Reviewers' ChoiceIn audio, a clock determines the times at which digital audio samples are converted into analog. Messing up this timing condemns the music to digital hell, where jitter causes distortion and harshness in music. Thomas Hobbes said that “Hell is truth seen too late.” For audio purposes, I suggest that hell is truth seen too early or too late.

Think back to the days of hand-cranked film viewers. Because it was extremely difficult to achieve a constant playback speed, the films looked jerky, unnatural, jittery. A similar phenomenon occurs when the conversion of a digital to an analog signal is imperfectly timed.

Clocks can be found in most digital audio components. Like the subject of this review, Stanford Research Systems’ PERF10 Rubidium Audio Clock ($3495 USD), such clocks can also operate as a single external “master” clock that controls all of the digital components of a home or professional audio system.

Stanford Research Systems PERF10

Interestingly, the PERF10 aside, SRS doesn’t make audiophile components. Rather, it makes a wide range of test instruments for research laboratories and various industries, including programmable temperature controllers, time-interval and frequency counters, melting-point apparatus, nitrogen lasers, RF signal generators, LCR (inductance) meters, and residual gas analyzers, to name just a few.

But SRS manufactures the PERF10’s key internal component: its PRS10 rubidium-disciplined oscillator. The word on the street is that SRS sells that oscillator to a number of high-end audio companies, which use it in their expensive master clocks.

Thinking that perhaps I’d stumbled on a hidden gem, I contacted SRS and obtained a review sample of the PERF10.


“Our clocks do not measure time. Time is defined by what our clocks measure.”
-- Worker at the National Institute of Standards and Technology

Quartz-crystal oscillators are often used to keep time in digital watches and clocks, and to provide a stable clock reference for the digital circuits in radios, cell phones, computers, and the like. Quartz is used in these devices because it is piezoelectric: If you squeeze it, an electric current is generated. The opposite is also true: If you pass electricity through quartz, it oscillates (i.e., vibrates) at a frequency of precisely 32,768 times per second. In a quartz watch, for example, a battery sends electricity to the crystal, causing it to oscillate at the above frequency. A sensor counts the number of oscillations and uses them to generate electric pulses, one per second. These pulses either power a numerical LCD or LED display, or drive a motor that turns the gears that spin the clock’s hands.

Quartz-crystal oscillators are typically inexpensive, and most of them have poor frequency stability and high phase noise -- that is, fluctuations in the phase of a signal’s waveform. Associated with jitter, phase noise is a killer for reproduction of high-end digital recordings.

Atomic clocks are much more complex than clocks that use quartz oscillators, and display much better frequency accuracy. Atomic clocks contain crystal oscillators made not of quartz, but of chemical elements such as rubidium-87 or cesium-133. They are used for demanding applications such as frequency stabilization for television and cell-phone signal transmissions, laboratory testing equipment, and global satellite navigation systems. If, for example, the frequencies of telecommunications switches vary even slightly, serious performance degradation can occur.

Cesium-disciplined crystal oscillators, which are even more accurate than rubidium ones, are not used for audio, and start at about $250,000 each. A clock with a cesium oscillator at the National Institute of Standards and Technology, in Boulder, Colorado, is used as the US time standard. It has been found to be much more stable than the earth’s orbit.

Rubidium oscillators, much smaller and less expensive than the cesium variety, are used for audio. In extremely simplified form, a rubidium lamp, which produces a 6.83GHz light (the time period of a transition between two specific hyperfine energy levels of rubidium atoms), is passed through a chamber of rubidium gas. It is then sampled by a photodetector, which measures photon flux by converting the energy of the absorbed photons into a measurable form.

A 6.83GHz RF signal is then applied to the chamber, and its frequency tracks higher and lower until the photodetector senses a decrease in the amount of transmitted light. At that point, the RF wave's frequency is identical to that of the electron transition between two specific hyperfine energy levels of rubidium atoms. When the frequency is locked via a phase-locked loop (PLL) to match this decrease, it is less than one part per billion within the frequency of rubidium, which is very accurate.

