Projects: My SFF Build

 A little bit of history...

As my parents were not very computerphile we had an own PC quite late. It was an 700Mhz AMD K7 with (I guess) 128MB RAM and some sort of Nvidia graphics card. At some point I was (together with friends from school) quite heavily into PC gaming so I had to replace this thing. I bought my first own hardware which was an AMD Athlon 64 3200+ on an ASUS board with an Nvidia GeForce 5200 which I later upgraded to a 6800GT. Woha.

When my friends went from Counter Strike 1.6 to MMORPGs I lost interest in gaming and used my PC for other things - such as running gentoo Linux. Which was nice but a problem at the same time: this still was the only PC we had and my mother wanted to use it as well - and yeah she was not really compatible with Linux.

One thing that changed the timeline a lot was when I started my studies - at university I got my hands on all these nice student licenses for Microsoft products. So I made sure my parents got their own PC bought another refurbished PC and installed Windows Server 2008. There it was. My own small active directory network. Including fancy thing such as booting and Windows installation via LAN.

At that time I also got something new and I wanted it to be small. So I put an Intel E8400 into an Shuttle XPC SG33G5M barebone and was happy for a quite long time until I somewhat outgrew the need of having a desktop PC and so it ended up in the attic.

Fast forward.

The 'server' (which was in the meantime replaced by something serious) is long gone, the barebone PC still in the attic and I got used to use my quite decent DELL Precision 5570 laptop from work for private stuff as well. But there were two reasons why this was not longer perfect for me:

• the IT department got a bit restrictive over time and I didn't want them to spy on me in private time
• I had no usable monitor anymore and the laptop screen is way too small (but the 4k IPS panel is quite nice) to do cool things such as PCB layouts

So I decided to have an own desktop PC again. 

My parents had a DELL all-in-one PC for 12 years which I recently replaced (because of Windows 11) with the 27" version of a newer HP model. I took the old DELL with me and while playing around with it I discovered a little bit of love for all-in-one PCs at my end. So I almost bought the 32" version of the HP which cost ~2200€.

Almost. Because I did a little research about what else I could get for the money - which is:

• Case:       Fractal Terra Jade                169,00€          
• Mainboard:  Gigabyte Aorus Z890I              300,99€
• CPU:        Intel Ultra 7 265KF               277,00€
• Cooling:    be quiet! pure loop 2 120mm        87,88€
• RAM:        Patriot 32GB DDR5-8200            188,90€
• Graphics:   Zotac GeForce RTX5060 SOLO        294,90€
• SSD:        Verbatim Vi5000 SSD 1TB M.2        59,00€
• PSU:        Seasonic Focus SGX 650W           129,20€
• Monitor:    Philips Evnia 32M2N6800M          665,98€
• SUM:                                         2173,55€

Actually everything fit nicely - just with the cooler I had the 'issue' that i needed to put the fan 'behind' it in order not to collide with the front USB socket and to get the pump of the cooler more to the inside of the case. The spine is in the 2 position. And yes, I turned around the CPU cooler block during the assembly ;-) 

Lovely!

Cheers,

P.

Class-D Tales: The D is For Denmark (a comprehensive overview)

Origins

The origins of switch mode power amplifiers (SMPA) date back well into the era of vacuum tubes [1]. Once there was a website listing some patents of the early days which was quite helpful in order to get an overview over SMPA history [2]. 

My personal impression is that there was not so much momentum in the early days. When switch mode power supplies (SMPS) started to become popular (late 70's to early-mid 80's - most likely fueled by the work of Middlebrook and Ćuk and by the advent of suitable semiconductors [3]) also Class-D technology saw a little boost (e.g. Brian Attwood at Peavey). The whole thing really took off in the late 90's to early 00's (not only but significantly) driven by a group of former PhD students of Michael A. E. Andersen at DTU.

Class-D is called Class-D simply because at the time it arrived the amplifier classes A, B and C were already known and D is the next letter in the alphabet. However, due to the fact that a power sage switching between (at least) two discrete states is at the core of the concept people tend to misinterpret (yes, it is still ongoing) the D as abbreviation for ‚digital‘. Which is horrifically stupid.

D is for Denmark - which is the Class-D hotspot.

