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Page Last Updated - November 2018

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Troubleshooting - part 1Troubleshooting and Repair Guide - Part 1 ... introduction, general principles and power amps
Troubleshooting - part 2Troubleshooting and Repair Guide - Part 2 ... preamps, crossovers and other opamp circuits

Projects 1-9   10-19   20-29   30-39   40-49   50-59   60-69   70-79   80-89   90-99   100-109   110-119   120-129   130-139   140-149   150-159   160-169   170-179   180-

No.Project DescriptionDatePCB
01A Better Volume Control A volume control using a linear pot that is much better than most log pots
02Simple High Quality Hi-Fi Preamp As it says - simple, high quality preamp.  Has all the facilities normally expected
0360W / 8 Ohm Power Amplifier (Updated) My old faithful power amp design. - See Project 3APCB
04Dual Power Supply A power supply that is adaptable for most power amps
05Preamplifier Power Supply Uses External AC Adapter.  Safe and effective (replaced by P05-Mini). 
05APreamplifier Power Supply All the features of the P05, but much more versatile.  Includes 'Loss of AC' detector. 
05CPreamplifier Power Supply All the features of the original P05B plus an improved muting circuit.PCB
05-MiniPreamplifier Power Supply Flexible, low-cost PSU for preamps or general test work.  This is a new PCB that replaces the original P05 board, because some builders just want a simple PSU. 2017PCB
06Phono (RIAA) Preamp Very high quality moving magnet phono preamp - few circuits will better this one.PCB
07Discrete Op-Amp Class-A Output.  Intended as an experimental device - works extremely well
082-Way Electronic Crossover  Conventional 3rd order electronic crossover
0924dB/Octave 2/3-Way Xover Linkwitz-Riley alignment.  This is the optimum choice for any active system - Rev-B boardsPCB
9aA Reader's Xover This is well worth reading if you can't decide on biamping
1020 Watt Class-A Power amp True Class-A power amp for low power systems or tri-amping
11Pink Noise Generator For loudspeaker and room acoustics testing
12Current Feedback Amp An updated version of a very old 60W / 8 Ohm design
12aEl-Cheapo This is the real El-Cheapo, presented in more or less original form
13Low Noise Preamplifier Simple unbalanced design, suitable for microphones - very low noise
14Power Amp Bridge Adapter A conventional adapter for bridging power amplifiersPCB
15Capacitance Multiplier Supply Extremely low ripple supply with less power dissipation than a regulator
16Audio Millivoltmeter For Testing Amplifiers (etc) - An analogue design, 3mV to 30V with dB scale
17A-Weighting filter For noise measurements.  Ideal for use with the AC Millivoltmeter above
18Simple Surround Sound Decoder A line-level version of the "Hafler matrix" decoder
19Single Chip 50W Power Amplifier Using the National Semiconductor LM3876/ LM3886 Power IC.PCB
20Simplest Ever Bridge Adapter Use this simple method, and avoid external circuits
21Stereo Width Controllers Two to choose from.  Expand or contract the stereo sound stage
22Simple Audio Oscillator For use with the millivoltmeter, for testing amps and speakers
23Power Amp Clipping Indicator A fast and accurate indicator to show an amp is clipping
24Hi-Fi Headphone Amplifier Contributed by a reader, this is a very nice circuit
25Phono Preamps For All Various circuits for moving coil and moving magnet pickups
26Digital Delay Digital delay  Retained for information only - The Mitsubishi delay IC is no longer available
26ADigital Delay Unit Digital delay using PT2399 - Note: PCB (with IC) available nowPCB
27100 Watt Guitar Amp (Mk II) The new and improved version of the original Project 27 guitar amp (now 27b).PCB
27b100 Watt Guitar Amplifier The original of the unit above.  Retained for posterity, and has speaker box details
28Parametric / Sub-Woofer Equaliser A simplified version, which performs surprisingly well
29Tremolo Unit A versatile guitar effect.  This is a simple circuit that gives very good results
30Stage and Recording Mixer Able to be built in modular form, allowing as many (or few)  channels as desired.
