Pro-Ject Pre Box S2 Digital D/A headphone amplifier Herb Reichert May 2019

Herb Reichert auditioned the Pro-Ject Pre Box S2 Digital in May 2019 (Vol.42 No.5):

Pro-Ject's "audiophile digital amplifier," the Pre Box S2 Digital, looks like something big that got shrunk down small enough that John Atkinson could carry one in his pocket on his way to my studio. "Herb, Ken Micallef reviewed this Pro-Ject DAC in our April issue. I'd like you to write a Follow-Up. I won't tell you what I think, but I'm curious what you'll make of it."

Then I remembered that Pro-Ject, an Austrian company, makes everything microsized—except their famous turntables, which are turntable-sized. On Pro-Ject's website, CEO Heinz Lichtenegger explains: "By shrinking the cabinets to a miniature size, we were able to save costs and offer our customers a true price to performance relation."

Yes, the Pro-Ject Pre Box S2 Digital D/A processor is small. A $399 giant killer? I wondered.

Auditions & Filters: You know that old vaudeville saying: Never follow a kid or a dog act (or Jimi Hendrix). Sequence is everything. If I were a tiny, fits-in-the-palm, $399 D/A processor and headphone amp, I wouldn't want to try replacing the ever-steady, pro-styled Mytek Brooklyn DAC ($1995, footnote 1) in my desktop reference system. I felt bad for the little Pre Box S2 Digital, but I had to start my auditions somewhere.

For me, the beauty of the Mytek Brooklyn is how it tames the ubiquitous but sometimes hard-sounding Sabre ES9028PRO converter chip. The Brooklyn, through its minimum-phase filter and with MQA disabled, sounds very un–sigma-delta, and remarkably nondigital. It plays with warm-blooded life, a good beat, and vivid sparkle. I've logged more listening hours with the Mytek Brooklyn than with any other DAC. Pro-Ject's Pre Box S2 Digital uses the ES9038Q2M standard version of the ES9038PRO chip; both DACs exploit the features and filters of this popular converter.

When the Pre Box S2 arrived, I'd just finished studying the stop-start rhythmic inventions and artfully planned tonal vicissitudes of French guitarist Noël Akchoté playing his arrangement of Meredith Monk's Nightfall (24-bit/44.1kHz FLAC, Rectangle/Tidal). The Mytek Brooklyn reproduces this recording with luxurious tone and vibrant, energy-charged, empty spaces. It produced a vivid see-into rendition of Nightfall's odd space. This brief work's six movements are performed in some sort of simple but clearly artificial acoustic space, with something like a vacuum cleaner running in the background. This unusual recording environment focuses the listener's attention on the weight, body, and percussive tones of Akchoté's acoustic guitar. The Mytek Brooklyn made all six tracks of Nightfall (total time 6:51) lush and exciting to listen to. Unfortunately, the Brooklyn sold for $1995 when new. I immediately wanted to hear if the $399 Pro-Ject could reproduce all the clever nuances of this mesmerizing recording.

When I played Nightfall through the Pre Box S2 Digital, I realized, for the umpteen-millionth time, just how completely different two DACs—even two DACs based on similar converter chips—can sound. The Pro-Ject conspicuously emphasized the attack of each note, way more than the Mytek. With the Pre Box S2, my ears—and eyes—were riveted on that sharp small place where Akchoté's pick touched the strings of his guitar. The Pre Box S2's stark immediacy dominated my Noël Akchoté experience, and its focused clarity let me hear more deeply than the Mytek had into the background—where, especially in Nightfall: No.5, P–R, that whining-hissing noise is most distinct. I was intrigued.

Through the Pre Box S2, the guitar and that strange background sound were equally clear and almost equally loud, but much farther apart in space than through the Mytek. With the Pro-Ject's Optimal Transient filter—Ken Micallef's favorite—engaged, Akchoté's guitar sounded closer to the microphone, the hissing farther in the background. (Isn't it funny how subtle changes in spectral balance can generate such radical differences in soundstage mapping?)

The Pre Box S2 Digital's reproduction of Nightfall emphasized the force and raw energy of this recording, while reducing the reverberant atmosphere. Bass had muscle, but midrange color was, to my painterly taste, insufficiently saturated. High frequencies seemed suppressed and/or nondescript.

