It could have slipped under the radar undetected...

[RedHouse]

...I think that maybe stray capacities play a role...

I would agree with that.

I once tried to reduce a FuzzFace into a 1"x1" PCB thinking I could fit that nicely inside a guitar control cavity without taking up too much space.... bbbbzzzzzttt! wrong. No matter what I did it wouldn't stop oscillation, I gave up on the idea but it was a nice lesson (to me) in how physical layout (trace and component proximity) can effect a circuit.

[R.G.]

I have a tried and tested layout (by GGG)

Yeah. Actually, that's my layout, sold under license. 

and have rechecked all components of the input buffer on the board. They all have the correct value according to the GGG PDF, even the 330p capacitor. At least, they were all new components and the color codes and descriptions match. Also, Mich P reported the same problem on page one of this thread. He also fixed it with changing the transistors. This means that at least two people have reported this now.

OK. Two of them have had this problem

There is a discrepancy though: The resistor at the base of Q3 has to be a 3.3k according to the information on your site. However in the GGG layout it is a 3.9k. Maybe not a spectacular difference but this does increase the feedback slightly. Was it was put there on purpose to have more similar gain from both sides of the phase splitter?

I am aware of the Nyquist theory. But, of course, the theorem is based on a perfect differential amplifier with some voltage gain A which is completely independent on frequency voltage and whatnot.

The idea of a perfect amplifier in the theory is just to allow you to model imperfections external to the "perfect part" of the model for separating them out. Imperfections in the amplifier do not invalidate the theoretical application. Rather the perfect amplifier part lets you model imperfections in the amplifier you actually have and correct or modify the imperfections.

So I lowered the "gain".

Yes. However, lowering the gain of any oscillating amplifier will stop always it from oscillating at some point. Obviously with a gain of less than one, no amplifier can oscillate. But simply fixing things by lowering gain also sacrifices the advantages that higher gain and feedback give you, at the very same time. Keeping the gain high AND making it stable at the same time is an advantage.

Since the circuit in question has worked properly and not oscillated with all higher-gain devices, this is a big suggestion that lowering one transistor's current gain is not the single and only way to fix it. It's like stopping your teenagers from talking or texting all their waking hours. You can do that by tying them to a chair out of reach of the phone/computer. But that may possibly not be the best of all possible ways to stop the behavior.

On the other hand, if it produced results you liked, that's fine too. If it works and you're happy with it, great. Making any one pedal work to your satisfaction is fine. But generalizing from one - or two! - pedals to every pedal is an easy to misunderstand how the electronics works.

I admit I used the term gain too freely. I know what hFE means, I've spend some hours explaining this stuff to undergraduate students . I'm not an expert but at least I have some idea what we're talking about .

OK. That's good!

Also true, but as you say this assumes that the circuit is designed well and that local feedback is used.

Yep. That was my point.

But just take a look at the emitter of Q2. What's that capacitor doing there? I smell a complete lack of local feedback there.

Yep. That smell is the deliberate use of a bypass cap to eliminate local feedback. My point is that not all circuits do this, or do it well. And not all circuits which have overall feedback need local feedback at every point. IC opamps for instance don't necessarily use local feedback on every point.

There is no substitute for knowing the details.

Clearly a part of the circuit where the voltage gain of the combination of Q1 and Q2 is directly related to the bare transistor characteristics (hFE is some sort)  .

Yep. That's where the raw voltage gain for the overall feedback around all three of the first transistors is developed, Q3 being a combination buffer/phase splitter. And in this circuit the combination of gains in Q1 and Q2 is what's making the raw voltage gain. And the parasitic capacitances of those two devices plus Q3, plus oddities in layout and wiring, make the parasitic components which would make the amplifier unstable if it's not properly compensated by gain/phase compensation techniques. The whole point of overall feedback from the resistors in the emitter of Q3 and the gain from Q1 and Q2 is to form an overall feedback loop around all three. It can be done so it makes the individual characteristics of all three transistors not matter much as long as there's enough gain, and as long as the compensation gets the feedback loop gain under unity by the time the phase shifts add up to enough to make the feedback turn to positive, as Nyquist said.

