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Sony Interactive Entertainment to Acquire Audeze
Sony Interactive Entertainment - the Sony division responsible for the PlayStation products - announced that it has entered into an agreement to acquire Audeze, the renowned planar magnetic headphone manufacturer, which has been increasingly also recognized in the high-end gaming headphones space. The acquisition, Sony says, will strengthen SIE's efforts to improve the audio experience of PlayStation games, while Audeze will continue to operate independently and develop multi-platform products. Read More
Volt Introduces 10" Radial Triple Suspension Midbass Driver
British OEM speaker manufacturer Volt Loudspeakers launched the new RV2501 Radial Midbass driver, its first 10" Radial model, designed to deliver into applications that need consistent performance with its larger counterparts. With 91dB/W/m sensitivity, a frequency response between 30Hz to 800Hz at 8 Ohms - which enables its use well into the midbass range - and rated for 250W AES power, the new RV2501 not only expands the Volt range for monitoring solutions in smaller studios, it could also find its way into home hi-fi designs. Read More
SB Acoustics New 7.5" MT19CP-8 Satori Coaxial Driver Expands High-Quality Options for Speaker Designers
The latest product in the growing SB Acoustics Satori high quality transducer range is the 7.5" Satori MT19CP-8 coaxial driver featuring an Egyptian papyrus fiber cone developed and perfected in its own cone/spider facility in Indonesia, matched with a dedicated 26mm high performance soft dome tweeter. A well-engineered driver for refined home audio designs that leverages on the extensive R&D that already characterizes the Satori speaker driver range. Read More
Røde Wireless PRO Expands Leadership on Wireless Microphone Systems
Røde has once again reacted quickly to the increasing competition in a market that it helped create, adding the most requested features to its just announced Wireless PRO wireless microphone system. The Wireless PRO is a compact dual-channel wireless microphone system using Røde’s state-of-the-art Series IV 2.4GHz digital transmission with 128-bit encryption, able to achieve stable audio up to 260m (850ft) in line of sight. It adds 32-bit floating point recording, advanced timecode, and much more. Read More
U-Turn Audio Reveals Next-Generation Orbit Turntables
U-Turn Audio, a small, independent American maker of turntables based in Woburn, MA, announced the first refresh of its Orbit Turntable lineup. This next generation of turntables adds enhancements to its Orbit Basic, Plus, and Special models, and the ability to personalize a product with Orbit Custom models. The turntables feature high-tech magnesium tonearm, electronic speed control, revamped drive system, and more options than ever before - starting at just $249. Read More
Bowers & Wilkins 600 Series Updates Entry Point to Famous Brand
When confirming a new production batch from a popular product series that has been in very limited stock, why not improve its features using updates created for recent products? Bowers & Wilkins is doing precisely that and building upon three decades of tradition in high-performance loudspeakers with the launch of its new 600 Series S3. The eighth generation of one of hi-fi’s most acclaimed ranges features some of the most comprehensive upgrades the 600 Series has ever received. Read More
Dotterel Technologies Introduces Konos Directional MEMS Microphone Array
It's not every day that we see a new type of microphone showing up in the market. Intent on revolutionizing audio capture, Dotterel Technologies, a startup from Auckland, New Zealand, introduced the Konos microphone system, a new 80-element, high sensitivity MEMS array, enabling adjustable directivity and optional real-time noise filtering to capture the clearest sound at a distance, even in the most challenging sonic conditions. Read More
Metaxas & Sins Ethereal Electrostatic Headphone Amplifier/Tourbillon Clock
Among the very exclusive number of highly recognized ultra-high-end audio builders, Metaxas & Sins, a Greek family affair based in Switzerland with headquarters in Amsterdam, is by far the most exquisite. All the company's designs are simultaneously design masterpieces and beautiful, unique art pieces. The most recent proposal is the Ethereal Electrostatic Headphone Amplifier, combining horology (the study of mechanical time-keeping devices) with reverie. Read More
Bryston Introduces the BP-19 Preamplifier
Bryston has announced the introduction of an ultra-high-performance BP-19 analog preamplifier. According to the Peterborough, Ontario-based manufacturer, the BP-19 leverages years of R&D and many design innovations that were achieved during the development of its best-selling BR-20 preamplifier. The BP-19 is meticulously handcrafted in Canada and comes with Bryston's legendary 20-year warranty. Why would you need another preamplifier, particularly if Bryston already offers great options? The answer is in the performance. Read More
Guest Editorial
Jan Didden
(audioXpress Technical Editor)
Audio Myths
Feedback Cannot Fix What Has Already Happened
First this: It was pointed out to me that my conclusion in the previous Newsletter was incorrect. The impulse marker signal cannot be used to conclude that there was no delay. It is more subtle and complex, and if you wish to read up on it, I suggest René Christensen's article "Misconceptions in the Audio Industry," audioXpress June 2023, which discusses in depth the related topics of group delay and phase delay.
If I would have gotten a dollar for every time someone mentioned that negative feedback cannot work, because you cannot correct something after it happened, I would be a rich man. Yet, all it takes to debunk that statement is to look around you. Someone riding a bike – negative feedback in action to ensure a smooth ride without crashing. Jet plane flying overhead – negative feedback making sure it stays overhead unless you deliberately want to return to mother earth. You - relaxed and breathing regularly, thanks to negative feedback that regulates your breathing based on an error signal – the amount of carbon monoxide in your bloodstream.

