Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Effects => Topic started by: VMS on April 13, 2016, 03:17:50 am
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Hi guys!
Having trouble with this one.
Which two of the four diodes in this circuit are doing the clipping?
Thanks in advance.
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All of them. D1, D2 clip positive peaks. D3, D4 clip negative peaks. Assume the diodes are all the same and that it takes 0.7 volts to forward bias each diode. So, a signal that is less than 1.4Vpeak will pass with no clipping. Any signal that exceed 1.4Vpeak will be clipped to a max level of 1.4Vpeak or 2.8Vp-p. IOW, a 5Vp-p sinewave input will become a 2.8Vp-p square wave output.
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Maybe I need to rephrase the question.
So if D1 and D2 clips the positive wave, which one of those does the clipping and which one is there just for the 0.7 voltage addition?
Reason I'm asking this is that if I was going to change one of the four diodes to germanium type I would like that diode to do the clipping and not just change the treshhold voltage.
....or am I thinking this all wrong?
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Maybe I need to rephrase the question.
So if D1 and D2 clips the positive wave, which one of those does the clipping and which one is there just for the 0.7 voltage addition?
Reason I'm asking this is that if I was going to change one of the four diodes to germanium type I would like that diode to do the clipping and not just change the treshhold voltage.
....or am I thinking this all wrong?
The answer is the same. You must put at least 1.4V across the two diodes before either one can conduct, so the clip level will be 1.4V. Clipping takes place when BOTH those two diodes are forward biased. When they are both forward biased, there will be .7v across each diode, for a total of 1.4v. If you use 3 diodes, the clip level will be 2.1V. If you use 1 diode, the clip level will be .7v.
The operation is the same if you use one germanium diode, but the clip level will be lower. If the forward voltage for germanium is .2v (I don't recall what the accepted voltage drop is) then the total clip level will be .9V.
You can create some very different clip levels by using zeners in combination with silicon or germanium diodes. Or you can use a pot connected to some reference voltage to fine tune the clip level to exactly what you want.
You can use a scope and a sig gen to easily verify this.
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The answer is the same. You must put at least 1.4V across the two diodes before either one can conduct, so the clip level will be 1.4V. Clipping takes place when BOTH those two diodes are forward biased. When they are both forward biased, there will be .7v across each diode, for a total of 1.4v.
Ok, so if I'm understanding this correctly both diodes go to forward bias state at the same time.
So if D1 is germanium and D2 is silicon diode the clipping would sound the same as if D2 is Ge and D1 is silicon?
I'm asking these questions more for the sound attributes than the actual clipping voltage level.
Differences in knee voltage and such....
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Ok, so if I'm understanding this correctly both diodes go to forward bias state at the same time.
Yes. You cannot have one diode forward biased and the other reverse biased. The reverse biased diode will be switched off (think open circuit, very high resistance). Since no current can pass thru the reverse biased diode, there can be no current thru or voltage drop across the other diode either.
So if D1 is germanium and D2 is silicon diode the clipping would sound the same as if D2 is Ge and D1 is silicon?
I believe so. The output waveform should look the same for either case.
I'm asking these questions more for the sound attributes than the actual clipping voltage level.
I don't see how you can separate sound attributes from clipping voltage. If you only clip a little off the peaks, you will have a slightly distorted sound. If you clip deep, you will have a very distorted sound. Since you can't control the clip level provided by a simple diode circuit (it is what it is), the amplitude of the applied signal determines the clip level. A signal level that is less than the diode forward bias voltage will not distort (clip) at all, but a signal that greatly exceeds the diode forward bias level will have severe distortion (clipping). That's why backing off the guitar volume will clean up the sound.
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So if D1 is germanium and D2 is silicon diode the clipping would sound the same as if D2 is Ge and D1 is silicon?
I believe so. The output waveform should look the same for either case.
Thank you so much. This answered my initial question.
Sometimes I struggle writing understandable questions in english but we got there in the end.
I wasn't ignoring the voltage level but if we keep the Vfb roughly the same by using either a single led or two silicon diodes in series or a string of Ge diodes in series the clipping would sound different. Right?
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Ge diodes don't turn on/off as abruptly as Si diodes. And they have a much lower front to back ratio. The sound will be different between the two types.
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Ge diodes don't turn on/off as abruptly as Si diodes. And they have a much lower front to back ratio. The sound will be different between the two types.
Many various diodes all have subtle sound differences as well as Vf's. Composition and Vf both matter as does total Vf and where/how it is used.