Theo Verelst Audio Activity Page
031 (0)6 266 24 284
Text: sept 2004
( March 2009 For the latest
info check this page
This page is to give an idea of my involvements in audio
related subjects, or at least directly related subjects. I've been very
interested and active in audio fields as long as I can remember,
certainly from the day I got a radio and cassette recorder when they
were just starting to get popular. Before well into teenhood, I'd build
high quality mixers, amplifiers, speakers, and lots of other audio
related hardware as a hobby, including computer audio, and led the
school technical committee for years. The activities and projects on
this page are an extension of these interests.
In principle this page is still in buildup, I think not all is on which
I planned, and it might be there are some errors, and it's not
extensive, I've done (a lot) more than is presented here, which is
mainly recent stuff.
A little background
I'm an Electrical Engineer, with a Delft University masters' degree
from Network Theory Section (currently Circuits and Systems), I
also was electronics practicum assistant for years at the same
university, and after I graduated worked in a computer graphics
computer hardware and software design project at the section where I
graduated, in cooperation with the computer graphics section at
Informatics, and TNO-Delft, through STW funding, interacting with
several companies including Hewlett Packard, Philips and Archavis. An
example article from the graphics side of that project is,
Challenges for Ray Tracing and Radiosity Algorithms).
As a student, and also later on I was often seen in the Delft
University Musisch Centrum (Currently in a new building under the name cultural centre), in all kinds of
band and project setups and performances, one of the main year long
projects being the Jazz (fusion) Combo, where I usually also was
responsible for the piano/synthesizer amplification primarily as a
musician, not a sound engineer. I also followed an extensive video
course there, and kept working with the equipment. On various
occasions, I used various multi track sound studio setups, too. Since I
built my own synthesizer mid high-school, I acquired or could use a
more than moderate setup of synthesizer equipment, including poly 800,
RX-15, DW8000, DX-7, REV-7, Bit-One, Teac/Tascam 244, mixer, moderate
quality condenser mic, Kaway spectra, HP9000 roland piano, TG-500.
Recent additions and work
March 2009 Check this page for the
latest audio developments I´m into.
The rest of the page is from some years ago, the latest additions until
3 way amplified main speaker system, with a new 4 channel amplifier with a even
heaver main channel high quality amplifier with very heavy supply (86
Volts with dual 10,000 microfarads) and high quality chips and wiring
at least a 100 Watts continuous per channel, combined in 2 unit 19 inch
rack with a equally good but lower power stereo amp for about 50 Watts,
with seperate 400 and 230 Watts toroid tranformers.
The main speakers have extra wiring for sperately driving the mid high
cone squakers, which get a seperate amplifier and electronic crossover
filter. The old main amplifier (2x100 High Q Watts cont. with 300 Watts
toroid) replaces the 500Watt car booster for the subwoofer for high
quality (bridged) amplification for the below 40 Herz range.
The 500Watt max. 15" subwoofer now has the desirable closed (pressure)
enclosure, with ´infinite baffle´, NOT a bass reflex, with
good acoustic foam damping and a carryable enclosure.
The synthesizer has received in addition to its generally usable very
high quality pre-amplifier based on the OPA627 BurrBrown opamp chips a
handfull more of those chips and a number of good capacitor based
electronics seperation filters with second order, non-resonating
crossover frequencies of 40Hz, 4kHz and 8kHz and 6 buffered outputs for
the end amplifier set. The quality unmprovement, also the result of
having seprate volume controls for each speaker part is considerable.
The system can perform extremely good HiFi sound reproduction at quite
high output levels, even compared with guitar amps, and a big PA in an
adjacent space, as became clear on the MILE 2006 musical instrument
fair, including in the sub low frequency range, so over the whole audio
spectrum, not limited to some peaks in the mid and low mid or so. Using
a Chesky audio reference CD or other good wav recordings (also 24 bit)
the HiFi quality is outstanding, very bright, clear, transparent and
deep, direct yet not indtruding when used with good microphones for
live sound, and QUITE loud while staying at low distortion levels.