While the above may be lost on most audiophiles, the take-home is that the accuracy of a rubidium clock is governed by the natural resonance of rubidium atoms.

The PERF10 features SRS’s own PRS10 rubidium crystal oscillator. SRS cuts this crystal on two axes using a process called SC-cutting. Much more expensive than typical cutting methods, in which a crystal is cut on only one axis, this process results in a precisely cut crystal that exhibits extremely low phase noise.

SRS then places the rubidium oscillator in a tiny oven that maintains a constant temperature of slightly over 200 degrees Fahrenheit. Without such an oven, SRS would have to use an oscillator that works over a wide temperature range but displays significant frequency instability.

While most atomic clocks that use rubidium oscillators are very accurate, they generally don’t have extremely low phase noise. This, SRS states, is what makes the PERF10 ideal for high-end audio.

The PERF10 came well packaged, and unboxing it revealed a component of utilitarian fit and finish. Certainly, the PERF10’s case will win no awards from those who prefer beautifully sculpted, museum-quality components. Also, the silk-screening of the front panel is a bit rough, a shortcoming apparent only from distances 2’ or less. The SRS weighs ten pounds, and comes in a black, rack-mountable case made mostly of cold rolled steel and measuring 19”W x 2.5”H x 10.125”D. On the front panel is its only control: a rocker switch, labeled Outputs, that activates same. This is not a power switch; the PERF10 powers the rubidium oscillator regardless of the status of the Outputs switch.

Stanford Research Systems PERF10

What the PERF10’s front panel does boast is an ample array of brightly colored LEDs. One group glows green when the outputs are switched on, red when they’re switched off. A second group of LEDs glows green when the PERF10 is connected to AC power. For those who select the DC power option, a third group of LEDs glows orange when the PERF10 is connected to a source of 12V DC power. A fourth group flashes yellow to indicate that the PERF10’s rubidium oscillator is heating up but that its phase-locked loop (PLL) is not yet operational. Finally, a fifth group of LEDs glows purple (yes, purple) when the PLL LEDs stop flashing, thus indicating that the crystal is warmed up and locked, and that the PERF10 is ready for use.

On the PERF10’s rear panel are eight 10MHz, 75-ohm outputs terminated with BNC jacks. This means that the SRS is compatible with up to eight devices that will accept a 10MHz input, including audio components made by Antelope, dCS, and Esoteric. Also on the back is an RS232 connector -- it can be connected to a computer to monitor the health of the rubidium oscillator -- and connectors for AC and DC power (the latter is covered when the DC option is not selected) and chassis ground.

Supporting the PERF10 are four permanently affixed rubber feet.

The only option SRS offers is a 12V DC power input ($495) that lets you run the PERF10 from a user-supplied battery. My review sample did not include that option.

When shipped, the SRS PERF10 is accurate to 0.05 part per billion (ppb) -- the same degree of accuracy claimed by Esoteric for their rubidium G-01 Master Clock ($23,000). Esoteric does not disclose the type of oscillator used in the clock of my Esoteric K-01X SACD/CD player and digital-to-analog converter, which is specified as being accurate to 0.5 part per million. However, I would guess that it is a nonatomic quartz design of very high quality.

The PERF10 has an annual aging deviance of 0.5ppb -- but don’t let that figure lead you to believe that its sound quality will audibly degrade in the months, years, or even decades after you purchase it. According to SRS, in 20 years, the PERF10 drifts from 10MHz to about 10.00000005MHz, a change of virtually zero, which the company states is inaudible.

The PERF10 comes with a one-year warranty, extendable to three years for $400. Although I’m less familiar with warranties for lab gear, I understand that for consumer products, such as computers and refrigerators, charging extra for a warranty period that exceeds one year is de rigueur. But one year is too short for an audiophile product costing thousands of dollars.


