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Poul Petersen

Design examples.


This page show some examples of designs I have been involved in through the years.

Transformer balanced audio input stage

The transformer, operating in current mode, is loaded with a negative resistance of same value as its winding resistance, so that the field in the core is close to 0. The circuit lowers the transformer's lower cut off frequency (defined by maximum acceptable distortion) with up to 2 decades.
An example of the performance of this circuit with an EI 19/5 transformer can be seen in the
PP IA18 datasheet

Logarithmic rectifier with large dynamic range

I designed this rectifier for use with a combined compressor, expander and noise gate using feed forward control.
The rectifier is designed around a single log element (a VCA ) that is placed in front of the rectifier circuit to reduce the gain-bandwidth required by the op-amp in the rectifier.
The circuit obtains a 1 dB accuracy from -70 dBu (250 µV) to +20 dBu (7.75 V) in the frequency range 1 Hz to 20 kHz.
Getting a feedback loop stable through a full-wave rectifier has a certain entertainment value, should you decide to try this.

Audio Precision System One / Microsim PSpice interface PP type APXPS

APXPS is a DOS based bidirectional interface between the Audio Precision System One audio test system and the Microsim PSpice circuit analysis software.
This utility enables you to combine calculated and measured data within the working environment of System One or PSpice.
Calculated data can be transferred to S1.EXE and used as a reference for measurements or as data for limit files.
Measured data can be transferred to PSpice providing real-life models of complex components like for example loudspeakers.
This software was later updated to run under APWIN.

Passive loudspeaker cross over network for high-power stage monitor

2 way 24 dB/oct. crossover network for a 15"/2" coaxial driver.
The filter is designed in PSpice with driver models generated from level, phase and impedance measurements on the drivers.
Measurements were made with Audio Precision System One and transferred to PSpice.
The filter is built with inductors wound on gapped toroid cores to save weight.

2 * 100 W class A High End power amplifier

Discrete components only class A power amplifier with no global feedback.
Some key features are: The audio circuits were designed by the manufacturer for best possible audio performance.
I have made some minor modifications to the audio circuits so they were easier to manufacture. I also designed the balanced input stage, the protection circuitry and the power supply regulators, designed the PCBs, developed a PC controlled test procedure and have tested 20 amplifiers.

2, 3 or 4 way PA loudspeaker crossover network, PP type CS01

19" 1-unit crossover network with 2, 3 and 4 way modes.
There are outputs for subwoofer, low-frequency, mid-range and high-frequency. On/off switches for the sub-woofer and high frequency sections selects the filter mode. When the sub-woofer is turned off, the low-frequency band will also reproduce the sub-woofer band. When the high-frequency is turned off, the mid-range band will also reproduce the high-frequency band.
Some key features are: I have designed the circuits, the PCB, the mechanics and a PC controlled test procedure and manufactured and tested 50 filters.

Battery powered microphone preamplifier, npn Elektronik type MA02

Transformer balanced microphone amplifier for use where the cable from the microphone to mixer is too long for the microphone to drive. This amplifier will drive more than 500 m of cable.
I have designed the circuit and the PCB and assembled and tested 25 amplifiers for the manufacturer.

Cable-, fuse-, and battery-tester, PP type CT01

A handy cable tester for test of cables with XLR and ¼" jack connectors. The tester has 4 keys and 8 high-intensity LEDs to provide a clear and unambiguous indication of the cable connections.
I have designed the circuits, the PCB, the mechanics and test procedure and manufactured and tested 50 units.
PP CT01 datasheet

Limiter for 8 channel Sony mixer

These mixers are used by journalists to edit news spots. After changing to digital tape recorders, there were problems with audible overloaded recordings. The problems were solved by installing limiters in the mixers' outputs. The limiters, that were developed for this purpose, have 3 time constants in the control path, and will allow for up to 10 dB overload with speech and pop music without audible abnormalities.
I have designed the circuit and the PCB and manufactured, installed and tested 7 units.

