Solid State Power Amp Reviews

For reasons that have as much to do with why the creek does or doesn't rise as anything, almost all the UK-manufactured electronics I've reviewed over the years were from dCS of Cambridge. Through 2023, no electronics from Linn or a host of other UK-based companies have crossed the threshold of my music room.

That situation changed when, soon after New Year's, a pair of Linn's 60lb Klimax Solo 800 monoblock amplifiers ($90,000/pair) arrived from Scotland. Right away, they delighted me with their ease of maneuverability, handsome, uncluttered look, and relative compactness. Given their impressive power output—400W into 8 ohms, 800W into 4 ohms, and a whopping 1.2kW into 2 ohms—the Klimax Solo 800s set a record for highest price per watt and per pound among class-AB monoblocks I've reviewed.

The Klimax Solo 800 had not yet been released when an early pair arrived for review. Information about it began to appear online only well after the review period began (footnote 1). There is no manual, because every Linn product is dealer-installed: It's the dealer's responsibility to make sure you're well-informed. Besides, "the amp is designed to plug-and-play, as it were," Brand Manager Joe Rodger told me in an email. For those who want something to read, a "Linn Docs" wiki for the Solo 800 can serve as what Rodger termed a "no-frills encyclopedia." (footnote 2)

To learn about the Solo 800, I Zoomed with Murray Smith, the electronics team leader who initiated the project that led to the Klimax Solo 800 and who worked on its Adaptive Bias Control system, and Linn Senior Electronic Design Engineer Nina Roscoe, the design lead for the Klimax Solo 800. Years ago, Linn sponsored Roscoe's attendance at Cambridge University, providing sufficient industrial electronics manufacturing experience to allow her to take her second- and third-year exams. When she finished her studies, she spent a year or two at Linn, doing some work on the long-discontinued Klout amplifier before moving on.

"It was a really good experience," she said. "It's a good educational system—one that actually gets a design engineer to spend lots of time in manufacturing. You understand how the products that you make are built and learn the implications of the design decisions you make. The process creates reliable products and makes for long-term customers."

New product, new design goals
At Linn, the Klout was followed by the Solo 500. Intended for the smaller power market where the majority of loudspeakers present 4–8 ohm loads, it remains in production after almost 25 years. Eventually, acceding to requests for a higher power monoblock that could drive larger speakers with lower impedances, Linn decided to create a new flagship monoblock that would utilize recent improvements in electronic and component design and architecture. By this time, Roscoe had returned to Linn.

"I think probably the biggest performance advance we were aiming for in the Solo 800—the thing we felt makes a huge difference to a power amplifier—was lower distortion," she said. "We went all out to bring distortion down from every source and choose topologies and components that give you the lowest possible distortion. We were really aiming for the purest signal reproduction. While distortion levels in the older Solo 500 are really quite good, there is room for improvement, especially when amps are driven really hard by large, lower-impedance speakers.

"We were able to do more with more modern architectures and more modern components. Previously, we hadn't really done a lot of the design work necessary to drive difficult speaker loads. Mostly, we'd just aimed at 8 ohm speaker loads rather than at much bigger loudspeakers that sometimes present really difficult loads. We wanted the amp to perform with excellence even when it was working really hard." Many engineers initially focus on measurements as they build prototype after prototype. Once their measurements are the best they can achieve, they turn their attention to sound quality, "tuning" the sound. Roscoe and her team proceeded differently.

"We didn't really feel we needed to tune the amplifier sonically," she said. "We designed for very, very low distortion levels, very low noise levels, wide bandwidth, and low output impedance, all of which are obviously measurable. Then, when we listened to the design, we felt that it sounded so clear that there didn't seem to be reason to do more. I don't know what we would have tuned really. We didn't feel like we could improve it by listening to it and changing something empirically. Instead, we heard what you would expect from an amplifier that measures as well as the Solo 800 does. It really seemed to pick out music better.

"Ultimately, when 'tuning' the sound by listening and modifying the design, designers usually select which distortion is least offensive to the ear. However, when there is so little distortion there, this becomes a more meaningless exercise."

Short of resorting to class-D (which obviously they did not do), how could Linn get so much power out of an amplifier that weighs a teeny bit less than 60lb, is relatively small, and manages to run at a stable, cool temperature without the use of a fan?

"The amp weighs enough and is big enough to ensure it runs at a nice, cool temperature with maximum volume levels and average kind of music," (footnote 3) Roscoe said. "The fins, which are machined from a solid block of aluminum, are vertical; this design allows a lot of air to move up through them. We've also got vents underneath the bottom plate and in the groove around the lid. We tried different heights and sizes of the heatsink until we perfected a design that sucks air up from underneath to help cool the transformers in the switching power supply and the driver transistors on top inside of the board."

A switching power supply
"We also use an efficient switching power supply. We at Linn feel quite strongly that switching power supplies are a superior way of powering an amplifier because all the power supply noise moves out of the audio band. Our switching power supplies maintain very high switching frequencies that can't come near the audio band. The 50Hz, 60Hz, and 120Hz [supply-related spuriae] are very low in these amps, and the power supplies are very efficient."