31Full Featured Transistor Tester Just the thing to check the transistors for any project
32Car Audio Preamp + Artificial Earth Especially for car audio installations, can be used for any single supply system
33Loudspeaker Protection and Muting Protect speakers from turn-on and turn-off transients and amplifier faultsPCB
34Guitar Spring Reverb Unit A high performance spring reverb unit for guitar amps
35Direct Injection (DI) Box An essential companion for the mixer for stage or recording work
36Death Of Zen (DoZ) An ultra simple, high performance Class-A power amp.  Revision-A boards now shippingPCB
37Death of Zen Preamp "Minimalist" preamp, designed to suit the DoZ (or any other) power amplifierPCB
37ADeath of Zen Preamp Updated "minimalist" preamp, designed to suit the DoZ (or any other) power amplifierPCB
38Signal Detecting Auto Power-On Unit For equipment to be turned on with the main amp.  Detects signal and applies power.
39Soft Start Circuit Designed for (large) toroidal transformers, limit the inrush current to a sensible value.PCB
3A60-100W Hi-Fi Power Amp 100W into 4 or 8 ohms.  Has excellent performance, and is easy to build.PCB
3B25W Class-A Hi-Fi Power Amp Up to 25W into 8 ohms.  Using the same PCB as P3A, the Class-A version is a good performer, and is easy to build.PCB
40Load Sensing Auto Switch Apply power to the system by turning on one item.  Superseded by Project 79.
41Opamp Design + Test Board Quickly assemble an opamp circuit for testing.  Very easy and intuitive to use
42Thermo-Fan For Amp Cooling Use a 12V computer fan to cool your amp.  Uses diode temperature sensor
43Ultra Simple Split DC Supply When you need a +/- supply, and only have a DC adapter
44Dual +/-25V Lab Supply Up to 800mA (typical) output current at +/-25V.
45Simple Bass Guitar Compressor An ultra simple compressor, ideally suited for bass guitar.
46Thermal Shutdown/ Protection Add this circuit to either turn the amp off or activate a cooling fan if it overheats
47WithdrawnDetails no longer available 
48Sub Woofer Processor Designed to operate a sub-woofer driver below its resonant frequency. 
48ASub Woofer Processor (Rev-A) New version of the sub-woofer driver has far greater flexibility.PCB
49Guitar Vibrato Unit A reasonably simple circuit, with results similar to the Vox AC30 guitar amp
50Mic Circuit Tester Small oscillator to check mic & line circuits during sound setup
51Balanced Line Drivers Use these to eliminate hum for long signal leads
52Distortion Analyser A simple distortion meter you can use with an oscilloscope or millivoltmeter
53Output Power Limiter Just the thing if you want to limit amplifier power to something 'sensible'
54Low Power FM Transmitter Useful to retransmit the hi-fi to another FM receiver nearby, or as a wireless mic
55PPM and VU Meter A versatile and useful VU meter circuit, can also operate as a Peak Program Meter 
56Variable Impedance Modify the output impedance of an amplifier with these simple ideas (Updated) 2012
57SIM - The Simple Version The smallest variation in amplifier performance is shown with this simple circuit
58Tone Burst Speaker Measurement Set Based on work done by Siegfried Linkwitz (reproduced with his permission).  See Project 143 for a conventional Tone Burst generator
59Self Oscillating Amplifier Allows you to make a power amp oscillate at a defined frequency for testing
60LED VU Meter Standard LED VU meter, includes simple rectifier circuit for full wave detectionPCB
61WithdrawnDetails no longer available 
62LX-800 Lighting Controller Light is always needed for theatre and live music, and this is just the ticket
63Multiple Feedback Bandpass Filter This is the basis of an expandable equaliser and analyser or vocoder
64Instrument Graphic Equaliser Designed especially as a guitar / bass equaliser, this unit is expandable
65Strobe Light Designed as a companion to the LX800, can also be used by itself 
66Low Noise Balanced Mic Preamp A discrete front end makes this balanced microphone preamp very quietPCB
67Fast Audio Peak Limiter This peak limiter is simple and very effective.  Uses discrete FET gain control 
68300W Subwoofer Amplifier This amp is designed especially for subwoofersPCB
69Switchmode Car Power Supply Low power +/-12V supply for car preamps and equalisers (etc.)
