Appeared in the August 2004 issue of...
Choosing Opamps, PART-2
©2004 by Eddie CilettiThere are no black and white answers to your upgrade questions. Even identical circuits will behave differently depending on the construction (circuit board layout) and components used. To paraphrase an old saying, learn to fish and you’ll answer your own questions. I encourage experimentation, but, you really need an oscilloscope to keep an eye on things.
For simplicity, opamps can be distilled into three application-specific categories — high gain preamp, general-purpose buffer and line driver (output amp). Opamps aren’t supposed to have "color," the output signal should be the same as the input signal with the least amount of additional artifacts. All opamps are designed to have low distortion, low noise and wide bandwidth — some turn out better than others.
PACKAGING
For our purposes, IC opamps typically come in two packages — 8-pin "DIP" for single and dual opamps, 14-pin "DIP" for quad opamps — that are easily socketed. Many chips are also available in the surface mount (SMT) package. Because the pin-outs are pretty much standardized, the interchangeability factor is a tempting tease that implies plug-and-play upgrade potential but comes with the warning "proceed with caution." With the Internet providing easy access to data sheets as well as a deluge of opinions, the only question left is "how to choose?"
The specs are mind numbing at first. With names like Open-Loop Gain, Bandwidth, Distortion, Noise, Slew Rate, DC Offset and Settling Time, some are tangible and some are not. Gain is a variable that effects each of these parameters and one more, Stability. Increased Gain and signal levels raises distortion and awareness of noise while decreasing bandwidth. What starts out as several mega-Hertz (mHz) might be reduced to the upper kilo-Hertz (kHz) when the gain is 100 times the original signal.
TAMING THE COWBOYS
In this age of kissing digital zeroes for fear of losing resolution, one short-term fix for what might seem like "bad analog sound" is to back off, take advantage of headroom and let noise be your dither. Seriously, no amplifier sounds its best when pushed toward the rails except when you are specifically looking for distortion. Opamps are not internally designed to emulate guitar amps.
It is nearly impossible to resist the temptation to compare specs. Yes, a modern opamp boasting a slew rate of 20-volts-per-microsecond (v/uS) is better than 30 year old, 2v/uS technology, but that doesn’t imply that 2000v/uS is going to be as noticeable an improvement. Such demon-speed may be a gift to those in search of "effortless sonics," but taking advantage of video-agile ICs is not a plug-and-play upgrade. That’s the domain of people with RF skills.
There are plenty of vintage consoles out there "limping" along at 2v/uS that just happen to sound amazing for that old-fashioned rock ‘n roll. You might prioritize your upgrade investigation by targeting the preamp ICs or, if outboard preamps are plentiful, focus on the mix buss and master module as Dan Kennedy did for his Trident 65 upgrade (featured in a Tech’s Files column in 2000).
DESIGNER CIRCUITRY
Despite the ease of ICs, some designers still choose discrete circuitry. Rather than initially obsessing over specs, their goal is simply to realize an idea in hardware. Each person has tests that weed out problems and assist in maintaining high standards — a type of "headroom" that translates to reliable performance in the field under adverse conditions. And yes, many designers do not have engineering degrees. This should be encouraging to all you "tweakers" out there. Now get to work! You’ve got some catching up to do.
When I asked Crane Song’s Dave Hill for an example, his discrete output amplifier had to pass an "acid" test of driving a 100kHz square wave into a 75-ohm load. If it still looks like a square wave and remains stable, the circuit is a winner. In this case, a square wave reveals an amplifier’s ability to drive a capacitive load (a potential cause for instability), which is what 1,000 feet of cable looks like to a piece of audio gear.
Dan Kennedy is using discrete amplifiers in the AC signal path and ICs in the DC servo loops for his new EQ-2NV, a vintage Neve inspired Equalizer. DC offset and DC stability (settling time) are two issues that can reach a critical mass in high Gain applications (in a mic preamp, for example). Such idiosyncrasies reveal themselves in funny ways — like scratchy pots and switches — and oscillations.
DC offsets are measured in milli-Volts (mV) and hopefully micro-Volts (uV), a rather intangible specification compared to the brute force transistor stages of the original single-ended Class-A Neve designs. The output amps are biased to operate around 12-volts (half the power supply), so blocking caps are used throughout. If a capacitor becomes leaky (develops a parallel resistance) DC from one stage can re-bias another stage to the point of compromising headroom or worse, nasty distortion. Similarly, a transient pumped through a Neve amplifier stage will typically bump all of the bias points. With an o’scope (wink-wink, nod-nod) you can easily watch the recovery, a snails crawl compared to the needs of Digital audio converters that rely on amplifiers with fast settling times (on the order of a few hundred nanoseconds).
At the other end of the spectrum, Jim Williams of Audio Upgrades typically chooses video op amps for audio applications. It is a different philosophy aimed at improving existing equipment and not recommended for your first DIY. I recently repaired a mic preamp in a Fostex portable DAT recorder, substituting an OPA2604 for an NE5532. You might view this as an upgrade, but I saw it — as some engineers also see it — as using what was on hand. This is a somewhat less romantic than some would imagine — Audio is not Fashion — designers know what works and what doesn’t and where the laws of diminishing returns come into play. You should know that the OPA2604 was totally unstable without additional de-coupling — a challenge in a cramped, surface mount village.
Supply Voltage Range, not detailed in this article, is one parameter that is self-explanatory. Here’s a short list of popular modern IC opamps along with a few new chips at the bottom. For more details, go to the manufacturer’s website, download the full data sheet and happy reading.