Roscoe said that linear power supplies require very large transformers, which are very inefficient. "The first problem with a linear power supply is that it's working at 50 or 60Hz, which is right in the audio band. When you try to filter the noise and get ready to power an audio amplifier, generally what happens is you get a certain amount of mains noise and frequency leakage. ... It delivers DC voltage to the output, which then needs to be converted/regulated down to the lower voltages needed to power a lot of the circuitry in the amp. Doing this throws away quite a bit of power. ... Switching power supplies ... upconvert the power frequency to create a very high-frequency AC signal that requires a much smaller transformer. With a lot less copper and much smaller cores, switching transformers generate less heat and operate faster, at higher frequencies.

"Some people in the audio world don't like switching power supplies because they switch very fast and, in their opinion, create a lot of switching noise. If that noise does end up in the audio band, it can make things worse. That's why we make sure the switching frequency is way beyond anything that the human ear is ever going to pick up, even when we deliver 1.2kW into a 2 ohm load."

"I think the reason switch mode power supplies have gotten a bad rep in the audio world is that they were originally used mainly for computers and other applications where their kind of electrical noise levels weren't that important," Smith said. "You can't just stick a switching power supply from a computer into an amplifier and achieve a good result. ... You really have to understand all the principles behind them before they can meet all the myriad audio requirements of being silent and noise free.

"Linn has been using switching power supplies for a number of years. Designing them properly for audio requires a lot of hard-to-find, specialized skills. ... We take the money that other manufacturers usually spend on the iron and copper in their big transformers and spend it on sophisticated electronics instead."

A problem with linear supplies, Smith told me, is that because they operate at mains frequencies of 50 or 60Hz, their noise falls right in the audible range: 60Hz and its lower-order harmonics. This is often heard as a low-level hum or buzz. Finally, "traditional linear transformer supplies also lack any form of regulation. As a result, their output voltage is at the mercy of the mains voltage in your house.

"Short of buying a large mains generator to keep voltage constant, a good audio switch-mode power supply can keep voltage constant without dumping audible noise into the audio band. Instead of taking power in short pulses, switching power supplies can be designed using a Power Factor Correction circuit, which draws power in an even and consistent way, creating no more disturbance than an old incandescent light bulb (albeit quite a big one). That makes the amplifier a much better citizen on your mains network. It's far less likely to interfere and interact with household appliances or other components in a hi-fi system. Switch mode power supplies also feed the output they generate back into a control circuit that maintains constant and steady output voltage, thus protecting your audio output from any disturbance on your mains supply."

Adaptive Bias Control
While developing the Klimax Solo 800, Smith helped develop Adaptive Bias Control to address the crossover distortion created in class-AB amplifiers when the set of transistors that generates the positive half of the musical waveform hands over the signal to the set that generates the negative half. As Smith explained during an extremely cogent discussion, "We have this handover problem as the signal crosses through zero while it goes from positive to negative."

All linear amplifiers apply a bias voltage across the output transistors to set bias current during crossover. The applied bias must constantly adapt to compensate for the changes in temperature, voltage, and current that transistors undergo as they react to the signal passing through them, while also accommodating aging effects.

"It's almost impossible to do that with exact correctness," Smith said. "If you have too much correction, then you can end up with thermal runaway, where the devices just get hotter and hotter until they destroy themselves. That's why most compensation schemes are deliberately 'detuned': to make them safe. In addition, as the amplifier warms up, distortion characteristics can change. Quite often, that's the reason for long warm-up times in amplifiers: They may need a certain amount of time to settle to a good performance characteristic.

"Because no two transistors are the same, transistor circuits need adjustment from amplifier to amplifier in order to maintain correct bias. ... With Adaptive Bias, we measure the bias condition of the transistor and then use a digital control algorithm to maintain bias at a completely constant level regardless of temperature or transistor type. ... It's not an easy thing to do in a purely analog sense because the signals that you need to measure aren't easy to extract. The bias signal that we want is mixed in with the music signal within the amplifier."

Linn monitors the current flowing in the output transistors and uses a digital algorithm to extract the bias signal from the music signal. "That allows us to control that parameter on its own," Smith explained. "It's something that you can only achieve by having good knowledge of traditional analog amplifier design, digital electronics, and digital signal processing. It's a kind of marriage of two different worlds. Digital signal processing is my specialty, so that's the part of the design I was involved in."

He emphasized, though, that the audio signal is not digitized. "The signal path remains completely analog. But we have digital assistance to ensure the amplifier performs consistently and runs at its best. It's almost like the engine-management system in a car. ... We have a completely linear transistor amplifier with a digital engine-management system that is keeping operating conditions constant and ensuring that performance is always optimal."

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Footnote 1: See linn.co.uk/us/power-amps/klimax-solo-800.

Footnote 2: See docs.linn.co.uk/wiki/index.php/Power_Amplifier/Klimax_Solo_800.

Footnote 3: When asked to define "average," Roscoe said, "What I meant by 'average' is really the standard 12dB 'crest factor' which is usually applied to music. It dictates the average power handled by a power amplifier when operating just below clipping. Some tracks might push the amp a bit harder, but so far nothing has really managed to get them really hot, even while driving challenging 'difficult' speakers."