70DoZ Headphone Amplifier The DoZ is ideally suited to headphone use.  Revision-A boards now shippingPCB
71Linkwitz Transform Circuit The Linkwitz Transform circuit - an equaliser to provide extended bass responsePCB
7220W/Ch  Stereo IC amplifier Based on the versatile LM1875 from National SemiconductorPCB
73Hi-Fi PC Sound System You have never heard your MP3 collection, CD or games sound so goodPCB
74Simple RF Probe Indispensable for any RF work.  Uses just 4 passive components
75Constant Q Graphic Equaliser Constant Q design.  Can be made using as many or as few sections as needed.
76Opamp Based Power Amplifier This is a contributed project, and has some interest as a learning exercise
7713.8V Power Supply / Charger A supply for testing and working on car amplifiers - up to about 500 Amps!
783-Way 12dB/ Octave Crossover Contributed project, simple 12dB/ Octave crossover network.
79Current Sense Auto Power Switch A current sensing switch turns on multiple devices from one master
80Reverse RIAA Equaliser Test phono preamps for correct equalisation, or convert unused phono inputs.
8112dB/Octave 2-Way Xover Linkwitz-Riley alignment and phase coherent (uses the P09 Rev-B PCB)PCB
82Loudspeaker Test Box Quickly and accurately determine the optimal impedance correction network
83MOSFET Follower Power Amplifier Another contributed project that may be of great interest
84Subwoofer Graphic Equaliser This is a constant Q design, eight 1/3 octave bands covering 20Hz to 100Hz.PCB
85Simple S/PDIF DAC Quite possibly the simplest possible S/PDIF receiver and DAC you will ever find
86Miniature Audio Oscillator The Miniosc - low distortion battery powered audio oscillatorPCB
87Balanced Transmitter / Receiver Updated and upmarket versions of the Project 51 balanced circuitsPCB
88High Quality Audio Preamp An audiophile quality preamp, with full switching and programmable gainPCB
89Switchmode Power Amp Supply A high power (350W typical) car power amplifier supply
90Dimmer Control Polarity changer Convert old dimmer units using a 0 to -10V control signal
9178 RPM and RIAA Phono Equaliser Multi Standard 78 RPM and RIAA Phono Equaliser handles all 'standards'PCB
92Guitar and Bass Sustain A simple, low distortion limiter for musical instruments
93Recording & Measurement Microphone A selection of projects for making your own mics for recording and/ or measurement (includes PCB Version of P93)PCB
94Universal Preamp/ Mixer A small and versatile mixer and preamp, suitable for many uses.PCB
94AUniversal Preamp/ Mixer Alternative wiring for the versatile mixer/ preamp, suitable for many uses.PCB
95Low Power -Ve Supply Low power -ve supply for car audio preamps and crossovers
9648V Phantom Supply Low noise 48V regulator and phantom mic supply circuits.PCB
97Hi-Fi Preamp Another preamp, but this one has tone controls, and all pots are PCB mountedPCB
98Battery Charger Automatic Charger for Battery Operated Hi-Fi Preamps
99Subsonic/ Rumble Filter A 36dB/octave 17 Hz high pass filter to remove ultra low frequencies (Rev-B PCB)PCB
100Headphone Adaptor A simple adaptor to provide a headphone output to an existing power amplifier
101MOSFET Power Amp 200W MOSFET Hi-Fi Power amplifierPCB
102Simple Pre-Regulator Use this to adapt P05 to a power amp supply for your preamp   - added 'enhanced' version 2012
103Subwoofer Phase Control A standard phase control for use with subwoofers.
104Preamp/ Crossover Muting Circuit Muting circuit suitable for all line-level applications. 
106hFE Tester An hFE tester with switched collector currents for the Device Under Test 
107Polarity Reversal Switch Switch the absolute polarity (180° phase shift) of signals with this simple circuit 
108Switchmode PSU Protection This contributed circuit can help prevent failure of your switching power supply. 
109Portable Headphone Amp Another contributed circuit, this is a complete portable headphone amp that features crossfeed. 
