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The Technology of the Univibe


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1/30/1998 10:46 PM
R.G.

The Technology of the Univibe
OK, my fingers have blisters and my eyes burn.  
I completed the first installation of The" target="_blank">http://www.eden.com/~keen">The Technology of the Univibe.  
 
Like the others in the "techology" series, this is a section by section dissection (oops...) of the vintage 'vibe circuit with my best guesses at how and why they work.  
 
It is, alas, only about the first half of the article. The rest will be input as I have time to type. Take a look and as always, suggestions and corrections are welcome.
 
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1/31/1998 2:18 PM
harlan stephens

cool rg Iam gona build one ! anyone build one with rg s p.c.b. ? comments ?
 
2/2/1998 10:27 AM
J Epstein

I built one. I have to confess I was underwhelmed at first - I thought the effect was too subtle. Also there was some 120Hz "buzz" I could not rectify (nyuk, nyuk) for the life of me. When I told RG about this, he offered to take a look at it, I sent it out to The Tone Lab, and he came up with a tweak whereby I could adjust the regeneration of the effect to intensify it.  
 
Neither one of us was ultimately able to get completely rid of the (slight) buzz. The effect sounds pretty good now - very "meaty," and organic compared to your standard phase shifter, but overall (sorry, R.G.!) my experience was mixed. Note that this WAS NOT RG'S FAULT AT ALL! The board is very high quality, and as you can tell he stood by it like the righteous dude we all know and love. I suspect my build quality/exact wiring placement; I'm gonna hook this puppy up to my o'scope when I get the chance and fix its wagon once and for all.  
 
-j
 
2/2/1998 11:18 AM
harlan stephens

Thanks for the imput Mr.Epstein.The regeneration mod sounds cool. what ( if I may ask) type of caps,resistors,photo cells and semiconductors did you use? I sent for the board today and want to start collecting parts, anyway Thanks Again chip
 
2/3/1998 11:57 AM
J Epstein

I would ask RG, he helped me source all the parts and I honestly don't remember. Nothing exotic, I assure you - probably everything was from Mouser or Digi-key. The regen mod is just a pot (I used the volume pot which I otherwise found to be useless to me) from the output back to the input; there may also be a resistor in series to limit the regeneration at the pots maximum setting.  
 
-j
 
2/3/1998 12:33 PM
harlan stephens

R.G. told me the board will come with recommended part #s the feedback mod is good to know thanks cant wait to see part two of (The technology of the univibe) anyone get a chance to look inside a fulltone daja vibe?
 
2/3/1998 2:37 PM
John Greene
I noticed that on the schematic there isn't *any* bypassing on the output side of the 7815. Shouldn't there be at least a .1uF??  
 
I think I have an idea as to why the capacitors in the phase shifter are the values they are. Allow me to toss this out for discussion...  
 
My understanding is that the Univibe was originally intended to be a 'vibrato/chorus' and not a phase shifter as we know them today. The purpose of the Univibe's phase shifter is to advance and retard the phase to increase and decrease the pitch of the sound. Basically the same way doppler shift works.  
 
Anyway, a 180 degree phase shift would result in a cancellation and some point in frequency and this would *not* be a desirable feature when vibrato is the name of the game.  
 
The capacitors in the Univibe provide a continuous 180 degree/decade phase shift. The first 'null' takes place at just under 100Hz. This puts the second 'null' at just under 10KHz. The phase shifter has a total shift range of approximately 90 degrees because of the 4.7K resistor in series with the LDR. The upper 'null' will come down to around 3KHz but no lower.  
 
So, my take on this is that the Nulls were placed on the outer edges of the intended range of use so that the phase could be shifted without cancellation of the desired frequencies.  
 
Possible tweaks to intensify the effect would be to reduce the series 4.7k and increase the value of the capacitor by a proportional amount. This should increase the amount of phase change to more than 90 degrees. Another tweak would be to increase the value of the 470pF. This will pull the upper Null down into the range of use. If you go much beyond 4700pF you will have to start increasing the value of the .015uF as well.  
 
--johng
 

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