Switched Micro-Power Supply

In this Article I present my Switched Voltage Converter for the low-power range.

For example it can be mounted on a small PCB with 3 pins that can easily replace regular 78XX regulators. That will become the “simple” Version.


The advanced Version in the image above has some quite impressive features:

  • The nominal output current is 250 mA, which is half of what the simple version can produce due to design reasons.
  • The module is absolutly overload and short-circuit proof.
  • In contrast to the simple version it can Buck and boost the voltage at the same time. The input and output Voltages range from 3V to 35V.
  • The negative output can even create a negative Voltage, though it must be stabilised with an externel Zener Diode.
  • Provides Control Pins for Enable Input and Power-OK Output

And the most important ones, and the reasons why I designed this Discrete Converter is:

  • The Design can be changed and adapted for high voltages using appropriate transistors.
  • The polarity of all semiconductors can be swapped with the opposite ones (NPN to PNP) to build the same converter for negative voltages!

For size reasons there are no output caps onboard and therefore the converter rely on external ones, like 3-pin regulators do too. These are mandatory, starting the circuit without them will immediately destroy it as the stored energy in the coil will induce extreme voltages. (Maybe I’ll fix that in an upcoming smd version of this converter)

I have already used two of these in my MPS-Stack. For example in my Preamp-Module it generates a very stable bipolar supply voltage of + and – 5 Volts independent of the Input Voltage that can be anything from 4 to 35 Volts.


Transconductance Preamp v2

My First Preamp has proofed its quality now for over a year. As I now plan to upgrade my record player with an integrated one, I worked on a improved and simplified version of my first prototype.

I already had the idea for this when my first version wasn’t even built, but I needed some time for basic design evaluation and performance simulation.

The most important change was to get rid of the bipolar supply voltage and a fast self-biasing upon startup while maintaining a low cutoff frequency. Waiting for more than 30 seconds until it gets operational while generating a loud “Plob” was not okay. Luckily I had an brilliant idea how to bypass this problem.

Of course it will again have an High-Z Current output that must be terminated with the proper 620 Ohms.

Here is the simplified schematic for Version 2 of my Transconductance Preamp:

Preamp Unipolar

Over the Xmas holidays I had enough time to solder the new prototype, it fits nicely into a corner of my Pioneer Record Player:


I am going digital…

Now that my Stack has grown bigger with the preamp-module, I decided to get some visual presentation of the Sound”. A LED-Bargraph for the RMS power with peak-hold would be nice!

I also planned to build a discrete logarithmic volume switch and found that it’s best to integrate both functions on one module as both need digital control.

First I researched the famous LM3915 IC only to conclude that a pure hardware approach is to limited and inflexible. I thought that a common Atmega-Micropocessor should have more than enough power to implement these functions and maybe some more later on.

The Display-Module had already been designed in autumn last year: An Active Bicolor-Led Display Module of 7 x 40 Points:


I has eight 5×7 matrices of the smallest size of 20mm height (OEM). As it is not possible to route 2 times 40 pins for the rows to external connectors, I needed to mount ten (!) 74HC595 shift registers in SSOP-package on the backside of the PCB. The craziness of this endeavour became clear to me as I needed 2 weeks to get the high-complex layout finished on a 2-layer PCB.

So it was now time to begin my first programming of AVRs. After the 2 Atmega32 and AVR-Progrmamer, that i have ordered, arrived, I build a small development board and wrote my first lines of AVR-Code in C.


4 weeks later I had acquired some very interesting Insights into AVR-Programming:

  • Don’t try to push the ADC-Clock beyond 1 MHz with changing channel every time if you don’t have a sufficient break between the conversions. Otherwise the cross-channel isolation can be lost completely due to the big time-constant of the Switch- and S/H-capacitators.
  • You can’t do a true and fast Root-Mean-Square calculation on an 8-bit uP. Instead I had to satisfy myself with a averaged rectified Signal. For this I implemented a 10Hz-lowpass filter using a 256-Sample Ring-Buffer.
  • All the available ports are occupied by the Special I/O-Pins. It is not possible to avoid port extenders!
  • ISR’s need an incredible amount of time!
  • Processing Power is running scarce!!!
    I already had to decrease the samplerate down to 2*32kHz (Stereo muxed) to get the calculation done without any line written for the display control yet. I used an available Portpin to visualize the time needed for every single Interrupt (Image follows…)

Can AVRs be overclocked?!