Measurement microphone amplifier, PP type MI02

A half-length PC ISA expansion card that contains a galvanically isolated power supply and a microphone amplifier. With this card and a signal processing card mounted in a PC, a complete, portable loudspeaker measurement system can be realized. The microphone amplifier can be powered from the PC or from an external 5 V power supply.
Some key features are: I have designed the circuits and the PCB, a PC controlled test procedure and manufactured and tested 10 amplifiers.
PP MI02 datasheet

Direct Inject box, PP type DI02

Compact, phantom powered DI box, that provides the impedance- and level-matching required for a high-impedance music instrument signal to be connected to a mixing console's microphone input.
DI02's high-impedance input and its symmetrical, low impedance output insure that almost any high-impedance signal can be tapped and fed to a long microphone line without loss of signal quality.
DI02's heavy duty rectangular enclosure guarantees a long life on stage and allows easy packing of any number of units for transportation.
I have designed the circuits, the PCB, the mechanics and a PC controlled test procedure and manufactured and tested 150 units.
PP DI02 datasheet

2 channel 2-way / 1 channel 3/4-way crossover network, PP type XO01

Half-width 19" 1-unit modular loudspeaker crossover network. Extremely compact high-performance crossover network for PA and studio applications. The function of the XO01 is determined by the modules installed in it. Some features are transformer balanced inputs, 24 dB/oct. high-pass filter, 24 dB/oct. crossover filters, up to 4 band fixed equalizer on each output, limiter on each output and up to 4 channels 2-way crossover network in a single unit 19" rack space.
PP XO01 datasheet

Equalizer for correction of frequency response in recording studio

The filter is based on my standard crossover network type XO01.
I have designed a new processor module for the XO01, based on the studio's measured frequency response and a plot of the desired frequency response.

Portable, 4 channel modular mixer for on-location recording

Dynamic microphone, phantom or AB powered condenser microphone, line or test tone can be selected for each input, that also have a 3 band equalizer, 1 aux. send and PFL.
There is a selectable limiter and a level and phase meter in each output.
I have developed a PC controlled test procedure and tested 50 units for the manufacturer.

Hi-Fi CD player

Hi-fi CD player with volume control, headphone output and remote control utilizing a low-cost mechanism.
This CD player has been in production in Taiwan in a volume of 5000 units/year for some years and was updated mainly for cosmetic reasons, but a few bugs, that had been discovered through the years, were also fixed.
I have designed a new PCB and built and tested a prototype.

Mixer for partially automatic program broadcast in a radio studio

A 19" modular rack mount mixer with 7 transformer balanced microphone and line inputs, 4 transformer balanced outputs, remote controlled signal routing functions and comprehensive monitor functions.
Each input is equipped with a 3 time-constant limiter that will allow more than 10 dB limitation without audible abnormalities.
I have designed the circuit and the PCB and assembled and tested 1 unit.

Telephone answering machine with external tape recorder, npn Elektronik type ATS1

The telephone answering machine records incoming messages on an external tape recorder to enable very long recording times.
The tape recorder is controlled via a remote-control interface or by powering it when the recording is to begin.
One of 4 electronically stored messages can be played to answer incoming calls. In order to be able to recover from a power failure, all the telephone answering machine's settings are stored in EEPROM.
I have designed the circuit, the PCB and the software, developed a PC controlled test procedure and tested 60 units.

Noise generator for loudspeaker power testing, PP type NG01

19" 1-unit noise generator that deliver white or pink noise with crest-factors of 3 dB, 6 dB or 9 dB.
The noise source in this generator is an ultra-low-noise op-amp, as this component has a very linear noise spectrum in the 20 Hz - 20 KHz frequency range and a very well defined and stable noise level.
I have designed the circuits, the PCB, the mechanics and test procedure and manufactured and tested 3 units.

RIAA amplifier with Feedback Canceling Filter PP type RA03

The feedback canceling filter reduce the acoustical feedback from the loudspeakers to the gramophone and enables up to 10 dB higher SPL before feedback becomes audible. The special differential filter adds the two channels to a mono signal at low frequencies. The idea of summing low frequencies into a mono signal to avoid feedback is not new, but all earlier implementations uses some sort of crossover network to extract low frequency information, adds the two channels and then add this signal into the stereo high-frequency signals. This topology gives an undesirable phase response and will in some cases make the acoustical feedback problem worse. The Feedback Canceling Filter uses a special differential filter that acts like a wire if it's input is a mono signal, but will add the low frequency content of the channels if they differ.
PP RA03 datasheet

Cinema surround-sound sub-woofer crossover network

The filter is based on a commercially available crossover network with plug-in filter modules.
The loudspeaker system has 2 satellite speakers with a single sub-woofer. The satellite speakers are able to handle the entire frequency range, but provides insufficient low-frequency output for the application. To maximize the low-frequency output, both the satellite systems and the sub-woofer reproduces the low frequencies. To obtain proper acoustical summation of the outputs from the sub-woofer and satellite systems, the filter contains a phase alignment for the satellite systems so that they are in phase with the sub-woofer over its operating frequency range.
I have designed the filter circuit and modified and tested 1 filter.

