Appeared in the August 2004 issue of...

Choosing Opamps, PART-2
©2004 by Eddie Ciletti

Go to Choosing Opamps and Capacitors, PART-1

There 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.

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1348 words
Click here to read PART-1 of this article that includes information on capacitors
DEVICE
description +

applications

# of

Amps

supply

current

Slew rate,

bandwidth

Manufacturer
           
INA103
Low noise, low dist, instr. amp (current feedback)

Grace Mic preamp

1
9mA ~ 12.5mA
15v/uS

Gain =1 = 6mHz

Gain =100 = 800kHz

Burr-Brown

Texas
Instruments

           
LT1358
Wideband amp, active filters and data acquisition
2
2.5mA / amp
600v/uS

25mHz

Linear
Technology

 
LT1468
16-bit accurate

distortion –96.5 dB for 100kHz

(Dan’s summing amp)

1
3.6mA ~ 5.2mA
22v/uS

90mHz

Linear
Technology

 

           
LM6172
Highs speed, low power, low distortion, video amp
2
2.3mA /amp
3000v/uS

100mHz (unity)

National Semiconductor
           
OP275
Bipolar / JFET
2
4.5mA 
22v/uS

9mHz

Analog Devices

www.analog.com

           
OPA132
High-speed FET input

Low distortion:

.00009% into 600ohms

1
4mA ~ 4.8mA
20v/uS

8mHz

Burr-Brown

Texas
Instruments

OPA627
Precision High-Speed DiFET

DAC output amp

(Dan’s sum buffer insert send)

1
7mA
55v/uS

Gain = 1 = 16mHz

Burr-Brown

Texas
Instruments

 

OPA2604
Dual-FET, low distortion

DA "I/V" conversion

 
6mA / amp

12mA total

25v/uS

Gain = 100 = 20mHz

Burr-Brown

Texas Instrument

OPA2227
High precision

AC and DC performance

DC Servo amp

2
3.7 / amp
2.3v/uS

8mHz

Burr-Brown

Texas 
Instruments

 

           
THS4012
Very High Speed + Low dist amp

(voltage feedback)

video

2
7.8mA / amp
310v/uS

Gain = 1, 290mHz @ -3dB

Gain = 1 70mHz @ -.1dB

Texas 
Instruments
THS4052
Low cost, high speed

Voltage feedback amp

video

2
8.5mA /amp
240v/uS

Gain = 1 = 70mHz @ -3dB

Gain = 1 = 30mHz @ -.1dB

Texas 
Instruments
THS4062
High Output drive

Voltage feedback amp

video

2
 
400v/uS

Gain = 1 = 180mHz @ -3dB

Gain = 1 = 70mHz @ -.1dB

Texas 
Instruments
TABLE-1: A short list of modern opamps