Category Archives: Op Amp

ADA4084: Can it get better than this?

      Electronics, Modula, Op Amp

Can it get better than this? Oh, I’m sure it will! I’m pretty sure we’ll see continuous improvements in Op Amp performance over the years. It is 2017 and we’ve come a long way since the early days of the Fairchild µA702 op amp, created in 1964 by Dave Talbert and designer Robert Widlar, and then a few years later: the μA741, also from Fairchild —the most popular op-amp of all time. I fondly recall the smell of solder (oh my… lead!) and a drawer full of LM741 DIP ICs as I was learning about these magnificent Op Amps with the Art of Electronics book at my side.

Check out our  Op Amp Shootout page for more Op Amps. If you need some background information, I invite you to read the series of articles by Douglas Self (EE Times) Op amps in small-signal audio design (part1part2part3part4).

So… I’m always on the lookout for low-power, low-noise Op Amps. Last time, we had a look at OPA1692. With a 4.2-nV/√Hz noise density and only 650 µA of supply current per channel, it is hard to beat. Alas, it is not available in singles (only duals and quads), which makes it a non-contender for the Modula pickups.

So, the search continues and as I moved outside my typical search area, I discovered a splendid Op Amp from Analog Devices —the ADA4084:

FEATURES

  • Rail-to-rail input/output
  • Low power: 0.625 mA typical per amplifier at ±15 V
  • Gain bandwidth product: 15.9 MHz at AV = 100 typical
  • Unity-gain crossover: 9.9 MHz typical
  • −3 dB closed-loop bandwidth: 13.9 MHz typical at ±15 V
  • Low offset voltage: 100 µV maximum (SOIC)
  • Unity-gain stable
  • High slew rate: 4.6 V/µs typical
  • Low noise: 3.9 nV/√Hz typical at 1 kHz
  • Long-term offset voltage drift (10,000 hours): 3 µV typical
  • Temperature hysteresis: 4 µV typical

The ADA4084-1 (single), ADA4084-2 (dual), and ADA4084-4 (quad) are single-supply, 10 MHz bandwidth amplifiers featuring rail-to-rail inputs and outputs. They are guaranteed to operate from +3 V to +30 V (or ±1.5 V to ±15 V).

These amplifiers are well suited for single-supply applications requiring both ac and precision dc performance. The combination of wide bandwidth, low noise, and precision makes the ADA4084-1/ADA4084-2/ADA4084-4 useful in a wide variety of applications, including filters and instrumentation.

Wow! 625μA and 3.9 nV/√Hz!!! Is this it? We’ll see… I have a few samples coming and I’ll share our test results.

Aside: At 1mA, you get the ADA4807 3.3-nV/√Hz noise density with vanishingly low distortion (−141 dBc/−144 dBc at 1 kHz). A fair tradeoff, I would say.

 

OPA1692: The perfect Op Amp for low power audio?

      Electronics, Modula, Op Amp

Our Op Amp Shootout page has always been the site’s top post. I’m always on the lookout for new and better Op Amps I can use in our products and I am continually updating the page as new products come along.

I’m never satisfied with the current offerings. Typically, you either have ultra-low noise OR very low power, but not both. Somehow, these are conflicting requirements. See the Shootout link to understand what I am talking about. For benchmark, in terms of low-noise performance, anything close to the 5532 (and its sibling, the 5534), with 5nV/√Hz noise figure (3.5nV/√Hz for the 5534) is wonderful. But the 4mA quiescent current consumption of these audio Op Amps is unacceptable for low power operation, especially when using batteries.

For some background information, I invite you to read the series of articles by Douglas Self (EE Times) Op amps in small-signal audio design (part1part2part3part4).

For applications requiring batteries, like that ubiquitous 9v battery found in typical active guitar electronics, I’m always searching for Op Amps in the 500μA range. My current favorites are 1) NJU77806, from JRC New Japan Radio Co., LTD, with a noise figure of 5.5 nV/√Hz at 1kHz while consuming only 500μA. But the NJU77806 can only go up to 5.5v supply, making it a nonstarter for 9v operation. So, for low power, my other favorite is 2) OPA180 from Texas Instruments, with a quiescent current: of 525 μA (max), but with a noise figure of 10 nV/√Hz at 1kHz. Looking at that spec, it’s not quite at par with the 5532 (or the NJU77806), but in reality, with very low 1/f noise, the 180 performed quite admirably in our noise tests. The 180 can operate from 4.0v to 36v. I’ve always wanted to release a very high headroom preamplifier for our pickups.

Now, there’s this new kid in town: The OPA1692 – SoundPlus Low-Power, Low-Noise, High-Performance Op Amp from Texas Instruments. Hey, Low-Power AND Low-Noise! Have your cake and eat it too?

