LPB2 problem.

[Xavier]

2 days ago I opened up my LPB2 to sub the 2n5088 by a 2n5089, and see if it provides some more gain.

Box it up again, use an AC adaptor and.......horrible sound. Gated, misbiased horrible sound........so ok, go back to the 2n5088 and same result .

I have triple checked everything ,wiring is correct.....c'mon it must be one of the smallest circuits in the world....

Measuring the AC adaptor it reads 15v (and that coming from a 9v adaptor !!). It can't be the voltage, right?

[PharaohAmps]

Measuring the AC adaptor it reads 15v (and that coming from a 9v adaptor !!). It can't be the voltage, right?

That's your problem, right enough.  Simple transistor preamps like the LPB1/2 are fairly sensitive to power supply voltage.  You'll hear a similar "gating" wound when the battery falles below about 8V.  There are a couple of ways to proceed here:

1.  Purchase or build a regulated 9V supply.  Your AC adaptor is basically a transformer, a bridge or full-wave rectifier, and a load capacitor.  If you were to build a small box with a couple of caps, a diode, and a 3-terminal regulator (LM7809 would do nicely, I use LM2940-10 all the time) you would get good results.  A Boss adaptor or a 1-Spot type switching supply would do great here as well.

2.  If you're only going to be running the adaptor, you could re-bias the circuit to fix the problem.  I usually install a pot from the base of the transistor to ground and adjust it until the collector is around half the supply voltage, but you could also use your ears.  You could even put this pot on the chassis to adjust from outside, but there is a pretty limited range where it will sound good.

3.  Stick with batteries.  The current draw of the LPB-2 circuit is VERY low.  A decent 9V battery will power one for over 100 hours.

I'd go with #1.

Matt Farrow

[Xavier]

Measuring the AC adaptor it reads 15v (and that coming from a 9v adaptor !!). It can't be the voltage, right?

That's your problem, right enough.  Simple transistor preamps like the LPB1/2 are fairly sensitive to power supply voltage.  You'll hear a similar "gating" wound when the battery falles below about 8V.  There are a couple of ways to proceed here:

1.  Purchase or build a regulated 9V supply.  Your AC adaptor is basically a transformer, a bridge or full-wave rectifier, and a load capacitor.  If you were to build a small box with a couple of caps, a diode, and a 3-terminal regulator (LM7809 would do nicely, I use LM2940-10 all the time) you would get good results.  A Boss adaptor or a 1-Spot type switching supply would do great here as well.

2.  If you're only going to be running the adaptor, you could re-bias the circuit to fix the problem.  I usually install a pot from the base of the transistor to ground and adjust it until the collector is around half the supply voltage, but you could also use your ears.  You could even put this pot on the chassis to adjust from outside, but there is a pretty limited range where it will sound good.

3.  Stick with batteries.  The current draw of the LPB-2 circuit is VERY low.  A decent 9V battery will power one for over 100 hours.

I'd go with #1.

Matt Farrow

I didn't have a battery clip but I have used another adaptor that had 11,89 volt and still horrible.This pedal is in my pedalboard usually, where I use a Boss adaptor (you already mention this...)

Another hint. I accidentally have grabbed the board by the soldered side and surprise surprise....when I cover the circuit with my thumb the sound comes back  . Lift the thumb and it disappears. The technician that was just beside me has told me "You are a big capacitor, bud.."

WTF?