Monday, September 14, 2009

An Op-Amp IC For Your Hi-Fi Needs?

Given that they’ve been used successfully in a number of excellent sounding hi-fi applications, is there really a right op-amp for your audiophile needs out there?


By: Ringo Bones


Yes it’s true, there really is such a thing as an audiophile grade integrated circuit operational amplifier or IC op-amp. And most of them are not necessary manufactured by Analog Devices like the AD845 and AD843. Or those by Burr-Brown which their dual op-amps that are specified to be fast enough to handle the RF energy present in Red Book CD digital to analog conversion are often used in bridge configuration in left / right analog outputs.

The quest for finding the best off the shelf IC op-amp probably started during the early 1990s. When major CD player manufacturers discovered – either by theoretical introspection or trial and error – that those high-speed op-amps made their 500 US dollar or so CD players sound closer to entry-level audiophile grade vinyl LP replay.

From the electronic engineer’s design standpoint, high-speed op-amps are a necessity in Red Book specification CD players. Sufficient slew rate ratings are a necessity to handle the quite large amounts of ultrasonic requantization noise - which is an unfortunate by-product of converting your 16-bit 44.1-KHz digital data into a reasonably smooth analog waveform that could sufficiently past muster as music. In my experience with the most widely used up-market “hi-fi” op-amps – namely the LM318 and the LF356 – which have very different personalities when used in an audiophile context. Although I used audiophile grade ceramic IC sockets with silver connectors to allow me to easily replace both op-amps for comparison.

Over the years, my countless experiences with the high-slew rate (50 volts per microsecond) LM318 suggests that this IC op-amp is well suited to audiophiles who like to listen to Classical Music - Or wants to reproduce the recorded hall acoustic of an opera recording accurately played back in his or her listening room. It even enhances – or exaggerates – the Classical Music-like hall acoustics of some tracks of The Gathering’s “How To Measure A Planet?” album.

One drawback of the LM318 op-amp though is that it doesn’t like very much the “relatively” high-capacitance interconnects often used in entry-level solid-state audio gear. Like Monster Cable’s mellow sounding M850i interconnect often used to tame the harshness of cheap solid-state audio systems. Resulted in a high-pitched squealing sound on rare occasions (guaranteed more than once) during turn on. Although easily remedied by turning off and turning on again your entry-level solid-state amp.

Even though from a technical standpoint, the LF356 has a much lower slew rate rating (12 volts per microsecond) in comparison to the LM318, it does audiophile-oriented things that the LM318 can only aspire to. The very high input impedance – about 1 trillion ohms - of the JFET input stage of the LF356 allows it to have a bass response that Rock Music aficionados since the time of Elvis strive for. The LF356 is also capable of driving large capacitive loads – up to 10,000 picofarad or 0.01 microfarad – with ease. Which makes it more suitable for driving high-capacitance mellow sounding interconnects used in entry-level solid state gear.

Sound quality wise, it is as if the designer of the LF356 op-amp want it to sound like what recording engineer Andy Johns wants the first four album of Led Zeppelin to sound like – i.e. the “John Bonham snare sound”. The LF356 also sound as if it is the first op-amp with a very musically ideal loudness control. It defeats the Fletcher-Munson contour curve characteristic of the human ear that makes us less sensitive to the bass and treble frequencies when listening at reduced sound volume levels. With the LF356, you’ll get the full works whether you’re playing at 65dB or 95dB sound pressure levels - not unlike the sound of Electro-Harmonix versions of 12AX7 preamplifier tubes.

Surprisingly, the LF356 does room sound too - Although not like the Classical Music concert hall portrayed by the LM318. The room sound produced by the LF356 is the “normal” unadorned type – typical recording studio or just a spacious venue. The LF356 also has better low-level sound retrieval in comparison to the LM318 because the LM318 tends to exaggerate the dynamic range of CDs that are recorded without Tom Lord-Alge levels of compression. Like Lunachick’s Binge and Purge album which the LF356 still manages to retrieve low-level details that are played back even softer by its higher slew rate counterpart.

Both can still benefit from a well-regulated plus and minus 15 volt split supplies though, given the inherent RF corruption of our contemporary power lines. Despite both IC op-amps often rated with a power supply rejection ratio of over 100dB at 50 to 60-Hz AC. Boutique capacitors like Rubicon Black Gate capacitors or Philips sourced French Blue capacitors also help improve sound quality to no end.

So there you have it, two op-amps that I have extensive experience with that could past muster as being audiophile certifiable. Although it is somewhat over simplistic to conclude that one prefers Rock, while the other op-amp prefers Classical. The sound quality of one is sufficiently different from the other that it is worth noting. Although the LF356 also has a gorgeous presentation with Orchestral Classical Music recorded during the Golden Age of Stereo.

3 comments:

Sisto said...

When it comes to entry-level Monster Cable interconnects, I think they should change / upgrade their "too tight" turbine RCA connectors because they tend to break / destroy the RCA jacks of entry-level (cheap?) equipment that these "value-for-money" interconnects often end up being paired to.
Monster Cable interconnects are known for their relatively high-capacitance, the LM328 high slew rate op-amp probably reacted unfavorably with them. In my experience, you're better off using JFET input LF356 IC op-amps. Surprisingly, mainstream audio manufacturers like Sony don't use the LF356 - as far as I know.

VaneSSa said...

I hope that you're not one of those folks who often confuse the DIY supplement of Hi-Fi World or the Audio Note catalog with Penton Publication's Electronic Design - or even the 1960s era Wireless World. Don't get me wrong, those Red Book spec CD players that are starting to sound like good old vinyl LP playback during the 1990s probably owe more to those high slew rate IC op-amps and Delta-Sigma DAC chips being scrutinized in Electronic Design magazine as the next best thing a few months after Operation Desert Storm.
You may like the high slew rate LM318 IC op-amp but it still can't beat my Michell Argo / Alecto combo. Just try using Veruca Salt's Loneliness is Worse from their Eight Arms to Hold You album. The Michell Argo / Alecto combo really captured the emotion of Nina Gordon's delivery on this particular song.
I do agree that the LF356 is more musical than the higher slew rate LM318 IC op-amp. THe LF356 does better portray the John Bonham snare sound of early Led Zeppelin. Probably due to the "lowly" op-amp's ability to drive difficult capacitive loads - including the Monster M850i interconnect cable.

Laurentz said...

The LM318 high slew rate operational amplifier was also used on the Musical Fidelity A1 amp.