Features

  • Low Noise: 4.2 nV/√Hz at 1 kHz
  • Low Distortion: 0.000045% at 1 kHz
  • Very Low Quiescent Current: 650 µA per Channel
  • Slew Rate: 21 V/μs
  • Wide Gain Bandwidth: 5.7 MHz
  • Unity-Gain Stable
  • Rail-to-Rail Output
  • Wide Supply Range: ±1.75 V to ±18 V, or 3.5 V to 36 V
  • Dual and Quad Versions Available
  • Small Package Sizes: Dual: SOIC-8, VSSOP-8, SON-8 Quad: SOIC-14 and TSSOP-14

The OPA169x operational amplifiers achieve a new level of performance for low-power amplifiers with a low 4.2-nV/√Hz noise density and distortion of 0.000045% at 1 kHz. These op amps offer rail-to-rail output swing to within 150 mV of the power supplies with a 2-kΩ load, which increases headroom and maximizes dynamic range. These devices also have a high output drive capability of ±50 mA. The OPA169x operational amplifiers operate over a very wide supply range of ±1.75 V to ±18 V, or 3.5 V to 36 V (on only 650 µA of supply current per channel), are unity-gain stable, and provide excellent dynamic behavior over a wide range of load conditions.

The OPA169x family of amplifiers uses a propriety technology to reduce signal distortion that does not increase the power-supply current. The distortion cancellation technique reduces odd-order harmonic distortion, which is produced by the input transistor pair of the amplifier.

I’m getting some samples for testing as soon as they are available!

 

Op Amp Shootout Updated

      Electronics, Op Amp

Time flies! It’s been almost 4 years since I last updated our Op Amp Shootout page. Now It’s 2017, and this page is still our top hitter. I guess that only means that people find this page helpful. And I am always happy to share information. I’ve updated the page to reflect recent developments. I always have a keen eye for new Op Amps and the new additions (shown in light purple) were taken straight from my notebook.

There are some interesting additions such as the OPA188 with a wide supply range of 2V to 18V, and very low 500μA current consumption. I also added a bunch of very Op Amps from Japan with rather respectable specs. Check out the NJ series Op Amps from JRC New Japan Radio Co., LTD. I found out about these cool Op Amps from studying Roland schematics. Have a look at the NJU77806, for example, with a noise figure of 5.5 nV/√Hz at 1kHz while consuming only 500μA.

There are also retirees. For example, the ultra-low noise LME49990 has reached its end of life. But there is certainly a new audio Op Amp king of the hill: the LMH6629 with a super impressive 0.69 nV/√Hz at 1kHz noise figure! The LMH6624 takes second place with very respectable 0.92 nV/√Hz at 1kHz noise figure.

Interesting tidbit: In that page, Mark Norton commented: “I feel sorry for all of you using op amps. Sterile squinching of sound imho.” He’s obviously not a fan, but then I had to reply: “I feel sorry for you believing such myths :-)”. Op Amps: Myths & Facts. The funny thing is, all forms of modern recorded music would have gone through dozens if not hundreds of Op Amps in the signal chain, through the recording process (EQs, compressor, limiters, etc.). If “sterility” means not degrading the purity of the sound (e.g. preamps), then I suppose that is good. But “transparency” is a better term.

Now go and check out the updated Op Amp Shootout page.

Op Amp Shootout Updated

      Electronics, Op Amp, Pickups, Six-pack

Our Op Amp Shootout has been updated to reflect recent research and development. The previous favorites are now replaced by better choices and there are more choices to choose from depending on requirements from highest performance and highest power, to moderate performance and low power.

Check out the the Op Amp Shootout page. The page is continually being updated. The Op Amp table is maintained as a linked Google Doc Spreadsheet.

Let it breathe (part 3): Frequency Response

      Benchmark, Electronics, Op Amp, Pickups, Six-pack

This is part 3 of the “Let it breathe” series. For proper context, you might want to check out Part1 and Part2. This third and last part of the series concludes with actual frequency response measurements for various coil configurations of the Hexaphonic Pickup Project and with the Fender Stratocaster middle pickup as benchmark to compare against. In addition to standard windings with 500, 1000, 1500, 2000, 2500 and 3000 turns AWG 42, we also tested bifilar windings (see Alternative Ideas) with 500 and 1000 turns AWG 42. Like before, the low-power LME49726 OP Amp is used in the differential preamp stage with a gain of 5.

Continue reading

Singles or Doubles?

