HiFiCritic compared the Bel Canto e.One REF1000, the NuForce 8.5 monoblocks and the Channel Island Audio D100LGR in the September/October 2007 issue. This is a magazine that accepts no advertising in order to be able to say what they think. They were not impressed with the performance of any of the amps. They were quite powerful but the actual sound quality was lacking. As they put it," Poor treble seems a feature of Class D amplification." Some other publications have hinted at this; I remember an Absolute Sound review of 2 Class Ds where the reviewer commended on the necessity of using a tube preamp to tame the top end[ I don't have the issue in front of me]. When I posted a reference to this article previously a mighty roar was heard from owners and SELLERS of Class D amps accusing both the Magazine and myself of being in league with entrenched commercial interests. The loudest roarer was a large audio dealer. The magazine takes no advertising and while I myself am an audio dealer I do not sell electronics. You may well see this repeated here; if so, remember that those who try to stifle debate have something to hide. To me the whole thing is reminiscent of the CD debacle where a new technology was rushed to the market before it was developed.The test is not on line but the HIDICRITIC site has excerpts from a paper presented at a professional conference concerning Class D amps. I would have a long listening session with any of them before I bought one.
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Look to the technology the amps are based on. ICE, Tripath, UcD, etc. I have used both Tripath and UcD based amps and I think UcD is better. Either amp is better than most mosfet,jfet, solid state amps. They have no "sound". They will, without adding their own sonic signature, amplify what they are fed. They are "neutral", which is what you want an amp to be. I have stated many times a tube pre with a "class D" amp is a great combination. I also believe any SS amp sounds better with a tube pre. I only know, that for me, "Class D" opens up the sound, highly detailed, very wide sound stage and with the tube pre, produces a warm musical experience. I have been in "hi fi" since 1971. The debate over this new technology has been anything but stifled. Just as SS rose against tubes, cd's replaced cassettes which came out against records; chips will overshadow mosfet/jfets. Each camp will have its champions and shrill voices. There will be product for each technology. Most will go with the new technology, some never will.
Excuse me as I listen to some Bach, produced by my system of cd through tube pre to a "Class D" amp. Debating over technology is not what is important, listening to music is. Less chat, more tunes.
I have a NuForce integrated I purchased used here on the Gon. It definitely had a nasty glare on top. I sent it in to NuForce (xlnt service) for a V2 upgrade - glare is all gone and it is a very sweet unit with great dynamics - and I only use it with digital sources.
The gentleman I bought it from is, among other skills an equipment reviewer. When we were discussin the upgrade (which was the plan going in) he wrote me that "I think the learning curve is still relatively steep on this kind of stuff which means upgrades not to far out from release (a few years at best) have meaning."
As noted, at least in my experience this has turned out to be true.
I am running the latest and top version of Nuforce, the Reference 9 V2SE. It is very wonderful accross the spectrum, particularly the bass and including the highs. The amp is reported to have high bandwidth, low distortion, near zero phase shift, a very high damping factor and a proprietary modulating signal versus Tri Path etc.
That said I can understand the difference of opinion about these amps. Using a tubed pre (Audio Horizons) my initial sound was amazing in the bass and harsh/glassey in the highs on 2/3 of CD's, noticeable at higher volumes in a very live room. I replaced my silver IC with gold and used Hyperion vibration control discs under the amps and voila! the highs are now sweet, extended and yet tame at high volume on about 80% of CD's. So the amps are sensitive to source material, wire, FRI, EMI, and vibration. They are also psychologically challenging. You hear this incredible extended articulate full bass and look down at the puny little things, knowing you paid less than half what would be usual for this good a sound, and it doesn't add up. But it is worth giving these amps a try. The detail, soundstage and musicality can be amazing if some time is spent on set up. Added to that is portability, low energy use, and space savings.
I agree completely with one thing that Stanwal said:
"I would have a long listening session with any of them before buying"
That is because switching amps -- which in most cases are analog devices and not digital) -- like any other technologies, have a sound that is much more characteristic to a manufacturer or an individual model than to a 'class'. Thus the sound can range from bright and distorted to exceedingly musical, from extremely extended to limited in bandwidth, from 'dry' to harmonically rich and complex, from quite rough to incredibly subtle.