Low cost PA loudspeaker crossover network

19" 1-unit 2 channel 2 way crossover network with sub-woofer output.
The crossover network is built on a universal PCB, that can be loaded as required. Rather than saving on component quality, cost savings have been obtained by omitting all features that are not absolutely necessary for crossover operation.
The following filter functions are available: I have designed the circuits, the PCB, the mechanics and a PC controlled test procedure and manufactured and tested 50 filters.

Recording studio monitor sub-woofer power amplifier

A 300 W power amplifier module with crossover network for installation inside a sub-woofer enclosure.
I have developed a PC controlled test procedure and tested 50 units for the manufacturer.

Demonstration test program for trade show

This demonstration shows the benefits of a new analogue audio tape type by comparing its technical performance to an established industry standard tape type.
The test setup consists of an Audio Precision System One audio analyzer, a high performance tape recorder, a small keypad for use by trade show guests and a PC with a 17" screen.
The PC controls the System One, the tape recorder and the keypad. Samples of the two tape types are placed on the reels of the tape recorder and the test program keeps track of which type is currently under test.
The test will show the performance of a 1 KHz sine wave FFT spectrum,  3rd. harmonic distortion versus level or A weighted noise for the two tape types. The test results are displayed graphically on the PC screen with results for the two tape types overlaid for easy comparison.
The keypad selects between FFT spectrum test, 3rd. harmonic distortion test, noise test, autorun or stop. All measured data are compared to predefined performance limits to avoid running the tests with worn out tapes. When a performance limit is exceeded, the program prompts the stand staff to replace the tapes.

Stereo 3 way loudspeaker crossover network experimental board

Experimental cross over network designed for use in a loudspeaker manufacturer's development laboratory. The crossover network's PCB is mounted on a 3 mm aluminum base plate and contains the power supply and amplifiers required to build a 2 channel 3 way 24 dB/oct crossover network. All filter components are mounted on solder pins to allow easy replacement.
I have designed the circuit, PCB and manufactured and tested 3 units.

6 * 150 W surround sound power amplifier

Surround sound amplifier in a 80 * 350 * 300 mm enclosure. The amplifier contain s 1200 W switch mode power supply for the 6 channels. Each channel has a separate signal controlled step down switch mode regulator to optimize the systems efficiency.
I have designed the circuits in cooperation with the manufacturer, designed the PCBs and built and tested a prototype.

High End class A loudspeaker crossover network

Active stereo 3 way crossover network.
Some key features are: I have designed circuits in cooperation with manufacturer, designed the PCBs, developed a PC controlled test procedure and tested 40 units.

Balanced preamplifier

Preamplifier with 7 balanced line inputs, 1 MM/MC pick-up input and 4 balanced outputs.
Separate buffers for each input provides a low drive impedance for the relay controlled signal source selector, so that a cross talk figure in excess of 120 dB can be achieved.
Some key features are: I have designed the circuits and the PCBs and manufactured and tested 2 prototypes.

Low cost preamplifier

Simple preamplifier with 6 line inputs, 1 MM/MC pick-up input and headphone output.
Some key features are: The amplifier is produced in Taiwan in a volume of 3000 units per year. A partial schematic were supplied by manufacturer. I have finished the circuit design, designed the PCB and built and tested 2 prototypes.

2 * 80 W integrated amplifier

Integrated amplifier with 6 line inputs, volume, balance and tone control.
This is a combination of an existing pre-amplifier and power amplifier.
Some key features are: The unit is produced in the Taiwan in a volume of 3000 units per year. Schematics for the existing products were available from the manufacturer.
I have finished circuit design, designed PCB and built and tested 2 prototypes.

Controller for motorized lamp

The lamp was used to attract attention to a shop window. This compact 4 channel controller mounts on the rear of the lamp and is powered from this.
The control of iris, color, lamp position and gobo is performed by a PIC microcontroller and a 4 channel DAC. A random like function is made by using a carbon track potmeter in one of two timers.
I have designed the circuit, the PCB, the mechanics and the software and manufactured and tested 10 units.

