Diode Clippers - How do they work?

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Hello All,

This is my first post in this forum. Some back ground info on me.

I've built two valve guitar amps. On single ended the other push pull and i'm pretty familiar with gain stages, biasing, phase inverters and power sections and I'm not terribly new to this but the idea of how diode clippers work seems to elude me.

I know most pedals seem to use them and i can pick them out of a pedal schematic but for the life of me they alway seem like a direct short to ground minus the Vf..... Can some explain them to me?

Thanks
 
Okay,

Maybe i've found the answer to my own question by reading some of the other threads. Correct me if im wrong but the do actually clip the signal to the Vf level of the diode....i.e .7 vf for normal diodes and the Vf of the particular LED used if going that route... What about zeners...... I've seen a set of zeners used a a mod (im assuming this is the "jose" mod) in the signal chain of a tube amp...?
 
A diode conducts when the forward voltage exceeds its junction voltage..So if your diode has a 0.7v Vf, and you place the diode from signal to ground, then yes, any part of the waveform that is more than 0.7v will be shorted to ground, clipped right off. Typically, you would use two diodes pointing opposite ways to clip both sides. it turns a relatively large sine wave into a smaller more or less square wave.

Not all diodes are the same. LEDs are diodes, but instead of 0.7v, they have a forward voltage of 1.2v or 2v or whatever. If you use them instead of a plain diode, then the clip voltage will be greater so your signal will have less shaved off the peaks.

And yes, if I used 10v zeners, then the effect would only occur on peaks over 10v.



Not every pair of diodes in an effect is a clipping pair. MAny solid state circuits will have clamping diodes on the inputs. That is from the signal line, there will be a diode to +15v and to -15v. But the diodes are wired reverse bias, so they ONLY conduct when the signal exceeds the 15v (in my example). That is done not to clip but to protect the ICs from signals that exceed their power rail voltages.

For example, in this amp are the clamps I just mentioned, and none of the diodes in this amp are clipping.
http://www.webphix.com/schematic heaven/www.schematicheaven.com/newamps/peavey_basic50.pdf


Now compare this amp:
http://www.webphix.com/schematic heaven/www.schematicheaven.com/newamps/peavey_audition30.pdf

CR1,2,9,10 are clamps, not clippers. Then CR7,8 could be clippers, but I think their main purpose is to limit the signal going into the power amp. The CR3,4,5,6 are in fact clipping diodes, and are thus part of the overdrive "channel" of this amp.
 
Now compare this amp:
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/peavey_audition30.pdf

CR1,2,9,10 are clamps, not clippers. Then CR7,8 could be clippers, but I think their main purpose is to limit the signal going into the power amp. The CR3,4,5,6 are in fact clipping diodes, and are thus part of the overdrive "channel" of this amp.[/QUOTE]

In the example above, CR7 and CR8 in parallel with the post gain pot looks as if they would limit the signal to U2 (which im assuming is the power amp) to the .7 volts or would it be 1.4 volts (.7 volts positive going and .7 volts negative going)

Is it common to limit input voltage swing to such low voltages for solid state amps or am I missing something?
 
It isn't the solid state-ness, they just want to limit the signal so it doesn;t overdrive the following stage. Common? Not particularly. For whatever reason they felt the need here. Note there is no equivalent in the other example amp I linked.

yes, U2 is the power amp IC.

1.4v peak to peak, but I leave peak to peak readings for people who only look at scopes. So nothing over 0.7v peak will get past those diodes. 0.7v peak equates to 0.49v RMS if we talk sine waves. So they decided the powr amp didn't need to see more than half a volt signal in the context of the rest of the circuit and the amount of gain the power amp is set up to provide.
 
Note also that Enzo said "more or less square wave".
LED and germanium clippers are less square than small signal silicon diodes.
I personally prefer the sound of rectifier diodes to small signal diodes.
And you can have a different type on each side for other tonal flavours.
And then there is the use of Mosfets instead of diodes.
Hours of experimental fun...
 
This circuit is better than diodes. Diodes do hard clipping.
This circuit is used for guitar to get a soft clipping like a valve amplifier.
I have used one for 30 years.

An externally hosted image should be here but it was not working when we last tested it.

very nice.
interesting how it's sounds.
to get the valve like soft clipping in pedals usually diodes are placed in the
negative feedback loop.
is that what you are doing there? using the transistors as limiters in the negative feedback loop?
 
You *still* have diode clipping there, the BE junction in each transistor.
Only difference , and which is an improvement, is that these are what´s called "amplified diodes" (which are standard in SS amp biasing circuits).
They work this way:
the actual clipping diode is the BE junction, which clips at, say, 0.7V .
you add a voltage divider: one resistor base to collector, the other base-emitter.
Now that transistor behaves like a "huge" diode connected from collector to emitter, which will now will a turn on threshold of 0.7V multiplied by the resistance ratio.

Practical example: both resistors the same, the transsistor will clip at 1.4V .... because when you have 1.4V end to end you have required 0.7V BE .
So far it looks easier to just use 2 diodes and call it a day.
Fine, but what if you need, say, 10 or 20 diodes?
Now it becomes *very* interesting.
And it will still clip rounded like a diode , while an equivalent Zener will be square shouldered and very buzzy.

One practical use is in +/-15V fed Op Amps, with a 20X resistor ratio (say 220K/4K7).
In that case, the Op Amp will never clip "square" but rounded, yet provide almost full rail to rail swing, because the amplified diode will stop it before doing that.

In biasing circuits, resistor ratio is adjusted with a preset resistor, and you can fine tune it and get, say, 2.6 diode or whatever intermediate value you need, and it will *still* track thermally.
 
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You *still* have diode clipping there, the BE junction in each transistor.
Only difference , and which is an improvement, is that these are what´s called "amplified diodes" (which are standard in SS amp biasing circuits).
They work this way:
the actual clipping diode is the BE junction, which clips at, say, 0.7V .
you add a voltage divider: one resistor base to collector, the other base-emitter.
Now that transistor behaves like a "huge" diode connected from collector to emitter, which will now will a turn on threshold of 0.7V multiplied by the resistance ratio.

Practical example: both resistors the same, the transsistor will clip at 1.4V .... because when you have 1.4V end to end you have required 0.7V BE .
So far it looks easier to just use 2 diodes and call it a day.
Fine, but what if you need, say, 10 or 20 diodes?
Now it becomes *very* interesting.
And it will still clip rounded like a diode , while an equivalent Zener will be square shouldered and very buzzy.

One practical use is in +/-15V fed Op Amps, with a 20X resistor ratio (say 220K/4K7).
In that case, the Op Amp will never clip "square" but rounded, yet provide almost full rail to rail swing, because the amplified diode will stop it before doing that.

In biasing circuits, resistor ratio is adjusted with a preset resistor, and you can fine tune it and get, say, 2.6 diode or whatever intermediate value you need, and it will *still* track thermally.

thank you very much.
that's very informative.
it will be interesting to implement this in a distortion pedal.
 
Music Man preamps use it a lot, check the amplified diodes clipper network in IC6A
GP3.GIF
 
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