“It’s not rocket science.”
-- Unknown speaker likely contrasting football to the work of former scientists (Germans) who assisted the US in developing military and space rockets after World War II

Not only was it not rocket science -- setup was a breeze. I placed the SRS PERF10 on three of Synergistic Research’s metal MIG footers. I then selected a Synergistic Element Copper power cord. That connected and plugged into the wall, the Outputs LEDs glowed red, the AC Power LEDs green, and the green Unlock LEDs flashed from left to right. The PERF10 was warming up.

The PERF10’s manual states that it takes about six minutes for the PERF10 to warm up and lock from completely cold. However, after what seemed like much less time than that, the Unlock LEDs stopped flashing and the purple Lock LEDs lit up. I turned the Outputs switch on, and the Outputs LEDs changed from red to green.

The PERF10 was now ready to be connected to my Esoteric K-01X SACD/CD player-DAC. Not having an audiophile BNC cable around, I used a generic one. But one thing I’ve learned is that with clocks, like pretty much everything else in audio, cables matter. If you use an external clock, make sure that you get a quality digital cable and power cord for it. I changed the K-01X’s settings to allow it to accept an external clock at an input frequency of 10MHz, an indicator light on the Esoteric confirmed a signal lock, and I was good to go.

I let the PERF10 run for hundreds of hours before I did any critical listening. At no time did it get more than barely warm, the far left part of the case being the warmest. I’m guessing that’s where the little oven is.

Knowing what I do about digital gear, I suspected that I shouldn’t turn the PERF10 off, and its lack of an On/Off switch seemed to only confirm that suspicion. However, there was a problem: Whether the PERF10 was in use or just sitting there, its front-panel lights were so bright and distracting that they drove me and others insane. So one evening, I went against my better judgment and turned the thing off. Every time thereafter, going from a cold start and waiting for the PERF10 to lock within minutes brought subpar results. I called SRS and was told that the PERF10 needs to warm up for an hour for its temperature to stabilize, and that they recommend that it not be turned off. Ever. So from that point I left it always on. As I should have done in the first place.

Apparently, SRS believes that audiophiles expect to see bright, garish lights. But don’t be dismayed. As explained below, there are several options from SRS that will not only cure this problem, but save you big bucks in the process.


“An atom must be at least as complex as a grand piano.”
-- William Clifford, mathematician and philosopher

The misinformation out there concerning external clocks is astonishing. Pretty regularly, some bozo attaches a master clock to his CD player, hears no improvement in the performance of his system -- which contains ill-matched components and/or is poorly set up -- and announces on an Internet forum that, particularly for one-box digital sources, external clocks are a waste of money.

Even many professional audio engineers are misinformed. An article from an audio-engineering publication concludes that, although an external clock will make a large professional audio system work more reliably, it “will not improve the audio quality achieved by the [DACs] in any technical sense,” and that “In most cases, the technical performance will actually become worse, albeit only marginally so.” (From Hugh Robjohns’s “Does Your Studio Need a Master Clock?,” published in the June 2010 issue of Sound On Sound.)

The PERF10 proved that such views are bunk. The results of using an audio clock are decidedly system-specific. However, particularly in highly resolving systems, the improvements in sound quality can be profound. In my system, they were impossible to miss.

In Tableau 2 (Moderato -- Allegro vivace) of Rachmaninoff’s opera Francesca da Rimini, with Neeme Järvi conducting the Gothenburg Symphony Orchestra and Chorus (16-bit/44.1kHz FLAC, Deutsche Grammophon), groups of violins, violas, and cellos join in a dramatic tutti that demonstrates how, in even accomplished systems, leading edges of notes can be compromised by clock jitter. With the addition of the PERF10, the coordinated leading transient attacks of these instruments were not only faster and more penetrating, but also somewhat more finely drawn.