Telephone hybrid for radio studio, npn Elektronik TH01

Portable telephone hybrid for recording of telephone conversations on tape and later playback of the tape via the telephone line.
I have designed the circuit, the PCB and the test procedure and tested 20 units for the manufacturer.

Optoisolator for the Philips I²C-bus serial interface, PP type II01

During the design of audio equipment, it is often desirable to use a PC to emulate the equipment's microcontroller so that new functions and control routines can be easily tested.
Many of the special ICs used for audio have an interface compatible with the Philips I²C specification and can be controlled from a PC with a I²C interface card installed.
The noise generated by a PC will often be induced into the device under test and prevent any measurements to be made, or in some cases (mainly with FM tuners) prevent the device from working at all. These noise problems can be solved by inserting a galvanic isolation between the PC and the device under test.

High performance D/A converter for CD player

D/A converter with Pacific Microsonics HDCD decoder, Crystal Semiconductor 24 bit DAC, low-jitter crystal clock oscillator module and class A analogue stages for balanced and unbalanced outputs.
This DAC has a 2. harmonic distortion of -105 dB at 1 KHz full scale output and will drive 2.5 V peak into 50 ohms.
I have designed the circuit, PCB and built a prototype for evaluation by manufacturer.

Fireworks firing box

Fireworks firing box with continuity test, battery charger and low-battery lock-out.
I have designed and tested the circuit and PCB and supplied 60 PCBs to the customer.

Critical speed indicator for high-speed ferry

Newer passenger ferries sails with still higher speeds and this causes some very high stern waves that can be very problematic for the surrounding environment. The size of the stern waves depends on the ferry's speed and the water depth. At a certain speed, the stern waves has a maximum, but decrease rapidly both below and above this speed. The Critical Speed Indicator calculates how far the current speed is from this critical speed and displays this on a LED bar meter. Input to the Critical Speed Indicator comes from the ship's GPS system and echo sounder in form of RS-232 signals using the NMEA 0183 protocol. The Critical Speed Indicator is built around a PIC microprocessor, that decodes the two NMEA 0183 signals, performs the required calculations and drives the display.
I have designed the circuit, the PCB and the mechanics, designed a PC controlled test procedure and manufactured and tested a number of units.

High performance Hi-Fi CD player

This CD player is designed around a Philips CD7 mechanism combined with a 24 bit D/A converter with class A analogue filters and balanced output stages.
Some key features are: I have designed the circuits and PCBs and manufactured and tested 4 prototypes.

High performance Hi-Fi pre-amplifier

Some key features are: I have designed the circuits and PCBs and manufactured and tested 4 prototypes.

Automobile subwoofer amplifier

The amplifier is designed around a dynamically self centering bridge circuit, so it will deliver almost rail to rail output with an unregulated power supply. The output power is 115 W into a 0.5 ohms load from a 12 V power supply. Another unique feature of this amplifier is a 10 mA idle current consumption over the -40 C to 85 C temperature range.
I have designed the circuit and PCB and built and tested a prototype. 3

Automatic, stepped frequency sine wave generator

This generator generates a 50 mS sequence of 4 tones and a pause for initial alignment of video recorders. When this signal is displayed on an oscilloscope, it is easy to see if the frequency response of the recorder is linear.
The hardware for this generator is a PC with a sound card. Each of the 5 tones used for the sound card was generated with Audio Precision's APWIN software and translated into WAV format (the pause signal is a 1 kHz, -120 dBfs tone).
The 5 tones was merged and expanded into a 10 seconds file with the Sound Recorder program, that is part of Windows 95.

External clock input for CD player

This small module, that can be installed in most CD players, allows the CD player's internal clock frequency to be synchronized to an external clock, so that digital data can be transferred to an external D/A converter without a sample rate converter.
The module consists of a crystal clock oscillator, a PIC microcontroller and a PLL. The clock oscillator uses the crystal that was originally installed in the CD player to provide the clock when an external clock is not present.
When an external clock is applied, the microcontroller calculates the ratio between the internal and the external clocks, programs the PLL dividers for this ratio and switches to the PLL clock.
I have designed the circuit, microcontroller code and PCB and made some prototypes for the manufacturer.

