Op-amps vs transistors.

[POTL]

Hello
I want to know how high-quality the replacement of the operational amplifier with a transistor will be.
1) Sallen-key filters is there any difference between BJT and op-amp? I noticed that BJT is used in Japanese effects, while op-amps are often found in American circuits.
2) Input buffer, I'm interested in comparing JFET and op-amp, how do they compare to each other? The reaction of the elements to a strong signal is interesting, can they clip? What JFET parameters are needed for normal operation, I heard that j201 are not suitable for these purposes.
3) Why is the output buffer almost always BJT and not JFET in many circuits? Does it have something to do with stability or output impedance?

[Vivek]

I read an analysis on ELLIOTT SOUND PRODUCTS, it compared impedance, noise, gain of Opamp circuit versus BJT.

One of the sentence was approximately "So we see that for most Audio applications, Opamps beat transistors"

[marcelomd]

3) Why is the output buffer almost always BJT and not JFET in many circuits? Does it have something to do with stability or output impedance?

BJTs have lower input and output impedance. The higher input impedance of the JFET is not really needed at that point and a lower output impedance can drive more current. Also, BJTs are easier to find.

[garcho]

When you say "op amp" it really depends on which one, which type, etc. There are FET op amps, BJT op amps, rail-to-rail, etc.

The main concern for input buffers in guitar circuitry is that they have input impedance that's as high as possible. That's because the guitar pickup's signal and the cable (and a host of other little gremlins) make a complicated mess of inductance and capacitance. When those elements combine with low resistance, audible, unwanted filtering occurs. Super high impedance means no current sneaks in, because it presents a high resistance, which means no unintended filtering. That's a simplification. Op amps, especially FET op amps like the TL07x, have much higher impedance than almost all transistors. The older Boss designs used transistor inputs for a number of reasons that aren't really concerns for the DIY maker. Many users also complained about "tone sucking" because of those realtively lower impedance inputs.

Output impedance also matters. If the signal isn't being "impeded" it can drive the next stage (the next pedal, your amp, another stage in your circuit) without any issue. The added bonus of low impedance output is that when it is connected to a high impedance input, that difference between the two - low out to high in - means none of that problematic filtering.

ESP articles are a really great resource. There's also this by Jack Orman if you're interested.

[Steben]

3) Why is the output buffer almost always BJT and not JFET in many circuits? Does it have something to do with stability or output impedance?

BJTs have lower input and output impedance. The higher input impedance of the JFET is not really needed at that point and a lower output impedance can drive more current. Also, BJTs are easier to find.

The way they are used makes for more consistency as well. jFETs are "finicky".

Opamps are fairly "easy" to use in entry level circuits and in those applications completely predictable compared to discrete circuits.
You don't need trimpots to bias a basic opamp stage. But they are the utmost linear choice, which means they are not suitable if "warm" "harmonic" amplification is demanded from a single component.

[Vivek]

"It should be fairly obvious that for small signal audio frequency applications, it's almost impossible to beat an opamp with any discrete option."

-- Follow The Leader - Voltage Followers & Buffers
© 2016, Rod Elliott (ESP)

https://sound-au.com/articles/followers.html

[niektb]

"It should be fairly obvious that for small signal audio frequency applications, it's almost impossible to beat an opamp with any discrete option."

-- Follow The Leader - Voltage Followers & Buffers
© 2016, Rod Elliott (ESP)

https://sound-au.com/articles/followers.html

Depends on what you're after, if you want 'color' then you should get a discrete implementation (or, you know, tubes). One of the selling points of the Boss Waza revisions is that they use good sounding buffers. Guess what? They're discrete... Strymon: uses a "Premium JFET analog front end" (from their website) for their digital (!) pedals...

[Steben]

BJT/jFETs do have the advantage of low power in some applications.
You can have a great booster pedal with nice harmonic contant with a cheap jFET and even more BJT with very low power consumption compared to a good common opamp stage.

[R.G.]

The best say to look at it is that transistors - BJT, JFET, MOSFET - have different characteristics from each other, and wild variations in their gain, input and output impedances, and so on. They're so widely varying that EEs introduce feedback >>even on discrete transistor designs<< to tame the variations somewhat. An emitter follower/source follower, for instance, has 100% negative feedback. Gain circuits have built in feedback to make the gain predictable.

Opamps are made from transistors. They use a group of transistors to get gain that they can, through the application of feedback, use to hide the variable characteristics of the transistors they're made from. In many cases, the feedback can produce better input impedance, gain accuracy and predictability, lower output impedance and so on. The whole point of opamps is to get away from the variability of individual transistors.