It is worth pointing out that the HiFi Critic article is almost 2 years old and that at least 2 of the amps reviewed there are obsolete and withdrawn. Furthermore, From a logician's point of view, the negative inductive reasoning used by the author to draw his generalized condamnation is essentially flawed. . . there may be close to 100 switching amps on the market today. . . he examined 3 and declared them to be wanting, then glibly proceeds to condemn the whole lot, based on his spectacularly limited observations. On the other hand his guilty virdict -- had been most likely decided 'a priori'.
If the same reviewer were to examine with a completely open mind current production of leading switching amplifiers such as the newest bel canto Ref 1000 Mk.2, JRDG 312, and latest Spectron monos (just to name the very few I am most familiar with)(, he may discover that the art and science of reproducing music is not the sole prerogative of traditional -- and sometimes nostalgic -- technologies.
Returning to the original question, within the very limited scope of my experience, one of the highest values in amplification -- regardless of class and technology -- may be the very moderately priced current crop of Bel Canto monoblock amps: the Ref 500 Mk.2 and the Ref 1000 Mk.2. As for which variant of underlying chip set may be best. . . it is a question for which there is no particularly valid answer. . . to paraphrase and old cliche. . . "Tubes and chips don't make music. . . designers with tubes and chips make music!"
Condocondor, I have no direct experience with the Bel Canto S500 stereo. Just looked at:
By reading between the lines, it seems that the S500 may lack the input AC current rectification circuit of the Ref 1000 Mk.2 and Ref 500 Mk.2. I can only venture to guess that in the end the S500 may sound slightly less refined than its mono brothers. Yet, without direct experience, my guess has only the negligible value of the electrons you see on the screen to paint it. G.
The two that come to mind that offer UCD is Channel Island (D-200) and Kharma (MP150). The Kharma is a little more money then the CI. You can also go to hypex.nl, order the parts and build one yourself. I have a pair of custom UCD700 made to monos. These output 350 wpc into 8 ohms and 700 wpc into 4 ohms. I am now building another pair. My past amps are; Adcom, GAS Ampzilla, Dynaco ST-70 modded, Melos MAT-180 and Karan KA-i180.
Of course I empathize with you Coffeey. . . it is really sad how things have the habit of being. . . and even more of changing and evolving. . . in spite of our better doubts, nostalgia for the safety of olden days, or tradition abiding skepticism. . . It's like we don't matter any longer to this mad mad World. . . 'Fore long we'll have to get out of the house and listen by wesselves to what's good with this hobby. . . because even the old tried and true "I truly believe it therefore it's true" does not sound so hot any longer among far too many young whippersnappers. . . there's no more Religion no more, I tell ya.
My very last refuge is in the stern wisdom of one of the greatest sages of our Age:
"Just don't fall into the category of those whose minds, like concrete, are thoroughly mixed up and permanently set"
Hi Coffeey, small design improvements can make large changes in sound quality in any design. Class D amps have always been good, maybe not great but good. UCD in the last year and a half has redesigned their boards and have chosen some better components. Class D amps use allot of surface mount parts and that is the reason most companies buy modules from B&O (ICE) or Hypex (UCD) and then design and build their own power supply. Now that class D production has picked up, many surface mount component manufacturers are willing to set aside the better parts for these designs. Most, if not all of the sound differences between tube and solid state amps are the parts and power supply. The same applies to Class D. Circuit topology for tube and solid state haven't changed much in decades. So how is it possible for a manufacturer of these amps to make their product sound better then last years or even 10 years ago? Parts, board layout, isolation and power supply.
Excellent points Stereo. I would like to add that at least 2 manufacturers (Bel Canto and JRDG) have started to add pre power supply current rectification in some models. This means that the power supply is essentially fed DC from a current reservoir of capacitors. The idea is the DC current is expected to:
1. largely eliminate 2 way AC line noise in and out of the amp.
2. Keep internal capacitors more optimally charged.
3. Be able to draw power for large transient from the DC reservoire regardless of the current state of AC 50/60 Hz phase.
4. Increase overall power conversion efficiency.
Some of these amps do sound amazingly musical to my ears. . . if and when more devices adopt various form of rectification prior to power supply, we may eventually be able to determine if there truly exists a correlation between input current rectification and sound quality.