110IR Remote Control Volume and mute are provided by this simple but functional circuit.  Suitable for all preamps.PCB
111PIC Speaker Protection This doesn't replace P33, but it is possibly the ultimate speaker protection. 
112Dummy Head Recording Microphone Surprise yourself with one of the most realistic recording processes known.PCB
113Hi-Fi Headphone Amp Yet another headphone amplifier, but this one has PCBs availablePCB
114PWM Power Amplifier Based on the ColdAmp BP4078 400W module, Class-D is here.  Definitely worth looking at!PCB
115GainClone Amplifier Complete GainClone amplifier, including full chassis details (2 part article)PCB
116PWM Subwoofer Amplifier Using a ColdAmp BP4078 module, this article has complete construction detailsPCB
1171.5kW Power Amplifier Just the thing for those for whom no amount of power is enough grin 
118PC Power Switch Simplest ever PC switching unit - turn on peripherals with the PC 
119Signature Analyser A component signature analyser that allows in-circuit parts tests 
120Crowbar Speaker ProtectionThe final answer to speaker protection.  Comes with many cautions and warnings!
121Reading InductanceUse your DC voltmeter or frequency counter to measure crossover coils.
122Ultra-Simple Mic PreampA single low cost IC and tiny PCB make a great utility microphone preamp.PCB
12318dB/Octave Crossover A selection of 18dB/octave (3rd order) crossover filters.
124Dummy Load High power dummy load for amplifier testing. 
1254W-LRX 4-Way Linkwitz Riley CrossoverPCB
126PWM Dimmer PWM LED Dimmer/ Motor Speed Controller.PCB
127TDA7293 Amplifier Dual power amplifier, using the TDA7293 power opamp IC.PCB
128VU Meter Bridge Build a pair of VU meters to monitor your recording or PA level.PCB
129Matrix Mixer Now you can build a matrix mixer to suit your exact requirements. 2010PCB
130Inverse A-Weighting Undo the rolloff caused by the A-Weighting filter 2010
131Light Activated Switch Little to do with audio, but a useful project that can be adapted to other uses.  Includes sound activation (added Oct 2018) 2010
132Air Bearing Tonearm More of a proliferation of ideas and techniques than a "real" project, the vinyl lovers should look at this 2010
133PA-PC Audio Interfaces Interfaces from the headphone output of a PC to PA mic input, and PA speaker line to PC recorder 2011
1344mA Current Loop Microphone Common for professional measurement microphones, but not well known outside the noise measurement field. 2011
135Phase Correlation Meter More of an experimental circuit than anything else, it should help anyone trying to build a phase meter. 2011
136Hardware Based RTA A real-time audio analyser - contributed project, based on the multiple feedback bandpass filter described in Project 63. 2011
137Powered Box Amplifier Complete Preamp, crossover & power amps, designed for powered PA speakers, Leslie cabinets, 'party' systems, etc.  Set of 3 PCBs. 2012PCB
138Mains Under/ Over Voltage Protection Some equipment is vulnerable to under or over-voltage.  Use this protection unit to prevent damage (Aug 2012).  Now includes 3-phase detectors 2016
139Mains Current Monitor A versatile, safe & accurate way to measure (and view with a 'scope) the current drawn by mains powered equipment 2012
139aSimple Current Monitor Much like P139 (versatile & safe), but a much simpler way to measure the current drawn by mains equipment 2012
140True RMS Adaptor The only way to measure non-sinusoidal waveforms is true RMS or errors can be significant.  Use this adaptor to get true RMS readings 2012
141VCA Based Preamplifier If you need a multiple channel preamp with a single volume control for all, this might be just what you're looking for.  Ideal for home theatre! Uses the THAT2180 VCA. 2013PCB
142Simple High Current Regulator Need higher than allowable input voltage or the need for more current than IC regulators can provide.  This basic design will be more than acceptable for many tasks. 2013
143Tone Burst Generator/ Gate Sometimes no other piece of test gear will allow you to run the tests you need.  Check amplifier overload recovery, perform non destructive high power speaker tests, plus many more. 2013
144Mains Power Sequencer If you need to turn on/off mains equipment in a preset sequence, this project will be just what you are looking for.  Suitable for large PA systems, recording studios, lighting, etc. 2013
145Silent Guitar Effects Switching How to use Vactrol® or DIY optocouplers to switch signals in guitar amplifiers.  No contact bounce or clicks, just virtually silent switching without any noises. 2013
146Overload/ Clipping Indicator Overload indicator for mixers, preamps or power amps.  Simple opamp comparator circuit gives fast response. 2013
