I know I really need an amp in need of repair. Trust me ive been looking.
Anyhow I was flipping through a book looking for something cool and there is an experiment for creating and then abating crossover distortion.
Here is the plagiarised schematic
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Ensuing rats 🐀🐁 nest
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I managed to throw this together pretty quickly and to my astonishment it worked. They called for 2.2 uf electrolytics the closest I had was 3.3 uf
I didn't even measure the input I just increased it from zero and BAM there it was on the output
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My instructions are to learn about the existing phenomena then take steps to cleanup the output by
-replacing one of the 1k resistors with a variable resistor
-replace both 1k resistors with diodes
But before I do that I need to try and grasp what's going on here :grr :lmao:
I cheated and read a little seems like biasing is the main deal here I'm gonna measure the Vbe on each transistor at present and see what's up
Hmm this might be over pretty quick
Maybe they are just trying to demonstrate that if you don't have enough forward voltage then stuff isn't gonna work right
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Seems like I remember threads where you were telling the noobies how to adjust the resistors until the notch disappeared
But seems like an elaborate circuit to demonstrate something that simple using two transistors and all.
I guess we'll see what's in store xP
The other thing that is messed up is there's no amplification/voltage gain. I went back and made the volts per division equal for both channels and the output is actually SMALLER :grr
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Hmm ok yeah I went to tear it down and disconnect and unplug everything I had an idea why not turn up the DC supply voltage so sure enough when i hit about 12.7 V
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And now remeasuring Vbe
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So seems kinda simple oh well xP
But I'll go ahead and proceed onward I anticipate they use the variable resistor to obtain needed voltages with the original 8 V DC but the part about the diodes has caught my interest :tu: should be cool
I went to redo this circuit with a variable resistor and forgot which of three terminals was variable and which was fixed 10k
So I stick it in and attached two leads and couldn't get any resistance
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So I pulled the trim pot out and saw sure enough 10k on outside pins :grr
Rechecked and messed around finally thought hmm what if it's one of the wires xP
Sure enough
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xP
Anyhow I replaced one of the 1k resistors with the 10k variable and was able to adjust it and find the sweet spot
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I did have a who's on first moment though
I had to ask myself if increasing the resistance of the variable resistor eliminated the crossover notch or if decreasing the resistance eliminated it.
Then I had to try and reason how the DC voltages at A and B would respond xP
Life is full of tough questions
Time for another side project xP
Btw one particularly annoying discovery
No crossover distortion when the bridge resistors are removed entirely
However it now looks like the poor little transistors are over biased and crying out for me to turn down the DC voltage
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Wow I was able to turn the DC down from 8V to 2V without a change in output
And get the Vbe down to about 0.65 on each
I still question why there is no voltage gain but this thread has already gotten out of hand divergence :'(
1. Yes, you have 8V going into a 10:1 voltage division, split between the transistors. So each one gets 8/11 ~ 0.36V. There is a dead band.
2. Removing the "bridge" resistors means that you have something very similar to "base bias" (except that there is no collector resistor) rather than voltage divider bias. You've eliminated the certainty of the voltage drop created by the middle resistors. The 10K resistors are now base resistors, limiting how much current flows to/from the bases, and that current, together with the temperature and BJT parameters, determines VBE. Current does flow, and so the transistors are turned on, and there is no visible crossover distortion. With no collector or emitter resistors, you risk instability: thermal runaway. The only thing limiting collector current is VBE in response to the base current, which is temperature and parameter dependent.
Quote from: saturated on October 01, 2026, 02:58:01 PMI did have a who's on first moment though
I had to ask myself if increasing the resistance of the variable resistor eliminated the crossover notch or if decreasing the resistance eliminated it.
Your voltage measurements between the bases (post #4) showed that 1V was not sufficient to remove the notch, but 1.2V was sufficient.
Increasing the value of the trim pot increases the voltage drop across it. So it was increasing the value of the trimmer that eliminated the notch.