December 25 2024                                                     Post Number 613

Ho, ho, ho
I don't like the idea of Christmas arriving without a post from me. True, I did post on the 18th of this month, but that seems oddly long ago. Besides, I love December's opportunity to post the photo of Jane Greer again.

Digital-Analog Hybrid Subwoofer
Back in my Post 407, I detailed how a digital electrostatic loudspeaker could be built that effectively transforms the electrostatic loudspeaker into a DAC whose analog output would be sound, not voltage or current. My last post was devoted to my staggered subwoofer idea, the one wherein a two subwoofer drivers would be used, one big and one small, with the small one handling all the output at low volumes or with music that didn't present low-frequencies, the big driver only switching on when needed.

Well, while I pondered this idea, the idea of making a digital subwoofer quickly filled my mind. By digital, I mean that the internally powered subwoofer could accept a digital stream of data and converts it into thundering bass. This idea also requires more than one subwoofer driver—but not 16 (or 24) of them.

Imagine using the most significant four bits of the incoming data stream as the signal to drive four woofers, while the remaining lessor-significant 12 bits would feed a DAC, whose analog output signal would drive an internal power amplifier that drove a small, high-quality woofer. To see how this hybrid arrangement could work, we must look at how the 16 bits of a CD can produce ±65535 levels of output signal. (Missing from the following table is a "sign" bit that denotes positive or negative.)

Click on table to see enlargement

Each successive bit doubles the value of the previous bit. By the selection which bits are "on," we can produce any integer between -65535 to +65535. If we limit ourselves to just the first four bits, 1 to 4, we can produce any integer between -15 to +15, including zero; if we limit ourselves to just the last four bits, we can produce any integer between - 61440 to -4096, 0, and 4096 to 61440—in increments of 4096; in other words, once again 31 integers. (Do not forget zero.) The four subwoofer drivers, therefore, can attain 31 possible arrangements of either not moving or of pushing out or sucking in. Each successive driver should offer half the emitting surface area of its preceding driver; say a 30in, 15in, 10in, 7in, as a circle's area is given by its radius squared against pi (3.14159). Since 30in subwoofer drivers are not made (at as far as I know), we simply double on 15in drivers for the MSB.

The four digital subwoofer drivers are each driven by its own three-state power amplifier that delivers only three output voltages, for example, -24Vdc, 0V, +24Vdc. In other words, these woofers can only be thrusted out or slammed in or left in a neutral center position. Imagine this digital-analog hybrid (DAH) receiving an analog input signal of 1Vpk from a line-stage amplifier or a DAC or a streamer. This signal would travel through a volume potentiometer and then travel through a steep low-pass filter, say 3rd-order or 4th-order at 60Hz to 120Hz. From the low-pass filter the signal would enter an analog-to-digital converter (ADC). The ADC sends the first 12 bits off to a DAC, which converts the digital back into analog signal to drive the internal solid-state power amplifier that drives the fifth woofer, say a 7in Hi-Fi woofer. Here is the overview.

Click on image to see enlargement

If a purely digital input signal is delivered to this subwoofer, via USB cable or WiFi, there's no need for the ADC, as the steep low-pass filtering can be performed in the digital domain through digital signal processing (DSP). The three state power amplifiers could be made from solid-state switches, such as solid-state relays, SCRs, triacs, transistors, MOSFETs. (If mechanical relays were not so expensive, bulky, slow, and noisy, they could be used.) These solid-state switches would dissipate very little heat, in spite of the high current flow, as they would experience very little voltage drop when fully engaged.

Class-D amplifiers use switching output devices that are either fully on or fully off, which also results in little wasted heat. The solid-state power amplifier that drives the analog subwoofer driver need not deliver all that much power, as the truncating off of the four largest bits means that this driver need only see 1/16th the peak voltage that the 16th-bit driver sees. (Actually, things are a bit more complicated here, as the 15in drivers might hold 2-ohm voicecoils, so the pair would draw 8 times the current than a 8-ohm woofer would. In other words, the analog solid-state power amplifier's gain would have to be adjusted to match the output from the digital drivers.)

