As an introduction to any discussion on audio amplifier measurements (as a means of comparison between different models/output topologies), it’s very important that they are not taken as “complete truth”, for several reasons.
The test signals used to do most of the measurements are nothing like the signals that the amplifier was intended to be used for. In the case of music/speech reproduction, these are dynamic signals with typical Crest Factors of 4 or above (CF 4 equates to an average of 1/8th power). Sometimes heavily compressed or synthesized music may even be as low as CF 2.8 (1/4er power) but most material is actually much higher than CF 4.
The measurements are usually made with resistive loads which give very different results to the reactive, and sometimes negative impedance loads, that a loudspeaker presents (especially those with complex passive crossover networks). Different makes/models of loudspeakers have a huge variation in the loads they present, and an amplifier that sounds very good on a particular loudspeaker may not be so good on another.
There are also many figures stated on data sheets that either have no relevance in assessing audio quality or are very ambiguous. The philosophy at MC² is to keep specifications brief/relevant and try not to baffle users with technical jargon. We take pride in designing some of the best sounding amplifiers available in the world, which is verified time after time in blind listening tests. This is the best way of qualifying sound quality.
Every person’s hearing ‘profile’ is unique, which is why many professionals have different opinions about the merits of one ‘set up’ over another. So, the conclusion is, as always, to engage in listening tests in the environment that the equipment is likely to be used in.
Slew rate is a limitation that applies to opp amps and linear Class A or Class AB amplifiers where the rate of change at the outputs is finite and potentially causes distortion overtones or other nonlinearities with very high amplitude, high frequency signals.
The most important factor affecting this is that the amplifier must be able to handle all the signals that are thrown at it. In other words, there must be enough filtering to ensure that the amplifier’s ‘Slew Limit’ is not exceeded.
In common with many manufacturers, we do not quote a slew rate since it is not helpful in comparing different designs. There are so many ways of determining slew rate that a direct comparison of quoted figures is impossible. Mostly any figures quoted relate to the output amplifier and ignore the effects of preceding stages, which are often much more relevant.
Our unique linear topology (Current Driven Sliding Opp-Amp) ensures that the output stage slew rate is largely dependent upon the op-amp and RFI filtering which we use to drive it. Unfiltered, an MC² S1400 current driven output stage has a slew rate of well over 100V/microsecond but the amplifier bandwidth is restricted to –3dB at 100kHz.
Our latest Ultra Sigma amplifier topology is effectively devoid of slew rate limitations so it’s not relevant or possible to quote a figure for it. That’s why we quote a rise time. It takes 10uS for signal to change from one voltage to another be it 1V to 2V, 1V to 20V or -40 to +40V. Assuming the preceding equipment has no slew rate limitations (i.e. produces a perfect voltage step change) then it’s always 10uS. That’s because the Ultra Sigma design is the filter (set at 40kHz) and cannot ‘Slew Limit’ like linear Class A, Class AB designs.
Many manufacturers calculate the damping factor from the components in the output chain, and do not consider the various conditions which can influence the figure. We quote a damping factor of >400 (8R, 1KHz) for our live sound products, and this is measured at the speaker terminals, so external to the amplifier which we see as giving the correct figure. We regard anything above 400 to be inconsequential because other external factors will have more influence on this figure, such as the speaker cable and the connectors between the cable and the speaker cabinets.
Over the years we have noticed some of our competitors have various ways of rating the damping factor of an amplifier, using irrelevant testing means to sell a product by putting this onto a datasheet. It’s almost like some manufacturers feel a datasheet is a sales tool above an actual physical demonstration, a notion we completely disagree with. We are more focused on real world ratings, and ultimately how these translate to the end user in the sound presented to them. We have seen spec sheets with >5000 quoted and it would appear that this is because the measurement has been taken internally in the amplifier, so by the time it gets to the loudspeaker it has passed through various components that reduce the rating dramatically.
To clarify the difference between class A and class AB amplifiers, they both operate in the same manner, the difference being that in the class A design the devices that are not driving current into the load do not completely switch off, whereas in an A-B design they
(almost) do. Some A-B designs have a very abrupt switching point, and some are gradual. MC² use a gradual approach which tends to have much lower distortion but does generate more heat.
