Chips in the old block

Mike McLane

Mike McLane

Active member
I did a search to see if there were any threads dealing with the use of alternate op amps in the OD/Dist/Fuzz category and didn't find anything. Is there a discussion to be had here from the experienced builders? I know if you're using a dual op amp you've got to sub another dualie, but what else is to be considered? Is it possible to generate a list of common op amps used in dirt pedal building and characterize why one might be used as opposed to another. . .or did Pandora just peak her head out of the box?
 
Typically, opamps do not make much difference when swapping them out unless there is a specific need for super clean results. Typically, swapping out opamps in, lets say a 'Tube Screamer', there is not going to be any real discernible difference in the sound. EXCEPT where the slew-rate is lower than what the human ear can detect - which is about 0.5V/µs or 0.7V/µs or less. Still using the Tube Screamer as an example, it traditionally uses a 4558 dual opamp - which has a slew-rate of 1.7V/µs. And while the 1458 dual opamp has a slew-rate of only 0.5V/µs, there is no real detectable sonic difference between them in a Tube Screamer. However, if the 4558 or 1458 is replaced with an LM358 (which has a slew-rate of only 0.3V/µs, some human ears can detect that sonic difference. But if the 1458 or 4558 is swapped with an LT1490 (which has a very low slew-rate of only 0.06V/µs), there will definitely be an audibly detectable difference. At least, I can hear a difference. The upper harmonic content seems to be reduced to simple 'noise' with the LT1490.

Fundamentally, the slew-rate specification of an opamp is the speed at which it can respond to an abrupt change of input level - the positive and negative swings if the audio signal it is processing.

There are numerous demos and "shootouts" of opamps on YouTube and other audiophile & DIY forums. It's a Pandora's box for sure - because everyone hears with different levels of distinction. They key is to conduct one's own experimentations. Swap different opamps out see for yourself. It's what many of DIY builders do and/or have done. And in my personal adventure down that rabbit hole, I've come to the personal conclusion that where any group of opamps I'm testing by swapping them out in any given circuit, when the slew-rate is above the audible threshold of around 0.5 to 0.7V/µs - I can detect no audible differences. That's where it comes down to what rest of the circuit is comprised. The biasing, filtering, and compensation components surrounding an opamp in any given circuit is the truer story to the tale of "which opamp is better".

Another and better example is the Rat circuit. Anyone who has built more than a few of them and tried different single opamps in that circuit know the 308 or OP07 chips hit the mark for achieving that traditional Rat sound. Both the 308 and OP07 have a slew-rate of 0.3V/µs. The 741 is a near-neighbor, but IMO, a 308 or OP07 is better.

Some opamps have fixed slew-rates while others are configurable or are dependent on the opamp's gain settings or current setup.

While it is not a complete list, here is a compiled list of some of the commonly used PIN-FOR-PIN OpAmp Equivalents with Slew Rates . . . .

Single Op Amp Equivalents:
CA3130 - Slew = 10V/µs (OTA)
CA3140 - Slew = 9V/µs
LF351N - Slew = 13V/µs
LF356 - Slew = 12V/µs
LM201 - Slew = variable
LM301 - Slew = variable
LM308 - Slew = 0.3V/µs
LM4250 - Slew = variable
LM709 - Slew = 0.25V/µ
LM741 - Slew = 0.5V/µs
LM748 - Slew = 0.5V/µs
LME49710HA - Slew = 20V/µs
MC1439 - Slew = ~6V/µs
MC33171N - Slew = 2.1V/µs
MCP601 - Slew = 2.3V/µs
MCP603 - Slew = 2.3V/µs
MUSES03 - Slew = 35V/µs (very expensive)
NE5534 - Slew = 13V/µs
OP07CP - Slew = 0.3V/µs
OPA134 - Slew = 20V/µs
OPA602 - Slew = 20V/µs
OPA1641 - Slew = 20V/µs
TL061 - Slew = 3.5V/µs
TL070 - Slew = 18V/µs
TL071 - Slew = 13V/µs