Well, the supply would be up to peak powers of up to a kilowatt in
total, and in fact even more, so that is no total surprise. The
speakers are not loaded with that much power in general by me, they are
made for hogh quality, not break-em-up boomboxing, and they all were in
perfect shape after the demonstration in a fairly large space, thanks
also to the seperate mid high cone driver amp with deliberately low
power and seperate volume control.
1080 HD video recording and processing with Studio 10.5 (Pinnacle) but
mainly reading in of mpeg2 transport stream with open source software,
processing and editing with Cinerella (and a 3 core 3GHz and 2 2GHz
processor cores renderfarm) with realtime HD viewing during edit with
up to 25 frames/seconds real HD viewing. HD reading and editing also on
Notebook with Fedora Core 5 (dual core) cinelerra. Transcoding to mpeg4
and other formats with ffmpeg.
Recording with Lexicon Omega and 2x AT2020 for video and studio audio,
with steinberg Cubase on a fast machine (pentium D), or with open
source software on Linux. I´ve recorded video with seperate high
Q audio but also a number of songs and song ideas with synthesizer and
electrical guitar, and singing.
Using the Lexicon Omage USB AD/DA unit with Linux Redhat Fedora Core 6
/ 64 (dual core) with programs like Rosegarden (midi and audio),
Audigy, Adoura, dssi and LADSPA plugins, Hexter, ZynAddSubFX, Qsynth
(with various free sound fonts like steinways and general midi set)
mostly run with jack and a graphical interface for it, with less then 5
milli seconds delay overall, from for instance analog in through
effects to analog or digital out.
I´ve remastered a CD full of studio recorded tracks from the 80s
to sound fresher and stronger from the original DAT masters with
digital refreshing, 24 bit equalisation, various compression effects,
some band chorusing and subtle but complicated reverberation additions
(Multiple Lexicon Pantheons Using Cubase), back to 16 bit convertion
and burning of CD by songlist. I used both monitos sets and an existing
(repaired by me) ananlog studio monitor setup for listening.
I´ve demonstrated the synthesizer with the biamped monitors at a
stand at the large FOSDEM Open Source conference in Belgium (its also
open source software and hardware), and I´ve demonstrated the
whole new system (the new amp was built only 4 days before and in full
use immediately) at the MILE2006 musical instrument fair in Utrecht, as
well as cinerella and dual screen Linux and a S90 synthesizer to
amplify with the system.
See also my latest string
simulator page and the Open Source additive synthesis software.
Audio related Research and Development
One of my main goals
for a longer time has been to design and built a (music/tone) synthesizer.
Both an analog, electronical design has my interest, for which I've
made various electronical units, as well as a more daring, computer
cycle eating synthesis form in the direction of complicated physical
modeling, a string
demonstration at Fosdem Open
Source conference, Brussels
Notice the below speaker set and amplifier being used, and
a notebook with the real time
simulator fitted with the USB high Q DA convertor.
See also my fosdem
I've also made a sample processing program in professional
Using a fast Z80 based design I experimented with simulated analog
synthesis methods on a very low budget.
To built some prototypes with, I've been able to use the blackfin DSP
on one and two core >1Gops/s EZ-LITE evaluation boards, for a
variety of preliminary tasks, from video signal generating through
audio delays and signal generator to a prototype for a self-contained
chorus unit/perfect VU meter.
Currently I'm prototype test-using a 5 stereo channel computer
controlled mixer, with very high quality gain control chips from TI,
driven by a tcl/tk script. A similar high quality microphone pre-amp
chip will be added.
For sound reinforcement, I've made various high quality
amplifier/speaker designs and prototypes, which all work(ed) very well.
See below for a portable set, and the latest big speakers and power
amplifier, and even an quality USB AD/DA unit I built.
Not only for sound synthesis, I've started a network with blocks
editing and network based program execution program called bwise.
Simulated and built electronics include parts of moog synthesizer building blocks .
Other web resources
putting things up for reading on the Tcl wiki, for instance about using
the tcl containing Maxima mathematics package to do symbolic signal reconstuction
from samples. Sound Envelope
Generator, "Audio volume under
tcl control through the parallel port", "Watching a remotely stored video",
"Windows WAVE volume
control using Snack".
