Electronics > Beginners
TL071 distortion - bad amplifier design?
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Circlotron:

--- Quote from: AngraMelo on December 10, 2018, 03:39:12 am ---By probing the output of the TL071, the wave became a whole new thing completely, it was distortion in the theory sense of the word but what I mean is that it did not resemble a sine wave anymore.

--- End quote ---
The TL071 output is inside the feedback loop so it may well look distorted. If the circuit is operating properly, the TL071 output will be whatever it needs to be to make the amplifier output "zero" distortion.
David Hess:

--- Quote from: HB9EVI on December 09, 2018, 11:22:38 pm ---First of all, you have the opamp inputs wrong; the feedback network goes on the inverting input and the audio signal on the non-inverting.
--- End quote ---

The first transistor stage inverts the output so feedback does need to go from the output to the non-inverting input.


--- Quote ---That what looks like a VAS stage, is not really needed if you use an opamp - but you have problem; the opamp cannot deliver full swing of nearly +/-40V, since its supply rails are limited by the zeners, so the whole concept has big lacks.
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The first stage has voltage gain so the operational amplifier can have a lower output voltage range.


There are better but more obscure ways to design an amplifier like this but it should still work and I have done it this way in the past.  The TL071 is faster than a 741 so it might require a separate feedback network from its output (not the amplifier's output) to inverting input to maintain stability.
Zero999:

--- Quote from: Bassman59 on December 10, 2018, 03:25:27 am ---
--- Quote from: oPossum on December 10, 2018, 03:15:26 am ---The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.
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no.

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Take another look. The second stage, A1015 and C1815, is in a common emitter configuration, which is inverting and has the voltage gain required to boost the output voltage from the op-amp. The third and final stages TIP41 and TIP42 and 2SC52EE and 2SA1943 are emitter followers, which just amplify the current.


--- Quote from: AngraMelo on December 10, 2018, 03:39:12 am ---I actually bought the boards. I got them on our local "ebay" kind of website.
Both boards were assembled exactly as shown on the schematic.

it did not matter how many volts I put in, the output would never swing more than 7.5Vpp. After that it would distort until it became a nearly square wave.

By probing the output of the TL071, the wave became a whole new thing completely, it was distortion in the theory sense of the word but what I mean is that it did not resemble a sine wave anymore.

How could the 1015-1815 and/or Tip41/42 be a VAS? They are both emitter followers. I dont understand.

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When the input is connected to 0V and the output open circuit, please measure the following voltages, all relative to 0V:

The +/-40V supply
Output of the circuit.
TL071: pin 2, 3, 4, 6 & 7.
HB9EVI:
Thanks for the feedback; you never stop learning. I've not yet met such an amp, so it looked all wrong to me;
not even in Self on Audio I found such an amp.

good to know!
mikerj:

--- Quote from: Bassman59 on December 10, 2018, 03:25:27 am ---
--- Quote from: oPossum on December 10, 2018, 03:15:26 am ---The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.
--- End quote ---

no.

--- End quote ---

Yes.  The drive stage after the op-amp is inverting, so the feedback connection to the non-inverting op-amp input is correct.

However "distortion" doesn't tell us much without an example of the output waveform.  e.g. could be the amp is oscillating, or incorrectly biased and clipping.
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