neovibe LFO tuning

[pedro]

All,
just completed neovoibe build and its good except that its basically a one-trick unit until I can tweak the LFO speed (and hum levels.....I'm swapping PS for DC 18v tonight)

The LFO seems to go from "nothing to everything" - If I try to slow down the sweep then I suddenly lose all the depth as well - I cant seem to get the LFO slow and still retain the effect.  The depth control seems to interact with the speed as well a little.  Frustrating !

I have meter , signal gen and a scope which Ive just bought second hand so I'm interested in faultfinding and tweaking the circuit till it works great even if it takes some patience.

What should I be looking to check around the LFO area ?- e.g. voltages, components to suspect etc. which might be leading to these symptoms. ??

Also I wondered whether the values of the LDRs may be leading to this _ I couldnt find th 50-330K values specified in the list so I used nearest I could find ( not sure of values )

Any detailed advice very greatfully received - I love the effect I just want it to perform 100% as I know it can....

Thanks
Pete

[R.G.]

Do you know your *pot* is not everything or nothing?

[pedro]

RG - many thanks for responding  ,  understand your point re the Pot  :oops:

I found your post from November suggesting swapping out the LFO transistors for more gain in the neo LFO section.
I tried this using a 1815 for the first transistor ( remebering to swap base and collector pinouts ) and now the pedal is working really well except for a hum which is due to an earthing fault somewhere.

I would still realy like to "tune" the circuit for ideal conditions so if your still minded to help out with any detailed suggestions then I'll try to keep up with you.
Thanks for a great project. Its a real pleasure to be able to buyild this stuff with help from more experienced people.

Rgds
Pete

[R.G.]

Don't be too embarassed - it may not be the pot. That was just a guess.

now the pedal is working really well except for a hum which is due to an earthing fault somewhere.

Possibly. A very, very common problem with Neo's is this: using an AC power source running into a *grounded* jack on the chassis. This grounds one side of the AC, which has really bad effects on the full wave bridge that makes DC on the PCB. Check for that on while you're at it. Neither side of the AC input should connect to chassis ground. I did this myself on the first prototype, and about one in every eight people that build this do the same.

I would still realy like to "tune" the circuit for ideal conditions so if your still minded to help out with any detailed suggestions then I'll try to keep up with you.

I'm not sure any keeping up is needed.

The biggest thing to tune is the dry/wet mixer at the "chorus" output. This is two 100K resistors in the stock circuit. A phaser (any phaser) really demands equal dry/wet signals on the mixer for best notch depth. A really good trick is to make the two 100K's be two 82K resistors feeding the ends of a 25K pot and taking the output signal off the wiper of the pot.

This lets you tune for the deepest sound effect by ear. If you like, you can then measure the pot setting with an ohmmeter, and put in fixed resistors for the best depth.

Stock univibes are subject to the same problem; some of them have great cancellation, and hence great sound. Some have less good cancellation, and don't sound as good.

[R.G.]

Sorry, I did the math in my head. Make that two 82K resistors and a 50K pot, not a 25K.

[pedro]

thanks RG
Ive made the output divider mod and get much better depth of cancellation.

I still have the hum though and dont have the grounding issue with one side of AC supply to ground which you suggested is a common cause.

I have a suitable DC adaptor I could try - should I just power the unit by putting the 18V DC stright into the rectifier or should I connect after the rectifier straight onto the board - where is the best place - after the caps or right before the filter caps ?

What else would cause the mains hum - Ive shielded the inputs and oututs but still have it.

The unit isnt yet completely enclosed - would this be the cuplrit ?
Or should I try DC powering to eliminate mains hum possibility - assuming that only poor grounding somewhere is the cause of excessive hum.

Unit sounds great apart from having too much hum to gig with - I'd really like to de-bug so I can get it on my rig.
Thanks for all the help so far - it really is appreciated when some key contributors help out newbies like me.
Rgds
Pete

[Chris Goodson]

The only complaint I have about mine is that when I cut the depth back to where I want it the tone starts sounding really thin.