Iron OOR
Stereophile|February 2022
GRAMOPHONE DREAMS
HERB REICHERT

I’m deep into audio power amplifiers because they remind me of race car engines. Both power sources are wildly inefficient, converting only a small percentage of their stored energy into work while dissipating the rest as heat and vibration. Mainly though, I love how race car engines sound. How they shake the air. Just like amps and speakers.

Audio-frequency power amplifiers take a small input signal-usually less than 1V-apply it to an input impedance of (typically) no less than 10k ohms, and make this tiny eyeblink of power (0.0001 W) into a waveform that's many times more powerful, capable in some cases of doing hundreds of watts of speaker-moving work. To grasp the magnitude of this chore, and what I am about to describe, it might be useful to imagine power amplifiers as energy adders.

The majority of audio amplifiers I've encountered were built-in closed metal boxes (with power cords), and every high school grad knows that energy is conserved: neither gained or lost. Therefore, if we want more energy at the output-if we want the amplifier to amplify-then we need to import energy from an outside source like our household AC line) and store it in something-capacitors-that's engineered to hold and release a rapidly varying stream of energy.

Audio frequency amplifiers are notoriously inefficient. Therefore, amplifiers of the highest fidelity must not only store (by necessity limited) volumes of stable standing energy in their capacitors; they must also have effortless access to a flow of fresh energy through their power cords (to fill and refill those capacitors) and, equally important, a direct, low-impedance path to an infinite source of free electrons from our good and gracious Mother Earth.

Importing and outflowing energy with some amount of reliable precision is a messy, variable-plagued business, which is why an audio frequency power amplifier can only be as effective, linear, and exciting to use as its power supply allows it to be.

The Ferrum HYPSOS power supply

When I saw my first two-page magazine ad (in our sister magazine Hi-Fi News) for the Ferrum HYPSOS “Hybrid Power System, I smiled and felt validated. The ad looked enticing, and its points were well-stated. I wondered where this Ferrum company came from and how the Ferrum people had the courage to make and market what appeared to be a fully formed hip’n’cool-looking variable-voltage audio power supply. They must have known I hated wall warts and that I think switching supplies are bean-counter ruses to save money on transformer iron. Ferrum's logo is Fe, which is the symbol for the element iron and the abbreviation for the Latin word “ferrum”; which is Latin for iron, which, in its usual metallic form, is a rather dense, crystalline substance capable of being magnetized.

The Ferrum HYPSOS Hybrid Power System and Ferrum's matching OOR headphone amp (auditioned below) are both made in Poland by the same company (HEM) that distributes Clarus Cables in Poland and until recently manufactured Mytek products.

On their website, Ferrum Audio says that the HYPSOS is their first product, calling it a “linear/switching hybrid power system” that accepts 110-240V AC in and can output a user-selectable DC voltage from 5V to 30V at up to 6A. It's priced at $1195.

On their website, Ferrum claims that their hybrid design offers the best of switching and linear supplies.” Via email, Roy Feldstein of VANA (Ferrum's US distributor) explained Ferrum's choice to make the HYPSOS a hybrid design: Linear regulators have low ripple and noise as well as fast transient response. Switching power supplies have higher efficiency. Ferrum wanted it all.”

The engineering team at HEM/Ferrum told me in an email that the design of the HYPSOS supply begins with a noise-reducing input filter to clean incoming AC. That's followed by a stepdown transformer, which feeds Schottky diodes and four 4700uf electrolytic capacitors (for smoothing and storage). That's all pretty standard. After that is where it gets interesting.

According to the HEM engineers, “That first linear supply feeds a low-ripple switching supply (SMPS) through a single-stage filter, and there is a two-stage filter on the output of the switching supply. The design and implementation of these input and output filters are critical to the performance of the HYPSOS. To further reduce noise, the SMPS has a spread-spectrum mode. This mode modulates the switching frequency by 20%. This technique reduces EMI because the spectrum of noise generated by multiple switching frequencies and their harmonics is lower in magnitude than those of a single switching frequency.

“Following the SMPS is a low-noise linear regulator. This final regulator is a low-dropout design based on an NMOS [negative channel metal oxide semiconductor] transistor. In the context of linear power supplies, the term ‘low-dropout’ means that the device can regulate the output voltage even if the input voltage is only slightly higher than the output voltage. In the HYPSOS, the output of the switching supply is less than 1 volt greater than the output voltage of the linear power supply. This attribute increases the efficiency and reduces the heat output of the supply. An ARM-based microprocessor adjusts the duty cycle of the SMPS and the reference voltage of the linear regulator to vary the output voltage. This processor also controls all the other functions of the unit.”

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