Of course, a good audio system must not only preserve the attacks of an instrument’s notes, but the notes’ dissolving sonic trails as well. In audio reviews, this most often comes up in regard to a component’s ability to fully capture the decays of metallic percussion instruments such as cymbals, gongs, triangles, and chimes. But poorly resolving components can not only truncate such decays -- they can also, ever so subtly, exaggerate them.

Take Muddy Waters’s “unplugged” album, Folk Singer (CD, Chess LPS-1483). The beauty of this 1964 recording lies not only in its simplicity, but in its many perfectly timed silences. Through the PERF10, the more accurately rendered and still fully reverberant sounds of Buddy Guy’s guitar notes and Waters’s poetic singing no longer subtly bled into the onset of the empty spaces. This caused those spaces to be more dramatic and increased the music’s poignancy -- which, of course, is what the blues is all about.

Stanford Research Systems PERF10

Perhaps not surprisingly, the PERF10 generally cleansed my system’s sound, making it more liquid, with better flow and pacing. Cole Porter’s 1928 song “Let’s Do It (Let’s Fall in Love),” recorded in 1958 by Ella Fitzgerald for her Sings the Cole Porter Songbook (16/44.1 FLAC, Verve), is also all about timing, and I seemed to be doing a bit more toe tapping with the PERF10 on the job. Fitzgerald’s voice also sounded a bit more luxurious.

Also perhaps not surprisingly, the PERF10 aced details that my system had previously not fully resolved, although I’d thought it had. In this category were the fine-spun vocal harmonies in “Lucky Seven,” from Chris Squire’s Fish Out of Water (CD, Atlantic ARC-8065 WQCP-1019). Also of this ilk were the countless faint and often overlapping sounds of bare fingers scraping and prodding guitar strings in any track from Three Guitars, by Larry Coryell, Badi Assad, and John Abercrombie (SACD/CD, Chesky SACD289). Yes, I’d heard such sounds before, but never with quite so much clarity and intricacy.

However, what some might not have guessed is that, in addition to the above improvements, the PERF10 also enhanced soundstaging and imaging. With Chick Corea and Gary Burton’s Hot House (16/44.1 FLAC, Concord Jazz), the PERF10 helped my system better delineate the microseconds between Corea on acoustic piano, to the left of the stage, and Burton on vibraphone to the right. However, it also made Corea’s and Burton’s images a bit more solid, and further separated their aural images on the soundstage.

With another Ella Fitzgerald track, “Over the Rainbow,” from Pure Ella (CD, Verve 539206), the PERF10 added size and depth, placing the singer more clearly in front of Billy May’s orchestra.

Sorry, folks -- I’m not suggesting that the PERF10 is the sonic equivalent of Esoteric’s G-01 ($23,000) or dCS’s Vivaldi ($13,499) master clock. It’s not. The use of an extremely accurate oscillator, while a great start, does not necessarily make for a state-of-the-art audio clock. When you shell out extra for a high-end clock, you get top-notch hardware that includes power supplies, output devices, PLL circuitry, capacitors, diodes, casing, and connectors. You also get design features such as a physical layout that optimizes electrical isolation and signal-path mapping. And in the case of the G-01, you get a multipoint suspension that isolates its rubidium oscillator from vibrations. Such features separate state-of-the-art clocks from the merely very good. Indeed, when the G-01 was in my listening room, it produced transient reproduction, soundstaging, focus, and realistic timbres that were all superior to and more realistic than what the PERF10 could provide.

Still, the PERF10’s value proposition is off the proverbial hook. The performance gains that I heard with it were undeniable, and when it was removed, much of the detail, precision, and focus, were gone. But keep in mind that my experience may not, and likely won’t, be the same as yours. Improvements, if any, will be highly dependent on not only such factors as your system’s resolution, but also on the accuracy of the internal clock(s) in your components.