Tube (valve) amplifiers are a different matter, these are high impedance devices which require transformers in order to drive low impedance loudspeakers. They tend to have quite high distortion figures, but it is usually 2nd harmonic which is quite melodic. They also handle transients in a different manner to transistors, and also introduce dynamic high frequency filtering. This sounds particularly good when used for musical instrument amplification such as guitar amps.
Class D amplifiers operate by directly switching power, on and off, at extremely high speed & frequencies using advanced MOSFET’s (or other high-speed devices). A modulator is used to vary the size or density of the switch pulses to achieve an average output power that represents the audio; and a low pass filter (typically a coil and capacitor) is used to remove the high frequencies generated by the modulation, leaving only the audio signal to feed the speaker.
Class D topologies inherently eliminates the midpoint cross over distortion associated with Class AB designs so that linear operation, similar to Class A, can be achieved; and yet the power switching devices are only ever operated in a linear mode for a fraction of the time making the amplifier much more efficient (reducing the need for massive heatsinks).
In early Class D designs the maximum switching speeds possible (then) could cause resonances or limit the audio performance at high frequencies, but modern designs from MC² (ULTRA SIGMA) using ultra-fast switching components (operating hundreds of times faster than audio frequencies) totally eliminate these drawbacks and often extend bandwidth beyond what is possible with linear designs.
When selecting amplification for a loudspeaker there are a few schools of thought when it comes to power requirements. Watts are often quoted in RMS or AES and relate to amplification requirements as Program Power. Headroom is the power available in the amplifier above the power rating of the loudspeaker. We have always advised that the amplifier should be approximately 1.5 to 2 times the power of the loudspeaker/driver rating to ensure good dynamics and a safe operating level for the amplifier.
Headroom is important and must be looked at carefully on different drivers/loudspeakers for both safety and sonic purposes. When listening to high resolution audio with high dynamics the amplifier should be able to successfully recreate the transient peaks with little or no effort, to ensure that there is no limiting/clipping of the signal. All the listening tests we have done with Studio monitors, PA, and Hi-Fi speakers has shown that there is a very discernible difference between the sound quality when an amplifier has lots of headroom, especially in passages with high dynamics. We would consider headroom as extremely important when specifying a system, particularly when using source material with a high crest factor.
A bullet proof, classic linear design was chosen with a low noise custom wound 500VA rms transformer, capable of delivering enough power for an amplifier 4 times the size! This ensures that the U500DC is never stressed and has next to no losses. In essence the stereo amplifier performs as if it were two separate mono blocks; in fact, from here on in it is…. Four 35A 600V rectifiers are usedbut each are only working at 80V, but able to deliver peak power with ease anywhere from 5 to 5,000 Watts! This is then delivered into a huge 27,000uF bulk storage capacitor bank… and that’s just for the left channel! The whole process is then repeated completely independently for the right channel ensuring no interaction between the two is possible and confirming ultimate L R separation and uncompressed powerful bass propagation.
Special attention to detail has been made with the component placement ensuring no 50Hz noise can be picked up from the power supply. This includes a physical separation barrier between amplifier/power supply but goes beyond that with use of separate ground return current paths and differential interstage coupling, ensuring total silence between music tracks.
A completely brand new push/pull output stage has been developed specifically for this product, and refined over the last two years through multiple listening tests. It uses supersonic switching techniques, 20 times higher than human hearing, to reduce resistive losses 100 fold compared to older class AB designs and totally eliminates crossover distortion (like a Class A amplifier). Overall control of the push pull power stage is achieved by a single feedback loop; this “ultimate in simplicity” is one of the mechanisms to the open, transparent, and expansive sound the U500DC delivers.
For the last 50 years or more it has been commonplace to have capacitors in between the separate stages of any audio product. This is because they eliminate small errors that are typically present which can cause clicks or pops or even damage speakers. But no matter how good the capacitor quality they can not improve the sound, they can only minimize how much they degrade it (skewing the phase of bass content in transients). To overcome this there are no interstage capacitors inside the U500DC; it only uses direct (copper tracks or wires) DC coupling, and it eliminates any errors that may be presented on its inputs using a single overall outer loop DC servo. This ensures total alignment of transients, amazing phase coherence across the entire audio bandwidth, and strong focused low-end (bass) response.