Yes, it can be hard to get your head around feedback. We see a circuit where a forward signal passes though some devices and passive parts that all cause phase shifts and distortions and then part of it is fed back to the input side and summed with the input signal, and we expect it gets there in time to correct the errors before they get created?
Figure 1: A first-order roll-off negative feedback amplifier used in the tests.
Then How Does It Work?
For a good illustration of the workings, I need a situation where we clearly see how the feedback corrects deviations from the desired output in real time, so to speak. Figure 1 above shows the circuit diagram. Let’s set the scene with a DC input signal of 1V. The amplifier has a gain of 1000 and rolls of at about 1.6kHz. The output signal is inverted by E2 before being sent to the feedback loop, to maintain negative feedback.

The nominal gain from R3/R2 would be 22k/1k = 22 times with an infinite open loop amplifier gain (E1). E1 only has a gain of 1000 though, so the closed loop gain will fall short of 22. LTspice tells me V(amp-out) = 21.5V. The amplifier input signal V(amp-in), which in the infinite open loop case would be zero, turns out to be 21.5mV DC. And that squares beautifully: that 21.5mV amplified 1000 times by E1 gives an output of 21.5V, which is indeed what we measure. But this is steady state – how about fast-moving signals or music?

Figure 2a shows the circuit of Figure 1, but with a modified input signal: a fast pulse exceeding the capabilities of the amplifier: the input pulse rise and fall times are shorter than the amplifier rise and fall times. Figure 2b shows the inputs and outputs. In the top pane of Figure 2b the red curve is V(amp-out), while the blue curve is Vsource x 21.5 (remember, 21.5 is the circuit gain). Ideally, the curves would overlap; V(amp-out) would be exactly Vsource x 21.5. And a bit down the line, they do. But the source rises faster than the amp can follow so there’s a deficit in V(amp-out) for the first dozen microseconds.

During the initial period, with V(amp-out) a bit too low, the voltage across R3 is a bit too high (Figure 2a). That means that the junction between R3 and R2 is a bit higher, which means the input to amplifier E1 is a bit higher, and that is beautifully shown by the green curve in the lower pane of Figure 2b. The input to the amplifier rises in response to the lagging output, to around 500mV at its maximum. Remember, in steady state with DC, that V(amp-input) was just 21.5mV; and down the line, after a few tens of microseconds, V(amp-out) gets to 21.5V and V(amp-in) gets to 21.5mV. Through feedback resistor R3, the deficit in V(amp-out) causes a rise in V(amp-in) which makes up, partially, for some of the output shortfall, although it doesn’t so completely. The feedback cannot totally cancel or compensate for the error – there still is a shortfall in V(amp-out) during the fast rise of the input.
Figure 2a: Fast rise time pulse applied to feedback amplifier.
Figure 2b: The response shows lagging output and temporary increase in amp input due to feedback.
So, What Did the Feedback Give Us?
Suppose we would have built an amp with a gain of 21.5 and no feedback, by taking R3 out of the circuit of Figure 2a and setting E1 to 21.5. The output signal is now as shown in Figure 3a, which has the same time scale as Figure 2 and it is very slow to rise towards the expected value. Figure 3b expands the scale twenty times, and yes, eventually V(amp-out) gets to 21.5V, but it takes ages without the help of feedback!

