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Classe Sigma Series AV Processor and Amplifier Interview

by October 21, 2014
Classes Sigma Series Family

Classe's Sigma Series Family

The folks at Classe had a little surprise for us at CEDIA this year with the unveiling of their new entry level line, the Sigma series. While we were able to get some of the basic information, i.e. power ratings, inputs/outputs for the Sigma SSP pre/pro, etc., the finer details were relatively sparse. As such, we thought it might be a good idea to ping our contacts to see what more we could learn. Imagine our surprise when David Nauber, President of Classe stepped up to the plate to answer our questions. What is Classe's new Sigma series all about? Keep reading to find out!

Audioholics: What drove Classe to bring the Sigma series processor and amplifiers to market? What were the design goals of the Sigma series?

Classe: People who want something better than what mass market brands have to offer find that the cost to step up to the next level can be a shock. That’s because high-end companies, with their higher manufacturing costs and smaller sales volumes, struggle to make a reasonable value proposition, even at two or three times the price of mainstream components. Classé is unique among high-end audio brands because we assemble our products in our own B&W Group factory in Zhuhai China. It’s just a fact that we can put more into the products because it costs us less to manufacture them. This gives us the opportunity to build components that are competitively priced above the top of the mainstream offerings and below the entry level products of other legitimate high-end brands.

The Sigma series is all about showing how high-end technology and performance can be affordable. The design goal was performance and quality at the most competitive price. We have focused on delivering exceptional value, which isn’t to say these are cheap products; they certainly aren’t. But what you get for the money is quite a lot more than what you’ll get from other brands’ even more costly products.

Sigma AMP2 & 5

Beauty shots of the Sigma AMP 2 and AMP 5.

Lastly, I would say that I think there are a lot of people out there with a TV between their speakers and they want a high-end stereo preamp that can also handle switching and processing for the signals from their AV sources like set top boxes and Blu-ray players. There are also a lot of people who can’t or won’t make a dedicated theater room in their house, so they want high-performance music and theater capability available in a multi-purpose room. In these scenarios fewer rather than more channels are used so we saw an opportunity to make an SSP which has fewer channels and emphasizes higher performance, particularly in the front L&R channels. We then packaged our new class D amplifier technology in stereo and five-channel versions to match. 

Audioholics: Where does the Sigma series fit in the Classe model lineup?

Classe: Sigma is our entry level, positioned at about half the price of our Delta and Custom Theater (CT) series models.

Audioholics: What are the retail prices for the Sigma series components?

Classe: The Sigma SSP is $5,000 as is the 5x200W amplifier called the AMP5. The AMP2 is a 200W stereo amplifier that sells for $3,500.

Sigma SSP Questions

Audioholics: Which version of HDMI is the Sigma SSP preamp/processor using? If it is using 1.4a or less, are there any plans to offer an upgrade to full bandwidth HDMI 2.0 with HDCP 2.2? Will early adopters be able to take advantage of such an upgrade?

Classe: The Sigma SSP uses HDMI 1.4 hardware on a separate video board, which can be upgraded at such time as an upgrade makes sense. We have already seen numerous 4K early adopters get burned and the carnage is not over. You mention full bandwidth and for HDMI 2.0, that means 600 MHz, and those parts aren’t available yet. As they do become available we’ll have to see if the advantages of running at 600 MHz are worth the additional challenges and limitations.

Sigma SSP Processor

Beauty Shot of the Sigma SSP.

We make expensive products with long lifecycles, so we look closely at these things and move carefully. Once the hardware and software are mature enough to give a few years of stability and we have real native 4K sources to switch, we will offer an upgrade. Our SSP-800 was the first high-end SSP with HDMI 1.4 and we offered customers with original HDMI 1.3 version units an upgrade path, so I’d say we have some credibility on the subject. You don’t want to buy an HDMI 2.0 equipped unit today only to find that it won’t work once you really get some native 4K sources to switch. And be forewarned that that is guaranteed to happen.  

Audioholics: Does the Sigma SSP have a true analog bypass mode? If so, does bass management engage in this mode if the user wants to use it?

Classe: Yes, we have a true bypass mode, although we now call it digital bypass, because the signal is bypassing all of the digital conversion and processing. With our CP-800 stereo preamp/processor it is possible to generate a sub out in this Digital Bypass mode, but since the L&R channels remain analog throughout you cannot do any high pass filtering on them.

Audioholics: Please tell us about the bass management options on the Sigma SSP. What options are available in the Sigma SSP for routing LFE and bass from channels set to small, i.e. will users be able to redirect material to “large” main L/R channels?

Classe: Yes, if you have no subwoofer enabled, then the LFE channel and the low frequencies from any channels set to x-over (small) are divided among the full-range (large) channels in your configuration. One powerful feature of the Sigma SSP is that you can create up to six speaker configurations to support numerous listening situations. For example, you might build configurations for stereo without sub, stereo with sub with its crossover point at 50Hz for music, stereo with sub crossed at 80Hz for TV and another for full 7.1 channel surround. You can tailor levels and delays in one configuration to suit music videos while another works best with conventional movies or TV. Each can then be assigned to a source so when you choose that source its configuration is loaded. You do a little work up front to make it all happen, but then you don’t even have to think about it. Just choose the source and the SSP sets the appropriate channels, crossovers, levels and delays automatically.

