How does Biasing effect sound?

[Leftrights]

Jay Doyles "a challenge"  and he said that you could tweak the type of biasing that you put on a transitor to get different sounds.  I always thought that as long as it was inside the range where it would amplify that was it.  I have a feeling that I'm missing something good here that I need to know.

So could someone mention some of the ways of biasing tranistors and explain how the biasing affects sound.  Or maybe lead me to some schematics to look at.

[bwanasonic]

I'm not sure there is a blanket description for all transistors in all circuits. I think in a lot of cases your understanding of it is probably the case, as long it turns on you're OK. But with something like Q2 in the Fuzz Face circuit (and I'm sure many others) , varying the bias within the working range gives a useable range of different tones. With my own Ge FF, I have twiddled the bias between the *ideal* 4.5v and about 7v. I would describe the lower voltage as more *classic* fuzz face and a bit fuzzier/woolier, and the higher bias a bit more *modern*, defined and aggressive sounding. I did fool a bit with *misbiasing* the Rangemaster, but found I preferred the *ideal* voltage of @6.8-7v. I should have taken notes, but I seem to remember lowering the bias voltage made for a less defined more fuzzy sound also. I would like more info on this subject myself, with more subjective descriptions of what various bias settings sound like.

Kerry M

[brett]

As far as I know, you're basically right that a transistor is usually biased into its linear (non-distorting) region.  Conversely, there are also regions where a transistor (or IC) will give some distortion.  In most cases that distortion sounds fairly terrible (in musical terms), so distortion is usually achieved by other methods, such as clipping signals with diodes or via some sort of feedback arrangement.

However, in some cases, transistors (and ICs) can be biased so that they operate close to, or just within their cutoff or saturation "regions".  In those situations, the sound MAY become coloured without sounding too horribly distorted.  All of this is somewhat dependent on the circuit, the desired effect and the type of input signal.  For example, in a "normal" amplifying transistor, it is much easier to operate close to saturation if the input signal is small (5mV) than if it is large (5V)

Hope this helps.
cheers

[aron]

and again, some people like the "horribly" mis-biased sound of silicon transistors.

I found that if you vary the voltage going to a FET, and vary the bias, you can get a different "feel" out of the transistor.

[Jay Doyle]

First and foremost biasing tells you how the output will behave. Take a two stage circuit. If you bias the output of the first stage close to V+, the output is going to be assymetrically clipped with the signal hitting the V+ rail before the ground rail. If you then bias the second stage closer to ground, because a single transistor stage inverts, that squashed part of the signal which was near V+ after the first stage, is now inverted out of the second stage so it is nearer to ground; thus if you bias the second stage nearer to ground you can squash the same side of the signal as you did the first.

That is the basic idea but within a specific stage you are biasing, you are not only choosing what the resting output voltage is but where in the transfer curve of the device you are operating. This is considerably more complicated but good information about it can be found on most tube sites. Essentially, with every change in bias you are changing the way the transistor operates from the gain to the current used etc. It may not end up being a big difference in the sound but then again it might.

Jay