Dual Op Amp Equivalents:
AD712 - Slew = 16V/µs
CA3240 - Slew = 9V/µs;
CA3260 - Slew = 10V/µs;
LF353 - Slew = 13V/µs
LM358 - Slew = 0.3V/µs (@ unity)
LM1458 - Slew = 0.5V/µs
LM2903 - Diff. Comparator
LM4558 - Slew = 1.7V/µs
LM4559 - Slew = 2V/µs
LT1490 - Slew = 0.06V/µs
LM4560 - Slew = 5.5V/µs
LM4562 - Slew = ~20V/µs
uPC4570 - Slew = 7V/µs
LM4580 - Slew = 5V/µs
LM833 - Slew = 7V/µs
LMV652 - Slew = 3V/µs ~ 0.12 mA;
MAE2741 - Slew = 1.6V/µs
MC3317B - Slew = 2V/µs
NE5532 - Slew = 9V/µs
NJM4565 - Slew = 4V/µs
OPA1642 - Slew = 20V/µs
OPA2107 - Slew > 9V/µs
OPA2134 - Slew = 20V/µs
OPA2604 - Slew = 25V/µs
OP275 - Slew = 22V/µs
RC4559 - Slew = 2V/µs
TL022 - Slew = 0.5V/µs ~ 0.13 mA
TL062 - Slew = 3.5V/µs
TL072 - Slew = 13V/µs
TL082 - Slew = 13V/µs
TLE2072A - Slew = 38V/µs
TLE2072C - Slew = 45V/µs

Quad Op Amp Equivalents:
LM324 - Slew = 0.5V/µs
LT1491 - Slew = 0.06V/µs
OPA1644 - Slew = ~20V/µs
OPA4132 - Slew = ~20V/µs
OPA4134 - Slew = ~20V/µs
TL074 - Slew = 13V/µs
TL084 - Slew = 13V/µs
TL064 - Slew = 3.5V/µs
TLE2074A - Slew = 38V/µs
TLE2074C - Slew = 45V/µs

Pandora's box is now yours to play with. ;)
 
Great Stuff!

There are a few other considerations as well when choosing opamps.

Noise
High-gain circuits amplify noise and you will definitely hear a difference between opamps. It's not straightforward to rank opamps by noise because noise comes in two flavors: voltage noise and current noise. In a high impedance circuit, like the input stage of a pedal, current noise will dominate. In a low impedance circuit, voltage noise dominates. To further complicate matters, noise depends on the circuit bandwidth. Amplification stages that limit the bandwidth have less noise, but also sound darker. The first stage in a circuit has the strongest influence on noise performance because every stage amplifies the noise from previous stages.

TL072, OPA2134, OPA1642, OP275, NE5532, NE5534, to name a few, are all low noise opamps.

Input Bias Current
Input bias current is the DC current flowing into, or out of, the opamp's input pins. Not a big concern if the resistors connecting the input pins to Vref, ground, or the opamp's output pin is 100K or less. If the resistance is 1M or more, then some opamps will have DC offsets large enough to consume a significant amount of the headroom. Opamps with JFET or MOSFET inputs (TL072, CA3130, CA3140, OPA2134, etc.) have bias currents that are so low we don't have to worry about it. Opamps with BJT inputs like NE5532 are another story.

Crossover Distortion
Opamp outputs are push-pull and are usually biased class-AB. There is a transistor that sources current from the positive rail and a transistor that sinks current to the negative rail. They take turns pushing the output pin high or pulling the output pin low. Near the halfway point, they are both conducting current. That's the class-A region. As the output gets pushed or pulled far enough from center, one of the two transistors turns off and now we're in the class-B region. The transition from class-A to class-B is not perfectly smooth because the gain of the output stage changes when it moves from class-A to class-B. This gives rise to crossover distortion. Same thing happens in a push-pull tube amp. Like some tube amps, some opamps are biased such that no transistor or tube turns off. This is true class-A operation and there is no crossover distortion. It's hard to accomplish in an opamp because it requires precise biasing and matching in the output stage. I believe this is one of the reasons that OPA2134s are so expensive. Crossover distortion is most prominent at low signal levels. In dirt pedals, it's difficult to differentiate crossover distortion from all of the other distortion products. There are some tricks we can perform to keep an opamp's output in class-A mode.

Transient Response
A big part of an opamp's large-signal transient response is slew-rate limiting. Another significant part of an opamp's transient response is overload recovery, basically it's what an opamp does once its output is driven to the rails. Some opamps are slow to recover; it takes time for the opamp to get back into a linear operating region and in the meantime, the output is more-or-less stuck against the rail. Some opamps take longer than others. That's the other difference we hear when comparing opamps in a circuit like the Rat.

So what do we do about all of this?
Slew rate limiting, noise and bias current are pretty easy to analyze and the datasheets usually provide enough information to do so.

Crossover distortion and overload recovery are difficult to analyze and the datasheets usually do not provide any useful information in that regard. Simulation tools like LTSpice aren't much help because the opamp models don't properly simulate crossover distortion or output saturation. Listening tests are our best option.
 
You must log in or register to reply here.
Back
Top