The four three-state solid-state power amplifiers might need dissimilar bipolar power-supply rail voltages. Why? Air is not linear, especially when the trapped air inside the sealed enclosures is small relative to the loudspeaker driver's VAS. In other words, it's harder to compress air than it is to expand it; thus, to get equal positive and negative cone travel, dissimilar power-supply rail voltages might be needed. At low volume levels, the digital woofers might not be called upon to deliver huge compressions and rarefactions of the air, leaving that task entirely to the analog woofer and linear power amplifier.

WiiM Ultra Review
I couldn't wait, so I bought a WiiM Ultra streamer, as soon as it was available, well before Black Friday or Cyber Monday. I needed it to join my existing WiiM Pro Plus. Why? So that I could time align the output from my subwoofers and main speakers. I have rearranged my listening room so that my two subwoofers no longer sit below my main loudspeakers, but now sit in two corners, which effectively increases their output by 12dB due to the two boundaries formed by the two walls meeting (think of it as semi horn loading). In front of them sit my main loudspeakers, which thereby get a jump start on having their sound hit my ears while the that of the subwoofers catch up 8 milliseconds later. By staggering the streamed digital signals, giving the subwoofers a head start, the sound sources fall into time alignment. Is that important? Big time. Before, the subwoofer bass seemed omnipresent, sloshing about the room. Now, the bass is localized to the music instrument that produced it. Mission accomplished, although I could have achieved the same results with another (cheaper) WiiM Pro Plus.

(The rectangle in front of the speaker is the cat's scratching pad.)

A time-shifting signal is easy with digital, but a nightmare with analog.

The assumption behind the WiiM streamers is that we could fill our house them, giving each important room its own streamer. To prevent the time delay from creating an annoying echo, the WiiM software allows us to adjust the timing between grouped units.

In fact, the software includes a utility to auto adjust the latency between WiiM streamers. The way this works is the software uses your smartphone or tablet's microphone and software-generated tone bursts to evaluate latency.

This is my setup, but I knew an audiophile who used his two giant subwoofers as end tables for his listening chair.

He would need to invert my setup, as he would have to delay the signal sent to the subwoofer amplifiers.

The optical SPDIF digital signal feeds my Schiit Gungnir revision-B multibit DAC, which in turn provides the analog signal for my two single-ended mono amps. Actually, the optical output from the Ultra must first pass through two iPurifer2 re-clockers in series before reaching my DAC.

In contrast, the analog output from my old WiiM Pro Plus is sent off to the subwoofers internal power amplifiers. My digital music collection resides in my PC in my workroom. WiFi connects the collection to the WiiM devices.

What about my LP collection? The Ultra holds a phono preamp that can accept either MM or MC cartridges. In my system, the LP phono signal, on the other hand (after it leaves my tube-based phono preamp) arrives at the analog inputs on the back of the new WiiM Ultra, where its internal ADC converts the signal to digital (at 24-bit and 192kHz). Why? This way the same time-alignment technique can be obtained, as the subwoofers need to get their signal about 8 milliseconds before the main speakers do. How well does this work? Amazingly well. The odd feature is the tiny time delay in hearing the LP's music when the needle is lowered and then still hearing some music when the needle is raised, as the newly created digital signal is stored in RAM within the WiiM Ultra.

In short, I now do my signal selecting and volume controlling all via my Samsung tablet. This time, less is more. In other words, I do not need a line-stage amplifier with its input selector switch and stepped attenuator. What about tube sonic glory and sweetness? I tried using my harmonic-restoration 12Vac Aikido unity-gain buffers on the DAC's output signal, as I do not need any extra gain or a volume control. How did the harmonic restoration sound? Voices became more human, more palpable and emotive. Unfortunately, the extremes of stereo imaging weren't as fine. My first thoughts were too much harmonic restoration; then, I thought it more likely that the coupling capacitors needed an upgrade, which will be hard, as the PCB is relatively small. The longer I pondered the situation, the less likely the coupling capacitor upgrade would do much good. Why not?