Of course the combination of the 47k and 330p should stop feedback

Yeah. Actually, it's a single dominant pole to cut the gain under unity before the other phase shifts make the thing oscillate. I found in simulation runs that it also makes for a resonant peak that's not quite oscillatory out at a couple of MHz. A 30pF cap from Q2 C to Q2 B is much more effective, and does not have the resonant peak. But both sound the same.

but I can imagine that if Q2 is completely open that the 'resistance' of collector/emitter pair will be much smaller. Definitely the transistor is loaded capacitively for positive input swings.

It's capacitively loaded for both positive and negative swings.

I think that you or probably anyone else here would have never designed that buffer like that.

I'm confused - Q2's not a buffer. Q3 is. No, I'd not do a circuit like this, given today's state of knowledge about components and circuits. But Mieda-san didn't have that advantage some forty-plus years ago.

I spent some time today performing circuit simulation of the input buffer. I could not change the gain freely in my spice program. I was not able to get the circuit to oscillate, even with darlington transistors so in 'theory' it should be stable. So that's one point for the Univibe.

Yeah, I get pretty much the same results. My simulator lets me turn up the gains on transistors at will for a given run, and I couldn't get the thing to oscillate until I put in current gains of nearly ten thousand. That is another thing that makes me think that changing the 2N5088 for a 2N3904 may have lowered open loop gain, and may have stopped the oscillation, but that it could be something about the transistor that was replaced or the solder joints or PCB condition that made the first one oscillate, not simply that it was a high gain device. It is quite difficult to ensure that simply because something got "fixed" when you changed something that it was the part *type* that you replaced that fixed it. Could have been a bad/nontypical part you replaced, could have been a number of other things.

But at least two people have reported oscillation of the input stage, both with the same solution.

OK. And more than two have reported all 2N5088s work fine. I've also found that all 2N3904s work fine. The sheer number of counter examples lead me to believe that there is more to it than using all 2N5088s is a problem, or all high gain devices is a problem. Not that you're not seeing and reporting correctly - just that I suspect that there's more going on than is apparent.

I think that maybe stray capacities play a role.

I agree 100%.

Just things that are harder to predict. My unit also sounded great except for distortion/clipping at relatively high input levels. That's how I found out. It could have slipped under the radar undetected...

Good that you caught it then.

[jasperoosthoek]

Thanks R.G. for your free lecture in Univibology! I'm going to need a phase splitter for the funnel diode fuzz I'm designing. I might breadboard this one and see if I can get it to oscillate again. 

[Brossman]

Hey all,

Glad to see everybody was so interested in learning more about the 'Vibe while I've been out!  School has been keeping me very busy (as in, very without a life other than studying, haha)...

I have recently received my trannys and perfboard via post, so I'm ready (and totally pumped) to begin laying it out.  Upon sitting down to lay this badboy out, I realized I don't exactly know where to begin...Yes, I want to do this on PERF.

Sometimes I get overwhelmed by the end product and can't take a step back and look one step at a time...anybody have some words of wisdom to help me out a little bit?  ...point me in the right direction, if you will...?

Thanks in advance,

- Britt

[R.G.]

Doing a 'vibe on perf is not too taxing. There are no high-gain sections where you get into trouble, with the possible exception of the feedback on the first three transistors.

Here's an off-the-top rundown of the commonest Neovibe problems as I remember them without an actual count.
1. Wiring errors, especially on the speed control.
2. Part orientation of transistors and electro capacitors.
3. LFO won't oscillate (Q11 and Q12) for various reasons, including miswiring, orientation, etc.
4. Bulb won't light up/flash right; this is usually from issues with the depth wiring, but includes driver parts, and the bias trimmer on the driver transistor.
5. Other stuff. This includes a few where the input triple oscillates.

