Electronics > Beginners
TL071 distortion - bad amplifier design?
AngraMelo:
Hey guys,
I just got a couple of boards and populated them for an audio amplifier.
After completion and checking to see if everything was in the right place, I powered it up, injected a 1kHz sine wave from the generator, hooked up the speaker to the output and was greeted with a enormous distortion.
Schematic attached
probing the output of the TL071 (pin 6) shows that the distortion is coming from there.
Now, the other thing is that by powering the amp with +-40V (unregulated but stable) I can never get a output of more than 7.5Vpp
That is very weird. I thought the whole point of the +-40V rails was to have the power transistors swinging accross that voltage.
I have no clue what is going on. Any help?
HB9EVI:
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.
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.
My tip would be to construct a proper long tailed pair input stage, a VAS and then the emitter follower as output stage; but also in the output stage as-is are several things to correct, for example you have far to many miller capacitors, which limit you the bandwidth far over the needs.
Mechatrommer:
--- 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 ---
its common to feed the signal in inverting manner, but the feedback path, it depends.
--- Quote from: HB9EVI on December 09, 2018, 11:22:38 pm ---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.
--- End quote ---
the opamp doesnt have to swing full ±40V, there's app note for voltage gain stage like this...
it seems the OP copied the design from somewhere else, he may want to post here the original diagram so somebody may check for him.
--- Quote from: AngraMelo on December 09, 2018, 11:10:12 pm ---Now, the other thing is that by powering the amp with +-40V (unregulated but stable) I can never get a output of more than 7.5Vpp
That is very weird. I thought the whole point of the +-40V rails was to have the power transistors swinging accross that voltage.
--- End quote ---
the TL071 will never be supplied from ±40V it will smoke otherwise. the zeners will limit the incoming supply to maybe ±15V. full swing is achieved in the 1st or 2nd stage of the transistor amplifer, you may check the swing at TIP41/42 emitter pins or after the 100R resistors.
oPossum:
The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct. That stage also has voltage gain, so the output can get near the rails.
I don't see anything obvious wrong with the circuit. It may be a build error. Pictures of the build and the distortion you are seeing would be helpful.
Mechatrommer:
--- 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. That stage also has voltage gain, so the output can get near the rails.
--- End quote ---
the feedback should stabilize/equalize the inverting input path. if inp goes -ve, out will goes +ve, if further voltage gain stages goes +ve, it should go to inverting pin, otherwise it can go to non inverting pin. but you might be correct. if probing the output is flat low or high, the feedback path is probably wrong, but if excessive swing, maybe too much voltage gain there or even TL071 goes phase reversal. ymmv..
edit: err.. your speaker output should be connected to the last emitter follower (current gain) stage 52ee/1943 after 0.22R. not the earlier stage tip41/42. there dont have enough current power, you load that node you'll have distortion. ymmv.
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