Sigma SSP Internals

Top view of the Sigma SSP with the cover removed.

Audioholics: What room correction system does the Sigma SSP utilize, i.e. Dirac Live, Audyssey MultEQ, etc.?

Classe: The Sigma SSP has a nine-band parametric Q for each channel. Gain, Q and frequency are adjustable with frequency adjustments being in 1Hz increments below 200Hz, 10Hz increments up to 2,000Hz and 100Hz increments above that. We don’t offer an automatic EQ in the Sigma or even in our $9,500 SSP-800 because we don’t believe they work consistently well enough. Our preference is to use EQ sparingly to tame low frequency room modes. With some inexpensive measuring equipment and human judgment you can achieve better results.

Audioholics: How many subwoofer outputs does the Sigma SSP support?  If multiple outputs are available, do they have independent delay and level adjustments?

Classe: There is a single sub out. We debated adding additional capability but we resisted the temptation. One of the important qualities of the Sigma SSP is that each channel is a high-performance channel with the L&R channels receiving special attention. We didn’t want to dilute the performance of the limited number of channels we have to add more subwoofer flexibility or anything else, for that matter. For a full-on home theater our SSP-800 and CT-SSP are available and they support up to three subwoofers with independent delay and level adjustments.

Audioholics: Will the Sigma SSP be upgradable to support new audio formats like Dolby Atmos, Auro-3D or DTS-UHD?

Classe: The Sigma SSP uses a Texas Instruments-based DSP that is contained on its own separate pc board along with other associated parts. This module may be replaced with a newer TI DSP with one or more 3D audio decoding features whenever it makes sense to do that. Customers will then have the option to upgrade if they feel they need to. We upgraded the SSP-800 from a single DSP to a dual DSP to accommodate HD Audio, so as with HDMI, we also have credibility offering upgradable DSPs.

It’s debatable how popular 3D audio will be among the target customers for the Sigma SSP, especially considering its 7.1 channel limitation. For dedicated theater rooms where a customer has four or five times the budget, it makes sense to pursue 3D audio. Why not? But similar to the 4K video situation, what you buy today isn’t likely going to be what you will want in a year or two anyway, so having patience and an upgrade path is a better long-term purchasing strategy and that’s what the Sigma SSP offers.

Audioholics: Can you please explain the design decisions behind omitting legacy video inputs in the Sigma SSP? Is any video scaling available for standard definition sources?

Classe: The Sigma SSP is differentiated as much by the connectors it doesn’t have as by those it does. The fact that it has an Ethernet port for streaming audio and IP control as well as USB host and device connectors but supports no analog video tells you that this is a forward-looking component. It will have the right connectors for your system for the sources and displays you’ll buy now and five years from now. 

Analog video carries with it a high cost, both for the circuitry if it’s of good quality and the back panel real estate its connectors consume. The Sigma SSP achieves its high performance/price ratio by ruthlessly eliminating extras in order to fit the essentials into a slim, high-performance package. If someone really needs analog video, then the Sigma SSP is not for them, but I’d question why someone would be playing at this level with analog video sources or displays anyway. There are ways to accommodate it if you have a real need for analog video so for the few who do there are options.

We do not include any video scaling capabilities for several reasons. First, there are very few sources today that are standard definition, so it’s of limited utility. For those few SD sources that do exist, the display can and should provide the scaling. No sense adding yet another scaler to the system. The SSP is an audio product and video is handled as efficiently and sparingly as possible. In general, and this applies especially to Blu-ray players with 4K scalers inside, you should let the display handle the scaling. To create all that extra data and burden the transmission system with it makes no sense at all, especially when the people who designed the display (assuming it’s a good one of course) ought to know best how to scale images to suit it.  


We’ve noticed that the Sigma SSP has an Ethernet port on the rear panel. Can you tell us what streaming audio options are on board?

Classe: The Sigma SSP works as a renderer for both AirPlay and DLNA sources, so it supports streaming of all types up to 192 kHz. There is also a master clock mode employed to use our own clocks for the data rather than the clock embedded in the Ethernet module. The result is higher performance than you will find in other components that support both AirPlay and DLNA streaming.

We take a specific view of streaming that should perhaps be explained. For streaming through a network you need three things: a Media Player (iTunes, JRiver, etc.), a Media Server (a computer like either a Mac or PC, or a NAS drive with Media Server software) and a Media Renderer. The Sigma SSP is a Media Renderer. We believe that these three things should be kept separate and not combined into a single product. By keeping them separate, consumers are free to choose whatever interface (Media Player) and storage device(s) (Media Servers) they want to use on the basis of their merits, and they are free to change any of them if and when they see something else they want to try. It is for this reason that the Sigma SSP does not include its own Media Player or support for Pandora, Spotify or other streaming services onboard. You can use all of those services with the Sigma SSP but a smart phone or tablet is the preferred interface device. It’s a fast moving world and how and what we choose to listen to should not be tied to the hardware that makes it sound its best.