I illustrated the workings of this circuit with a fast-rising pulse, but the exact same reasoning is valid for a signal that is distorted internally in the amplifier. For example, with some third-harmonic distortion, the amplifier output positive peak might be a bit too high, and the negative peak a bit too low (or vice versa) in amplitude. Same difference – the output voltage deviates from the target, and the feedback divider causes a change in V(amp-in) to counter the error. And the error is not nulled, but decreased by an amount determined by how strong the feedback is, which ultimately depends on how much larger the E1 gain is than the target gain. Any deviation in V(amp-out) from the wanted value, whether it is due to distortion or inability to follow a fast signal, is called the error. Feedback decreases the error but cannot null it.
Figure 3a: Fast rise time pulse applied to non-feedback amplifier shows extended amplifier rise time.
Figure 3b: This figure shows the signal for a 20 times longer time duration.
Phase Matters
I used a separate inverting stage (E2) to make sure the feedback signal is negative with respect to the output signal. In a real amplifier you cannot maintain perfect inversion from DC to lightspeed. Eventually, phase shift will cause the feedback signal to become in phase with the output signal, and the correction applied to the amp input will be the wrong polarity. We then have a positive feedback amplifier, and the result can be seen in Figure 4. V(amp-out) increases, which increases V(amp-in), which increases V(amp-out), ad infinitum.

Those voltage scales in Figure 4 are not realistic; in a practical amplifier the output will be limited by the supply voltage, and depending on conditions, the output will slam to a rail or start to oscillate at a frequency determined by the circuit time constant(s). And before the feedback has turned fully positive, you will already start to see overshoots on the input and output signals as the amplifier becomes increasingly unstable.
Figure 4: Input and output signals with the feedback turned positive.
The Verdict
As long as the feedback signal remains negative in polarity, it has a clear beneficial effect on the performance of the complete amplifier. And there are many more beneficial effects I have not mentioned. A gain variation of 10% of the 21.5 times gain of the non-feedback amplifier will immediately have the same 10% variation in the total circuit gain. In contrast, a 10% variation in the gain of the x1000 amplifier in the feedback circuit will only have a 0.23% variation in the circuit gain.

There’s much more, but not enough space here to keep going. Negative feedback cannot make things perfect, and sometimes turns your amplifier into an oscillator. But it is not something that comes too late to correct what has already happened. It’s a tool, and as every craftsman knows, tools must be learned and their limits understood before they can be applied to their advantage.

Comments and contributions are always welcome.
Fresh From the Bench
WiiM Pro Music Streamer
Smart Network Streaming System
By Oliver A. Masciarotte with measurements by Stuart Yaniger
Linkplay Technology is recognized for its widely adopted wireless audio streaming modules, smart home, and connected AI products. The WiiM Pro reflects Linplay’s efforts to combine all the latest and best audio technologies available in a single design that provides users with an easy and affordable way to connect existing audio systems. The new WiiM Pro is a versatile music streamer that combines all the latest technologies and protocols, making it the ultimate (and most affordable) home audio hub available. audioxpress received a pre-launch unit to review subjectively and objectively. This article was originally published in audioXpress, July 2023. Read the Full Article Now Available Here
Voice Coil Patent Review
Double Coil Speaker
By James Croft
This article discusses a patent originally claimed in 2013 on a Double Coil Speaker design, which was granted in 2017 to Friedrich Reining (Vienna, Austria), on behalf of Sound Solutions International from China. The abstract from the patent details an audio system comprising an electro-acoustic transducer having two stacked voice coils mechanically linked to a membrane. The voice coils are oscillatingly suspended in the magnetic field of a permanent magnet focused by a pole plate and are mechanically arranged symmetrical to the pole plate while in a rest position. The audio system further comprises two driver circuits connected to the electro-acoustic transducer. James Croft discusses multiple approaches in similar concepts, which apparently have never resulted in practical applications, but notes that the current state of technology, and notably the evolution in Class-D amplification, might finally create a way to explore it in commercial products. This article was originally published in Voice Coil, March 2018.  Read the Full Article Now Available Here
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