Neovibe volume drop and distortion

[martinbertrand]

Hi,

I finally finished my neovibe from the GGG/Geofex site.
I have a problem with the volume drop. the volume seems at the right level on both parts ( wet and dry) just before the 100k mixing resistors. after that, the volume is just too low and the volume pot is useless since it is always maxxed out to be usable.  I changed R8 and R9 to 300 ohms and 4,3K. that cranked to volume to near ok but now the sound is distorted too easily.

can I get rid of the 100k resistors ? or any suggestions ?

Martin Bertrand
:?

[R.G.]

I have a problem with the volume drop. the volume seems at the right level on both parts ( wet and dry) just before the 100k mixing resistors. after that, the volume is just too low and the volume pot is useless since it is always maxxed out to be usable.

Welcome to the real, original univibe circuit. It does that.

can I get rid of the 100k resistors ?

No, not and still have it phase. Those resistors are what makes the mix that causes notches for phasing (see "Technology of Phasers and Flangers" at GEO).  Take those out, no more phasing.

or any suggestions ?

You could raise the 100K volume pot to 250K, 500K, or 1M and get marginally more volume out. You could add a booster stage to raise the volume at the output. You could convert the output mixer to an active mixer with either transistors or opamps to raise the level. Which of those sounds interesting?

[martinbertrand]

Hi Mr Keen,

Glad to hear from you.

First, I want to thank you for the neovibe project. A real piece of art. the pcb is very tight and it put my soldering skills to test.
Am I right when I think that the increase in gain at the preamp that I did at the split resistor is responsible from the distortion ? ( I use humbuckers)

You could raise the 100K volume pot to 250K, 500K, or 1M and get marginally more volume out.

I must now understand that as hi as I set the gain on the preamp, i'll never match the bypassed volume... then I'll change the vol pot to 500k.

You could add a booster stage to raise the volume at the output. You could convert the output mixer to an active mixer with either transistors or opamps to raise the level. Which of those sounds interesting?

That seems a good idea... do you think the rangemaster could be good? or the minibooster ? Can I run the rangemaster on 15v ?
maybe a 15v LPB-1 could be suitable ... I could set a trim pot or a pot to control the gain at R4. what do you think ?
http://www.generalguitargadgets.com/diagrams/lpb2_sc.gif

I used this trick to boost the output of my pulsar tremolo a while back but again, lots of unwanted distortion. The circuit was a lpb-1 placed after the pulsar. In some cases it could be useful since it sounded like some REM sounds. now it's just dead as my Neovibe sits in its enclosure!

[martinbertrand]

Here is an update from my neovibe build.

problem 1: volume too low

the volume after the preamp was similar to the volume after the phaser but the mixed outputs of the two gave an unacceptable volume drop.

I fiddled with the split resistor values on q3 but the distortion generated at this point was too much. I settled for an active boost after the circuit. I compared the mini-booster, the LPB-1 and the Dallas Rangemaster and chose the LPB for its simplicity and transparency.

I run the LPB-1 on 19V, the output from my charge pump. The whole unit works on any 9v walwart !

problem 2: phasing more like a tremolo

This problem was due to the bad biasing of q2 by a 4,7K in place of a 47K for R10. This puzzled me for a couple of hours because I was too confident that after 10 builds I could not make that kind of mistake :oops: . Once more the facts proved that there's an asshole hidden deep inside of me.

I'll be posting pictures of the graphics soon. Dont expect to see the guts.. it's a mess.

Thanks to R.G., and JD Sleep for the sharing the project files.

Peace.