A gain boost switch on a jfet boost to push weaker guitar signals into clipping?

[kimelopidaer]

I installed a switch that toggles between 470nf and 10uf on the Source bypass cap.
My favorite testing guitar is a Gibson Marauder. Some of my guitars give weaker signals and don't trigger the clipping diodes i've placed after the output cap.
When i switch to the larger cap using those guitars, I get clipping.


Thoughts about this?
Is this the shortest distance between two points, so to speak?


Thanks,
K

[teemuk]

Well... Isn't it obvious? The higher value source bypass cap introduces higher gain at lower frequencies so I can see how it easily will introduce more clipping to the signal. With higher source bypass and higher low frequency gain you are not only boosting the high frequencies above the clipping threshold but the low frequenciess too in much greater extent.

In addition to enhance the effect, the high frequencies will often "ride a top" the wave of a lower frequency signal component, a wave which has much larger wavelenght. So if you clip that wave you'll introduce plenty of high frequency clipping at the same time, some of which might be totally devoid from a system with lower low frequency gain. The amplitude of the high frequencies might not exceed the clipping threshold alone but riding atop a low frequency signal exceeding it you'll end up clipping both.

But still, the clipping threshold does not change and you are not boosting the lower amplitude signal content. You are just feeding more bandwidth to the clipping stage. If you want to clip the lower amplitudes in a greater ratio you'll need to introduce a compressor/limiter circuit of some type before the clipping stage. A plain high-pass filter will do no good for that.

[petemoore]

When i switch to the larger cap using those guitars, I get clipping.
 Capacitors can impede signal at certain frequencies more than others. In this case a source bypass capacitor provides AC gain to happen @ frequencies determined to some degree or other by the value of the source bypass cap, the 10uf should let the amp work like the source is grounded [22uf for sure would] for AC, but the DC bias is still determined by the resistor values.
 Thoughts about this?
 Small enough source bypass capacitor cuts gain on the lower frequencies, smaller yet cuts out more bass amplification.
Is this the shortest distance between two points, so to speak?
 I'm not sure I understand the distance or what two points...perhaps a second boost transistor before this would allow clipping when 'fed' by the AC bypass cap, only higher frequencies allowed through the smaller value source cap.

[jafo]

Even simpler explanation: the source bypass capacitor creates a treble boost, since it's basically a high-pass filter into the boost. For higher frequencies, it is indeed "the shortest distance between two points, so to speak." A bigger cap means that lower frequencies are passed into the FET, which means that they, too, are boosted more -- which leads to more overall volume and hence more clipping, since more of the total signal is boosted. Frequencies below the cutoff point don't get the extra boost.