Literature

For long years there has been no comprehensive treatise on Class-D technology. Some days ago I found the book of Robert N. Buono on the interweb [4]. 

My first thought was: "woohoo, finally...". Then I read the name of the author and realized two things:

• I've never heard of that guy
• Apparently he is not danish

So my second thought was: "please, please, please don't be bullshit". I downloaded the masterpiece on scribd.com. And it was utter crap. Honestly, don't waste your money on this! It is frightening to see that there are people spending years on the topic who afterwards still have no clue about it. Another good example is the dissertation of Verena Grifone Fuchs [5]. 

You could simply read the respective chapters in Bob Cordell's (otherwise very precious) book about linear amplifiers [6] and reach the same level of enlightenment easily - and Bob is clearly no Class-D guy.

After all this page is meant to contain a list of all literature that is relevant according to my impression:

• Some of the links get you to archive.org - this is the case for everything that I could find somewhere on the internet.
• The other links get you to a google drive - the files you get from there are password protected. Send me a message and I'll help you.

 After all, there is nothing like a step-by-step guide around a 'blameless' solution.

People

• People around Michael A. E. Andersen
• Karsten Nielsen
• Founder of ICEpower [7] - most likely the first supplier for off-the-shelf Class-D modules.
• Publications:
• [NIE98] Audio Power Amplifier Techniques With Energy Efficient Power Conversion
• [NIE00] A novel Audio Power Amplifier Topology with High Efficiency and State-of-the-art performance
• Lars Risbo
• Founder of Toccata which was later acquired by Texas Instruments - they made up the Class-D department there. Now he works at Purifi. 
• Publications:
• [RIS05] DISCRETE-TIME MODELING OF CONTINUOUS-TIME PULSE WIDTH MODULATOR LOOPS 
• [RIS06] PWM Amplifier Control Loops with Minimum Aliasing Distortion
• [RIS08] A Versatile Discrete-Time Approach for Modeling Switch-Mode Controllers
• [RIS09] Suppression of Continuous-Time and Discrete-Time Errors in Switch-Mode Control Loops
• Mikkel C. W. Høyerby
• Founder of Merus Audio which was later acquired by Infineon.
• Publications:
• [HOY05] Derivation and Analysis of a Low-Cost, High-performance Analogue BPCM Control Scheme for Class-D Audio Power Amplifiers
• [HOY06] A small-signal model of the hysteretic comparator in linear-carrier self-oscillating switch-mode controllers
• [HOY07a] Self-Oscillating Control Systems
• [HOY07b] A Comparative Study of Analog Voltage-mode Control Methods for Ultra-Fast Tracking Power Supplies
• [HOY09] Carrier Distortion in Hysteretic Self-Oscillating Class-D Audio Power
• Lars P. Petersen
• Started at ICEpower, founded UpCon and Audiobricks now back as CTO of ICEpower. I had a tremendous time meeting him (and others such as Patrik Boström) during my time at d&b.
• Søren Poulsen
• Worked for Texas Instruments nowadays for Purifi.
• Publications:
• [POU04a] Simple PWM modulator topology with excellent dynamic behavior
• [POU04b] Towards Active Transducers
• [POU05] Hysteresis controller with constant switching frequency
• Niels E. Iversen
• [IVE19] A high power switch-mode power audio amplifier
• Khiem Nguyen-Duy
• [NGU14] Constant Switching Frequency Self-Oscillating Controlled Class-D Amplifiers
• Kaspar Sinding Meyer
• A student who passed away shortly before handing in his PhD thesis.
• Publications:
• [MEY06a] Minimizing distortion in self-oscillating switching amplifiers
• [MEY06b] Small-signal modelling of self-oscillating switch-mode amplifiers

