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Vintage threads from the first ten years
More Uni-Vibe stuff...
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 2/11/1999 5:58 PM |
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Dave B. |
More Uni-Vibe stuff... Hi, Ok, more questions. First, is the preamp and the output mixer essential to the Uni-vibe sound?? Like, could I use a different preamp and still have a sound that is recognizably a Uni-vibe?? Also, is the signal boosted much in the circuit?? If not, I'm going to omit the output control. Oh yeah, one more. The speed control. I can get a dual 250k pot but not a reverse taper one. What will happen if I use a normal taper one?? And what is the most preferred and effective way of controlling speed??? Thanx, Dave |
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| 2/12/1999 9:59 AM |
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R.G. |
Good question. I've dinked with opamp replacements for the preamp, and it sounded good to me, but of course it's not "original". An aside about originality and vintageness. My hot button is clearly the preservation of the technology of vintage stuff with no mysticism about magic mojo parts. I find that often, you can replace a given function with newer parts, and that section or part may be as good or better than the original. However, musicians are conservative to a mania. While opamps might well be just fine there, to a majority of musical DIYers it's just not "exactly the same circuit and parts" and so it must be at best a second rate imitation and at worst a semi-fraud. When I used a three terminal regulator instead of multiple RC sections for filtering in the audio path of the vibe clone, I got some very caustic and public comments about how that was just not the same so it couldn't sound the same. Silly. So I try to produce information about what is and is not "original" sounding and let people make their own choices. And, since a persons mental perceptions color what they perceive, a person may really perceive that something they know contains only super-vintage carbon-comp-tropical-fish-Fasel-germanium whatevers sounds better, although no meter can show it. In that spirit, I have heard snide comments that opamps in the vibe preamp make it sound "sterile". I find that hard to believe in general, and I do encourage you to experiment. I would use a noninverting gain of three opamp and a gain of one inverter to make up the two phases of the preamp and see how it sounds if I were you. The magical "vibeness" is probably not hidden there. Also, on the output mixer. This is one of the magic secrets of the univibe, and is not widely known. The phase path of the univibe is not a well-behaved allpass filter, and there is some loss of level with each of the phase stages. If the wet signal is not *very* close in amplitude to the dry signal in the mixer, you don't get good cancellation and deep phasing notches, so the effect can be somewhat anemic. This, too is "original", as some of the vintage vibes I've worked on have poor cancellation. You may have to diddle the values of the two 100K mixing resistors to get good cancellation on the output mixer to make up for the signal losses in the signal path. This will greatly improve the sound.
It is actually a slight loss of level. I personally would omit the output level control and kick the gain of the preamp up to about four or five in the stock circuit, or add a modest variable gain to the input of an opamp-derived version. The loss of level shows up when you do a true bypass. The stock 'vibes just kill the LFO driver, so there is no change in level there, but the tone sucking remains. I believe that the Fulltone DV's actually have the gain of the input modestly enhanced for exactly this reason, although I have not seen inside one. Of course, that's not "original" either 
The input resistance is low, you might also want to do something about that...
Yep, that's the problem with the vibe. By the way, I need to do an update of the materials. The stock vibes came with a dual 100K pot, not 250K, at least the ones I have been able to measure. In practice, there is not much difference, as the 250K just enables the LFO to go a bit slower, about half the slowest speed of the 100k. TAPER: if you use a dual linear taper, the speed control will be kind of bunched up at one end of the pot rotation. There will not be an even speed increase per unit of pot rotation. I find that this does not bother me much. If you use an audio taper speed pot, in one direction of hookup, the speed per pot rotation is REALLY bunched up at one end, unusably so. If you use an audio taper pot hooked up the other way, the speed per pot rotation is now even per unit pot rotation, but the fast end is counterclockwise, so it's a "slowness" pot.
I prefer to use a dual 100K audio pot in a crybaby shell. When you've done the somewhat demanding mechanical work to get the pot fitted correctly, you find that the rack and pinion mechanism works an audio taper pot in such a way that the toe-down position is the highest speed, and that it's the even speed-per-rotation hookup on the speed pot, which is what you want. If I am using the one with only a panel pot (I have three of them running right now...) I use a linear pot. There is another alternative. You can use an LED/dual LDR to replace the dual pot and just control the LED current in a way that linearizes the pot/speed relationship. I've done this, works OK. | ||||
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| 2/13/1999 3:55 PM |
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| Dave B. |
RG, About the speed control, on your web site you said that the reason for the Reverse taper was to have an increase in speed while turning clockwise. Cant I just use a normal tapered pot and reverse the leads?? Dave |
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| 2/13/1999 4:02 PM |
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| Dave B. |
In addition to the preamp question: How do you determine the gain of the op-amp?? What is changed in the circuit that is different between a gain of 3 opamp and a gain of 5 opamp?? Thanks, Dave |
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| 2/13/1999 4:17 PM |
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| R.G. |
Go read the preamp section of the technology of the univibe. The two series resistors in the emitter of the third transistor in set the feedback gain. To make it higher gain, increase the top/emitter one and decrease the bottom/ground one. When the gain is determined, make sure that the collector resistor of the same transistor is equal to the sum of the two emitter resistors to keep it a good phase inverter. |
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| 2/13/1999 4:13 PM |
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| R.G. |
This does not reverse the physical distribution of the resistance on the pot wafer versus mechanical rotation, so it does not give you the desired resistance-per-rotation that you want. If you mounted a bushing on the back of the pot AND reversed the leads, that would work fine, as the mechanical rotation of the shaft would now give a reverse audio characteristic per rotation. If you think about it, you can figure that one out from an engineering/economics standpoint - if simply reversing the leads gave the required reverse audio characteristic, then reverse audio pots would not exist as a separate kind of part at all. If they exist, they must be necessary. Engineers are too cheap to use a special part if a common one works. Well, OK, *bosses* are too cheap, the engineers would use solid gold and platinum. | |
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