Finally, I have my triaxis effects loop back

[tubeydude]

Yes, this is just an experiment. Socketing the IC's will allow me to go back to stock without any problems.

Op-amps, while having some different inherent characteristics were somewhat designed to be more sensitive to the circuit around it than to their internal properties. They are more predictable that discreet transistor circuits by design. All active devices have ranges through which they will work. Some of op-amps require more or less compensation than others. Some will give different results at the same levels of feedback. Op-amps differences are most pronounced when run with minimal negative feedback. However, this is almost never done. The amount and type of feedback employed has vast effects on the op-amp.

I'm just going to try two different op-amps the TLE2082CP and the OPA2134PA These both have values not too far off from the operating parameters of the TL072. The major difference is the bandwidth and slew rate. You are correct that this could result in the net circuit going into positive feedback, but at a couple bucks each, it is a cheap and non-destructive experiment.

Sometimes technically perfect circuits are not the best either. Tubes are nasty, dirty, non-linear devices that have orders of magnitude more THD that any self-respecting solid state amplifier. However, tubes sound better. Some of he the best sounding amps just dripping with character have a weak power supply that sags when called upon to deliver more power. Some even have NO global negative feedback. Talk about non-linear. Why on earth would somebody do that? Because it sounds better sometimes. If it sounds better and it is stable and not near self destructing, why not use it?

This could end up being an exercise in futility, but at least I will learn something out of this. Might even sound better too.

I'm not going to go messing around with the operating point of the op-amps as 1) I am not all that great with optimizing circuits and 2) I just don't want to get that deep into it.

One of the reasons I'm doing this is that I just got a Studio pre and the recording outs of that unit are quite a bit more dynamic than the TX. Just got me thinking about where the differences are. The Studio does not have an op-amp driving the rec out circuit. Is that the only difference? Of course not, but it is one that is relatively easy for me to vary and see if there is any difference.

Nothing wrong with curiosity and exploration...in the right hands and with a bit or research and knowledge. If someone is willing to put in the effort and time to make changes and are then prepared to deal with the consequences, why discourage them. Granted some non-technical people should NEVER open any electronics, but others might get a kick out of it and learn something.

I'm not trying to say your wrong at all. You are right on all counts. I just say it might be good to encourage a bit of learning rather than shut the door on any tone quest.

FWIW,
Erik

 

[fatboy135]

Sometimes technically perfect circuits are not the best either. Tubes are nasty, dirty, non-linear devices that have orders of magnitude more THD that any self-respecting solid state amplifier. However, tubes sound better. Some of he the best sounding amps just dripping with character have a weak power supply that sags when called upon to deliver more power. Some even have NO global negative feedback. Talk about non-linear. Why on earth would somebody do that? Because it sounds better sometimes. If it sounds better and it is stable and not near self destructing, why not use it?

yes but the matter is not about if it sounds well or not, I´m talking about since a totat tecnical point of view, for example don´t think now in audio
think in a smith trigger circuit with a histeresis, that kind of circuits are typical used on situations where we want trigger a determinate event. for example think that we have have a microwave oven, and we want an alarm on when the temperature is over 28 ºC, so if our temperature oscilate from 27.9 to 28.1 the alarm is on/off constantly causing a posible failure for extenuate use of the alarm, ok so we use a smith trigger circuit it preserves the threshold of the alarm to be on/off continously. Well when you design that circuit you use a determinate opamp, and a determinate design with components and values to minimize the faulties in opamp manufacture, well your circuit could perfectly work, now you replace that opamp from another one, with closest features and suddenly it generates a high frecuency oscillate, if you don´t detect it, it could trigger your alarm avoiding the benefits of the smith trigger and overloading the alarm.
So the first matter is not if the opamp is better or not, the first matter is to test if our opamp could works on our design, because if it doesn´t works well I don´t mind if it sounds the best of the world, because that opamp is not working for my design
On the other hand as you said having a socket is the easiest way to test them, but remember to test the circuit when you replace it.

 

[tubeydude]

Ok. That is certainly valid, but we are not talking about microwaves and Schmidt Triggers (I think that is what you meant. I'm not aware of a smith trigger). Those circuits rely on well defined and stable thresholds to operate correctly. Audio circuits have no such requirements and are a completely variable signal.

For an audio circuit, the only thing that should matter is the sound. The original designers of tube guitar amplifiers would likely be horrified to know that we regularly push the entire tube audio circuit into full distortion...transformers included. That's not what they intended, but that's the sound that was desired at the time.

You are correct that the operating points will be different though. Just curious...did you find any sound difference when switching out the TL072? Which op-amp did you end up sticking with?

Erik

 

[fatboy135]

For an audio circuit, the only thing that should matter is the sound

No no no. wrong my friend I will try to give you an example, about a year ago somebody gives me some tl audio stuff a lot of equipments they were demo products and he wants to improve them, ok after recaping with better caps changing the tubes for better and mod some design parameteres I decided to change the opamps, ok we switched it on and we hear the product, that guy saids to me " hey dude it sounds real amazing it is music for my ears " ok now let me do some test, my first test was an overheating of the opamps, I was real amazed. That opamps were running on a class A but not enough to heating like a car turbine, ok I hear the machine and it sounds completely well, I connect my scope and we have a mighty oscillate on 1 MHz. The dude saids to me " oh man not problem is 1 MHz we can´t hear it " so I think maybe... but what about with the heat??? so I comment him what about changing with other opamp looking for something more stabilizate??? he saids ummmm but I like that sound, give them a 24 hours test ok I said come back tomorrow if the opamps are working good we can forget the oscillate, after 24 hours 2 of the 4 opamps was totally damage. So I replace with another model, the guy said to me " hey dude but I really like the first opamps you put " and I said I know me too, but if we want to not to being change opamps every month we need that other opamps that sounds worst but works real fine with that design.
Of course we sacrifice sound for a more stability circuit

Just curious...did you find any sound difference when switching out the TL072? Which op-amp did you end up sticking with?

ok regarding that issue I must said that there is not a night and day change, the improvements are general in better S/N ratio, because you could get a lower noise opamp with a rail to rail that allows you to increase the headroom, other improvement is that now I could get higher volumes without the nasty opamp distortion, I take advantage over +/-1.2 volts about 2.5 volts headroom extra thanks to the rail to rail opamp.
Now I mounted a burr brown opa2134

 

[tubeydude]

Cool, thanks. I get mine on friday and I'm curious to see how they sound.

Do you know of any simple, hopefully free circuit simulation software? I'm just looking mainly to see what the frequency response of different LRC filters would look like. I don't need anything too complicated.

Thanks,

Erik

 

[fatboy135]

the classical on this way is the orcad, you can download a free trial from the web page