147BJT Muting Switch A little known technique that doesn't look like it could ever work - using bipolar transistors. 2013
148State Variable Crossover Perfect for loudspeaker system development, or can be used as part of a biamped or triamped system.  12dB/octave continuously variable filters. 2014PCB
149Musical Instrument Graphic EQ Guitar, Bass or Keyboard Equaliser.  A greatly improved version of Project 64. 2014
150Wien Bridge based equaliser A functional building block that can be used in mixers, preamps, guitar and bass amps, etc. 2014
151High Voltage DC Supply If you want to experiment with valve ('tube') circuits, you need a power supply for the B+ and DC for the heaters. 2014
152-1Bass Guitar Amplifier (Part 1) First part of a project with the circuits for what might be one of the world's best DIY bass amps. 2015
152-2Bass Guitar Amplifier (Part 2) Part Two of the bass amp project describes the compressor, soft-clip and crossovers, and discusses power amp needs. 2015
153Frequency 'Isolator' EQ 'Isolator' EQ is very common amongst the DJ fraternity, but they can be rather expensive.  Now you can build your own, and with all the features you need. 2014
154PC Oscilloscope Interface PC sound card oscilloscopes can be handy, but you need this circuit to make sure that it doesn't get blown up if you connect it to more than a few volts 2015
155Variable High And Low Pass Filters These circuits are common in mixing consoles, but you might find them useful elsewhere as well.  The frequency ranges can be adjusted to suit your needs. 2015
15612V Trigger Switches Many home theatre receivers (aka audio-visual receivers or 'AVRs') have a 12V trigger output, and the circuits shown can be used to switch on equipment when the trigger voltage is present 2015
1573-Wire Trailing-Edge Dimmer You can't buy these easily, so the only option is to build it yourself.  This is the first (and only) fully tested and working design you'll find anywhere. 2015
158Low-Noise Test Preamplifier Every so often, you find that you need to listen to or measure signals that are well below the noise floor of your bench amp or 'scope.  With 20, 40 & 60dB of gain, this is what you need. 2015PCB
1593-Wire Leading-Edge Dimmer You can't buy these easily either, so again the only option is to build it yourself.  This is also the first (and only) fully tested and working design you'll find anywhere. 2015
160LM386, LM380 & LM384 Power Amplifiers Sometimes, you only need a small amplifier to use as a signal tracer, or as a workshop monitor amp.  Nothing special, but useful. 2015
161High Impedance Input Stages This is a combination of a project and an article, so is listed as both.  When you need an input impedance of up to 1GΩ, this shows you how to get it. 2015
162Voltage Controlled Oscillator A voltage controlled oscillator (VCO) isn't something you need every day, and you may never have thought that you need one.  You may be right, but some things are just too interesting to ignore. 2015
163Preamp Input Switching Using Relays How to use relays for input switching, including several designs for logic controls to allow push-button input selection 2016
164Signal Tracer A version of this project was shown in the troubleshooting pages, but it's now a project in its own right.  The unit presented here is simple, cheap, and runs from a 9V battery. 2016
165Valve Tester for Service Techs If you service valve (tube) amplifiers, you need to be able to test output valves under the conditions used in the amp being fixed.  Note that it is not a 'general purpose tester. 2016
166Push-on, Push-off Mains Switch You can add a little 'bling' to your project with this simple design.  Instead of a toggle or rocker switch, you can use a momentary push-button to turn your gear on and off. 2016
167MOSFET Follower & Circuit Protection Many people like their valve (tube) preamps, but if connected to opamp circuits the voltage 'surge' at power-on may cause damage.  A MOSFET follower and a muting circuit are also provided.  2016
168Low Ohms Meter Most people don't need to be able to measure down to 10 milliohms or so, but sometimes there is a genuine need to do so.  This project shows how it's done. 2017
169Battery Powered Amplifier There seems to be some mystique about amplifiers that don't connect to the mains, and are therefore considered (at least by some) to be more 'pure'.  However, you don't need to shell out a fortune 2016
1706dB/ Octave Active Crossover Some people like the idea of 6dB crossover networks.  While first order networks provide little by way of driver isolation, there might be a few readers who'd like to experiment. 2016