Other SRS clocking devices

“Why should I need more time? The job’s done.”
-- James Bond to MI6, Casino Royale

By now, it should be clear that the PERF10 will do a great job at an incredible price. But two other SRS products, the FS725 Rubidium Frequency Standard ($2695) and the PRS10 Rubidium Oscillator ($1495), also warrant serious consideration. Unlike the PERF10, these are not intended for the audiophile market, but for industrial and laboratory applications. Accordingly, they do not come with blindingly bright LEDs.

Like the PERF10, the FS725 uses the PRS10 rubidium oscillator. However, it provides some additional functionality over the PERF10 that is not required for audio use. It comes in an attractive case of warmish gray that is a space-saving half-width (8.5”W x 3.5”H x 13”D), and sports only a few mutedly tasteful LEDs. At $800 less than the PERF10, the FS725 has no audiophile downsides, and is a great option.

The PRS10 is the small (3”W x 2”H x 4”D) rubidium oscillator contained in the PERF10 and FS725. However, unlike the PERF10, the PRS10 comes sans power supply, heatsink, or connector hardware, all of which are available on the cheap from SRS. If you’re a DIY type, add these items to the PRS10 and you’ll have one kick-ass atomic clock for under two grand. This option also permits you to choose an audiophile-grade power supply and the other necessary hardware from a third-party supplier, which will likely boost performance even further.


“Give me time and I will give you a revolution.”
-- Alexander McQueen

The bottom line is that the PERF10 and its lower-priced siblings, the FS725 and PRS10, will virtually eliminate clock jitter at a price -- $3495 or less -- that is nothing short of revolutionary. For the one-percenters out there, that may be no big thing. But for the rest of us, it’s one of the rarest of high-end audiophile bargains: an atomic clock for the people. ¡Viva la revolución de sincronización!

. . . Howard Kneller

Associated Equipment

  • Amplifier -- Esoteric A-03
  • Preamplifier -- Esoteric C-02
  • Sources/DAC -- Esoteric K-01X SACD/CD player-DAC, Hifidelit music server, Windows 7 laptop running JRiver Media Center 17
  • Speakers -- YG Acoustics Kipod II Signature, JL Audio E-Sub e112 subwoofers (2)
  • Interconnects -- Synergistic Research Element Copper-Tungsten-Silver (components, bass modules of active speakers)
  • Digital cables -- JPlay JCAT USB, Synergistic Research Galileo LE USB
  • Speaker cables -- Synergistic Research: Element Copper-Tungsten-Silver (tweeter) and Element Copper-Tungsten (midrange)
  • Power cords -- Synergistic Research: Element Copper-Tungsten-Silver Analog (preamplifier, amplifier) and Copper-Tungsten-Silver Digital (disc player, DAC), Tesla Precision AC SE (speakers), Element Copper-Tungsten (Powercell power conditioners), Element Copper-Tungsten-Silver Analog and Digital (Enigma power supply fed by two power cords), Tesla Hologram A (QLS Lines strips with Galileo MPCs)
  • Power conditioners and distribution -- Synergistic Research: Powercell 6SE (digital only) daisy-chained to Powercell 10SE Mk.II
  • Isolation devices -- Silent Running Audio VR fp Isobase, Symposium Acoustics Osiris racks and RollerBlock Series 2+ equipment support system, Synergistic Research Tranquility Bases and MIG footers, Custom Isolation Products amp stand
  • Room treatments and correction -- DSPeaker Antimode 8033 subwoofer equalizers with Channel Island Audio linear power supplies; Synergistic Research Acoustic Art System, HFT and FEQ room-treatment devices, XOT Crossover Transducer
  • Misc. -- Black Discus Audio System Enhancer, Synergistic Research Galileo Universal interconnect and speaker-cable cells

Stanford Research Systems PERF10 Rubidium Audio Clock
Price: $3495 USD.
Warranty: One year parts and labor.

Stanford Research Systems, Inc.
1290-D Reamwood Avenue
Sunnyvale, CA 94089
Phone: (408) 744-9040
Fax: (408) 744-9049