      Electronics, Op Amp, Pickups, Six-pack

 

Humbuckers

dimarzioInvented independently by Joseph Raymond Butts and Seth Lover, the humbucker pickup “bucks” hum. It is ingeniously designed to passively cancel noise by having two coils with the magnetic poles reversed (i.e. the first coil has its north at the top while the other coil has its north at the bottom) and with the coils wound in opposite directions (one clockwise and the other counterclockwise). By connecting the coils together either in series (typical) or parallel, the common mode signals (signals common to both input coils —the noise) end up 180° out of phase and cancel out. The coils can either be placed side by side (typical) or stacked (to fit single-coil guitars such as the Fender Stratocaster).

Continue reading

Let it breathe: Frequency Analysis

      Benchmark, Electronics, Op Amp, Pickups, Six-pack

The following FFT graphs show the harmonic spectrum of a couple of wave samples taken from:

  1. A Fender Stratocaster with the middle pickup
  2. A Gibson Les Paul Studio with both pickups on
  3. The same Strat with the hex pickup between the middle and bridge pickups
  4. The same Les Paul with the hex pickup in the middle

The samples were taken from a high impedance audio input port direct to Logic Pro. The samples were recorded raw (no EQ, no effects). All samples are normalized. Currently, I sampled only the E string. I like the heavy E string with rich harmonics. For fun, I also added the FFT graph of a Bösendorfer from the stock Logic Pro sample collection.

Note: This is the initial test and is continually being updated. See Let it breathe (part 2): More Frequency Analysis and Let it breathe (part 3): Frequency Response for latter installments.

test-lpThe test pickup in this experiment has 2000 turns AWG 42 magnet wire with about 260 ohms D.C. resistance. The differential Op Amp has a gain of 5 and generates a peak-to-peak voltage of around 2 volts (moderate pick attack). For this early prototype, I am using the plain TL072 Op Amp. Already, I am quite pleased with the very low noise even without any shielding at all (image at right). Next time, I’ll have some more graphs comparing the noise floor of the hex pickup compared to the passive humbucker and the very noisy single coil pickup. I also intend to experiment with better Op Amps and even lower impedance coils. This initial analysis will serve as a good benchmark for comparison.

The 60 Hz hum and its harmonics is quite evident in the Stratocaster spectrum. Interestingly, the same 60 Hz (and 120 Hz) shows up as a smaller bump in the Les Paul graph. The so-called humbuckers do not fully cancel hum due to the irregularities in the construction of both coils. It is impossible to manufacture perfectly matched coils. Any irregularity in construction will degrade the common mode rejection of the humbucker. Differential amplifiers have better common mode rejection and are easily balanced by using matched resistors with 1% or even 0.1% tolerance.

As can be seen in the graphs, the lower impedance test pickup is very rich in harmonics. It has a lot more harmonic content than even the bright, but very noisy Stratocaster pickup. The difference in harmonic content between a 24¾” scale Les Paul and a 25½ scale Stratocaster is also quite pronounced, even when using the same test pickup on both. Most of the higher frequencies in the FFT graph of the Stratocaster is just the noise from electromagnetic interference.

Obviously, all guitar samples here don’t come quite close to the rich harmonic content of the Bösendorfer with its long and heavy hammered strings. But so far, I like the tone and clarity of the hex pickup. Leo Pedersen, at sevenstring.org comments: “Nice sparkly clean sounds”. I like that characterization 🙂

strat

Fender Stratocaster

lp

Gibson Les Paul Studio

Hex on Stratocaster

Hex on Stratocaster

hex-lp

Hex on Les Paul

piano

Bösendorfer Grand Piano

New stuff from Digikey

      Electronics, Op Amp, Pickups, Six-pack, Workshop

digikey-stuff

New stuff from Digikey. We ordered a bunch of precision SMD resistors (1% and 0.1% laser-trimmed), a bunch of low-power, low-noise Op Amps, gathered from this table here, plus a bunch of general purpose low-power cheapos for non-critical applications. We will need these for prototyping. The precision resistors are needed to have good common-mode rejection ration (CMRR) for the differential amplifiers.

The one at the upper left is C5000, ultra-low-power DSP Booster Pack for the MSP430 launchpad from TI (below). The Booster Pack contains a TMS320C5535 fixed-point DSP chip with two multiply-accumulate (MAC) units capable of up to 200 Million Multiply-Accumulates per Second (MMACS) and a tightly-Coupled FFT Hardware Accelerator. At $10 a piece (per 100 at Digikey), these DSP elves are very capable.

ti-dsp

Choosing a good Audio Op Amp

      Electronics, Op Amp, Pickups, Six-pack

Question: What’s a good Op Amp for audio? The answer: depends. There are many factors to consider and there’s no single best answer. There was a time when I was quite happy with the cheap and simple TL072 for most tasks. If I needed something better, then there’s the more expensive (at the time) 5532. It’s notable that after all these years, these critters are still quite capable…

Note: this blog entry has been superseded by the Op Amp Shootout page. The page is continually being updated.