Why is my opamp slow?

if i compare to this one

there’s nothing in pin 4 & 5 of your AS

Is that necessary? I’ve based mine off of this schematic (found here):

Pins 4 and 5 are the output and CV input for the on-chip pulse wave. Since you’re making your own pulse wave externally they’re not used. It’s not obvious to me whether it’s best practice to leave them disconnected or tie them to ground or what. But they presumably have nothing to do with your op amp problem.

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I’m all in favour of innocense until proven guilty. Making remarks like this may give people a bad reputation that they do not deserve. Until you can present proof that all of the hundreds of traders on ebay and aliexpress (or the like) sell fake stuff, I think this is rather misleading.

The response to your post supports that feeling:

No further response.

This post reads (to me) simply that we have seen many people complain about sellers from both of these sites for exactly this reason.

There is being charitable, and there is defending things that have been shown to have a pattern of risk.

I hardly think anyone is saying every seller on those sites are peddlers of problematic products, but to ignore the actual real risk isnt doing anyone any favors either.

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To settle it, cross over distortion is clearly visible when the wave passes zero volts. This is a key indicator that you have lm324s (or similar) marked as tl074s.

Unfortunately getting remarked IC’s from these sites is now all too common. I’ve seen a bunch of people waste a lot of time on remarked chips sold as a relatively uncommon audio codec ic, because it was on back order with the usual trusted vendors.

There are good parts to be brought from these sites though, you just need to be wary!

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Interesting, learned something new today and it’s only 7 am!

The output stage of the LM324 has low output impedance when it sources output current (when the ‘push’ half of the push-pull stage is active), and when it sinks output current (when the ‘pull’ half of the push-pull stage is active). It turns off (goes high impedance) at zero output current, and that causes a ‘dead spot’ in the transfer characteristic.

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Starting in 2018 I’ve made some 950 purchases via aliexpress and so far I’ve never received any fake stuff.

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How do you know?

Seriously, 1) fakes have always been a thing (cheap things recycled but sold as “new”, more expensive things relabeled), and 2) there’s a major semiconductor shortage right now making it lucrative to relabel also cheap stuff, and 3) multiple forum members have gotten fake TL07x ICs from Aliexpress over the last year. I got recycled chips from a usually reliable eBay seller just a few weeks ago.

You don’t have to defend the Aliexpress and eBay sellers, they’re not your friends, and their supply chains don’t provide much traceability (usually zero traceability once it reaches the store).

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I hope you understand that most of the points you raise do not necessarily hold in the case at hand.

You are assuming that what is shown in the video is correct and not fabricated by the person making the video. And you extrapolate from that to the current situation.

This is at best supposition / a hypothesis but that does not necessarily make this a reality.

Hmm, have you checked this or do you just believe their statements are correct? In a court of law this kind of reasoning is called “hearsay” and not admissible.

I did not say this because I thought I needed to defend anyone. Ever heard of slander and being sued for it? Aliexpress and Ebay could maybe feel that the suggestion made could be slanderous.

[edit] my original response was (in part) meant as a warning having this last point in mind.

Wow. ㅤ ㅤ ㅤ ㅤ ㅤ ㅤ ㅤ ㅤ ㅤ

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Sir this is a Wendy’s.

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This problem is still bugging me because I want to be ableo to make a good sounding square-wave oscillator with PWM and a comparator is the best way to implement this AFAIK.

I managed to reproduce the problem using a very simple experiment. Basically, I’m using my signal generator to feed a triangle wave directly into a comparator, which is also getting a steady voltage configurable using a pot. I send the output directly to my scope and output module. Check out the video:

To my surprise, when I send a square from the signal generator into my comparator, the output is a perfect square wave that has nice squary high-end to it. When I send my triangle wave into the comparator, it sounds different and more muffled and I can see slow rising edge on my scope too.

The TL074 chip is one I got from Germany so it should be decent. Can it be the capacitance in the cables? Should I add some caps next to the TL074? I’m really at a loss as to why this is happening.

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I did this simulation

image

It claims to be simulating a TL08x. How accurately, I don’t know. But the fact the triangle wave rises slower than square wave on input means the output will rise more slowly too. This is what I get:

So about 2 µs for the (100 Hz) square wave and about 28 µs for the triangle. I can’t tell from the video, is that much faster than what you see?

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@analogoutput Thanks for the simulation. I repeated the experiment in circuitlab and I’m getting similar results. :slight_smile:

If you pause the video around 40 sec, you can see that the rising edge takes about 0.25ms, which should cause distortions around 4Khz so that is clearly audible. This is an order of magnitude slower than what circuitlab suggests. See pic below:

rising_edge

However, when I look at the datasheet, I see that the TL074 should have a slew rate of 13V per microsecond, which is over 250 times better than what I’m seeing in practice.

I ran a test in circuitlab and putting two comparators in series will result in a nice square wave. However, I rather don’t want to add yet another opamp to my VCO and first I want to understand what is happening here.

Can any expert shed some light on this?

The slew rate would apply to the case where you apply a square wave to a comparator, not a triangle wave. It corresponds to what I see in the simulation where it goes -12 V to 12 V in about 2 µs with a square wave input.

I grabbed a TL072 out of my stash and took a look at it with a ~100 Hz square wave and triangle wave going in. The other op amp input was connected directly to ground:

That corresponds pretty well with the simulation, about 2 µs for the square wave input and about 30 µs for the triangle wave. It does look like yours is too slow though I’d be more confident of that if I saw it on a shorter scope time scale.

(I guess it’s similar to what you show in the first post of this topic? Is that the same op amp?)

I noticed the non inverting inputs on the unused sections of your TL074 are floating — in principle they should be grounded, with the inverting inputs connected to outputs (as yours appear to be). I doubt that has anything to do with what you’re seeing though.

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:raised_hands: :raised_hands: :raised_hands:

I managed to fix the edge rise/fall by swapping the inputs to the comparator opamp!

First, I had the PWM reference voltage (from the knob) connected to the inverting input and the audio input to the non-inverting input and this resulted in rising falling edges that take 150+ microseconds. When I swap them around and connect audio to the inverting input instead, I get about 30-ish microseconds and a much crisper tone as well.

So apparently, a TL074 does not completely operate in a symmetrical fashion. :thinking: Looking forward to hear your theories about this. Circuitlab does not simulate this behaviour so it is probably not very well known.

Thanks for all your help in this topic!

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Unfortunately when I do that (and I switched from a TL072 to a TL074 just in case it mattered) I still see 30 µs either way.

Have you tried it with the reference pin tied to ground instead of a control voltage? I don’t know why that should matter but it’s the only thing I can think of that’s different between our setups.

I just wanted to weigh in here on possible fake TL074s from Amazon. I built a VCF some weeks ago that had some resonance and distortion issues that I chalked up to a mistake I probably made, but when I made a VCO around the AS3340 recently, I had a TON of distortion and clipping.

I rebuilt the VCO circuit on a breadboard and had the same issues. I swapped the TL074 IC with every one in the batch (I think I got 10) and had the same problems. I then had issues getting my CV voltages where I wanted them from a teensy DAC and PWM pin, which confused me because I used the same circuit from another project, but that one used a digikey TL072.

With all this, I decided to order 15 TL074 ICs from mouser and when I swapped the IC in the VCO with the mouser op-amp and the clipping and distortion went away. The output signal was very clean. So I went ahead and swapped out all the TL074s for the mouser chips and it resolved all of those problems listed above.

I’m not sure what chips I got from Amazon, but they are either faulty, or rebranded.

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