October 2012

The Herniator Amplifier
Build your very own high quality Class D amp.
Article By Jeff Poth

Difficulty Level

  Class D is a technology that has been used for a few decades, but really only has taken hold in audio in the past 20 years or so. Spectron was an early example of a quality Class D amplifier used in hi-fi, with President John Ulrick building a low distortion commercial Class D amp way back in 1974. Spectron leverages a post-feedback filter; with connection points and specialized cables that allow post-speaker cable feedback (Hypex has this available on some of their latest products). Later on, OEM manufacturers began releasing modules and boards similar to what are used by DIY'ers today. Tripath and Bang & Olufsen had their respective Tripath and Icepower technologies used in this market, and supplied a number of major manufacturers. These include Bel Canto, Jeff Rowland, Motorola (in their very cool but ugly DCP-501 receiver), and Sonic Impact. Another player was Phillips, who paid an engineer named Bruno Putzeys to develop a technology, now known as Universal Class D, or UCD. Phillips retains the right to this technology, but Bruno's company, Hypex, licenses the rights to it from Phillips.

I've been working with Class D since about 2003. I started with LC Audio, and then moved from the Zap Pulse modules from that company to Hypex's UCD. Class D for DIY'ers generally takes the form of small compact modules from a supplier, with case and power supply by the DIY'er. This is a sort of hybrid format of DIY, where the heavy engineering is done professionally, and the time-consuming main construction done by the builder.

Basics Of Class D
Class D is an amplifier topology. In audiophillia we often hear about Class A, Class AB, Push-Pull, and other terms. Class A means that the output devices (tubes or transistors) are run at full-power the whole time the amplifier is powered on. This generates a large amount of wasteful heat, which limits the amount of power you can push through a Class A amplifier, and requires more power supply than other more efficient topologies. Accordingly, Class A amplifiers are fairly rare, though single ended Triodes, or SETs, have a dedicated following in audiophillia. Solid-state, or transistor based, Class A amplifiers typically range from 10 to 50 Watts. Pass Labs is a key proponent of using Class A transistor output stages despite the limitation in power. One good reason for this is that a type of distortion called "crossover distortion" is eliminated by Class A biasing. This biasing can be applied along with other details, so "Push Pull" isn't necessarily Class AB, though it typically is. Some push-pull amps are Class A, most OTLs are Class A, and so on.

Class AB is more common, being the topology we see in typical solid-state or push-pull tube amplifiers. This is where the amplifier runs power through the output device on an alternating basis. The output devices are in pairs, with one of the two handling the positive side of the waveform, and the other the negative. This allows one of the two devices to take a break while the other is pulling duty. There's overlap to allow the handoff from one tube or transistor to the other to be clean. (Techies please don't mind the layspeak). By giving the devices a chance to power down (and cool down), they're able to handle much more power, and achieve higher efficiency, hence its popularity. The "A" in class AB represents the overlap region. A class B amplifier doesn't have this overlap and thus is more distortion prone, particularly the distortion at the 0V output point, aka crossover distortion.

In Class D, the amplifier's output devices are switched on and off rapidly, typically several hundred thousand times per second (300 to 800 kHz). The louder the sound should be, thus the longer each pulse lasts. In other words, it is wider (and Class D is also known as Pulse Width Modulation). This series of pulses isn't an audio signal, but when run through a low-pass filter, the pulses are smoothed into a waveform like that one is used to seeing on an oscilloscope. By modulating pulses in this way, Class D can have more "downtime" than a Class B/AB amp and achieve very high efficiency. With that high efficiency comes higher output as the two major limitations to power output, power supply and heat, are both improved by efficiency. Every watt of output from an amplifier must first pass through the power supply, so a more efficient amplifier makes the demand on the power supply lower, and thus a given power supply can provide higher amplifier output with Class D vs. other amplifier topologies. Likewise, the power wasted by the amplifier circuit is wasted as heat. Because the wasted power is low (the definition of efficiency), the amplifier doesn't get as hot, and as such can have higher output before temperatures become problematic. Also, one can use smaller heatsinks and that represents a space and cost savings for manufacturers. Additionally, Class D can run at very low distortion levels, including the dreaded "crossover distortion".