The above comparison was made with the Mytek and Pro-Ject DACs feeding Rogue Audio's Stereo 100 power amplifier, which drove the spatially precise Harbeth P3ESR loudspeakers.

In hopes of a closer view, I switched to a higher-powered magnifier: HiFiMan's HE-1000 V2 headphones ($2999). Right away, they demonstrated that the Pre Box S2 Digital's headphone amp was incompatible with low-sensitivity (in this case, 90dB/mW) headphones—even if said headgear has a ruler-flat 32 ohm impedance, as do the HE-1000 V2s. The Pro-Ject's low gain forced me to listen to the HiFiMans with the Pre Box S2's volume turned all the way up. The sound was basically good, but Noël Akchoté's guitar became softer sounding. Formerly sharp note attacks were now blunted and less prominent. My mind's eye shifted its focus from pick and strings to the guitar's body and neck. That mysterious background noise became dull and distant. Obviously, the difficult load presented by the HiFiMans took the sharp edge off Akchoté's picking, but strangely, the Pre Box S2's headphone amp made its most enjoyable sounds at low volumes with just such expensive, insensitive headphones.

However, in terms of gain, punch, and boogie, the Pro-Ject preferred sensitive, easy-to-drive headphones such as AudioQuest's NightHawks ($299), with their impedance of 25 ohms and sensitivity of 100dB/mW. This combo made a wide variety of recordings sound lively, smooth, and very musical—even through the S2's Brickwall filter.

Through the NightHawks I listened to a remastering of the Julie London chestnut Julie Is Her Name (24/44.1 FLAC, Liberty/Rev-Ola/Qobuz). I was taken aback by how soft and supple her voice sounded through the S2 Digital's Brickwall and Fast Rolloff (Linear Phase) filters. That worried me—since when does a brick wall feel soft?—so I switched to the headphones I use most with the Mytek Brooklyn: Sony's MDR-Z1Rs. With this album the Sonys sounded much less soft and more transparent than the NightHawks—but Pro-Ject's Brickwall, Fast Rolloff (Linear Phase), and Minimum Phase Slow filters sounded almost identical.

Julie London gave the most pleasure through my beloved Audeze LCD-X headphones ($1699). The very sensitive LCD-Xes (103dB/mW, 20 ohms) made the cleanest, most tuneful, most lively bass of any headphones I tried with the Pro-Ject. Midrange detail was most specific and natural. The LCD-Xes gave me an enhanced view of the Pre Box S2's transparency. But the sound still leaned slightly toward the gray and detached.

The Pre Box S2 delivered its most open, transparent, and flat-out pleasurable sound with (don't laugh) JPS Labs' Abyss AB-1266 Phi headphones ($4995), which have an impedance of 42 ohms and are totally insensitive at 88dB/mW—which meant that I could play them only at low volumes. But drive and dynamics were surprisingly good, and the Abysses' extremely natural and detailed sound gave me a more revealing view of the Pre Box S2's eight filters. I tried them all, but even through the electron-microscope resolution of the AB-1266 Phis, I occasionally struggled to detect when, precisely, I'd switched filters, let alone which filter was doing what to my music.

The only filter that caught my attention in a positive way was Pro-Ject's own Optimal Transient, which I used most often. Fast Rolloff (Linear Phase) gave music the most solid body, while Slow Rolloff (Linear Phase) added a hint of suppleness to Fast Rolloff's stiff corpus. Minimum Phase Fast brought that background noise in Nightfall somewhat more to the fore, but made it softer, less odd-order-harmonic sounding. Minimum Phase Slow increased the gentleness factor, but only by a teensy-weensy bit. For some reason, of all the filters, Linear Apodizing made Nightfall's motor-fan noise sound loudest and closest to the mike. The Hybrid filter seemed more unaffected (or less effective) than the rest, but I don't know why. Brickwall sounded as artificial as Fast Rolloff (Linear Phase). Switching from Brickwall to Optimal Transient was a slight but noticeable step in the direction of relaxation and naturalness.

These struggles force me to ask: Why does a DAC need eight filters? What purpose do they serve? Have filters become the digital equivalent of analog tone controls? Which filter measures best? Which is the most accurate? If one of these filters is right . . . which one am I supposed to like? Most important, why am I being forced to choose?

