Voltage limiter for a guitar distortion circuit

[ngdbud]

Hey guys, I'm trying to design my first guitar pedal. Distortion is
created when an AC wave's amplitude is greater than the input voltage
of the amplifier. (imagine a sine wave with the top cutoff, basically
planing it) I'm wondering if there is a simple circuit that will
conduct without resistance up to a set point and then hold the voltage
at that level until the input voltage drops below that level. For my
purpose I'm looking for this point to be around one volt from a 9 volt
input. I'm comfortable calculating component values if i could just
find a starting point.

 

[Rich Grise]

On Wed, 16 Sep 2009 12:25:57 -0700, ngdbud wrote:

Hey guys, I'm trying to design my first guitar pedal. Distortion is
created when an AC wave's amplitude is greater than the input voltage of
the amplifier. (imagine a sine wave with the top cutoff, basically planing
it) I'm wondering if there is a simple circuit that will conduct without
resistance up to a set point and then hold the voltage at that level until
the input voltage drops below that level. For my purpose I'm looking for
this point to be around one volt from a 9 volt input. I'm comfortable
calculating component values if i could just find a starting point.

A clamp. Put two diodes in antiparallel with each other, in shunt from
the signal path to ground. That will limit the voltage to about .6V peak;
for 1.2V peak, use series diodes.

Good Luck!
Rich

P.S. Do googlies ever come back to look for answers, or do they expect
them to be lying on the desk in the morning?

 

[Jon Slaughter]

"ngdbud" <ngdbud@gmail.com> wrote in message
news:17ab7bd0-5f98-4d9b-b03d-1abaab0027af@x25g2000prf.googlegroups.com...

Hey guys, I'm trying to design my first guitar pedal. Distortion is
created when an AC wave's amplitude is greater than the input voltage
of the amplifier. (imagine a sine wave with the top cutoff, basically
planing it) I'm wondering if there is a simple circuit that will
conduct without resistance up to a set point and then hold the voltage
at that level until the input voltage drops below that level. For my
purpose I'm looking for this point to be around one volt from a 9 volt
input. I'm comfortable calculating component values if i could just
find a starting point.

Diodes are as close as you are going to get. Note that this distortion is
not good sounding. Most analog "distortion" pedals either are transistor
based(overdrive a transistor so it clips) or based on diodes but with op
amps and some filtering to smooth it out.

The reason is that semiconductor based clipping is very good... too good.
The cliping is almost perfect and creates very distorted sounds(even when
just a little). This creates a ton of harmonics on the signal. This is more
used for metal type of distortion. Generally it is called "distortion".

Tubes, OTH, clip gradually and tend to be much more pleasing(they sorta
round the edge of the sin wave rather than a sharp clip). This is generally
the "overdriven" sound. Generally called "overdrive".

You have digital distortion methods now that emulate the above and can vary
between both extremes. Generally it is not as good as tubes but better than
silicon because silicon is easy to emulate since it is so perfect. Tubes are
not perfect and are extremely complicated when you consider all the other
circuitry that works together in complex manners(each tube section and the
EQ and stuff all interact in a complex way that may or may not have been
designed that way and hence it's sometimes hard to emulate). Good digital
distortion can come close to tube's but tubes offer a feel that nothing else
gives. It's probably the headroom that one gets but I'm not even sure that
can explain it.

In any case, there are many circuits online for making distortion boxes.
Some are very simple and some are very complex. The general idea though is
always the same.

 

[Bob Masta]

On Wed, 16 Sep 2009 14:19:23 -0700, Rich Grise
<richgrise@example.net> wrote:

On Wed, 16 Sep 2009 12:25:57 -0700, ngdbud wrote:

Hey guys, I'm trying to design my first guitar pedal. Distortion is
created when an AC wave's amplitude is greater than the input voltage of
the amplifier. (imagine a sine wave with the top cutoff, basically planing
it) I'm wondering if there is a simple circuit that will conduct without
resistance up to a set point and then hold the voltage at that level until
the input voltage drops below that level. For my purpose I'm looking for
this point to be around one volt from a 9 volt input. I'm comfortable
calculating component values if i could just find a starting point.

A clamp. Put two diodes in antiparallel with each other, in shunt from
the signal path to ground. That will limit the voltage to about .6V peak;
for 1.2V peak, use series diodes.

Be sure to use some series resistance ahead of the
diodes, to limit current.

You can also use two LEDs instead of regular
diodes. This will not only give a higher
peak-to-peak (depending on diode color) but will
also give razor-sharp clipping for more
harmonics... if that's what you want.

The amount of gain you use ahead of the clipper
will determine the point where clipping starts.
You may want to use a lot of gain here to get full
distortion even on fairly low levels.

You can also use an op-amp wired as a comparator,
perhaps with a bit of positive feedback for
hystereis so it stops in silence and doesn't
trigger on background noise.

You can mix the clean and distorted signals
together using a pot as a balance control.
Use a 10K to 100K pot, connect one end to the
clean signal and one end to the distorted signal,
and use the wiper as the output.

Best regards,


Bob Masta

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