Various pages are available on a variety of synthesis subjects (most of
them from more than a few years ago), such as a software drumcomputer, my web
page with DFT jave applet and examples "The
Wave Laboratory Page",
Laboratory page" for less technical synthesis subjects (page not
Various recordings of mine are on the web, such as a string simulator demo (wav file),
software drumcomputer use of my sample software with tcltk/bwise
graphical interface demo
(wav file) with linn drum samples, a page
with example audio files,
a few files on vitaminic,
A quickly-made realplayer video/audio playing
of a S90
by me. A short
piece I played at Herman Brood's piano some years ago, and a rock-like S90
multirecording, recorded in a short time.
The 30 MHz Z80 based microprocessorboard I made, driven by
windows 3.1 PC dedicated additive synthesis and filter simulation
program, played on a small keyboard made this
piece, which had
first been recorded on cassette tape.before being digitized into a
windows media sound file. Various audio related projects I did are on
my diary pages, see a list of the
latest ones, and for instance Page
19 for a later version (with grills) of the speakers, and some
keyboard (rock) performances of mine.
Pictures of some of my prototypes
2 major ampifier
speaker systems, and various electronics are shown.
Buildup of a 2x100 Watt
RMS amplifier, with 0.005 % of distortion and heavy supply.
No SMD's inside, though that's not a matter of principle, but two high
quality MOSFET based power opamp type chips, on heavy heatsinks, and
regular soldering. It's not likely the amplifier breaks down: the
supply is up to 300 Watts continuous power, and probably can do double
for some time, the rectifier can handle that, too, and the chips have
built in 10 Amps current limitation, and will gradually reduce power
when getting over 150 degrees Celsius, which doesn't happen when the
outputs are driving several 50 Watt halogen lights ....
Slew rate, frequency range and damping are such that with appropriate
speakers, a transparentness and brightness and clearness of sound can
be achieved which is uncommon.
100 Watt , big, 4 way
PA/personal use speaker prototype
One of the main design criteria was to create a system which is both
suitable for high-end HiFi use, and for Public Address, or live
purposes, which has happened to a fair extend. A hundred watt isn't a
big PA system, but efficiency isn't bad, even though this is a fully
closed (silicone kit, and cloth contained glass-wool (to prevent lose
glass particles from getting in the speakers during transportation) on
all sides) design, and a good quality is more interesting than only raw
power. The many long scews make the design stable while hobby-buildable.
There are only first order seperation capacitors of non-electrolitic
quality, which probably limits the mid/high range power handling
capacity, but that gives no additional impedances to let the high
amplifier damping keep the softly suspended cones in control, and also
adds the least phase shift, always an annoying seperation filter side
effect. The four way design came from practice, the chosen speakers
otherwise don't easily do justice to the whole spectrum (though I have
no facilities to do dead chamber frequency range or distortion
analysis). There is one pronounced reconce peak at exactly one
half-note, probably from the woofer (which is supposed to be it's own
high filter, but the cone is luckily very stiffly built) which even
gets the amp into it's 10 amp current limit at higher volumes... Apart
from that, response is too high for higher mid range which is of course
not really a problem, but the sound is wonderfull, straight,
uncomplicated, almost without any of the 'normal' speaker limitations
and heavinesses and slacknesses, fairly well spread, and up to very
loud volumes levels with very good quality. I've tried them before the
window on Queens day from non-ground level, and was able to reach near
concert levels various houses down the street away, trying it out with
a Chuck Berry live recording. High sound levels were equally reached
with music kinds from classical (very clear yet unintruding sound) over
disco to Eagles, John Scofield, Chicago, Rolling Stones, Earth Wind and
Fire, Kraftwerk, and even the electronic pounds of Jan Michel Jarre.
box-sized, NiCd/NiMH battery pack charged high Q amp
Of similar quality as the above big amplifier, with less cooling, and
more moderate supply, this portable amplifier, smaller than a car
radio, reaches high quality, for quite some running time (a day easily)
on a rechargable battery pack. Depending on the quality of
rechargeables, high currents and (peak) powers are realistic (up to 10
amps per channel, with good NiCD's with fairly low internal
resistence), at least tens of effective watts (about 20 volts top top
votage swing). Probably hard to beat high quality portable power.
