Neovibe is working, but...

[OverDriveManiac]

I have been lurking for a while but this is my first post.

I recently built the Neovibe. I had some problems with it but I was able to trouble shot through the problems by doing some searches on the forum.

Right now, my Neovibe works but it does not sound that smooth. What I mean by that is that the effect (the chorus/warble, or what ever you want call it) is uneven sounding.

I biased the lamp per the instructions, i.e. "the bulb is glowong dimly orange when the amplitude pot is all the way down" by using the trip pot (I used a 500-ohm trim pot).

However, when I max out the amplitude pot, the bulb does modulate but there is a point at which the bulb goes completely off and I think that is what is causing the uneven sound. I am not sure if I am explaining this correctly.

From what I have been able to research on the forum, it looks as if I need to do the "JC Maillet bulb driver offset mod" as outlined in the "Forum Vibe" documentation.

Does makes sense to anyone?

If I can figure out how to upload pics and record audio into my PC, I will upload so that I can better explain my problem.

Thanks in advance for any help.

ODM

[jasperoosthoek]

I think mine does the same. I remember it going completely off at maximum amplitude. With the amplitude maxed out there is always a trade off between the bulb turning off or drawing too much current during pulsing. How bright is it at the maximum of the sweep? I personally prefer the milder settings with a subtle vibe so I don't have that problem.

What bulb did you use? I have small bears 25mA bulb but recently there was a report that the 40mA bulb sounded better.

[OverDriveManiac]

I used a 12V 60mA bulb from Radio Shack.

I forgot to mention that the driver transistor (Q13) gets VERY warm.

My friend helped me with getting my PC setup to record and if I get a chance in the next few days, I will post an audio sample.

Best regards.

ODM

[R.G.]

I used a 12V 60mA bulb from Radio Shack.
I forgot to mention that the driver transistor (Q13) gets VERY warm.

It does get much harder on the driver transistor with higher current bulbs. The best solution may be to get a lower current bulb.

The Neovibe PCB is outfitted with a space to use a TO-220 power transistor as a bulb driver if needed. This can also be a darlington type if you are so inclined to mod it from the original design.

[DNomis]

I'm using a 12V/60mA lamp bulb in my Forum-Vibe build,and the lamp driver transistor is an MPSA14 Darlington-Pair transistor,it gets pretty warm to the touch also,you could probably substitute a TO-220 power transistor as the lamp driver,you might need to tweak the biasing though,but it should run cooler....


The modulation in my build is a bit lop-sided as well,so I think that's something of a characteristic of the circuit,also,the modulation gets more intense as the speed increases,maybe to compensate for the drop off in response of the lamp?.....

[jasperoosthoek]

The modulation gets more intense as the speed increases,maybe to compensate for the drop off in response of the lamp?.....

It's because of the two 10uF capacitors connected to the amplitude pot. I'll just link R.G.'s article again  : http://www.geofex.com/Article_Folders/univibe/uvfrindx.htm

It's basically a high pass filter with the first corner frequency at 0.32 Hz. The first meaning the leftmost 10uF and the 50k pot alone. Turning the amplitude up will increase the corner frequency a bit as the first cap is loaded a bit more. The first cap removes DC so only AC is changed by the pot, the second will add DC to the bias level of the transistor. This looks quite like the pot in the crybaby wah circuit.

The problem can be compensated by replacing the caps to say 22uF. If you make them much bigger it will take ages for the circuit to settle but in principle it will work.