Conclusion: No matter which filter I did choose, the overall sound quality of Pro-Ject's Pre Box S2 Digital was mainly affectless. It did nothing noticeably wrong. Many audiophiles would call this "neutral."

The Pre Box S2 Digital's clarity was distinctly good, its punch and drive distinctly strong, its ability to separate sounds and instruments on a soundstage obvious, its bass resolution above average—all very good, right? Unfortunately, in my floor and desk systems, the Pre Box S2 substituted a digital-artificial blankness for transparency. It diminished atmospheric effects. It never fully "disappeared" from the sound of the recording, or stopped being a digital device. Its headphone amp was gain-challenged.

I know—the Pre Box S2 costs only $399 and is USB bus-powered when used with a computer. But folks, no matter the price, I have a strong need for audio equipment to disappear into some amount of music-like beauty. Most important, I need some sense-oriented (as opposed to measurements-oriented) reasons to like a digital product.

The Pro-Ject Pre Box S2 Digital's main sense-oriented effects were its impact and its beat-emphasizing drive, both of which I regard as distinctly masculine traits. What was missing was a radiant, sensuous, feminine side. To me, the Pre Box S2 Digital is simply a good DAC-headphone amplifier at its price point but it didn't kill any giants.—Herb Reichert

Footnote 1: In 2017, the Mytek Brooklyn was replaced with the Brooklyn DAC+ ($2195).
Pro-Ject Audio Systems, Audio Tuning Vertriebs GmbH
US: Sumiko Audio
6655 Wedgwood Road N. Suite 115
Maple Grove, MN 55311-2814
(510) 843-4500

Lorenzo-Italia's picture

Ladies and Gents
With 399$ You get “almost state of the art...”
If you really need more than “almost”, maybe “full” state of the art just multiply almost by a factor of 100!

Is this the way you manage your everyday life ? Think before you buy
Lorenzo from Italy

allhifi's picture

Lorenzo (from Italy): What exactly are you saying ?

Your comment is difficult to decipher/decode ! lol

It's been my long-standing experience to note that 'Digital' source's
(when competently engineered) ALWAYS sound excellent. Unlike 'Analog' gear, very short signal paths and very near-by, 'controlled' circuit power supply 'draw'appears to reward handsomely. Consider 'Chord' Electronics for example.

In any case, the Pro-Next Pre Box S2 appears a thoroughly engineered product. I suspect it will offer up superior SQ. At the very least, it can be used as an evaluation tool with the very desirable option of the multitude of Digital Filter choices.

Presently, I'm using a couple mid-level Network Player's/Streamer's and as far as Internet Radio is concerned, my two separate SBC's (Rasp. Pi, Odroid C2) running Rune Audio/MPD/Linux via USB cable to DAC sounds considerably better than the 'commercial' Player's ;even using the modest Schiit Modi-2 Uber DAC.

later this week, I'll have one to evaluate (PB S2), and shall report back.

Seeing as Pro-ject's Pre Box S2 has garnered favorable reviews, it's surely a fine DAC. It certainly has earned its retail price by offering the latest premium DAC chip (32/768 KHz), careful USB implementation, Display Window, Filter Choice -gotta love that feature/option, MQA capability, premium parts/circuit board along with a pair of spdif Inputs along with USB.

Shall report back,

peter jasz

YankeePhile70's picture

I just bought this little beauty and I can attest to its CLASS A rating! After having owned Audioquest, Grace, and Chord dacs, this one is a keeper. Voices in particular offer outstanding clarity. None of the typical, "venetian blind" affect one gets, where voices don't lock to the center of the soundstage. Subtle shifts in soundstage perception are very audible. The highs are clean and liquid, without sibilance, typical for dacs in this price range. Dynamics are ELECTRIFYING! THIS DAC IS A REAL WINNER!

Bogolu Haranath's picture

What filter(s) are you using for listening? :-) .......

Kblade's picture

The fact that this reviewer only used the Optimal Transient response filter. Shows me he had no clue of what he was doing. It is the worst filter on the project. This filter pushes the midrange way to forward causing it to sound very dry with no air no sparkle. Awful review! ONE HELL OF A DAC FIVE STARS!

