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Vintage threads from the first ten years

Univibe clipping

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1/10/2000 4:27 AM
Rich	Univibe clipping Ive built a clone of the univibe and it works but I find that when I hit the strings somewhat hard there is this harsh clipping sound. Can anyone suggest what could be causing this. I am using nte123ap's for the transistors in the preamp section(cause maybe?). Thanks!

1/10/2000 4:45 AM

Clipping in an amplifier not otherwise intended to do that is always a result of either (a) the input signal being too big for the amplifier/power supply or (b) the bias being wrong somewhere.

The Univibe is designed not to clip as far as possible, so its stages are biased for the maximum swing before bumping into the power supplies.

Do the following: On the circuit board itself, measure the power supply voltage and the pin voltages for all the signal path transistors. It's likely I can tell which is at fault from this information. One or more stages will have voltages that are different from the design values, and that one will be the one that is clipping.

... er... unless you have a bionic picking arm or a guitar that puts out 110VAC of signal 8-)

8-)

1/10/2000 5:45 AM
Rich	Ok, Im getting a 14.86V B+ and about 22V after the diodes. The first preamp Trans. has 1.36V at B, 1V at E, and 1.94V at C. The second preamp trans has 1.94V B, 1.5V E, 4.26V C. The third preamp trans has 4.26V B, 3.66 E, 11.1 C. Does something seem strange with these or are the other transistor voltages needed to find the problem?

1/10/2000 4:18 PM

Those look OK - the third transistor has about 1/4 of Vcc on its emitter and about 3/4 of Vcc on its collector, about right for max swing on a 15V power supply. The others look OK as well, base about 0.5v higher than the emitter and the collector somewhat higher than the base and emitter.

Check the phase stages. The phase stages should have about 4.6-4.8V on the base of the first darlington transistor; the emitter of the second darlington transistor should have about 4 to 4.5V and the collector 10.6 - 11V for the first three phase stages. The final phase stage should have about 6.1V on its base and about 5.5V on its emitter.

1/12/2000 4:38 AM
Rich	All these voltages check out. Any other suggestions? Also, I have noticed that the slower the flashing of the bulb, the smaller the range of light to dark. Is this common?

1/12/2000 3:10 PM

Hmmm...

OK, here's a way to check for where the distortion's coming from. The gain through the 'vibe is pretty much unity or a bit less. Pull up the inboard end of the 100K resistor that feeds the "dry" signal from the emitter of Q3 to the output.
Temporarily connect the (-)end of a 1uF capacitor to the free end of the 1uF cap and then attach a clip lead onto the (+) end of the cap; then probe the emitters and collectors of Q3 and the first three phase stages, and the emitter of the fourth phase stage with the free end of the clip lead.

This will let you listen to the sound at each stage and find which stage is distorting.

Yes, the range of the LFO changes with speed.

1/13/2000 6:27 AM
Rich	Ive discovered something that stops the clipping. If I lightly touch the 1uF cap (the first cap in the preamp after the input) the clipping goes away and all is well. I tried replacing the cap but the same thing happens, so I figure its not the cap. Any light to shed on this weirdness? Rich

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