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Changes in bias resistors

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5/10/2000 9:20 PM

Changes in bias resistors

R.G. and others,

When we see basically the same circuit such as the tone benders etc... and they all have different values for biasing - some close, others very different, are these just differences due to the different transistors are they really changing the basic character of the sound?

I'm wondering how different these all sound.... To me, fooling around with the biasing seems to make it sound half-wave rectified or something and sometimes gives a gating effect which can be cool or really irritating.... :-)

:-)

5/10/2000 10:21 PM

Biasing 50.5 (half of 101)

quote:
"are these just differences due to the different transistors are they really changing the basic character of the sound?"

Both or neither depending on the skill and intent of the circuit designer. Biasing, input and output impedances, and rolloff frequencies all interact.

The "tone" of a stage depends on
(a) biasing, to the extent that it is or is not letting the transistor distort
(b)input and output impedances and their combined effect with the capacitances in the circuit to roll off or allow through treble and bass
(c) individual transistor characteristics may require some peculiarity of the circuit - the best example being the Rangemaster.
(d) feedback if used

Usually, a good designer will attempt to make (c) not matter.

It's not generally possible to just fool with one resistor or another in a biasing setup and generalize about what happens. Case in point - the stock four-resistor bipolar gain circuit.

The input signal loading is the parallel equivalent of the two biasing resistors and the emitter resistor times the Hfe. This is what determines the input bass cutoff. Change one biasing resistor and you also change the bass rolloff as well as changing the DC bias point. Change BOTH biasing resistors up or down by an equal amount, and over a broad range, the DC bias is not affected, but the input bass rolloff IS affected unless you make an inverse change in the input cap.

The only way to make this come out right for messing around is to have an idea what a change does to the DC bias point, which will affect the biggest undistorted signal the stage can pass.

quote:
"fooling around with the biasing seems to make it sound half-wave rectified or something and sometimes gives a gating effect which can be cool or really irritating."

You're running it into saturation or cutoff. When that happens, the signal has to push the transistor back into the amplifying region in spite of where the bias network is trying to put it. That accounts for the gating and sputtering effect.

I'll see what I can come up with for a better Biasing 101.

R.G.
...er, I mean "GEO"...

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