I probably won't be adding much to the discussion by simply suggesting (as another poster did) that it depends on the manufacturer. For the past 2 weeks I've been listening to an ARC 150.2 which I swapped into my system in place of a tubed ARC D90B. The 150.2 is a tripath class T amp, which is really just another name for class D. I'm feeding it with a tubed ARC SP-8, and it sounds as good as, if not better than, the tubed amp it replaced. I also have the TAS issue which compared a number of class D amps, and if you read the review of the ARC 300.2 (the bigger brother of my amp) you see two opposite opinions - the assigned reviewer loved it (as I understand, he actually bought one after reviewing it) but Jonathan Valin, another TAS reviewer, definitely didn't like it. It seems to me that when you have two professional reviewers for the same publication disagreeing, it certainly points up that there is no "easy answer" to the question of class D sound.
"Immediately good/refreshing sound" (how Class D is commonly described... at first) can be a very superficial thing. How many times was the sound of some gear (or music) you are long term happy with NOT necessarily so good at first ?
B. NEW TEAM - SUSPICIOUS NEWBISM
"Good" (sic) Class D is a massive Analog/audio/RF/Digital engineering tangle problem, & analog engineers are sage enough to know to leave such a vipers' nest alone. Seriously clever people and the biggest corporations have all thrown "brains and bucks" at, and into Class D, since the early 1960s. Over 40 years of trying! With still very few results on the high end stage, and, very late results all round.
People playing with overly-mathemeticised plans of reality may need to write out 1000 lines :
"Digital knowledge is not power amplifier engineering." !
Not a single master of analogue power amplifier engineering has turned to Class D. Shouldn't one know this, and, ask 'Why?
By their nature, electronic engineers are usually eager to "migrate into the future technology".
C. Because the people who make Class D amplifiers are somewhat "ignorant", let alone their advertising copy-writers, they forget that the ACTUAL efficiency of an amp has to be multiplied by the power supply's efficiency, to arrive at the nett, realistic sum. This down-plays their rather spurious/overstated claims of high efficiency, once the comparison is made on an apples/apples basis.
In other words, taking best case practical round figures, an analog amp (70% efficient) is only a tad less efficient than a Class D amplifier (90%), whether they both use a mains frequency power supply of nominally 70% efficiency (70% x 70% = 49%; 70% x 90% = 63%); Here, Class D's 63% is only "28% ahead" of 49%.
Or, if both amps use a high-frequency (switching) power supply, then 70% x 90%, and 90% x 90%, are 63% & 81%. Again, 'D' is only some "28%" better. Do you wreck high-end sound for that? But, even the average 63% (of both) is FAR more than 200% more efficient than the most efficient car or indeed, power station.
If we dare include the power station efficiency (circa 28%), then any eco benefit from just one final part of the total energy loop, acting efficiently in Class D, is further reduced!
D. To anyone with an understanding of what makes existing good high-end hi-fi amplifiers, it is hard to see WHAT Class D achieves, that is useful. No one has ever announced it or written it down. It is not a logical next step. ONLY IF manufacturing convenience, cost, and material usage were put at the head of the list, before sonic quality.
This is from Ben Duncan , prominent audio designer.
"Digital knowledge is not power amplifier engineering." !
Class D amps are NOT digital. There is no limit to resolution. Time (duty cycle) is as analog as voltage. Analog modulator used in class D Icepower is pretty much same thing as sigma-delta DAC without filtering. Also SACD is class D (you like it or not) as well as DSD recording.
"Not a single master of analogue power amplifier engineering has turned to Class D."
Not true - Jeff Rowland for instance not only turned to class D but also stopped making anything else. This alone should suggest something.
As for efficiency - It depends what you compare to. Efficiency of class AB is not 70% - it is around 40% or less. Class A is complete disaster with 7/8 of power wasted. Just imagine true class A 1kW amp. By definition it dissipates about 15kW. Now lets look at 1kW Icepowers . They have total efficiency of 79% (including power supply). In addition music power is only a few percent of average power.
If you don't understand what class D achieves - I will try to explain. In traditional class AB nonlinear characteristic of output transistors is corrected by negative feedback. Bandwidth is increased and THD as well as IMD reduced. Unfortunately TIM distortions are introduced causing not only unpleasant sound but fatigue as well. When feedback is not fast enough to respond amplifier goes momentarily into saturation and charge is trapped on semiconductor junction of output transistors making them non responsive. Small TIM sound pretty bad with sharp sound and expanded odd harmonics but more of TIM might not be so audible. Small gaps in sound are created and our brain fills missing pieces - causing fatigue after even short listening. In Class D there is no TIM since time and not the voltage is an analog quantity.