Sigma SSP Rear View

Rear panel of the Sigma SSP pre/pro.

There is no storage of music files inside the SSP (it’s not a Media Server) nor is there any Media Player software built into it. The reason is that whatever you put into the chassis is only really right for a small percentage of potential customers. How much storage is enough? What if I don’t need any storage because I have all my files on a NAS drive? And what about the interface? There is no such thing as the perfect Media Player interface. What works well and seems intuitive for one person might be a nightmare for another. Keeping these things separate from the expensive high-end audio components just makes sense.

As the Media Renderer, the Sigma SSP is seen by the Media Player and the consumer selects the Sigma SSP in the Player (using AirPlay in iTunes, for example) and streams to it. The idea is that we are the experts at processing and amplification, so you give us the data however you want and we’ll make them sound their best. We devote our resources to delivering the performance and we leave the customer free to choose the best interface and storage for their needs. I hope that all makes sense! It’s an important subject and one that takes a bit of explaining to get across.


Sigma Series Amplifier Questions

Audioholics: Why did Classe opt to go with a Class D amplifier topology with the Sigma series? Can you also explain the decision to utilize an SMPS power supply? Can you discuss the pros and cons of this approach versus a conventional Class A/B design with a linear power supply?

Classe: The Sigma AMP2 and AMP5 designs are based on the Delta series CA-D200 class D amplifier introduced earlier this year, so what I will say about these Sigma series amplifiers applies the to CA-D200 as well.

The short answer is that we felt that our own class D amplifier solution with Switch Mode Power Supply could offer the best performance including sufficient power for the intended application in this price range.

Sigma Amp2 Internals

Top view of the Sigma AMP 2 with the cover removed.

 Design is a process of choosing the best tradeoffs given the overall goals, so the choices you make at one price are often different than the choices you’d make at another price. For example, you can build class D amplifiers with power ratings of 100W or more at very low prices as evidenced by the choices made by designers of mainstream AVRs. Once you get above the lowest entry level price points though, linear amplifiers tend to have fewer sonic compromises. They are physically larger and less efficient than their class D counterparts however, so if factors other than price and power are important, class D may still win out. As you go higher in price it becomes possible to implement class D designs that address the sonic shortcomings of the less expensive class D implementations and at this level a real competition exists with the sonic merits of good linear amps.

The Classé design uses proprietary circuitry and DSP to effectively eliminate dead-band-time, which is the Achilles Heel of class D amplifiers. This is the time when both halves of the amplifier are off; it is similar to switching distortion in a linear class AB amplifier but more insidious. Even small amounts of dead-band-time results in high distortion, which leads to other design choices to try to control the distortion, which in turn begins a downward spiral of performance. By addressing dead-band-time up front we can have low open-loop distortion and apply only a modest level of negative feedback to optimize performance.

The SMPS with Power Factor Correction allows us to provide rock-steady supply voltage to the amplifier circuit and over 1,000W of power in an efficient and relatively small package. To achieve anything like this from a linear supply would cost far more, be quite a bit larger and not as efficient. The SMPS with PFC also has the advantage of pulling power from the wall more smoothly than linear supplies, meaning that any audio components sharing the same AC Mains circuit won’t even know it’s there. In other words, it helps the other components sound their best by not causing the AC to sag with demand and by avoiding high frequency harmonics that a linear supply would generate, polluting the line.

Our class D amplifier solution with SMPS and PFC give the best overall performance and power at this price while bringing other class D benefits such as small size, low heat and efficiency to the table. If you have double the budget or more, then we would see sonic advantages of linear amplifiers emerging to edge out the class D design, but even there it’s not an easy, obvious choice.

Audioholics: Is the frequency response of the Sigma series amplifiers independent of load impedance, or is it optimized for an 8 or 4 ohm load?

Classe: Class D amplifiers use passive devices at the output to remove the switching frequency and tailor the frequency response to some degree, so their frequency response can vary depending on the load. In this respect they share some common characteristics with tube amplifiers. The Classé amplifiers are stable into whatever speaker load you want to drive and frequency response characteristics will depend on the speaker load itself, but variations are small compared with other class D designs. It’s the interaction between the output filter and the speaker load, which changes with frequency, and how they may or may not influence the sound of a given speaker that matters. For a resistive load in a test setup, the amps measure best just above 4 ohms, but that’s not indicative of anything in particular as it’s the reactive performance with a real speaker that tells the story. Have a listen and see how well it works with whatever speaker you choose.


About the author:
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Steve Munz is a “different” addition to Audioholics’ stable of contributors in that he is neither an engineer like Gene, nor has he worked in the industry like Cliff. In fact, Steve’s day job is network administration and accounting.

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