Bogolu Haranath's picture

Another DAC/headphone amp to consider is Chord Mojo ($570) ......... Stereophile Class-A listed ........ Mojo is self powered with built-in battery :-) ........

er1c's picture

I have jumped back and forth between them as well. I too did not hear much difference scanning through the filters at first but now, 6 months in, with great NOS tubes in my Rogue Sphinx 2, I can clearly hear differences between a few. My preference, a significant preference, is the Linear Apodizing filter. With this filter mids on my LS50s are richer and have more weight. Close second (in a mood, a preference) is Hybrid, which on this gear gives some edge to the transients and I hear more energy in high frequency sounds. I did buy it for MQA on Tidal, for which I can offer no strong opinion at this time, just share that it is case by case, sometimes exciting but other times my experience is that I feel something in the music like the way you might get a feeling from food that's too processed, a sense that something is added that's subtly uncomfortable. S2 also doubles as DAC for my Cambridge CD transport, a very enjoyable detailed sound from my CD collection, though I plan this year to move up the food chain with a more sophisticated DAC. Film at eleven. Nice review thanks.

ken mac's picture

for your comments.

rt66indierock's picture

This review is an example of why you measure first and listen second. I measured it, Audio Science Review and John Atkinson measured it. We all came to the same conclusion that it measures well. Amir and I both found second harmonics which would have been worth discussing.

But if Tom Petty sounded louder in the MQA version you didn’t volume match. This renders many of your sound comments invalid. Finally, MQA like any format seems to be normally distributed with the majority either no difference or a different sort of different (thanks Kal).

ken mac's picture

you measure and listen independently. There are many many instances where gear measures poorly and sounds great and vice versa. A story as old as the hills. MQA sounded louder and I adjusted volume. And MQA sounded better on every available track. Your comments are invalid. Cheers.

rt66indierock's picture

I appreciate you documenting your lack of understanding of volume matching. Fortunately, someone I’m frequently at odds with can help, your new editor Jim Austin.

Ken you measure first to simply verify the component is quiet enough to take full advantage of the 96 dB on a 16/44.1 file. If it doesn’t, I box it up and return it as unacceptable.

As for every MQA track sounding better, those of us who tuned live systems with The Nightfly find the MQA version to be worse.

As for your comments about measurements and sound I think you are misinterpreting what Daniel von Recklinghausen meant so I’m going to write an article about it.

ken mac's picture

Which began with J. Gordon Holt, continued through the first JA and is now upheld by the current JA is to listen first, then measure. Sorry if our methods don't agree with your's. Plenty of mags/sites/forums where your views are in vogue...hey, you can even submit your article...Nightfly as barometer? No wonder.

allhifi's picture

Ken Mac is right; your comments (vol.level matching) are naively invalid.

AND, as Ken correctly points out again, listen, measure -comment. Only with considerable experience can one come to the realization that impressive "specifications" and SQ are not particularly related -again as Ken Mac has stated).


Graham Luke's picture

...model worthy of consideration is the excellent Gilmore Lite Mark 2 by Headamp. The Gilmore does not have a built in DAC but pairs incredibly well with a Dragonfly Red. The beauty is that you can mix and match DACs and upgrade in the future.

RaimondAudio's picture

There are many other very good cheap DAC's like: Topping DX7s, Topping DX3PRO, Topping D50, Aune X1s, Khadas Tone,SMSL SU-8V2 etc. Over 1.000$ we have Okto DAC, RME ADI-2, Gustard X26, Yulong DA10, SMSL D1, etc.

dc_bruce's picture

"Class A," they said.

If these classifications have any meaning, then this little box is just as good as, say, the Chord Quetest or the Schitt Yggdrasil. Admittedly, the test scores are nice; but the aural comparisons in the review are with other similarly priced budget DACs, like the Dragonfly. And, it was not even a clear winner, subjectively, over the Dragonfly.

For that matter, after Mr. Atkinson's glowing, enthusiastic review of the Quetest, I was surprised it was not in the "A+" category. Would that be because the much more expensive Chord DAVE is in that same group?

Having never had the opportunity to compare a slew of DACs against each other, I can't say whether meaningful (i.e. audible) differences between DACs exist. But it seems to me that if Stereophile is going to be in the business of ranking them-- Class A+, Class A, Class B, etc. -- then some kind of explanation is in order . . . and more than the vague generalities that precede the rankings.

Or, maybe I should just risk a few hundred $$$ and find out for myself.
So, is it just going to be about the numbers from now on?

allhifi's picture

dc_Bruce: For someone as yourself who doubts any "meaningful" distinctions exist between DAC's has no place here (experience/interest) to cast opinion.