Convenience, cost, material usage have nothing to do with class D. It just sounds great. I know - I have one.
Audio research, Jeff Roland, Nuforce, Spectron, Channel Island, Kharma, Bel Canto, Pioneer, Rotel, Panasonic, Acoustic Reality, Flying mole, Digital Amp Company, Sonic Impact, Peachtree and Carver are all companies that quickly come to mind that now make a type of Class D amp.
Another benefit is Class D dissipates much less heat.
I use the Bel Canto 300 integrated amp (primarily with Sonus Faber Cremonas) and would observe that the amp is clean, powerful, and otherwise unremarkable. I have observed no real issues with the treble, but I've definitely heard more "texture" out of these speakers as well as better imaging. However, the better performance came in a different room, so I can hardly pin these issues on the amp with any confidence. If you need high output at reasonable cost and your speakers do not present an exotic load, I suspect that switching amps are a good solution. If you have more funds on hand, don't need the power, or have quirky speakers - proceed with some caution.
Marty, the good news is that John Stronczer's designs have evolved quite dramatically from the BC 300. . . you'd be amazed. . . I definitely was so, last night, with the $6K BC R1000 Mk.2 having broken the 400 hour mark.
Stanwal, have you considered quoting a little less 'old trombones', and relying on your ears a little more? If after that you still do not enjoy some of the top flight switching amps we are discussing on some of these threads, you will at least be speaking out of personal experience, rather than nostalging over the anachronistic pontifications of some old anchoret whose better times have long come and gone.
Stanwal said this and much, much more:
""Not a single master of analogue power amplifier engineering has turned to Class D. Shouldn't one know this, and, ask 'Why?
By their nature, electronic engineers are usually eager to "migrate into the future technology"."
You spout these things as if you have some special knowledge, or, perhaps, thinking that if you say them with a deep enough voice and authoritative enough delivery then people will believe you. Actually you weaken your overall arguement with such drivel. Kijanki mentioned Rowland and there are others.
If anything, my guess is that a very large proportion of designers stick with what they know and avoid migration into future technology. Othewise, how can you explain the overflow of SET tube amps on the market today? That's only my assumption, but I think that resistance to change is highly prevelant, including among posters here at A'gon.
I have found through experimenting with just one amp, the H20, that it can sound like anything from bad to terrific. They take on the personality of the preamp, wires, speakers and source.
I solved the preamp problem when I purchased the H2O's stable mate, the Fire preamp. That helped enormously. Preamps are terrible coloring agents. The Fire is as neutral as it gets.
Then, through experimentation, again, I solved the wire problem by making my own naked ribbon SCs.
The speakers are Apogee Scintillas. These speakers can sound like anything too. They respond gratefully with every positive change to my system.
The source is more important than can be imagined. The old adage, "crap in, crap out," is frightfully true. My H2O modified Audio Note DAC is a thrilling addition. Here is what a neighbor wrote me recently. It supports my notion how system support means everything. He brought a CD player that he liked to my place, and found on this revealing system it was grainy, and bright. It didn't fail as bad as another visiting SACD 999 Modright player did.
"What many of the anti-ICE amp people don't understand is the amps (H2O) are like Apogees, very neutral. If it dosen't sound right, too harsh or bright, add tubes. IMO (Apogees) don't have that problem, but CD decks, preamps.. amps, do. Henry's amp OTOH are, from all the reviews I've read, much like many Damps, have such a "black" background, (no noise) they show off everything."
So, whenever I read the same old criticisms, like the publication quote above," Poor treble seems a feature of Class D amplification," I have a laugh every time. For one thing all publications depend on advertising for their existence. Another thing is they do not care to find complimentary components, choosing to use their usual review system. But, more importantly, countless class D systems prove it is a fib,"
I was able to spend a few weeks directly comparing a pair of ICE-based monoblocks with a Mark Levinson ML-9. I found the ICE amp to be a very good amp overall, with no obvous weaknesses. I find it quite puzzling that there is such vehment opposition to "Clas D" from people who have either never done a direct comparison or only listened in passing years ago.