As shown, it is made heavy by a speaker magnet, and rests on two
additional heat sinks, to use in stationary situation with a bit more
power, where the supply comes, via the thick plug-in wires on the back,
from a commercial high power car-booster. Nornally, a wall wart can
supply basic power for low power and a battery pack charger is built-in
to charge while the amp plays on mains power.
A car booster modified
to output it's upconverted voltage
A (rather cheap) unit like this on a car battery should be good for
1000 Watts of bridged peak power, though that may be optimistic (I
didn't measure) I use is as extra TV channel amp, but mostly it
has been modified to ouput it's internal heavy 2x26 volts voltage
converter to feed the above high quality portable amplifier with. That
quality is nearly uncomparable...
That setup is fed by the 12 Volt ouput part of a moderately heavy
regular computer switched supply unit, which works up to moderately
loud sound levels (the supply is good for over a 100 Watts). When the
power fails, or on location, a car or bit smaller battery can be used
NiMH power pack for
portable high quality amp
Made of a cheap set of 20 penlight NiMH rechargable batteries, this
accupack can give loud, outdoor useable sound with the high quality
amplifier shown above, for up to a full day's time easily, before
needing to recharge. Alternatively, the pack can be filled with nickel
cadmium cells, which when of high quality (expensive) can probably
deliver even higher peak currents to the amplifier, but their capacity
is about 2.5 times smaller. The cells can be (slowly) chagred by the
wall wart and the amplifiers' built in charger.
hifi speakers for moderate power, with added dome tweeter
Especially made to cooperate with the high quality portable amplifier
from above, these speakers are just a bit bigger and heavier than
a large ghetto-blaster, but the quality is uncomparibly better. In
outdoors situaltion, the combination give exceptionally clear high
quality sound, for instance for monitoring, or as portable radio/CD (I
used a sony walkman/radio and a philps one-bit protable CD player with
good results) amplification set. A large shopping bag can fit them, but
maybe they're a bit on the heavy side for a long carrying field trip...
The main speaker is a Pioneer dual cone broadband car-speaker, up to
about 50 Watts, of which I'd probably prefer to cut off the small high
frequency cone, which adds undesirable directionality and probably
distortion to the treble range. The additional (large) dome tweeters
are of very god quality, and make the whole sound attractive. The
enclosure is not of bass reflex type, and does not have false air
leaks, and contain damping material.
533 and BF561
dual core DSP EZ-LITE evaluation board.
These boards from Analog Devices
offer current state of the art Digital Signal Processors at 600
MegaHerz, with over a giga operation per second a piece, high quality
(24 buts, up to 96kHz) AD (4 channel) and DA (6 channel) converters for
audio, and a number of video connections, with megabytes of RAM, and a
flash memory, which can be used to make the board into a standalone
operating unit: it starts up within a second with for instance a audio
They come with a C compiler and integrated
development environment, to develop efficient DSP programs. I've
made a working Chorus unit of one of then as an example of the lower
board, where stereo oudio in and out wires are seen, and where the led
bars in the lower right corner are used as fast responding, perfect
peak measuring VU meters.
USB Based 16 bit, 48kHz
AD and DA converter
Based on a Texas Insturments PCM
2906 chip (see inlay for the small PCB tracks carved out to adapt
to more human connection wire pitch), the anti-interfearence conducting
cooky box contains a fairly good PC connected digital to analog and
analog to digital stereo converter, sounding uncomparibly better than
average sound cards. On top of built-in digital filtering, the DA
converter is followed by a high speed, low distortion opamp based
reconstruction filter/buffer (comparable with this article from elektuur).
The prototype also works fine on a notebook, and also on Linux. When
used with a software signal pass program, a sound being passed through
the AD and then back through the DA is not exactly the same, but the
quality degradation is quite limited. The cookie box lid is normally
closed, to reach AD converter 90 dB signal to noise levels without hum
interfearence... To improve supply volage properties, there is a
seperately stabalized 5V supply which can be used through a USB hub.
Delay times for the DA appear to be no less then about 50MilliSec on a
windows XP system, which is not too good for synthesis applications.
The codec chip with very
small pins with a PCB gettig hand prepared for connections
The chip used in the ADDA. For hobby electronicists it's quite hard
(almost inpossible) to make prototypes when pins come with 2 per
millimeter... I've low-budget got around that by hand cutting with a
specially sharpened small hobby knife a pattern which widens from that
pitch to a larger pitch. This was before I knew that could work, and
before I checked and cross checked and rechecked all connection before
and after soldering and using solder suck lint....