Overall, the 'vibe is a very forgiving circuit.

If I were doing one on perf, I would use a drawing program to lay out where each part and wire goes first, and print out both top-side and bottom-side drawings as a cheat sheet for the actual wiring. I would then build it one section at a time, and then power it up and test each section as I went. That way, nothing gets fried, and errors/problems are limited to whatever you did last. So I'd do the perf layout entirely as a drawing, either on a drawing program or what I personally did decades ago, on graph paper, in pencil, so I could erase and move things. As an aside, figure out where your wires come out and where the mounting post holes are going **before** you start putting parts down. You'll be a lot happier in the end.

When you get to soldering, wire up the power supply first, and get it working 100% before you do anything else. That's because you'll use the power supply in testing all of the remaining circuits. Then do the LFO, Q11 and Q12, the speed and depth controls, and the bulb and driver. These are completely separated from the audio path except for the light going into the LDRs and the power/ground, so when they work, they work and can be ignored for the rest of the build.

Then, place and solder the input preamp, Q1, Q2, Q3 and surrounding parts right up to the capacitors that come off Q3's collector and emitter. And solder up the volume pot and input/output jacks. You can now put a guitar signal into the input jack and temporarily run a wire from either of the the Q3 collector capacitor or emitter capacitor to the volume pot and hear the audio at both places by moving the wire from one to the other. This makes the circuit into its own audio probe.

When the first stage is working OK, build the first phase stage, Q4/Q5. Do the same "audio probe" trick with the outputs from the collector and emitter of Q5. When this works, do Q6/Q7, etc. till you get to the emitter of Q10. At this point, all the circuits work and all you need is the wiring to the output switch and volume pots updated.

This all relies heavily on already knowing where each part goes to come out right. It's also a good, foolproof way to build up the PCB for the project as well. It takes longer to build, but much less time to debug.

[Brossman]

Thanks, RG!

I'm actually doing the layout by hand on paper.  I custom made graph paper on EXcel with thin translucent lines.  I have it matched with the exact size of my perf, by way of hole X hole dimensions.

I will take your approach for doing it sectionally, thanks.  But...does this require a voltmeter? I have several analog multimeters...these aren't voltmeters are they? haha..

[R.G.]

You're all over it. Go man.

[Brossman]

I see in the old schemo where the cancel switch is, but I cannot determine where the other side of the switch is...however, in the old pedal, there was no true bypass, so is this the spot where signal was shunted with a SPST switch?

Also, taking a side-by-side of this and the Neovibe schemo on GEOFEX, I cannot seem to figure out where the option of having the speed controlled pedal is...were there not some Univibes that had the foot-controlled speed as an option?  I assume that would redirect the signal to a jack, where upon signal would be routed to (and then from) the rocker pedal via a stereo cable (likely hardwired to the rocker itself).  I guess the trick the would be getting the signal to use a knob on the pdeal as standard, then bypass it when the footpedal is plugged into the jack.

Now as for pulling this one off, I'm completely clueless.  I mean, I could use a DPDT switch to go between the two different speed knobs, but that would be...let's just say, a terrible inconvenience and not worth the effort (especially at a live gig)...

Any hints?

[Edit:] here are the links to the different schemos...

http://www.univox.org/pics/schematics/univibe.gif

and apparently, nvm about the neovibe schemo...I cant find a link, but I think you can find it at geofex somewhere...

[R.G.]

I see in the old schemo where the cancel switch is, but I cannot determine where the other side of the switch is...however, in the old pedal, there was no true bypass, so is this the spot where signal was shunted with a SPST switch?

Kind of. The original Univibe had no bypass, true or otherwise.
What it did was to cancel the sweep by shorting the LFO signal to ground, forcing the bulb to stay at a constant light. The signal was always beset with the frequency filtering that was left over by the driver not getting any LFO signal.