Truth be told, I returned the ICE amps because of two minor things my friend and I both noticed. One was that the ML-9 had tighter bass. No, the ICE amp wasn't "boomy" by any means. I'd say the ML-9 probably has tighter bass than most SS amps, so this shouldn't be an indictment of class D.
The other reason was that there was a slight veil of the high frequencies, as compared to the ML-9. I want to emphasize that it was very subtle and not noticeable most of the time. If I had not been doing a direct comparison, I probably would not have noticed it. Judging by the popularity of the various ICE amps, I'd say most people do not notice these issues.
Btw, based on a few days of diect comparison between the ML-9 and a Spectron Musician III SE Mk 2, I'd say the Spectron compares very well with the ML-9. My friend and I are not able to say for certain if there is or isn't high frequency veil, and the bass extension may be slightly better with the Spectron. It weighs 20 lbs less and uses less than half the electricity of the ML-9, which to me are "good" things. I think I may have found a keeper and can finally return my father's ML-9 :)
I want to reiterate that the class D amp did NOT have "loose bass". In fact, I would not be surprised if its bass is tighter than most other amps.
The ML simply was a little "better". That's it.
I have not had a chance to compare all the various ICE amps out there, but I am a bit dubious that the various tweaks different manufacturers claim to do actually make much of an audible difference, especially as far as the veiled highs go. That is something that seems to be an inherent problem with the ICE modules.
Has anyone been able to compare different ICE amps?
Thank you Nospam for the clarification. I would still like to know which ICEpower based amp you evaluated. Your assertion that recent production ICE amps have veiled highs across the board is an interesting proposition which I have not been able to verify. There are certainly some ICE-based amps that I heard at shows which sounded shrill to my ears, like there are SS and even tube amps that I feel the same about. There is one amp (Red Dragon) which I perceived to have limited yet ragged treble. On the other hand, I did not experience these to be generalized problems across all ICE amps.
I am currently evaluating a pair of Bel Canto Ref 1000 Mk.2s, which I experience to be extended in the treble and harmonically textured without sounding 'in my face'.
It is worth pointing out that ICE modules are not amps per se. . . they can be used to create very subtle sounding amps, or very basic ones, depending on the designers preference and philosophy. This is really the same as designing amps around 64550C or KT88 tubes. . . you do not tweak around these tubes. . . you use them as component pieces of a whole.
To give you an idea of the sonic differences I perceived within a single manufacturer, here are some capsule observations about a few Rowland amps based on ICEpower:
201 monos -- graceful but a little matter of fact and can run out of steam quickly on congested passages, in which case they turn a little 'glassy', particularly in the treble. They are fine on smaller speakers (Maggie 1.6, Vienna Baby grands), but run out of steam quickly on larger ones (Maggie 3.6, Vienna Mahlers) even on very moderate 'forte' of a chamber music piece.
501 monos -- No problem with power and authority here. . . no chance of them clipping and sounding glassy, even on difficult loads like the Vienna Mahlers. On the other hand, 501 may not yield the ultimate harmonic texture, nor the ultimate low level nuance, as such it is fair to say that they do not always draw me into the music.
302 stereo (withdrawn) -- same power/authority as the 501, but with grace and refinement in low level harmonic texture, giving the impression of an extended and musical treble. With this amp I also noticed a great deal of micro dynamics, which yield very musical string vibratos, even on pianissimo. Where this amp did not quite shine was paradoxically in macro dynamics, which IMO did not sound quite as extended as the 501, giving me an overall impression of 'politeness'. It draws me into the music somewhat, but does not quite excite me.
312 Stereo (current model) -- All the positive traits of the older 302, but with excellent macro dynamics and transient response, which make for me this amp both musically subtle, involving, and exciting to listen to. 312 is one of my very preferred amps, regardless of underlying technology. I have written a short article on the 312 for issue 188 (December 2008) of The Absolute Sound. It is a sidebar in the Vienna Mahler speakers review. . . I believe on page 88.
Hope this helps, G.
Kijanki, I have seen JRDG 102 but have never heard it. I only know that Jeff R. does like its sound a lot and is very proud of what the little critter can do for living (smiles!)