The downside of this is the need for some additional complexity in the circuit, needing additional complexity to run the high frequency switching, and an output filter (typically a two pole LC filter) to limit noise from the high frequency switching components. In some designs this filter creates phase shift and bandwidth limits at high frequencies. This is a major problem in some designs, but the UCD is run differently, with feedback in the circuit being applied after the output filter, rather than before. This is the same way feedback is applied in the Spectron amplifiers, which are renowned for their high sound quality. It takes additional effort to ensure that the amplifier stays stable, and that's where the engineering comes in -- a poorly designed Class D amplifier with post-filter feedback will be unstable, potentially damaging the amplifier... or the loudspeaker. Also critical in Class D is board design- small, compact boards are able to control switching noise, and dramatically reduce the challenges associated with the switching behavior.

Herniator
As mentioned, I've built a number of Class D amplifiers. Four of them have used Hypex modules; the most recent of these is the Herniator. Back in maybe 2004 I was commissioned to build a powerful amp for a friend of mine. I ordered a custom toroidal transformer with interwinding shield and flux strap, as well as four secondaries. This provides one winding for each the positive and negative power supplies, per channel. Not only is it custom made for the job, but more importantly very high quality and low noise. One of these transformers was used for my friend's amp, the other set aside for mine. When working with the hypex modules, or other Class D modules, the rest of the build is pretty straightforward with only chassis, connectors, and a power supply. With B&O's ICEpower modules, some even have onboard switchmode power supplies, so they just need to be wired up. Hypex also makes switchmode supplies, which are on separate boards from their amp products, but are nearly as simple to build out as the ICEpower product.

The Herniator is based off the UCD400 module, providing 400 wpc into 4 Ohms with 1% distortion at 400 Watt (cutoff point for measurement). At 350 Watt, distortion is closer to 0.05%, very clean, and gets better as power drops down into lower ranges. My speakers are more like 8 Ohms and so you only get half as much output current, but at still lower distortion due to the lower current demands placed on the transistors. 175WPC of very clean power... not too shabby, my setup is likely achieving 0.03% THD or less from the UCD amp. It could also do nearly 600 WPC or into 2 Ohms, before the power supply runs out of zoot. Alternatively, a couple additional channels could be added... Hmmmmmmm... Stay tuned!

Power Supply
Hypex offers power supplies for so-called "linear" power supplies, where a transformer drives rectifier bridges and a bank of capacitors. More recently, they've added switchmode power supply boards to their lineup. The Class D amplifiers I've built have all relied upon linear power supplies rather than switchmode. Ironically, even when building a switchmode amplifier (Class D bears some significant similarities to switchmode power supplies), many people are apprehensive about the use of switching power supplies. Much of this is related to the low quality of many switching supplies as a bad switchmode supply can be noisy. This is not only for the device using it; it is also noisy for the power line supplying it. Bad switchmode supplies are horrendous noise sources, contaminating everything they're wired to, and spraying airborne RFI all over the place. A typical or poor switchmode supply is worse from a sound quality and noise perspective than all but the worst linear supply. Have you ever noticed that many power supplies nowadays (aka wall warts) are very light and compact? These are switchmode supplies and probably making a mess of your already dirty wall power.

Modern switchmode power supplies of high quality, like those from Hypex, do not suffer from these same issues. They can be extremely low noise, low impedance, and high quality power supplies. Since I had a very high quality power transformer in hand already, I built a high quality linear supply. I utilized a quad of high current "Hexfred" rectifier bridges, low noise diode bridges whose job is to turn the alternating current (AC) to direct current (DC). After this is a high capacitance, low resistance, CRC supply. The resistance allows the first set of capacitors to better suppress power supply noise, but at the same time it limits the ability of the supply to rapidly "refill" the operating energy storage capacitors that are connected to the amplifier boards. Given the size of the supply and low ongoing current demands of the high efficiency Class D amplifiers, this was determined to be a good tradeoff in this case.