Why in the world would you be interested in Stereophile's rating grade when you yourself don't even own a DAC and quake in your shoes at the thought of shelling out/"risking" a few hundred dollars.

Stick to your 'Gun's' and Sears stereo.


helomech's picture

based on JA's measurements. I had been wanting to try a more modern architecture but figured I'd wet my feet before shelling out 3X as much for a Benchmark or Mytek. After spending a few days with the Pre Box S2, it's hard to imagine much can be gained with those pricier units. While it may not be a giant slayer, it's quite an upgrade over my old Topping PCM DAC. The Pre Box also outperforms my Line Magnetic 502 CA when compared to the latter's SS mode. That DAC had an MSRP of $1800 just a few years ago.

allhifi's picture

Great to hear you are enjoying the S2 -it wouldn't surprise me at all. I also have no doubt it easily outperforms the other DAC's noted.

It would be informative to hear from owner's (or qualified listener's) experiences between the S2 and the Mytek 'Liberty', 'Brooklyn +, Benchmark 2/3 or even the Chord 'Qutest'.


Kblade's picture

What is a qualified listener. Do you have to have dog years or do you have to write for a magazine. What a dumb comment

allhifi's picture

Yoh, Chooch: For 'normal' (i.e. no-moronic) folk it's rather self explanatory.

Perhaps you should spend more time listening to your Candle (brand) radio as opposed to imposing your 5th grade education/mentality here. Good luck -not really.


allhifi's picture

helomech: It's unlikely you/we have "specs" for 'cabling' (or are looking at the right ones -lol) but you be'd really surprised at the SQ distinctions noted if you exchange/experiment with the 5V Walmart power cord/USB-Micro cable -providing the 5V DC power to S2.

AND, without exception, give your DAC it's own space/ shelf/ space -as opposed to perched upon something else.

Finally, if possible feed your 'Digital' gear AC-Regen power or at the least Balanced/Symmetrical AC power; gains in SQ/performance are considerable.



Bogolu Haranath's picture

helomech, what filter(s) are you using for listening? :-) ........

helomech's picture

had this to say about the Pre Box S2:

miketk's picture

radiant, sensuous, feminine missing?

I have just purchased a Pro-Ject Pre Box S2 Digital to sit between my desktop PC and main hifi system - Prima Luna Dialogue Premium HP and Monitor Audio Gold 200 (ribbon) speakers.

Forget all about the headphone amp part. This is one of the best DACs I have heard so far for resolving voices and instruments and placing them in a huge sound stage. I think the secret may be to connect it to a valve amplifier. This adds that hint of natural warmth to the mesmerising detail retrieval.

(PS I also have a Cambridge Audio DACMagicPlus, Chord Mojo and and Audiolab M-DAC+)

In the UK the PBS2D has been described as 'Budget Esoterica' in the top quality HiFi News mag (August 2017). Just forget the headphone amp section. It is not as good as the dual mono ES 9038 DAC section.

The DAC section is another tiny piece of John Westlake magic and I think it may become a hifi classic.

As one example, The Opera Band Amici Forever album has never sounded better than through the PBS2D in my system. The separation of the five powerful operatic voices (two female, three male) and deep resonant bass is exceptional. To me this is radiant and sensual and feminine.

Bogolu Haranath's picture

What filter(s) are you using for listening? :-) .......

helomech's picture

I keep returning to the Slow Rolloff filter.

Bogolu Haranath's picture

Thanks for your reply :-) ........

bknielsen's picture

I don't think the pre-emphasis flag is properly handled on the COAX input of this DAC. Anyone with the same experience? Maybe Stereophile could check, and should I be right make a warning for folks with an old CD collection.

I discovered this playing the Dire Straits Alchemy album, with my old Sony CD player used as a transport. It sounds harsh/bright.

jimtavegia's picture

I just bought the Project S2 Dac to use with one of my old Sony DVP NS 755 DVD/SACD players and every filter bettered the Sony, which should come as no surprise given a 2003 player. It still sounds very good with SACDs, but my newer Yamaha players does better it.

It is amazing what $299 buys in digital these days and I bought the unit without the headphone amp. I can't imagine that there is any new 2020 CD player that could beat this in sound quality, given all the filter options there are.