Also, taking a side-by-side of this and the Neovibe schemo on GEOFEX, I cannot seem to figure out where the option of having the speed controlled pedal is...were there not some Univibes that had the foot-controlled speed as an option? 

It was not an option. The original Univibes all had a foot controlled speed rocker. They did not work *at all* without that pedal. The pedal contained the speed pots that made the LFO run. No option. Some of the units may have oscillated at a very slow fixed speed with the pedal not plugged in, but none of the ones I've messed with do.

I assume that would redirect the signal to a jack, where upon signal would be routed to (and then from) the rocker pedal via a stereo cable (likely hardwired to the rocker itself).  I guess the trick the would be getting the signal to use a knob on the pdeal as standard, then bypass it when the footpedal is plugged into the jack.

As I said, the rocker was a fixed part of the LFO. No rocker, no sweep. And the signal never went to the rocker. The cancel switch was activated by rocking the pedal fully back. That shorted the "cancel" pad to ground and killed the sweep internally.

Now as for pulling this one off, I'm completely clueless.  I mean, I could use a DPDT switch to go between the two different speed knobs, but that would be...let's just say, a terrible inconvenience and not worth the effort (especially at a live gig)...

There are ways to pull it off, but none of them are how the originals worked. You can use special switching jacks to switch between the internal and an external speed pot, but the number of wires gets you quickly. All of the three speed pot terminals you need are floating - none of them is ground. You have to run a ground wire to the rocker/cancel switch to work cancel, as well as yet a fifth wire to actually short the cancel pad. You're up for at least five wires to the rocker. A DPDT won't do it unless you're willing to float the whole rocker pedal sheet metal on the common line of dual pots. That's not a good idea from a hum and noise standpoint.

Your only mistake is thinking that the Univibe had some of the modern conveniences (mod cons in the UK) that we think are normal for pedals today. It didn't.

[Brossman]

>Your only mistake is thinking that the Univibe had some of the modern conveniences (mod cons in the UK) that we think are normal for pedals today. It didn't.

Taking a few second glances at the schemo, I see how that lays out now.  However, do I need the "cancel" switch if Im modding for DPDT?  Wouldn't I just wire the switch to go between itself and the board in/out?

And what about the transformer in the original circuit? do I need to include some kind of inductor into this casing, as well? Or am I better off with LM317 (or the like) acting in place of this?

[R.G.]

However, do I need the "cancel" switch if Im modding for DPDT?  Wouldn't I just wire the switch to go between itself and the board in/out?

My preference is for a real bypass, not the frozen-in-place filtering that the univibe does when it's supposed to be cancelled. I'm not sure what mod you're doing for DPDT; if it's true bypass, yes, do that and ignore "cancel" entirely. I would.

And what about the transformer in the original circuit? do I need to include some kind of inductor into this casing, as well? Or am I better off with LM317 (or the like) acting in place of this?

Putting a mains transformer in a low-level effects pedal is a sure way to get the possibility of hum where there was none before. The original Univibe transformer was solidly encased in thick steel end bells and frame to help shield it, and the enclosure was big enough to get a little distance from the circuit. Transformers are for making DC power (in the way we use them mostly) and don't have anything to do with inductance in this case at all.

My advice: use an external 15-18Vac or 18-24Vdc wall adapter and wire it so it powers only the vibe clone, wired into the power in pads on my schematic. Do not try to use its power supply for anything else in your pedal chain.

If you insist on wanting to run a univibe clone from the same 9V adapter as the rest of your pedals, you can do that with an LT1054 charge pump converter to make 17-18Vdc from 9V and feed that into the 7815 that regulates the power for the rest of the circuit.

And while we're on power supplies, don't use the R-C filtering from the original schematic. You get better performance from a full wave rectifier bridge and a 7815 three terminal regulator, in terms of both hum and noise. That is, use the Neovibe power section, not the "vintage" Univibe power section.

IMHO.

[Brossman]

where exactly does the power section end? after the diode/resistor/cap array? before the lamp and tranny 13?