Since opamps largely make what's inside the opamp not matter, only the feedback elements - resistors, capacitors, diodes, etc. - set up the operating conditions. As long as you're not running them into situations where the feedback gain just can't
"cover up" the quirks inside the transistors in the opamp, the opamp will be simpler to design with, and have better general performance, with few exceptions.

The exceptions are where you need a lot of gain at high frequencies or where there is some minor burble that the feedback can't quite cover up. Some filter circuits need a lot of gain at high frequencies, so you have to pick opamps that can do that. There are >>video<< opamps, by the way, so that one is solved by selecting special opamps and/or circuits. The minor burbles account for nearly 100% of the non-imaginary differences in opamps that purists like to point to. Grossly violating the operating conditions of the opamp's internal circuit is one other issue. If you violate the common-mode input range, min and max power supply range, or just bang the outputs against the power supply limits, the feedback can't cover that up and you get gross distortion.

This is a long winded way of saying again what has been mentioned - for nearly all situations in audio, an opamp should be considered as possibly the best candidate. There are places where discretes can do better, but the expertise needed to know these situations is far beyond the level where one would ask the question whether transistors or opamps are better.

[POTL]

Thank you very much for the answers, I'll read them later after work.
I was interested in transistors with the possibility of compact arrangement. They can be installed anywhere, whereas the 2 halves of the amplifier are always connected. And most single-channel amplifiers in the dip-8/so-8 take up more space.
The goal where I want to apply them is the input buffer of the circuit and standard filters in analog modulation/delay effects.
I tried designing effect boards in the spirit of CE-2/ DM-2 with the addition of several mods and digital controls, and thought it would be worth trying to replace the input buffers and sallen-key filters with op amps, but tracing the boards has become noticeably more complicated.
Okay, as for the input and output buffers, we found out what you say about the sallen-key circuits that are located in the middle of the circuit, will there be a real plus from using amplifiers instead of transistors?

[Fancy Lime]

+1 to what has been said before. If you want linear response, e.g. for filters and so on, use opamps. Discrete designs are mostly interesting for guitar effects in situations where their non-linearity provides a pleasant distortion. In my experience, designing with opamps is much easier than with discrete designs because it is much more predictable.

Andy

[marcelomd]

I find that opamps, on average, use less space than discrete circuits. I absolutely hate how the pins are arranged (try to route the four opamps of a quad opamp package, like the TL074 in sequence).

[R.G.]

I never use quads for that reason. As I noted here and in "PCB Layout for Musical Effects", I will prefer two duals to one quad because the outputs can be all pointed in the same direction for signal flow.

[ElectricDruid]

I never use quads for that reason. As I noted here and in "PCB Layout for Musical Effects", I will prefer two duals to one quad because the outputs can be all pointed in the same direction for signal flow.

Yep, me too. The quad package is an aberration! And the power pins are the wrong way around to boot!

[11-90-an]

I never use quads for that reason. As I noted here and in "PCB Layout for Musical Effects", I will prefer two duals to one quad because the outputs can be all pointed in the same direction for signal flow.

Yep, me too. The quad package is an aberration! And the power pins are the wrong way around to boot!

You can though, flip it 180 degrees... nothing really changes except for the power pins... 

[garcho]

nothing really changes except for the power pins...

...and some extra debugging time

[11-90-an]

nothing really changes except for the power pins...

...and some extra debugging time

naw.. magic smoke is easy enough to spot...

[Rob Strand]

Anyone remember the RC4136 quad opamp?   They had a different pin-out to the TL074, the pin-out was like a dual-opamp with a second story renovation.



The RC4136 was used in a few magazine articles in au in the late 70's and early 80's

IIRC there was also a TL075,  a TL074 with the same pinouts as the RC4136.

[11-90-an]

Anyone remember the RC4136 quad opamp?

This was the original op-amp in the "Woody" pedal right? making a TL074 alternative was hard so I ended up with 3 TL072's 

Why do you think they decided to just flip the power pins around and change the pinouts from the RC4136 type to TL074 type? anyone with ideas?

[Vivek]

Anyone remember the RC4136 quad opamp?

This was the original op-amp in the "Woody" pedal right?

Woody is very interesting

If I remember correct, it is

A) Take original signal and split into 3 parts
B) First part do nothing
C) Second part boost the bass with a resonance
D) Third part Aural Exciter (high pass, distortion, filters)

E) Add all 3 paths back

Maybe use external reverb

Are there any other ways to do Acoustic Sim / Sitar Sim than above idea ?