• Other honourable mentions
• John Vanderkooy & Stanley P. Lipshitz
• These two guys wrote a lot of audio papers, but there is especially one which I need for further reference
• [V&L79] Feedforward Error Correction in Power Amplifiers
• Stefan Wehmeier
• Self-proclaimed inventor of the AIM. We'll talk about him later in this post.
• [ELB98] Selbstschwingender Digitalverstärker DE19838765A1
• Bruno Putzeys
• He went from Philips to Hypex to Purifi.
• While most engineers tend to be comparatively silent persons this guy seems to have a rather pronounced sense of mission and self esteem. Let me come up with some remarkable examples:
• "I would discount any attempts by BA [remark: Brian Attwood] at filter feedback as hopeless cobbling. I have been told he still hasn't discovered the usefulness of the integrator in loop control and so far hasn't managed to put feedback around a class D amp that actually resulted in a reduction of distortion." [8]
• "When the UcD circuit was developed in 2001 the aim was to build, in a minimum of time, a simple circuit addressing the shortcomings of contemporary class D solutions sufficiently to make it a drop-in replacement for linear amplifiers in cheap consumer goods. It’s fair to say that the result quite overshot the target. 10 years on there are still no competing technologies [...]" [PUZ11a]
• "Apparently the Philips TV department had gotten nowhere with a Class D chip the research lab in Eindhoven had been working on for years. So I got a month to do a discrete circuit that could do what that chip couldn’t. Thirty days later I presented them with a small 25-watt self-oscillating amp that could be made for $5 or so. I’m still wondering if the intention hadn’t been to hand that young whippersnapper a loaded gun to shoot himself with because, looking back, one month was perhaps a bit short for a project of this nature." [9]
• "So one sleepy afternoon a Philips colleague walked through the office shouting “has anyone got something clever to present at the DSP conference in Eindhoven?” So for a laugh I tried and found a new PWM algo in two hours. Mind you, I’d been trying for years by then, but always under some pressure to deliver. Now that I’d basically given up on the idea I was relaxed and that helped." [10]
• "You can understand my bemusement at being asked to provide commentary on what’s supposed to be an amplifier revolution happening right now. To me, the revolution was over when the entire competing peloton settled on self-oscillating amplifiers with global feedback. [...] Technologically, class D is hard. Just the PCB layout is a nightmare to figure out, let alone the circuit. All truly high-performance designs are made by a handful of people who’ve made a career out of it. And they’re not exactly publishing how-to guides." [1]
• Honestly, I truly admire this attitude! The very cool thing about this dude is that he talks and writes about his work - I grew tremendously by consuming and digesting his output. Unfortunately (and despite the fact that I was part of the Class-D world for a while) I never managed to meet Bruno.
• "The author would like to thank the following people for their contribution towards this project: [...] Bruno Putzeys for his endless insights and eagerness to share his knowledge" [KEM12]
• Publications:
• [PUZ02] A True One-Bit Power DA Converter
• [PUZ03] Design techniques for high-performance discrete AD converters
• [PUZ04] Effects of Jitter on ADDA conversion
• [PUZ05] Simple Self-Oscillating Class D Amplifier with Full Output Filter Control
• [PUZ06a] All amplifiers are analogue, but some amplifiers are more analogue than others
• [PUZ06b] Simple, Ultralow-Distortion Digital Pulse Width Modulator
• [PUZ07a] A Universal Grammar of Class D Amplification
• [PUZ07b] An EE’s Guide to Survival Between Microphone and Voice Coil
• [PUZ09] GLOBALLY MODULATED SELF-OSCILLATING AMPLIFIER WITH IMPROVED LINEARITY
• [PUZ11a] Ncore Technology White Paper
• [PUZ11b] The F-Word
• [PUZ13] The G-Word
• Hendrik du Toit Mouton
• Students around him:
• Jason Quibell
• [QUI11] Digital Control of a Class-D Audio Amplifier
• Carel van der Merwe
• [MER17] Digitally Controlled Class-D Audio Amplifier
• Pieter S. Kemp
• [KEM11] High-Order Analog Control of a Clocked Class-D Audio Amplifier with Global Feedback Using Z-Domain Methods
• [KEM12] The Design of an Analogue Class-D Audio Amplifier Using Z-Domain Methods
• Himself:
• [MOU09] Digital Control of a PWM Switching Amplifier with Global Feedback
• [MOU12] Understanding the PWM Nonlinearity: Single-Sided Modulation
• [MOU13] Modelling and design of single-edge oversampled PWM current regulators using z-domain methods
• [MOU18] Small-Signal Analysis of Naturally-Sampled Single-Edge PWM Control Loops - Version A
• [MOU18] Small-Signal Analysis of Naturally-Sampled Single-Edge PWM Control Loops - Version B
• Stepehn M. Cox
• [COX05] Class-D Audio Amplifiers With Negative Feedback
• [COX15] Ripple Compensation for a Class-D Amplifier
• [COX17] Analysis of a hysteresis-controlled self-oscillating class-D amplifier
• [COX18] A third-order class-D amplifier with and without ripple compensation
• Paul van der Hulst
• [HUL02] An asynchronous switching high-end power amplifier
• Sven Klass
• [KLA16] Ein digital geregelter Klasse-D-Verstärker