A Lithium-Power Charger Circuit on the fly…

A good friend of mine called one day and asked me if I could build him a Li-Po Charger. His Girlfriend wanted to go hiking for some time and needed a way to accumulate solar power for her electronic gadgets.

So I designed and build the circuit in just a few days, though I needed about 3 weeks for it as my free time was a little low.


Measuring of voltage and current is done with the help of TL431 Precision References, wired as PID-regulators, and tied to a Switched Buck Regulator for high efficency. I also implemented an electronic switch for shut-off with integrated discharge and under-voltage protection.

The whole assembly found its way into a plastic cycling-bottle, so it can easily be mounted on a bike:

(Image following!)


Transconductance Preamp

After I have designed my Line Transceiver, i wondered how much gain these kind of circuits can achieve. My aim was to build a discrete preamp that has:

  • High Forward Gain without any feedback loop
  • High Bandwith
  • Low noise
  • and of course with an Current Output Driver

During my experiments i had to learn the hard way what temperature coefficient does mean. Surpassing 20dB of Gain makes the circuit extreme sensitive to any change of the surrounding air temperature. The DC-Operating point of my circuits swinged across the whole range only by blowing some air over it! So at least it was necessary to implement a DC-Feedback path to compensates the temperature drifts. The graphic below show the circuit in simplified form:


The Preamp is now build for a maximum gain of 86dB, sufficient for tape cassette replay.
The base-voltages at the Inputs lie about 2 Volts below the operating point due to the high input current and the 1 megaohms resistors. This has the advantage that the cascode stage (not shown here) can be tied directly to ground. The pot between the input transistors controll the sensivity of the Input stage. A higher impedance permit higher Input signals without raised distortions!

The passive EQ network can be adopted for all kind of equalizing curves, like reel-to-reel tape machines, vinyl players and others…

The following Image shows my implementations for the MPP-Stack. It uses one of my Switched Micro-Power Supplies to generate the necessary bipolar voltage of +/- 5 Volts. The free space on the right  is the recording amplifier that has not been mounted yet.


In the picture you can see it connected with a disassembled cassette tapedeck. But I also have used it to digitize some Vinyl records…


Discrete Transconductance Line Transceiver

After all my experiences with buidling switched ampliufiers, i felt the need to venture into the field of discrete analog circuits. I’ve read a lot about guys on the net that still today, in the time of ICs, build Opamps out of discrete parts.

They say because it sounds better,
it provides more flexibility, and higher bandwidths.
They say that feedback loops should be avoided at all.
Listening via an OpAmp is like watching an orchestra through a key-hole!

I was researching circuits to convert an symmetrical input into a non-symmetrical one avoiding the classic OpAmp circuit all together. Trying to think like one of these audiophile maniacs I thought that a 4-resistor bridge does no “feel” very good: in regard to the floating reference levels, and especially the limited input impedance against ground.

On  the other side are memories of frustating tries to eliminate the ground hum between amp stages, weather for recording sound from the tv-set or wiring multiple amplifiers on partys that draw their power from different sockets.

One way that this can be avoided is by using symmetrical or at least quasi symmetrical signal Transmission.
Another way can be found by force-feeding pure current through the line:

I’ve got the idea that you can have XLR-like quality on standard lines when the receiver does not perceive any noise voltage that can occur along the cable. This can be achieved by transmitting current instead of voltage, as long as the correct terminating impedance is kept maintained, like f.e. 600 Ohms.

This is the result:

All you’ve got to do is buffering the input into an defined impedance and mirroring the resulting current into the output line. Just have a look on the performance:
• THD: < 0.01 % (can be improved by raising bias current)
• -3dB Bandwidth: 6.5 MHz
• Slew Rate: 200 V ⁄ μs
• Output Impedance: 440 kOhm

For simple unipolar signals there is an even more simple way to build a current driving output; just take a self-biasing transistor with an current source as pullup:

This is a cheap way to upgrade your stereo with an current line-out, won’t cost more than using an OpAmp as output buffer!

