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Uni-vibe LFO questions

1/30/1999 6:40 PM
David Britton
Uni-vibe LFO questions
Hey people,  
I have a few questions here. Is it possible to use a different LFO. Like, w/ less circuitry?? Let me rephrase that- I know I can replace the LFO w/ a smaller one, but will it make any difference in the sweep??  
What about the pre-amp?? Could I just use a small op-amp pre w/o any dramatic change in sound??  
Thanx, I appreciate any help.  
2/1/1999 6:00 AM
Paul Perry

If you change the SHAPE of the LFO, yes, it will sound different.  
Maybe, even better (heresy!)
2/1/1999 8:47 AM

There have been several tries at this, with various degrees of  
The LFO on the univibe is a special animal. The current-mode phase shift oscillator has a rising amplitude versus speed that does something of a compensation for the sagging response of the incandescent lamp with speed and the sagging response of the LDR's with speed. I've experimented with LED/LDR setups and you have to some special waveshaping to get close to the same sweep. Even all light bulbs don't sweep the same - I hypothesize that the differences in thernal inertial and current-to-light-output between bulbs have an effect. The LFO/bulb/LDR has no effect on the tone, but you have to diddle the waveshape to make the sweep come out similar, and this probably means that the waveshaping needs to change with speed, becoming more of a pulse at higher speeds if you use LEDs. Getting good results will mean tuning by ear, I think.  
If what you're after is getting rid of the dual reverse audio pot, you can do that by using the VTL54C3/2 opto isolator and controlling the LED by a single pot. The dual sections make a good dual speed control, and you can then externally "taper" the control pot to make the speed control approximately linear.  
As for the preamp, there are a couple of commercial retro-"vibes" that use opamp preamps. Opinions are divided. There is some small distortion in the low-gain discrete "opamp" that the original 'vibe uses, and that seems to be part of the tone.  
On the other hand, there is no reason not to experiment. You could just stick in a gain-of-three amp and a gain-of-one inverter and drive the phase line from that. If you go to that trouble, you might as well mess with the input to present a high impedance to the input signal, which is a flaw of the original univibe. However, using a dual opamp and the associated biasing resistors and caps and gain setting resistors comes up to almost as much circuitry as the discrete preamp - do the design andcount the resistors. You'll save a few, but not many. This is one thing that surprises me a lot. When I first ran into this, I started comparing opamp and discrete designs, and to replace a gain stage or two at low gain, the saving is very small. The big advantage to opamps is when they can replace many transistor stages or do something that discrete designs can't do easily, some special function.  
You should probably leave the phase line alone, with the possible exception of making the last stage the same as the others and taking off the inverted phase from its collector to add to the "dry" signal for a stereo output. The characteristics of the phase line *are* special, and you'll lose the characteristic tone for sure if you make it into opamps.  
Hmmm... I sound like I've thought about this a lot, don't I?
2/1/1999 1:39 PM
Dave B.

Yeah, getting rid of the dual pot is my nain concern. So, you're saying to replace each half of the pot w/ one half of the VTL54C3/2 and control the resistance w/ the LED??  
What value pot should I use??  
What do I tune in the circuit to change the waveform and what type of wavs shape idid the origial 'vibe have??  
Thanx again, Dave
2/1/1999 2:55 PM

If you look at the original 'vibe, two of the "outside" lugs of the dual pot are tied together, and the two wipers are connected to the circuit, the other two outside lugs shorted to the wipers. A centertapped variable resistance replaces this beautifully. I did this in my all-tube Univibe and it works well, although I did have to tune the 220K parallel resistances to keep the LFO from actually stopping at max slowness.  
What you want is any way to vary the current through the LED in a controllable manner. I forget the exact circuit I used, but using a current limited opamp to drive the LED with a voltage and driving that with almost any pot value would give a linear-to-exponential response the LED. I'd have to dig out the circuit to find exactly what I did. Seems like I used the pot on the base of an transistor and the LED in the collector of the transistor, and a resistor in the emitter, then calculated the voltage ranges so the max pot value gave about 20 ma on the LED. As I say, I've forgotten the exact stuff, have to dig it out. Your experimentation is probably as good as my digging.
2/1/1999 11:19 PM
Mike Irwin

I built an LED + LDR phaser some time ago (not a Univibe clone) for operation on + 6 volts. A single supply op amp + 2N3904 transistor was used as a voltage source to drive up to 40 mA into a large high brightness red LED. This gives an exponential light out vs. control voltage in response. Basically a TLC272 op amp output connected to 2N3904 base, collector to +6V through 150 ohms, LED anode to emitter, LED cathode to battery minus (ground), 47K feedback R from emitter to op amp - input, LFO variable output capacitively coupled to op amp -input through 47K, manual adjust voltage to op amp -input through 47k, op amp + input to 1/2 supply voltage (fake ground, 2 10K resistor voltage divider with 0.047 cap). For operation on +15 volts the 150 ohm resistor should be replaced with about 680 ohms to limit the max. LED current. Since it is exponential, adjustments have to be done with extra care to get good results.
2/2/1999 8:45 AM

Sounds very reasonable. I've seen similar setups with just an opamp driving the LED, but with a series resistor from the output of the opamp to the LED, and feedback from the resistor/LED node, so the opamp forces a voltage onto the LED but the resistor limits the current that the opamp can supply when the opamp output is banged against the supply.

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