GRANDPA’S AXE

 

Dick Pountain /Idealog 330/ 07 Jan 2022 02:10

Grandpa’s Axe is a figment of American folk mythology: “had it nigh on 60 years, never give me no trouble exceptin’ fer two new heads and three new handles”. Well my hi-fi system is very much like that. I’m very far from being a hi-fi buff but I do listen to a lot of music and like reasonably good sound. I bought an ex-review Sansui system back in the days when Dennis published Hi-Fi Choice, but the only component remaining from that is a pair of wood-cased Castle speakers that I love. Nowadays all my audio feeds – Smart TV, Spotify and YouTube on Chromebook, Sony CD player and Dunlop vinyl turntable – play through them. However just before Christmas my latest amplifier, a Denon which also acted as the hub, expired with a horrid death rattle. Not being up to diagnosing and repairing it, I went online to look for a replacement.

It appears ‘separates’ hi-fi amps are a dying breed and accordingly are subject to ‘Veblen Pricing’ as rich folk’s toys, but I became intrigued by the new breed of tiny power amps meant for bookshelf systems, and found myself buying a Fosi bt20a. When I first unpacked it I thought it was a joke, not much bigger than a packet of cigarettes, but then I plugged it in and reeled in amazement as it drove my massive speakers just as loud, and with superior, more open sound quality than the Denon…

Now since this isn’t an audiophile magazine (we don’t even have audio devices in the A-List) how can I steer this miracle around toward my digital brief? Well, I assumed the Fosi must be digital inside, but I was wrong – it’s even more interesting than that. It turns out to be a ‘Class-D’ device, a technology which exists in a grey area between analog and digital which I’ve covered here before, for example in the silicon retina chips designed by Carver Mead or AI chips that use analog adders to perform convolutions. Hybrid analog-digital circuits can often be faster and less power-hungry as they don’t need to analog-to-digital convert their inputs and then digital-to-analog convert the outputs. The price is loss of the precision that digital brings, but for some non-numerical applications that may be worth paying.

Class-D amplifiers are a nice case in point. They work by chopping up an analog input signal into a very high-frequency stream of square-wave pulses whose spacing represents the analog values at each tiny interval. This stream then switches twin output transistors at a similarly high frequency, then filters out frequencies beyond the audible to directly produce an amplified analog output signal. This process can be 70-90% efficient: because the two output transistors are never both on at the same time little current flows and little power is lost as heat. The Fosi uses a phone-style 24-volt power supply and barely gets warm even when playing loud. It’s still a wholly analog technology as no bits are involved, but like a digital technology it works on a stream of discrete pulses, and can therefore be supported by cheap, miniature digital chips.

The efficiency and power saving such hybrid processing confers is becoming ever more desirable to designers of the neural networks on which Deep Learning AI depends. Implementing a simulated neural network in digital technology is enormously power hungry. Training a network involves storing billions or trillions of weights in memory cells, and then performing multiply-accumulate operations on them and new inputs when the network is eventually deployed to analyse new data. Back in issue 301 I described how the US firm Mythic’s IPU chip contains an analog computing array of memory cells that are actually tunable resistors: computation happens in-place as each input voltage gets turned into an output current according to Ohm’s Law, with resistance representing the stored weight value. With data and computation stored in the same place less energy is wasted and less silicon real-estate is needed for A-to-D conversion. 

Carver Mead’s silicon retina chip similarly employed an array of photosensitive cells connected by a network of resistors, with Kirchoff’s Laws achieving the necessary ‘computation’ as currents flow between them to achieve behaviour similar to that of the human eye.

Fitted with its second new head and third new handle, my audio environment is now immensely satisfying, even if it is so hybrid that it would cause a real audiophile to grind their teeth. The little black box omnivorously devours analog feeds from TV, CD player and turntable (through an equally tiny analog mixer) just as effectively as Bluetooth from my Chromebook. And it passes my own very analog test regime: Wayne Shorter’s ‘Footprints Live’ album makes my remaining hair stand on end via either route.

[Dick Pountain thought DAC was a brand of trousers until he discovered Smirnoff]