Loudness Scan

Do you hate brick-walled music? Do you hate over-compressed sound?

Me Too!

Surfing the web about the development of Music Mastering, like the Loudness War on Wikipedia, I stumbled over Jim’s Audio page about HDCDs, where a tool was used that analyses the loudness spectrum of audio files.

I just had digitalized some LP recordings using my new Transconductance Preamp when I thought it would be a nice idea to get this Dynamic Plot for the recordings so I can compare it against other albums. Unluckily I had to find out that the original program was not able to handle larger files, so with the help of the original source code from Keith Howard I was able to code my own console-app called “Loudness Scan” and a tool to generate the corresponding emf graphics:

LSshot Jethro Tull - Minstrel In The Gallery

The images show the Volume Spectrum of my digitized recording of “Jethro Tull – Minstrel in the Gallery”. A Spectrum like it should be. The spike at the lower end contain all values below -40dB.

Link to the file:

This is the first Version of the tool, only for analysing 2 x 16bit Wave-files. The png-write is not (yet) implemented and there are some bugs with rendering the emf inside emfexplorer… But for now it does what I wanted it to do.

Modular Power System

…or simply “MPS”:
MPS Complete Stack

It consists of (from top to bottom):

Stereo Amplifier
– Discrete Digital Stereo Amplifier, providing 2 x 120 Watts Sinus
– Operating Voltage: +30 / -30 Volts, Efficiency of Conversion > 95%
– Protection against overload and overvoltage
– Remote Activation Input

Voltage Converter
– Can be used for either Bucking or Boosting
– Inverter for Upper (“Primary”) Voltage
– 4 to 30 Volts Range
– Build for High Current; limited by safety fuse only
– Voltmeter Display for either of the 2 voltage sides, Display of operating mode
– Remote Activation Input; Power OK Output for Boosting Mode

Battery Module
– 2 x 2 Li-Ion 18650 Cells, nominal voltage 7,4 V
– CCCV switched charging circuit
– active balancing
– external battery connector available

Finetuning is finished and Long Term Burn-In Test provided very satisfying results.
Due to the high efficiency of all power conversions the average power drain is quite low even at high speaker volume. The Maximmum Current Draw for 2 x 8 Ohm Speakers  seldom exceeded 4 Amps, calculating to about 30 Watts of total Power consumption. Fully charged Batteries can therefore provide many hours of offline listening experience.

Several more Modules are planned:

Preamplifier for Tape/Phono/Mic
– Preamplifiers with 3 selectable Equalizing profiles
– Current-driving Recording Amplifiers for Tape/Cassette included
– All gain stages consist of Discrete Transconductance Amplifiers: differential voltage Input transform to current Output with Pure Forward Gain while providing quite low distortions and noise

Analog line receiver/transmitter
– All gain stages consist of Discrete Transconductance Amplifiers:

Digital line receiver/transmitter
– Optical Toslink for Input and Output
– 24bit and 96 kHz Audio codec
– Transconductance Input and Output

Discrete Volume Control
– 8bit resolution with hardware logarythmic curve
– Nominal Value of 600 Ohms for Termination of current transmission
– All switches made of small n-channel mosfets for low power consumption.
– Can be controlled by remote-set
– 7×40 Dual-Color Dot-Matrix Display

Booster/Controller for Modell-train
– High Efficency Switched Power Converter
– adjustable Output Voltage
– USB-to-serial interface for digital operation
– 50hz generator for analog Operation

Electronics is my profession


This is my site dedicated to my main interest: electronics and some other stuff.

But over the years i’ve had so many ideas and projects and some became so big that they had to be publiziced.

When i was 10 years old i had my first success in electronics when i build an amplifier that could playback an Audio Cassette from a disassembled tape deck. With 18 years of Age i decided to develop an power amplifier in switching Technology. I thought that would be easier than an common analog one….

It took me 10 years until it finalised in my

500 Watts Digital Power Amplifier.

I also learned to build switched power converters to supply it and audiophile gain stages and  now i integrated all into my new

Modular Power System (MPS)