171Infrasound Translator Infrasound (between 1Hz and 20Hz) is normally inaudible, but this project allows the sound to be heard using a voltage controlled oscillator to move the LF to the audible range   2017
172Wattmeter for AC Power Measurements For all service and development work, it's useful to know the current drawn by the system, and it's also now easy to measure the power consumed. 2017
173Constant Directivity Horn Equalisation Constant directivity (CD) horns are unique amongst high frequency reproducers.  They need a 6dB/ octave boost for high frequencies, as provided by this project. 2017
174Ultra-Low Distortion Oscillator Ultra-Low Distortion Sinewave Oscillator, a contributed project with both exceptionally low distortion and lightning-fast settling time2017
175BTL Amp DC Protection Single Supply BTL (bridge tied load) amplifier speaker protection circuit, used when P33 cannot be used due to the amp's DC Offset.2017
176Fully Differential Amplifier P87A & B have been around for years, but sometimes you need the best possible common mode rejection ratio (CMRR).  This circuit does just that.   2018
177Constant Current Transistor Tester Test transistors using constant collector (actually emitter) current.  Ideal for matching small signal and power transistors (Bipolar types only)   2018
178Low Voltage Power Amplifier Techniques you can use to build a low-power, low voltage power amplifier.  Ideally, it should have much better performance than the common LM386 and its ilk   2018
179Sinewave Oscillator A Filament Lamp Stabilised Wien Bridge Oscillator   2018
180Amplifier 'Power Meter' Add this meter to your power amplifier for some bling that (unlike most) is not simple 'eye candy', but actually shows how close you are to clipping   2018
181Accelerometer Audio Accelerometer For Speaker Box Testing (amongst other things)   2018
ABXABX Comparator Full ABX testing unit, without microcontrollers or a PC
XA-B Switch Box Can you really hear the difference between amps or cables?

Note Carefully

Although I am happy to provide assistance to prospective builders, I cannot (and will not) be drawn into prolonged e-mail exchanges if the project does not work as expected.  I can say with complete confidence that all projects presented will work if properly constructed according to the published design.  This is not to say that no help will be available - I will always help where I can.

It is inevitable that in some cases (due to component tolerances, for example), a project may require a different value resistor, capacitor (or whatever) to correct for an unexpected variation.  Since I cannot control or predict the quality of components sourced by readers, nor the standard of workmanship in assembly, it is not possible to allow for every contingency.

Please do not attempt the construction of any project which you do not fully understand, or if you do not feel completely confident that you can build the project without further assistance.  Do not expect me to be able to diagnose an obscure fault remotely, and especially if the project has been modified in any way whatsoever.

Under no circumstances should any reader construct any mains operated equipment unless absolutely sure of his/her abilities in this area.  The author takes no responsibility for any injury or death resulting from, whether directly or indirectly, the reader's inability to appreciate the hazards of household mains voltages or other voltages as may be present within the circuitry of a project.  Please read the disclaimer now if you have not done so already.  Also read the warning on the main projects page. This is important!

Not all projects that involve mains wiring are so marked, because some mains wiring is required in most projects that include a power supply.  Appropriate warnings are generally placed within the article itself, and it is the reader's responsibility to ensure that all regulations that apply where you live are adhered to.  I cannot provide detailed information for each country, so you need to know the regulations, colour codes and any other requirements as necessary to ensure safety.


Copyright Notice. All projects described herein, including but not limited to all text and diagrams, are the intellectual property of Rod Elliott unless otherwise stated, and are Copyright © 1999 - 2018 (see article for copyright details).  Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws.  The author / editor (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project.  Commercial use in whole or in part is prohibited without express written authorisation from Rod Elliott and the owner of the copyright in the case of submitted articles.