What if I actually want my power amp to distort?

[Zapper]

Class D makes me wary (I'm an amateur dabbler in electronics, and Class D essentially means buying an IcePower module and essentially set it up the way it wants to be set up, because there is no way you can design one at home)

You can make your own out of op-amps, a PWM controller chip, a couple of gate drivers and power MOSFETs. It's not necessarily harder than a Class-AB amp.

 

[ampetrosillo]

I used to design and build guitar tube amps when I was younger, and even though I never pay electric guitar anymore these days, I can still appreciate a good guitar sound and still know the schematics from the top of my head. I also filled around in electronic stimulation software trying to grasp a more detailed understanding about where tone comes from. The important thing is that it's more art than science: you need to work with your ears and hands and use that as basic guidance. In my opinion you're approaching this too theoretically. When designing/building a guitar amp it starts with inspiration: which tone and feel are you after? Build a basis and start tweaking (simply changing out capacitors, resistors and pots) until you get it at the sweet spot. Also: don't forget that a lot of the tone is made by the speaker cab, in case of distorted sounds, the cab even determines more of the tone than the amp. Then my advice of you are looking at getting tube like distortion from solid state: base your design on overdriving FETs. Look for example at the schematic of the zvex box of rock: that's an overdrive pedal with tube like behaviour and a good sound (considering it's not real tubes).


Also watch this video if you haven't already

The preamp is, in fact, based around opamps (for clean gain) feeding into a soft diode clipping network (2kohm series resistance, as is) into a JFET configured to clip asymmetrically.

This is the basic "building block" (with lots of room to tweak). Input is a 50mV -> 500mV peak signal varying by 50mV steps and gain almost maxed. The THD rises essentially linearly up to a point (about 6-8%), and then starts rising a bit faster (and then, of course, slowing down to a plateau). This is not exactly tube-like but it should be good enough for touch sensitivity. By increasing the clipping threshold and/or increasing/reducing the series resistance you can get the stage to clip a bit differently (faster or slower). Without it altogether the JFET clips harshly once reaching its Vgs limit.

 

[ampetrosillo]

Of course, the really important ingredient in good amplifier response is in the tone filtering and/or bass cut. Which is why I have included a tweakable pre-emphasis (and de-emphasis) circuit, a simple tilt control centred at around 500Hz, so that if you want more "Fender-like" tones you use it flat and at lower gains, and if you want more distortion, you crank up the treble (by up to 20dB) so as to make the distortion tighter, while keeping the bass frequencies low in amplitude (similarly to what happens in certain Marshall amps).

 

[er|κzvio1in]

The preamp is, in fact, based around opamps (for clean gain) feeding into a soft diode clipping network (2kohm series resistance, as is) into a JFET configured to clip asymmetrically.

View attachment 351768

This is the basic "building block" (with lots of room to tweak). Input is a 50mV -> 500mV peak signal varying by 50mV steps and gain almost maxed. The THD rises essentially linearly up to a point (about 6-8%), and then starts rising a bit faster (and then, of course, slowing down to a plateau). This is not exactly tube-like but it should be good enough for touch sensitivity. By increasing the clipping threshold and/or increasing/reducing the series resistance you can get the stage to clip a bit differently (faster or slower). Without it altogether the JFET clips harshly once reaching its Vgs limit.

Opamps with soft diode clipping is just going to sound and feel like any run-off-the-mill 60$ solid state overdrive/distortion pedal which wouldn't make me happy (at all). Why do you think your amp is going to be special and sound good? I don't get it.

 

[er|κzvio1in]

Of course, the really important ingredient in good amplifier response is in the tone filtering and/or bass cut. Which is why I have included a tweakable pre-emphasis (and de-emphasis) circuit, a simple tilt control centred at around 500Hz, so that if you want more "Fender-like" tones you use it flat and at lower gains, and if you want more distortion, you crank up the treble (by up to 20dB) so as to make the distortion tighter, while keeping the bass frequencies low in amplitude (similarly to what happens in certain Marshall amps).

Sorry to be pissing on your parade, but this is distortion design 101 and nothing special.

 

[ampetrosillo]

Opamps with soft diode clipping is just going to sound and feel like any run-off-the-mill solid state overdrive/distortion pedal which wouldn't make me happy (at all). Why do you think your amp is going to be special and sound good? I don't get it.

It's not an opamp with soft diode clipping. The opamp is actually used "clean". The opamp then *feeds* into a diode clipping network to ground which is only there to soften the eventual clipping of the FET. It appears to work well; up to the diode's thresholds, it's the FET doing most of the colouration, then the diode clipping network softens the transition of the FET into its non-amplifying regions (but with the FET providing rising distortion and compression, especially for negative half-waves). I don't care much for opamp soft clipping either, if not as an effect. It's quite bland.

 

[ampetrosillo]

Sorry to be pissing on your parade, but this is distortion design 101 and nothing special.

Guitar amplification is nothing special, it's decades old technology, and it was simplistic technology even when it came out. (I smile when I think at how revered the Centaur Klon - "designed with the help of two expert physicists" - is among guitarists...). Still, I can only name one or two non-modelling solid state amplifiers that actually sounded convincing to me (one of which is the Orange Crush line). Apart from the Roland Jazz Chorus 120, of course, which essentially is unapologetically "solid state" and is not intended to sound dirty.

To be honest I have no intention whatsoever to actually commercialise what I would like to make. It's just to scratch my own personal itch.