Hard to recognize, the
central-right white breadboard holds the electronics for a 10 channel
The desk contains a supply in the upper right, a connection board for a
parallel port cable (top centre), a digital buffer (middle), and a
board with electronics and 3 chips which in total offers a 5 channel
stereo computer controlled
mixer. Unlike older and maybe more common approaches, this solution
uses some rather new (90 NM CMOS, IIRC) chips to do the volume control
job in quite high quality: about 0.0002 percent distortion, and 120 dB
dynamic range.There is one normal DIL Burrow & Brown PGA2310
in the breadboard, and two little hand-cut circuit board adapters (one
horizontal, one vertical) with SMD PGA4311
(BB has been acquired by TI). The wires connect the early breadboard
prototype up to various inputs and ouputs, such as two amplifiers, the
USB DA converter, a synthesizer, a sattelite receiver, a video
recorder/tuner output, or a microphone amplifier or the output of a
EZLITE DSP board's DA converter.
Audio mixer control
panel in Tcl/Tk, on windows XP
The above panel is the analog
mixer control pannel,.which adjusts the volume of all of the mixer
channel pairs in 0.5 dB steps, over a > 120 dB range up to mute.
Note that the mixer is not based on digitizing each channel firtst, and
then DA converting the mix, the mixer is analog, has no delay or
sampling problems, and is of very high analog quality. This pannel is a
Tcl/Tk script, which can be adapted to store mizes, gradually change
between settings, and which allows remote (internet!) real time control
of the mixer.
An experimental stereo
microphone setup with low distortion preamp
The two blobs are foam covered condenser microphone capsules of medium
quality (but very cheap). The blue aluminium box conains a battery or
seperate supply powered high speed opamp (OPA2353)
based pre-amplifier, at short wire distance from the microphones. Most
of these equipment pieces use various 3.5 mm stereo jack plugs, which
are small to use for portable setups.
A 100 watt high quality
versatile audio amplifier chip on small board
Enlargement of the chip in the large amplider enclosure, fitted on the
heatsink below. On the small board, the parts needed to make a
audio amplifier are fitted to get a very low space amp. The led is
connected to the mute circuit, which automutes the chip for a second
during startup. The chip is DC coupled, aliviating the need for large
output capacitors and for excellent audio quality and damping factor.
The portable amplifier uses the same chip.
A Yamaha S90 synthesizer
with samples and virtual analog synthesis, and powerfull effects unit
A fairly recent yamaha synth available for use as musical instument and
goog for testing various pieces of audio equipment in practice. Since
the early days of synthesis, this is an advanced piece of equipment,
very well suited as home piano for domestic use, but containing
powerfull and editable synthesizer sounds and drum samples, too. Comes
with an official Yamaha midi and sample sequencer for windows computers
and a lot of presets. The outputs (and single input for effects
processing) are of high quality, and up to professional studio
A setup with computer
It's important to be able to listen to a modern synthesizer in good
stereo image, preferaby also when working with a computer, apart from
having high quality amplificatio to begin with. Here, the computer is
in the middle of the speakers, just outside the direct paths to the
listerner/operator. The table has later been made smaller, and the
second (smaller) screen dropped from the setup for better results.
There's no mixer or surround sound in this setup... It appeared as I
built the USB DA converter above, that quite some lack of stereo image
and depth in the setup was not because of limitations of the speakers
or otherwise, but that the AC'97 buildin standard PC soundcard simply
generates too much high fequency range smpling artifacts to be very
usefull for that sort of high end audio reproduction, which showed
especially with these 4 way speakers with excellent and high defintion
tweeters and amplifier, which are probably even reproducing sounds
quite a bit outside the audio spectrum (on the higher end, not on the
The large amplifier in
use in a studio setting (lower middle, with the red light on)
As I've tried also in another studio setting, the amplifier sounds
bright, clear, and not very comparably better then an older studio amp
(though that one had some more wattage), in this case on Bose speakers,
with 70's minded material.
home page 
email: email@example.com  My Wiki