With the DAC directly feeding my mono power amplifiers, I have the ideal grounding setup, as no right-left ground current flow admixture occurs. If I used a stereo power amplifier, admixture would occur within the amplifier and not sound as good as the two mono amplifiers. Adding my harmonic-restoration 12Vac Aikido to the mix undid the absolute ground separation. In other words, the coupling caps might not be the issue. Ideally, the best solution would be two separate tube-based buffer circuits, each with its own isolated power supply and, possibly, its own enclosure. See my Post 434, which is one of my finest efforts, as I strove to make the invisible visible.

Okay, back to the WiiM Ultra, it has a USB jack on the back that can be used either to drive a DAC or to accept an external USB hard-drive or SSD. I experimented with several 256G USB thumb-drives. Not all of them worked perfectly, however; faster storage was definitely better as far as the WiiM app on my Android tablet was concerned. I then tried a Crucial 2T 6X-speed SSD (tiny and light, only 2 2.5 inches and thin) with MP4 movies on it and to which I added about 100 albums. It worked perfectly, sending all the album info and cover art to the tablet. I then bought a Crucial 2T 9X-speed SSD and filled it with about 1.6T of my favorite music, which according to the WiiM app totaled 2,942 albums, 1,439 artists, and 32,962 tracks. In other words, my computer is now out of the loop. WiFi is still used to communicate between the tablet and the WiiM streamers and between streamers. Crucial 2T 9X SSD connects to the WiiM ultra's USB jack through a short USB cable that came with the SSD.

Interestingly enough, I can see the USB SSD connected to my WiiM Ultra in my PC Window's File Explorer—all share the same WiFi network after all.

I can copy and download files from this SSD to my PC, but I cannot paste new files or delete existing files on the SSD from my PC. Bummer. This would be a great feature, as I could prune and add to it as new albums arrive. The WiiM streamer holds a CPU and runs on the Linux operating system. I am going to write to WiiM, asking them to make a change in the file access permissions for the external USB drive, which would allow users to transfer new albums to the USB drive via WiFi from their PCs.

I listened to the WiiM Ultra as a streamer, with WiFi streaming data from my PC, listening to its own internal DAC (ESS Sabre ES9038Q2M) through its line outputs. Not bad, not bad at all. In fact, 99% of normal people would be thrilled; and about 50% of audiophiles would be as well. Very spacious sounding. WiiM states that the Ultra offers lower jitter than the Pro Plus, which might explain the better sound over the older unit. (Apparently the Ultra's DAC is linear to 21 bits, which is astounding.) My guess is that to achieve better streamer sound would require moving up to the iFi Audio NEO Stream, which cost almost four times more than the Ultra.

Much like the 5000-page bills that the US Congress must pass before it knows what the bill contains, you must install the WiiM app to see the huge array of features it offers—fortunately, unlike the 5000-page bill (and Roon), the app is free.

In short, the WiiM Ultra is an amazing bargain. It comes with cables and a remote—all in fancy box worthy of Apple, but without the $$$ price tag. It can do just about any audio goal, both digital and analog, you desire. In fact, if WiiM had decided to house the unit in a fancier box with a heavy, milled, metal knob, and fancy RCA jacks on the back, audiophiles would gladly pay over $1,000 for the unit; add a bigger touch screen, $2,000.

PS, 2800 CDs
How do you store 2,800 CDs without using a forklift or dolly? Simple: throw away the execrable plastic CD cases. That is what I did. My daughter and I long ago removed the CD, the front and rear cover sheets and any booklet, and placed the CD(s) in a paper sleeve and stuffed all of it into a clear plastic sleeve 5 by 6 inches. A 3-inch-tall stack of CDs now takes up about one inch.

Two CD shelves are pressed together, with each holding seven rows, and with each row holding about 200 CDs. (In their old plastic cases, only 80 would fit in each row.) If I had retained the the grim plastic CD cases, five of these cases would be required to house all 2800 CDs. By the way, this is not my complete audio CD collection, as I have boxes filled with audio-book CDs.

//JRB

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