troubleshooting the easy drive help...

[jk-fm]

Continuing on from my post on the gain pot...

Without the clipping diodes, the basic circuit is an NPN common-emitter amplifier.

http://en.wikipedia.org/wiki/Common_emitter
http://230nsc1.phy-astr.gsu.edu/hbasees/electronic/npnce.html#c2

This is the type of boost used by the LPB-1.
http://www.beavisaudio.com/techpages/HIW/hiw1.gif

Note, the LPB-1 has no gain pot or bypass capacitor. It is running at a set gain, and the volume control reduces the output.

Also, all these examples are using a Voltage divider bias, where the base has a separate path to power than the resistor. The Easy Drive uses collector-to-base bias, or "self bias". The base voltage has to be raised - "biased" a little above zero for the NPN transistor to function.

http://en.wikipedia.org/wiki/Bipolar_transistor_biasing

The 220k resistors are allowing a fraction of the collector voltage to bias the base. But the common-emitter amplifier is an inverting amplifier. When the voltage at the base goes higher, the voltage at the collector goes lower. So the input goes up, the collector drops, and a fraction goes back to the base and cancels the input. This is negative feedback. It won't totally cancel the output, but it will limit the gain.

The capacitor to ground between the resistors is acting as a low-pass filter. It eliminates high frequencies from going from collector to base, so high frequencies work at full gain. But lower frequencies "feed back" and stat to cancel out. If you don't want any of the bass cancelling out, just put a 1uF or more cap in there (with the negative to ground).

The diodes in the feedback loop are similar to the Tube Screamer or Boss OD-1, although those use op-amps instead of a single transistor. When the output goes more then 0.6V from average, the diodes start to conduct and limit the gain by negative feedback, flattening out the peaks of the wave. The Boss OD-1 has two diodes in series going one way, and one going the other way for asymmetrical clipping.

Note the diode capacitor is needed in the Easy Drive but not the opamp overdrives. Op-amps will have a small capacitor in parallel to cancel very high frequencies, but an NPN boost doesn't need it.

A different type of clipping is used by the Boss DS-1, ProCo Rat, and MXR Distortion Plus. These have two diodes placed after the output capacitor going to ground. This is a harder sounding clip.