TL072 replacement

[Randall]

I have a Mesa Walkabout amp in that I suspect has a bad TL072 in the preamp board. (It is hissing and the TL072 reacts a lot to a squirt of freeze spray). Is there a suitable replacement op amp that could be popped in just for troubleshooting purposes? I have 4558 and a LM741 on hand, would either of these work?

[nosaj]

I have a Mesa Walkabout amp in that I suspect has a bad TL072 in the preamp board. (It is hissing and the TL072 reacts a lot to a squirt of freeze spray). Is there a suitable replacement op amp that could be popped in just for troubleshooting purposes? I have 4558 and a LM741 on hand, would either of these work?

Maybe this will help
http://music-electronics-forum.com/t19942/

nosaj

[Randall]

Thank you. I subbed a 4558 for the suspected bad TL072 for a quick moment, and suspicions are confirmed. Noise is gone, and new TL072s are ordered.

[Enzo]

So how does the 4558 sound? Any reason not to just leave it there and consider the repair done?

[Randall]

It sounds fine, Enzo. I just do not know the finer points of why that would or would not be preferred. At less than $1 a pop, I chose to put in the factory spec'd part and sleep well at night knowing I can now not be judged by someone else further down the road for installing "the wrong part". If I had your breadth of knowledge and experience I could venture maybe some swagger, but alas, I don't.

[Dave H]

Is there a suitable replacement op amp that could be popped in just for troubleshooting purposes? I have 4558 and a LM741 on hand, would either of these work?

You picked the right one. 741 wouldn't have worked, it's a single, TL072 is a dual.

[Jazz P Bass]

It sounds fine, Enzo. I just do not know the finer points of why that would or would not be preferred. At less than $1 a pop, I chose to put in the factory spec'd part and sleep well at night knowing I can now not be judged by someone else further down the road for installing "the wrong part". If I had your breadth of knowledge and experience I could venture maybe some swagger, but alas, I don't.

Here is some MEF food for thought on the subject of interchangeability.

http://music-electronics-forum.com/t19942/

[Enzo]

It was never about the money, it was about getting the amp back to the owner sooner so he can use it instead of his backup.

[J M Fahey]

NickB´s post and tests nail it.

Many read datasheets and labels, nothing wrong with that of course, but tend to take them literally.
Hey, if one Op Amp is labelled as "low noise" and the other says nothing, certainly the second must be far noisier or even unsuitable, shouldn´t it?

Fact is that, besides labels, we should try to put some numbers into it , under real use conditions, of course.

If a bipolar input Op Amp has, say, 5M input impedance and a FET one has 500 Megohms, considering they are referred to ground or bias voltage through a 1M resistor and the pickup driving them is also driving TWO 250k or 500k pots in parallel (at mid/high frequencies both volume and tone are in parallel) real World effect on pickup response is about the same.

Also if pickup impedance is complex (which it is) meaning it changes with frequency, before jumping to conclusions we should see what input voltage noise (only spec most check) and input current noise (which most ignore) can contribute to total noise.

So, as said above, each case must be analyzed on its own.

FWIW when I commercially made PA mixers (impossible to compete today with Asians) , had less (audible and measurable) noise using RC4558 than using any FET input one , when making transformerless input Mic preamps.

For the better ones, I used a PNP transistor differential front end, bipolars of course, which was also the choice of many big name PA and Recording mixer manufacturers.

Not dissing TL072, my all time favorite Op Amp, which I use today for everything.

NickB´s excellent analysis and post:

Hopefully, for the benefit of everyone, I have some real data (spice simulation) I'd like to present. I'll leave the interpretation of the acoustic effect to others. The test circuit is slanted to using a TL072 in place of a RC4558 as I contend that due to the much lower currents and noise you are asking for trouble if you try to put a 4558 in place of a TL072 but not the other way around.

I've attached files (I hope!) for the frequecy response, transient response, output impedance, input impedance, noise, THD and DC offset. Surely that's enough for anyone ( I know it won't be) !

The attached test schematic is supposed to represent a 'typical', non-inverting x10 stage. I know typical is up for debate but I doubt that it would affect the conclusion significantly.


Input Z: TL072 vs RC4558 Zin.pdf
Frequency: TL072 vs RC4558 Freq.pdf
Transient: TL072 vs RC4558 Transient.pdf
THD, Noise, DC: TL072 VS RC4558 perf.pdf
Schematic: TL072 vs RC4558.pdf
Outoput Z: TL072 vs RC4558 Zo.pdf

You can see that for most parameters this is little difference as the traces are overlaid.

[Randall]

Not in this case. Time was not an issue. He is 1.5 hr drive from me, and didn't need it for a month.

[dai h.]

NickB´s post and tests nail it... [snipped for brevity]

NickB´s excellent analysis and post:

unfortunately, NickB's files seem to be missing.

(I'm far from expert but from what I vaguely understand) "conventionally good" (as in low distortion, clean ("neutral") performance (amplification)) out of the common (and sometimes mis-perceived as cheap and poor) opamps, can be obtained IF the implementation is well done (good layout, clean low impedance power supply, correct bypassing, grounding, don't ask for too much gain).

There also seems to be a mis-equation of the inexpensiveness of the mass produced types (TL072, 4558, 5532, etc.) with "cheapness" and "shoddiness" which seems to be a ignorance about their inner workings and configuration (and their strengths due to factors like the differential input which helps with noise and distortion, plus having lots of gain so they can apply feedback in a brute force manner (i.e. using lots to force linearity)), plus perhaps seeing them in cheaper lesser performing gear where a big reason for their use was due to economy (hugely important to a maker trying to make "value" products) and a good implementation wasn't carried out for cost reasons (sort of like confusing/equating the mere presence of PCBs in a guitar amp with lesser quality instead of recognizing that PCBs are more likely to be found for mass produced items where labor cost is high).

And in addition, economies of scale that must be there for these sophisticated parts to be sold so inexpensively. If a manufacturer can get excellent results from a common op amp (probably better than when they were originally produced due to cleaner manufacturing) which is very low cost (and even lower when purchased in large quantities) without having to buy something newer which is much more expensive, why would they bother? (It seems (again, my vague non-pro understanding)) marketing is one factor, plus a lack of understanding from some designers of how to use, how to get the best out of parts.

Anyway, my perception of op amps is quite different today from when I started DIY tinkering.