The power amplifier is fed directly from a Class A driver stage that collects differential signals from the input connector PCB and adds voltage gain and soft clipping protection. For those wanting to maximise power delivery (for bigger systems) the gain can be increased by 3dB and the soft clipping circuits removed using an internal link. Additionally, for those wanting a more vintage type of sound delivery a “TIMBRE” switch on the rear changes the input circuits from “Pure” to “VINTAGE” to operate more like traditional class AB amplifiers. This is a very subtle change, but it can be heard. Our in depth blind listening tests in development of the U500DC gave a 50/50 split as to what was preferred!
WHAT PEOPLE ARE SAYING
Dynamic, perfectly controlled bass
Turkey, Aug 2020
“Before I used many amplifiers…Because my ATC’s are difficult to drive (power hungry!). Now I drive them with an MC2 S1400 and I am discovering what the ATC 20 SL PRO speaker is really about! Dynamic, perfectly controlled bass, transparent and detailed. The MC2 S1400 with ATC Speaker is perfect matching. You can listen all day long with no fatiguing. Now the (…) amplifier is up for sale!”
A massive difference to the quality of my output
UK, July 2022
“When I first installed the U500DC II was absolutely blown away by the detail that became apparent in my mixes. I run 2 PMC TB2s passively crossed over through a PMC XB1-P. PMCs are notoriously power hungry and my old Quad 909 struggled. Just didn’t have the real power to drive them without issues. I eventually put in an active crossover and powered the tops and the bass unit separately. Not ideal but at least it worked. When I went back to the recommended way of powering this rig with your amplifier the difference was night and day. Effortless reproduction of every part of the programme. I was knocked out! This unit REALLY shifts some air and I am looking forward to it making a massive difference in the quality of my output.”
You should hear it to believe it!
Hungary, December 2012
“MC² Audio S1400 in a very serious company: Devialet D-Premier as wifi streamer, MSB Technology Platinum IV Signature DAC/preamp and Focal Grande Utopia Speakers – a mere 160k – 170k Euro system. This is a setup where the S1400 can show how good it really is. If you want something even more realistic and better, you can use two S1400s to bi-amp the Focal Grande Utopias. You should have to hear it to believe it.”
The difference is crazy
UK, November 2019
“The difference is crazy; it sounds like I have been listening to a broken amp for the last 6 months!!??. The S800 is so clean sounding. I left all the EQ’s the same, did not feel it needed to be changed which was not what I was expecting. It’s so interesting listening to the difference in sound presentation. The bass is not so present, as in full… but I would say that is because the S800 has far greater grip and control over the drivers, so different bass frequencies are much cleaner and tightly controlled. The different low frequencies are presented as individual tones / notes rather than a bit of a mush. I did get to turn it up a bit when the neighbours went out, and errrm… Liked That!! I have not heard those speakers so well controlled at that sort of volume before, which was really lovely to hear.”
What really stood out was the stereo imaging
The Netherlands, Feb 2022
“I did a comparison between the S800 (my reference amp in my demo room) and the new U500DC. Loudspeakers used were the Danley SM100 and SM60F. No processing used. Nice, clean and tight sound. Just like I am used to with the S Series. I then switched to the new amp… Both Rick (my colleague) and I were stunned by the even better sounding U500DC. More detail in LF, hearing more detail generally in the song, and what really stood out was the stereo imaging, it was much better. This is best to describe as Full HD to 4k! My opinion is that the S Series are already very good sounding amps. I have compared these to many competitors over the years and they always came out on top, BUT: the new (U500DC) is from a different planet! Congratulations to you.”
An enlightening experience
Charles Teofil Aryee
Switzerland, Feb 2020
“Using the S-800 in my system is an enlightening experience and a very healthy reminder, that it is always worth to consider each and every option available, however unusual they may seem at first. Using a quite industrial looking, beefy studio amplifier in a domestic setting is usually not the first choice, if at all. I consider myself lucky, because I had been able to experience the renowned MC-1250 amplifier 10 years ago in a very expensive audio chain and went away dumbfounded. ”
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If you have any questions or require information about any of our products, please don’t hesitate to get in touch.