2 * 20 W integrated amplifier

Low cost integrated amplifier with 5 line inputs, volume, balance and tone controls. This amplifier has been in production in the Taiwan in very high volumes for several years and was updated mainly for cosmetic reasons.
I have made a few circuit modifications, mainly to allow for the use of more modern components, and designed a new PCB, so the entire unit could be assembled without use of any wires.

Low cost universal power amplifier circuit

Universal cross-over and power amplifier module that will operate with supply voltages from 30 V to 80 V.
To lower the manufacturing cost, all components, except output transistors and driver transistors, are SMD types.
To simplify the module's integration in a product, short circuit protection, thermal protection and soft turn on is part of the module. Electrically the module's use is simplified by a 120 dB power supply rejection ratio and balanced input.
I have designed and tested the circuit.

Automatic production test software

Audio Precision test software for a CD-player, a pre-amplifier and a power amplifier.
The software checks the measured data against limits and saves the measured data.
When the units are re-tested after burn-in, the software compares the measured data with both the previous measurements and checks against limits.

Low power photocell with modulated infrared light

This photocell design is used to count the number of visitors passing through the gate to a bird preserve.
The design is based on detector that is highly sensitive to short infrared light pulses and at the same time rejects ambient light, so that the detector works in direct sunlight. This has been achieved through careful electronics design rather than with expensive optical components.
Power saving is achieved by transmitting with a very low duty-cycle - a 10 µS pulse every 200 mS.
The IR transmitter is a separate unit that is powered and controlled from the detector unit. To ease installation, the control signal for the transmitter is modulated onto its power supply line, so that only 2 wires are required between the detector and transmitter.
The IR photo diode amplifier is a bandpass filter design with a transresistance of 3 V/A at DC and 60 MV/A at 1 MHz.
The circuit is controlled with a PIC microcontroller and a 200 mS timer. The microcontroller is normally in sleep mode to save power. Every 200 mS, the timer issues an interrupt to wake up the microcontroller, that powers up the detector, waits for the detector circuit to settle (around 1 mS), sends a trigger pulse to the transmitter and checks if a pulse is received.
If the IR beam has changed state to interrupted, the microcontroller increments its counter.
When the gate to the bird preserve is closed, all circuits are disabled reducing the supply current from the normal 1.5 mA to 70 µA.

Power supply for 80 W valve amplifier

The power requirement for this amplifier is a little unusual as it requires one ground referenced 80 V supply at some mA and two 400V floating supplies capable of delivering 200 mA peak ( don't ask me about the amplifier - I have never seen the schematic ).
The low voltage supply was easy - a rectifier and an RC filter.
The common way to design a high voltage supply for a valve amplifier is to use a rectifier followed by an LC filter, but this takes a lot of space, so we made this regulator in stead.
It is based on a high-voltage MOS-FET with a current limiter at around 500 mA.
The current limiter is required as the regulator sees a short until the output capacitors are charged.
The voltage regulator is a current source loaded by a shunt regulator driving the gate of the MOS-FET.
The regulator has no overall feedback as there was no requirement for a low output-impedance.
I have designed and tested the circuit and pcb and built some prototypes.

Front-end for a sub-woofer amplifier.

The circuit automatically compensates for changes in the frequency response and driver sensitivity due to variations in the voice coil temperature by measuring the voice coil temperature with a NTC.
This was a re-design of an existing product, but using more modern components for improved performance and lower cost.
I have re-designed the circuit, designed the PCB and PIC software.

Gramophone control software.

Software for a reference clock to a PLL control of a core-less motor in a gramophone.
The software is designed to be easily adaptable to different gear ratios and tacho-frequencies by changing a few constants in the assembler code.
Some additional features are:

Gramophone control software II.

Complete gramophone and motor control in a PIC (just to see if it could be done).
The circuit is based on a PIC16F1713 as this device has OP-AMPs (for the motor driver amplifier), zero-cross detector (for the tacho) and Configurable Logic Cells (for the PLL).
Precision tacho-motors (as used in gramophones) are very expensive so the design was done with a tacho-motor from a worn-out tape recorder.
Having no data on the motor, I had to measure and simulate the motor's electric and mechanical parameters to get the circuit stable.
Apart from the PIC, a MOS-FET, a transistor and a voltage regulator are the only active components in the circuit.

Copyright and disclaimer.

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Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: pp@poulpetersen.dk

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