This is me in 2018 taking a little walk through the evening hours of Copenhagen after a fruitful day at the ICEpower office - a time I remember with great joy and pleasure (read [7] !)

Dichotomy and Archetypes

There is this ubiquitous myth that within self-oscillating topologies there is a dichotomy between phase shift controlled (PSC) circuits and hysteresis controlled (HC) circuits. However, Bruno showed that these can be analyzed using one unified framework [PUZ09]. Yes, there are differences when it comes to details but essentially PSC and HC are the same.

The only true dichotomy in Class-D is clocked vs. self-osc. Period.

Still, this circuit below is praised as the archetype of all Class-D amps basically everywhere. 

However, practical implementations of this tend to get quite complex due to several reasons:

• When you go full digital (which indeed has some powerpoint appeal) you need ADCs, DSPs and what not to in the end still have an analogue amplifier [PUZ06a]
• When you go analog you need a precision triwave generator.
• One very well known solution to this problem is a circuit which is known as "Rechteck-Dreieck-Generator" in German. It comes with a nice feature that might be worth to chew on for a while:
• By adding just one resistor to the circuit which forms an input node this thing starts acting as a full-blown HC-type Class-D modulator in itself. 
• A fact Stefan Wehmeier thought he figured out himself [ELB98] but I guess the patent office had a different opinion so the application got stuck in A1-phase. 
• Question: does it really make sense to hide an SMPA inside an SMPA? 
• In both cases the fed back signal parts will cause the beautiful triangle to be disfigured and so the PWM process itself is not linear anymore (-> distortion). Ripple compensation [PUZ06b] is a way around it (doesn't matter whether analog or digital) but this again adds complexity. 
So while it is inevitably true that this circuit is the archetype of clocked SMPAs practical design  it is absolutely no technical minimal system.

What is a 'technical minimal system'? 

I first heard this term during a lecture held by this guy. Actually the concept is fairly easy to understand. Think about an AC supply circuit. All it needs to supply a load is two wires (often referred to as 'live' and 'neutral'). The big benefit over DC (which also required 'just' two wires) it that with AC the voltage can be stepped up/down using transformers so to overcome large distances. But there is one remarkable drawback: as power is the product of voltage and current and both have periodic zero crossings also the transmitted power oscillates between zero and twice the average value (assuming a pure ohmic load). A more steady power at the load can be achieved by adding a rectifier and a smoothing capacitor. But this comes at the cost of high peak currents in the AC line (deformed power / Verzerrungsblindleistung).

Now comes the magic: by adding just one wire it is possible to go from single-phase AC to a three-phase system (a neutral wire is not really needed if the load is well-balanced). Transformation is still possible and the transmitted power is steady again. For sure it is possible to add more wires anb phases - but nothing gets better by doing so.

For my LinkedIn profile I once described it like this: "What drives me is the eagerness to find „minimal systems“ which is the state of a device, circuit or system where removing or altering a single component severely worsens the overall performance but adding components does not lead to significant improvements."

Final words

Finally one might ask: "what is the archetype of self-osc. SMPAs"?. Well (cliffhanger incoming), we'll clarify this in an upcoming post. However, I'm, 100% convinced that the following circuit shows (following the argumentation in [1]) the technical minimal system of an SMPA in general:

Yes, there is a first-order closed loop response - but there is no circuit with less components that yields (at the same time):

• the same order of robustness
• the same order of load-invariance
• the same order of loopgain at higher frequencies (check [PUZ11b] and [7])

Cheers,

P.

Misc: Hello World!

Hi!

In case you're curious who is writing here let me put together some facts about myself.

