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One Slew Over the Cuckoo’s Nest… and other quippings about Slew Rate

As we continue our blaze down the path of discovery, we’re frequently tasked with the laborious effort of comparing and contrasting spec sheets and running tests for various components in applied circuits, in search of that hidden “Holy Grail” component that outperforms all the rest. Then, when our eyes are glowing two shades of blood-shot red from near endless nights of reading notes and spec sheets and staring without blinking at a computer screen as the Audio Precision test equipment spits out it’s report, we’re often left with a pile of data that still means relatively little to us until we ultimately plug the gear in and listen to it. Occasionally, we’ll find something that truly stands out above the rest, with an heir of “almost too good to be true” clouding our minds with suspicion. It behooves us to dig deeper for truth, even as we case the joint for any Nazis that may have snuck in to try and rob us of our hard-fought quarry (did I mention we recently moved into an abandoned Aztec temple teaming with booby traps). So we tend to keep things under lock and key for some time until a suitable amount of trial runs and beta-testing clear our conscience enough to venture releasing such sacred and forbidden knowledge into the world.

One thing that has blown our head-gaskets of late, is the elusive topic slew rate and our studies into its impact on audio signal processing. Slew rate is defined as “the change of voltage or current, or any other electrical quantity, per unit of time.” In other words, it’s the measure, in volts per second, of how quickly an electronic component can react to and re-produce a shift in voltage for an audio signal that’s being passed through it. Typically, a faster slew-rate represents a higher fidelity and a more accurate recreation of the original source audio that’s being processed through the component or circuit. However, in application, slew-rate isn’t always maintained with results you would expect. And in general, the spec is widely misunderstood by the consumer market. Thus, we felt it might be time to visit this topic for the sake of steering customers in the proper path regarding modifications.

For the sake of this blog post, we’ll draw our attention to the TL072, which is a low-noise dual JFET-input operational amplifier available from Texas Instruments. This op-amp has been a work-horse in the world of solid-state signal processing since the late 70s. It’s been used in many applications from mic pres to summing mixers to guitar pedals to amplifiers to you name it. It boasts, a slew-rate of 13 V/μs (volts per microsecond), meaning it can effectively recreate a shift from 0 volts to up to 13 volts (on average) per microsecond (1 one-millionth of a second). The human ear has been documented to possess a hearing range of approximately 20Hz to 20kHz, which means it can react to and discern sound waves that measure 20 cycles or oscillations per second to up to 20,000 cycles per second. More realistically, the spectrum is slightly narrower in range for some people only going up to between 15kHz and 18kHz. The range tends to meet its upper limit sooner as the aging process and hearing loss from overexposure to high sound-pressure levels (SPLs) reduces our hearing sensitivity over time (but all those KISS concerts and excessive jamming in the garage with amps dialed to “11” were probably worth it... not to mention one time you took your dad’s .44 magum out for target practice and forgot to wear hearing protection). So, for every 13 volts of swing the TL072 is capable of reproducing within the interval of a millionth of a second (1,000,000 Hz or 1MHz), the human ear is only really able to discern a small portion of the chip’s processing capability (roughly 1/55th based on a 15kHz to 18kHz average range). It’s largely subjective as to how much we perceive otherwise as our body and central nervous system may perceive on a broader spectrum that’s not entirely understood. Not to mention, it’s also likely the amplifier and loudspeakers or headphones being used to reference said audio are limiting the picture way before that even...

Realistically, the rest of the circuit really can’t be forgotten, as we find more often than not, an op-amp that clocks a faster slew rate will often introduce high-frequency oscillations, often ultrasonic or above the normal human hearing range. These oscillations may also be termed Radio Frequency (RF) oscillations as they occupy a range occupied by radio wave communications (20kHz - 300 GHz). Regardless of what you call these oscillations, they not only pollute the signal pathway with noise but eat up precious headroom that lowers your mix ceiling and brings unwanted color from distortion into the equation. In fact, most circuit designs with op amps that create issues like this, must feature what’s known as compensation capacitors in the feedback loop between the op amp output stage and one of its differential inputs. These compensation caps, running in parallel with a feedback resistor in the loop, create what’s known as an resistor/capacitor low-pass filter (R/C LPF).

As most of you may know, a low-pass filter (LPF) effectively filters or suppresses frequencies above a certain threshold, while allowing all other frequencies below the threshold to pass by unhindered. This is much like a high-cut (low-pass) filter in an equalizer circuit or plug-in. The actual frequency threshold in the LPF is largely determined by the value of the capacitor, as capacitors introduce capacitive reactance into the equation. Capacitive reactance is the measure of a capacitors opposition to alternating-current (AC) which comprises the audio signal. This reactance is also a function of frequency, meaning that depending upon the capacitive value (in Farads), the relationship with allowed frequencies and opposed frequencies may vary. In other words, compensation caps are meant to tame the oscillating beast and make it more presentable at the “dinner table.” They filter and suppress unwanted oscillations like those caused by a faster slew rate. Usually, the smaller the cap value, the higher the frequencies it reacts to and opposes. In the example pictured, they’re using a 100 nano-Farad cap in parallel with the 100kohm feedback resistor. Unfortunately, in the process of taming the beast, they also tend to negate most of the benefits of a faster slew rate as the high-frequency content the chip may be capable of reproducing due to fast slew rate, can never go higher than what the R/C LPF is filtering off. Not to mention, these R/C LPFs also negatively affect things like phase response. Phase response, which is a function of frequency, largely has to do with how accurately an audio processor can reproduce signals from input to output without letting one range of frequencies slip out-of-phase and end up sounding too “loose” or “flubby” (these are scientific terms… at least I didn’t say “too warm”). This has a big impact on our perception of transients. So you start trying to tie bows around things to make it sound prettier, you may end up strangling something else. All us first-time parents and/or dog-owners have to learn that lesson the hard way, but our later children and pets will thank us for it (I’m joking, of course). Luckily, the one thing adding a compensation cap or R/C LPF will improve is your headroom, because those high-frequencies, whether audio or not, can still consume precious current that depletes your amp circuit of headroom, and distortion may not be too far off.

All that being said, a fast slew rate may sound ideal, but it’s not always going to make a world of difference you think it will, and it may even hurt your overall specs unless you know how to balance things out by making additional changes. In general, we’ve found that unless a circuit is specifically designed around a fast-slew rate chip, with the intent of reducing the RF oscillations without tossing the benefits out the window, it’s hard to warrant using a faster chip that’s going to break your bank to get there. I mean, it’s kind of like building a rocket car capable of breaking the sound-barrier, but then putting a governor on the engine that limits its speed to 55 mph, and then restricting it to public roads where the speeds are even more strictly enforced. Now you almost know how Batman feels, sans the emotional anguish of losing your parents to cold-blooded murder amidst addrenaline fueled attemps to curb those mood swings through masked, viligante justice. But, hey! I’m more of a Spider-man guy myself...

There’s lots to consider… so you must carefully consider these options before making your choice in designs and/or modifications, otherwise the aging knight in the temple may say of you:

He probably couldn’t hear too well through that chain-mail cowl anyway, but it’s always best to defer to the judgement of elders when forging a path of discover through a forbidden temple.