Unfortunately, I have not heard the Bel canto S300 either. . . what I can tell you is that at 475 hours of break in, Bel Canto Ref 1000 Mk.2 sounds too me very very sweet indeed while having magnificent staging and imaging. Today I listened to a CD of piano/violin/cello trios by my usual favorite author (Antonin Dvorak). The violin was somewhat to the right and closer to the backwall of the venue with all the soft phase interplay of an instrument in that position. The cello came from the very front of the stage and somewhat to the left with a clear, crisp and sweet sound. The piano sounded like a large Beckstein grand, powerful and dark and a little romantic. The performance sounded emotional while the virtual image was transparent, the instruments finely separated with a real sense of 'air' and venue around them. Should I call this perhaps an overall sense of involving musical crispness, with a subtle hint of warmth? Guido
None of the Rowland amps are comparable? That's surprising.
If can be bypassed, fine, but I would not see the point in paying for a high end integrated and only using the amp section.
My local dealer only carries the 102 and its the more powerful, high current class Ds, that compete against large class a/b monster sized amps, that are of interest to me personally.
I have the PSAudio GCC250, also an ICE based product. The difference here is the 'gain cell' inputs, the claim being it is now a variable gain amplifier, not fixed gain with an attenuated input. My Magnepan 1.6s seem perfectly happy. I believe the ASP modules have an ON-BOARD PS...a fairly small and efficient SMPS.
All amps run off rectified AC with the exception of stuff like the Emitter and some preamps, which are powered by banks of batteries. The purity of pure battery power is tough to argue with.
I work for International Rectifier and we make a class 'd' entry, of which I have a reference board which I intend to make the core of a 2nd system. Power supply issues will be obviated by use of....you guessed it.....4x12 batteries on EACH rail. That yields +-50v or so. The amp is asking for +-55v maximum, so I should be safe. If, driving normal, mid sensitivity speakers, I use an average of 12 watts, that is a play time of 6 or 8 hours, using 10amp hour batteries.
Nospam is right in my opinion. ASP modules cannot compete with a very good analog power supply. What is the advantage of introducing a digital power supply into an analog amp? The answer is, to lessen the cost of production by the amp builder.
The HF from my speakers are silky sweet. It never brings attention to itself.
I have owned the CIA UcD D200s and have borrowed a Rotel ICE amp for a while, but these are both out-performed by the Digital Amp Company's Cherry and DAC4800. DAC uses proprietary ClassD guts - Not UcD or ICE.
Disclaimer - I know the DAC designer so take my comments as you will, but it won't hurt to check them out.
"All amps run off rectified AC with the exception of stuff like the Emitter and some preamps, which are powered by banks of batteries. The purity of pure battery power is tough to argue with."
Actually, the Power Factor Correction module converts AC to DC in the 312, Continuum 500 and most other current prodcution units that are used in connection with the outboard PC1 PFC module.
When I asked Jeff Rowland if the conversion to DC provided the Continuum 500 with the same benefits as battery power (removal of AC noise) he said, "exactly." Beyond that, all I know is the Continuum 500 is, by far, the best integrated amp I've heard for powering dynamic speakers like my Vienna Acoustics.
"What is the advantage of introducing a digital power supply into an analog amp?"
First of all it's not digital supply - it is as analog as class D. In fact IT IS class D (class D was invented when designers of SMPS were demonstrating that it has such fast response that it can even play music.
Advantages are few: It is line and load regulated (while linear power supply is not) and quieter (50kHz is much easier to filter out than 120Hz). Just take as an example Jeff Rowland's Capri linestage - it uses SMPS instead of linear power supply (that would be small and inexpensive) and Jeff Rowland is not a "noisy" guy (he used batteries in preamps before). Don't be fooled by size of SMPS because 2" in diameter torroidal transformer can pass as much power at 50kHz as huge 10" torroidal transformer at 60Hz.
You made already one step accepting class D. Make another one - it is the same thing.
Mapman, unfortunately, unlike with Concerto, JRDG has not yet made an amp-only of the Continuum series. There exists the 312, which is a more complex amp only, based on the same ASP1000, and the 501 monos, which contain the same boards in the Continuum 500, minus the PFC circuit, minus the additional network of bulk capacitors, minus the Capri linestage boards.
501 monos + a pair of PC1 PFC devices should yield a sound similar to the Continuum if perhaps with slightly less authority, but perhaps with even greater channel separation. Bear in mind that C500's PFC circuit is a single 1500w device, while PC1 is a 750W device, hence you need 2 of them for the 501s.
Hope this helps, G.