I can only guess who happy Chord DAC owners are and will enjoy their purchases for many years. It is good to know that for all of us there is a way to improve our digital experiences.

I enjoyed the increased clarity of JA1's recordings of the Portland State Chamber Choir on CD with a greater sense of space. I also pushed the envelope with a HD tracks download of Eric Whitacre at Kings College in 2496. I feel that a real test of clarity of a DAC is how well it can keep sorted all the mass voices and can you make out the lyrics, which is also a part of the cohesiveness of the members.

This may be the best $299 I will spend this year on audio. Thanks to Stereophile for reviewing very affordable gear.

jimtavegia's picture

filter 2 is the Linear Phase fast Rolloff, and 5 is the Apodizing Hybrid filter. I did not buy the one with the headphone amp function, so I am wondering if the naming of the filters is different for some reason. Should be the same I would think. Great sounding little DAC. I have not gotten into DSD downloads as of yet.

jimtavegia's picture

Now two of my players, one a Sony DVP NS 755 (2003) has one and is now part of a headphone station, and the other is on my Yamaha S1800 player from 2007 in a main system. I bought a Silver S2+ so I can easily know which is which. A true bargain.

dclark2171's picture

Back when I had to send in my Rogue RP-5 in for repair (later traded in for my current BHK Pre), I used the Pre Box s2 as a pre amp (digital inputs only). I must say....if I did not need analog inputs ...I could be quite happy as a pre. Dead quiet. Great Soundstage

neurotopia's picture

Hi Ken (and everyone else). I noticed in your review the Pre-Box S2 is specced with Sabre Pro ES9038 chips. But the unit I got is specced with dual ESS9038Q2M chips (on the packaging) and online I've only seen reference to one model, all of which are also specced with the ESS9038Q2M. Are these the same chip (and I'm just an idiot) or was there a hardware change?

-edited for grammar.

nidaje's picture

KM introduced a fault in his presentation of the Pro-Ject S2 Pre Digital on page 1, section ”Design”, paragraph 1, line 2-4:
(it's first dual-mono DAC using ESS Technology's flagship Sabre Pro ES9038 dual chip,...)

KM got it right though in Sidebar 1: Specifications:
D/A-converter: two 32-bit ESS9038Q2M.

The fact is:
The Pro-Ject S2 Pre Digital is not housing the ES9038Pro Dac to be verified at:
Technische Daten: D/A-Wandler 2 x ESS9038Q2M 32-bit.

None of the Pro-ject dac's are housing the ES9038Pro. Depending on model they use:
ES9038Q2M (one or Dual-Mono 32-bit), Cirrus Logic 4344 (Delta-Sigma type), PCM 5102A, AK 4490 32-bit, 2 x TI PCM-1792.

The fact is:
The Complete ESS Sabre Pro Series of Dac's include:
ES9038PRO Flagship SABRE 32-bit 8-Channel Audio Dac with 140 dB DNR (64-Pin).
ES9028PRO SABRE 32-bit 8-Channel Audio Dac with up to 133 dB DNR (64-Pin).
ES9026PRO SABRE 32-bit 8-Channel Audio Dac with up to 128 dB DNR (48-Pin).

ESS produce other 8-Channel Dac's but none designated PRO.

The ES9038Q2M is a member of the SABRE 2-Channel Audiophile Dac's:
ES9038Q2M 32-Bit Stereo Mobile Audio Dac with 129 dB DNR.
It is a smaller 30-QFN package (30 pin), has ultra-low power consumption though very respectable data.

As the Pro-Ject founder Heinz Lichtenegger says in his interview with Hi-FiNews Nov. 2019 on The Pro-Ject CD Box RS2 T and Pre Box RS2 Digital review: (You realise very fast that the highest costs are in the cabinet and the mechanical parts, not the electronics – the only way to reach a sensible price is to reduce the size of the box, also improving it's feel and rigidity.)

A real SOTA Pro-Ject Dac based on the ES9038PRO or anything comparable with or without a valve section is hardly their aim. The money are in the segment below.

helomech's picture

Replacing the oem wall-wart with a Topping
P50 power supply makes for a substantial improvement with the S2. Compares favorably to the Chord Qutest in this configuration.

Also, the oem wall-wart is one that induces noise in adjacent components. It was causing a nasty buzz in my phono preamp.