Transistors and OpAmps

Why do similar effects sound so different
when using Transistors vs OpAmp IC chips?

 

The answer is simply because a circuit must be implemented differently to use OpAmps which "usually" causes a change in sound/tone.

Lets just say right up front here, there is nothing "wrong" with OpAmps, or the implementation of them in audio gear, it only becomes an issue when musicians expect an OpAmp based effect pedal to replicate the nuances of a transistor based effect circuit, that is when the differences show up.

Think about this for a moment, when music is recorded the sound gets processed through much gear before it reaches your ears, most of this gear has litterally dozens of OpAmps inside it and the audio path travels through these OpAmps. If OpAmps were "bad" they wouldn't to the job of processing the music as transparently as possible, so we should say they are "good, ...for a certain purpose".

Unfortunately when it comes to instrument effects pedals, many/most times the effect pedal circuit is not made with transparency, or hi-fidelity in mind, and it is not usually made with top-shelf top-quality components. Usually guitar and bass effect pedals are made with parts and circuitry that satisfies the design and economic goals, and almost always the total sound effect of the effect pedal is caused by these design and economic decisions. Strangely enough it is most often these things that cause an effect pedal to have it's unique sound which is so desired.

BTW, if you are an electronics guy you likely already know this stuff so you can move on, there's nothing for you here.

This web page is meant for "musicians" and the "average guy" who knows little-to-nothing about electronics, but has ears to hear, and they hear things sounding differently and wonder why. Those poor souls who try to sift through all the advertising hype, internet ooze, and mojo BS going around.

 


 

The Up-Side...

So let's start by declaring the up-side of using IC OpAmps, they are wonderful devices, many things can be done with them that are difficult to do with discrete transistor circuits, they can and often do provide superior capabilities, lower noise (hiss etc), and hi-fidelity sound capabilities, yadda yadda, ok we're done with that.

The Down-Side...

The down-side to using OpAmps is that very few OpAmp IC chips are actually optimized for the job you use them for, audio in our case. To begin we should understand that an OpAmp (IC chip) is not a single device in the way a Transistor is a single device, an OpAmp chip is a bunch of devices in the same box, a building block of stuff, usually a collection of transisitors and other parts all placed inside a small package with easy-to-use terminals (In, Out, Power etc).

 

We can identify some fundamental reasons why a sound effect pedal can sound so different with Transistors -vs- OpAmp IC chips, even though it may be the same type of sound effect pedal. In the following discussion I'll be using the vintage Uni-Vibe and Dunlop Roto-Vibe examples, but the info is applicable to other distinctly characteristic sounds effects pedals that once used discrete transistors and now use OpAmps.

 


 

To simplify, one can think of an OpAmp IC chip very much like an electronic "LEGO" block, you use the blocks to build bigger complete "things". OpAmps were originally designed as, and are referred to as, "electronic building blocks". Intended to be used in a huge wide variety of ways, by all kinds of people, doing all kinds of things with them. And just like LEGO blocks OpAmp IC chips tend to make a design modular and easy to put together but the trade-off is a chunky all-around generic "thing" that has features not essencial to the sound we are after.

When it comes guitar effects and more specifically to something like a Uni-Vibe pedal, why does a pedal built with OpAmp IC chips have such a different tone/sound than one built with discrete transistors?. Many times it's becuase of the OpAmp IC chip bringing extra baggage with it.

In the case of simple effects pedals like the Uni-Vibe an OpAmp IC chip brings in much "un-necessary stuff" into the circuit, then more stuff has to be added to tame/force that extra stuff (that we don't need) into working right. Always keep in mind that most OpAmp IC chips are not designed strictly for audio use so they have other aspects built into them for the other purposes we do not need.

OpAmp IC chips most often used in guitar/bass effects pedals (ie; TL072 and JR4558) are like 20-50/ea and so compared to OpAmp IC chips optimized for audio purposes which are like $2-$10/ea (or more) so you can see how the economic aspect can dictate which OpAmp chips would be used.

 


 

Lets look inside an OpAmp for a second and see some un-necessary (to us) stuff, I've doctored-up these pictures so the average guy can see and understand things so looking at the two most commonly used OpAmp IC chip's used in most common guitar effects pedals, the TL072 and the JRC4558.

 

In the picture below, circled in RED are the transistors inside each IC OpAmp chip:
(deep breath, stay with me average-guy, don't fall asleep just yet)

 


Figure-1. TL072 data sheet

 


Figure-2. JRC4558 data sheet

 

Notice there are like 13 or 14 transistors in there and like 13 additional extra additionl parts to "tame" or "control" the curcuit.

So what's the point? ...well the Uni-Vibe circuit actually uses only a total of 13 transistors to make it's very cool sound, using just 1 OpAmp we already have the ammount of transistors that should be present, but only have one (1) stage done and we need 3 more stages for our Uni-Vibe!.

 


 

The following pictures will show you how an OpAmp brings too much stuff to the table in a Uni-Vibe effect pedal circuit.

Here is a small section of a real Uni-Vibe circuit which is a 4 stage phase shifter, the picture has been cropped to show only the 4 phase stages so it's easier for average-guy to understand, notice there are 7 transistors in these four phase stages:


Figure-3. Uni-Vibe's four phase stages

now compare the picture above to the picture below (Dunlop RotoVibe and MXR Phase-90) notice the four red triangles (OpAmps):


Figure-4. RotoVibe and Phase-90's four phase stages

 

Wherever you see a Red triangle above, just imagine there are 13 transistors inside each of those triangles plus the other "stuff".

Now instead of just the 7 transistors in the original Uni-Vibe circuit's phase stages we can see our 4 phase stages times (x) the 13 transistors in each OpAmp IC stage now we have ~52 transistors where we only needed 7, now adding in the 50-or-more extra "other stuff" parts the OpAmp brings us in the extra baggage department we now have more than 100 extra ( and un-necessary to a uni-vibe circuit) parts in there!.

That much of a change to a vintage effect pedal circuit absoultey does alter the sound, and there's no way around that fact!.

This does not mean we can't build a decent effect pedal from OpAmp IC chips, oh yes we can, we just can't expect the same vintage nuances (hiss/low-headroom/etc) to appear, we will have a better, low-noise, hi-fidelity effect pedal which in some cases can be a better effect pedal, only you can decide if you want the old-n-raw or new-n-sleek.

 


 

Classic Vibe CV-2TM and Vibe-Baby CV-2TM pedals use the original vintage transistor circuitry to achieve the vintage tone we're after.

Classic Vibe CV-2x ProTM pedals use a mix of Transistors and OpAmp IC chips for high performance studio quality use.

 

 

 


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