The power supply has four bridge-rectified power supplies, one + (positive) and one – (negative) supply for each channel (so multiply the above schematic by four, and put two supplies per channel in series so as to create a bipolar supply.) Solid state amplifiers are typically arranged this way as it allows the DC bias on the output of each output device to be cancelled, resulting in a net 0 VDC output. Equal but opposite, you see. These rectifiers then feed into a CRC supply with multiple parallel capacitors and series resistors in parallel, as well as film bypass capacitors to ensure good high-frequency behavior. Wiring is 14 AWG, and arranged to give a low impedance path from amp board to capacitor supply. Additionally, the UCD modules have bypass capacitors mounted on the board.

Build Details
The modules are mounted to heatsinks, though in many cases this is unnecessary due to the high efficiency and thus low heat dissipation of the Class D circuit. Most builders simply mount them to a "normal" aluminum project box and call it a day, but I like to be very conservative about heat, and used heatsink compound between the on-board heatsinks and the chassis heatinks. The thick aluminum chassis heatsinks had to be retrofit to the chassis, which is an old Odyssey Stealth enclosure, repurposed several times both by its previous owner and now by me. I made oak rails to adapt the screw pattern of the chassis to that of the heatsinks I had on hand from Apexjr. For safety, the heatsinks had to be grounded to the chassis, so that's accomplished with some lockwashers and a pigtail.

XLR connectors and Cardas binding posts are utilized. Since UCD are inherently balanced mode (and properly balanced- many amps aren't), much of the benefit of balanced mode noise cancellation can be achieved through a properly constructed RCA to XLR cable. This requires the negative and ground connections to be tied together at the RCA connector. Easy enough for an old wiremonger like myself, so some nice adaptor cables were tossed together.

As with many of my projects, this is not a "recipe" but rather a "how I did it." No doubt you'll wind up with a different chassis and other construction choices if you build a UCD amplifier. Remember to keep the construction safe. If you're not following "double insulated" construction details, you will need to ground the chassis to "safety ground". You'll need to properly fuse the power input wiring, at least. I used no insulator between PS boards and chassis, and a modest airgap, so I chose to ground the chassis to safety ground. If in doubt, this is a good choice for safety. I will be refining my layout to shorten the power supply leads, and improve the layout in some other ways, but that's a tale for another day.

Sound Quality And Final Notes
I like the UCDs, which should be obvious from the fact that I've built several amps around them. They are very listenable, with nothing "jumping out" as a detraction. If I had to complain, I'd probably point to an overly mild top end, but really, that's if you're looking for trouble. Built properly, these amps (as with most modern, well made amps) are far from the limiting factor in a proper high-end system. More personality is contributed by even extremely good preamplifiers. They're quiet, powerful, and well-behaved, and as such can be used with a huge variety of loudspeakers and preamps. They present an easy load to the preamp, and a powerful, low impedance output to the loudspeaker.

I'm not a believer in an "amp for all seasons". With many single driver designs, a higher output impedance amplifier, or even a current source amp a la Nelson Pass' "First Watt" designs, is a better match, bringing the bass and treble up like a built-in EQ. Some good SETs have a vitality, whether it is the presence of something or the absence of something else, that is hard to deny. OTLs are very cool, and can have a great sound quality- I own a Transcendant SE-OTL, which is a very good amplifier.

All that said, the UCD amp represents a very high quality execution of class D, with high power, low distortion, low heat and power supply demands, and low noise. Most modern loudspeakers would be very happy with this amp behind them. I have not yet had a chance to work with the latest and greatest Hypex flagship "Ncore" technology, though the accounts I've seen speak exceedingly highly of it, even compared to the UCD tech. That speaks volumes to me. Hypex has already begun trickling down their Ncore tech into some of the UCD products, so that's pretty cool. All told, this is a terrific amp, and UCD is a technology well worth building around.