I was born in 1990, my parents and me were living in a small town in southern Germany. In the neighbouring town (also small) the was a library and they had a book called "Elektronik gar nicht schwer, Bd.2, Experimente mit Wechselstrom". And this (while still being in school) was my first contact with electronics.

Most likely the first thing I ever built was an multivibrator with LEDs and a little battery holder that I glued to an old tie pin and gave it to my father (who wore ties at that time quite often). I should ask him whether he still has that thing...  

However, I spent a lot of my time at the computer back then and so I did more SW stuff (especially php) than HW but there were two topics that was always present: 

• audio technology - I was obsessed with amplifiers and speakers and built some of these myself
• high voltage stuff - at some point I tired to build a DRSSTC but it failed horribly (today I know exactly why)

At the end of my time in school the "what to do for a living" question boiled down to two options:

• becoming a teacher
• becoming an electrical engineer

A little bit of reflection led to the idea that not all pupils might have a good time with me (as I imagined myself clearly being more focussed towards the smarter ones while ignoring the needs of the others) so I opted for the latter.

The university I studied at (KIT) had a department of electrical engineering and information technology with (as far as I remember) 17 separate institutes specialized in different topics. After passing the first three semesters (unfortunately during my time at school I never learned how to learn - so I had to figure this out the hard way...) it became evident that my topic is power electronics & control theory (which essentially is quite equal to developing audio amplifiers - at least in my little world).

So for my bachelor's thesis I developed a (surprise) linear amplifier meant to power an Epstein frame. during that time I basically had a workplace in a lab with some other (already more experienced students) which was a great time of learning for me. After everything was finished my supervisor offered me to be hired as a student assistant so to keep my workplace and  to continue the project. 

I started my day at 8:30 in the lab, went to some lectures during the day and ended at 16:30. And despite the fact that I was just payed for 40h per month I did that every day as this was a very insightful time for me - especially because of the discussions with other students being on a similar path than I was. One guy I'll definitely never forget: Benjamin Weschenfelder

During my master studies it was obligatory to do an internship for some weeks - but for me it was clear that:

• I wanted half a year
• I wanted to do it in my second last semester (the last being blocked for my master's thesis) because I hoped to end up at a company that wanted to hire me afterwards
• I wanted still be close to my small town (which I never really left - and will never leave)
• I wanted to do audio stuff

Yeah and this is how it happened 🙈. 

I spent 6 months at d&b working on a new power supply concept and they wanted to have me back as soon as possible. After doing some 100kW inverter things for my master's thesis (and yes, Bernd, I'll also never forget you) which even culminated in a nice paper I was prepared for audio stuff again.

Shortly after I joined in Nov. 2015 the colleague who was responsible for developing the actual amplifier (we called it "Switch Mode Power Amplifier" - SMPA) inside the 19" amplifier device (which comprised also a SMPS, DSP and other things and was called "amp") left the company and I took over his topics. 

A while later Christoph joined and we were some sort of dream team. While I was juggling fancy ideas all the time he was deep in the details making sure that each part of a circuit was really doing what it is supposed to do. And yes, we both looooove transistor level circuitry. In the five years I spent with d&b I learned so many things. It was mindblowing, fulfilling and entertaining. I truly miss that time.

This was me in 2017 (top) and 2020 (bottom).

And this was our masterpiece: click me.

At d&b we had a little wiki space developers used to document their work and this was something which I loved from the beginning: structuring things and putting them together in a litte page that could be interesting for other as well - maybe written with a little sense of humor.

When COVID struck the company went into emergency mode including shutting down engineering. Something I really was not happy with so I quit - and ended up at instagrid in August 2020. As of now I'm in a kind of double role ("Head of Power & Controls Engineering"):

• half of the time I'm doing something that could be called system engineering - which means that I create concepts of how to split devices into functions and assign them to assemblies including topics such as safety and EMC
• the other half I'm leading a team of SW and HW people who develop the core pieces of the instagrid devices (such as charging/discharging functionality)

At instagrid there is no wiki. But it gives me the same feeling and I still love it. As you might have guessed this is also the reason for this blog. I want to write a bit about the things I'm doing :-)

Cheers,

P.