The Audio Pages

Numerical Index of Projects
Page Last Updated - August 2024

Main Index Projects Index Projects By Category Pricelist

Troubleshooting and Repair Guide - Part 1 ... introduction, general principles and power amps

Troubleshooting and Repair Guide - Part 2 ... preamps, crossovers and other opamp circuits

Projects ...

0-9   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   180-189   190-200

200-209  210-219  220-229  230-239  240-249  250-259

The Date column usually indicates the date of initial publication, and there have been updates to many of the projects that are not reflected by the date shown.  Where possible, the date of initial publication and the update are both shown.  Because there are so many projects, it's proved difficult to ensure that the last update is shown.  Many of the projects are 'timeless', and a very early publication date doesn't mean the project is obsolete.  Most are in chronological order, but there are a few exceptions.  Use 'CTRL-F' to search within the page.

There are currently two projects that will/ might get PCBs (shown as Pending).  PCBs for these won't be available if there's not enough interest.

No.	Project	Description	Date	PCB
00	Opamp Bypassing	How (and why) to apply bypass caps to audio circuits using opamps and/or discrete circuitry (A 'must read' article)	2023
01	A Better Volume Control	A volume control using a linear pot that is much better than most log pots	1999
02	Simple High Quality Hi-Fi Preamp	As it says - simple, high quality preamp.  Has all the facilities normally expected	1999
03	60W / 8 Ohm Power Amplifier	(Updated) My old faithful power amp design. - See Project 3A	1999	PCB
04	Dual Power Supply	A power supply that is adaptable for most power amps	1999
05	Preamplifier Power Supply	All the features of the original P05 boards plus an improved muting circuit.  (Revision D)	2007	PCB
05-Mini	Preamplifier 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.	2017	PCB
06	Phono (RIAA) Preamp	Very high quality moving magnet phono preamp - few circuits will better this one.	1999	PCB
07	Discrete Op-Amp	Class-A Output.  Intended as an experimental device - works extremely well	1999
08	2-Way Electronic Crossover	Conventional 3rd order electronic crossover	1999
09	24dB/Octave 2/3-Way Xover	Linkwitz-Riley alignment.  This is the optimum choice for any active system - Rev-B boards	1999	PCB
9a	A Reader's Xover	This is well worth reading if you can't decide on biamping	1999
10	20 Watt Class-A Power amp	True Class-A power amp for low power systems or tri-amping	1999
11	Pink Noise Generator	For loudspeaker and room acoustics testing	1999
12	Current Feedback Amp	An updated version of a very old 60W / 8 Ohm design	1999
12a	El-Cheapo	This is the real El-Cheapo, presented in more or less original form	1999
13	Low Noise Preamplifier	Simple unbalanced design, suitable for microphones - very low noise	1999
14	Power Amp Bridge Adapter	A conventional adapter for bridging power amplifiers	1999	PCB
15	Capacitance Multiplier Supply	Extremely low ripple supply with less power dissipation than a regulator	1999
16	Audio Millivoltmeter	For Testing Amplifiers (etc) - An analogue design, 3mV to 30V with dB scale (superseded by Project 236)	1999
17	A-Weighting filter	For noise measurements.  Ideal for use with the AC Millivoltmeter above	1999
18	Simple Surround Sound Decoder	A line-level version of the "Hafler matrix" decoder	1999
19	Single Chip 50W Power Amplifier	Using the National Semiconductor LM3876/ LM3886 Power IC.	1999	PCB
20	Simplest Ever Bridge Adapter	Use this simple method, and avoid external circuits	1999
21	Stereo Width Controllers	Two to choose from.  Expand or contract the stereo sound stage	1999
22	Simple Audio Oscillator	For use with the millivoltmeter, for testing amps and speakers	1999
23	Power Amp Clipping Indicator	A fast and accurate indicator to show an amp is clipping	1999
24	Hi-Fi Headphone Amplifier	Contributed by a reader, this is a very nice circuit	1999
25	Phono Preamps For All	Various circuits for moving coil and moving magnet pickups	1999
26	Digital Delay	Digital delay  Retained for information only - The Mitsubishi delay IC is no longer available	1999
26A	Digital Delay Unit	Digital delay using PT2399 - Note: PCB (with IC) available now	1999	PCB
27	100 Watt Guitar Amp (Mk II)	The new and improved version of the original Project 27 guitar amp (now 27b).	1999	PCB
27b	100 Watt Guitar Amplifier	The original of the unit above.  Retained for posterity, and has speaker box details	1999
28	Parametric / Sub-Woofer Equaliser	A simplified version, which performs surprisingly well	1999
29	Tremolo Unit	A versatile guitar effect.  This is a simple circuit that gives very good results	1999
30	Stage and Recording Mixer	Able to be built in modular form, allowing as many (or few)  channels as desired.	1999
31	Full Featured Transistor Tester	Just the thing to check the transistors for any project	1999
32	Car Audio Preamp + Artificial Earth	Especially for car audio installations, can be used for any single supply system	1999
33	Loudspeaker Protection and Muting	Protect speakers from turn-on and turn-off transients and amplifier faults	1999	PCB
34	Guitar Spring Reverb Unit	A high performance spring reverb unit for guitar amps	1999
35	Direct Injection (DI) Box	An essential companion for the mixer for stage or recording work	1999
36	Death Of Zen (DoZ)	An ultra simple, high performance Class-A power amp.  Revision-A boards now shipping	1999	PCB
37	Death of Zen Preamp	"Minimalist" preamp, designed to suit the DoZ (or any other) power amplifier	1999	PCB
37A	Death of Zen Preamp	Updated "minimalist" preamp, designed to suit the DoZ (or any other) power amplifier	1999	PCB
38	Signal Detecting Auto Power-On Unit	For equipment to be turned on with the main amp.  Detects signal and applies power.	1999
39	Soft Start Circuit	Designed for (large) toroidal transformers, limit the inrush current to a sensible value.	1999	PCB
3A	60-100W Hi-Fi Power Amp	100W into 4 or 8 ohms.  Has excellent performance, and is easy to build.	1999	PCB
3B	25W 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.	1999	PCB
40	Load Sensing Auto Switch	Apply power to the system by turning on one item.  Superseded by Project 79.	1999
41	Opamp Design + Test Board	Quickly assemble an opamp circuit for testing.  Very easy and intuitive to use	1999
42	Thermo-Fan For Amp Cooling	Use a 12V computer fan to cool your amp.  Uses diode temperature sensor	1999
43	Ultra Simple Split DC Supply	When you need a +/- supply, and only have a DC adapter	1999
44	Dual +/-25V Lab Supply	Up to 800mA (typical) output current at +/-25V.	2000
45	Simple Bass Guitar Compressor	An ultra simple compressor, ideally suited for bass guitar.	2000
46	Thermal Shutdown/ Protection	Add this circuit to either turn the amp off or activate a cooling fan if it overheats	2000
47	Withdrawn	Details no longer available
48	Sub Woofer Processor	Designed to operate a sub-woofer driver below its resonant frequency.	2000
48A	Sub Woofer Processor (Rev-A)	New version of the sub-woofer driver has far greater flexibility.	2000	PCB
49	Guitar Vibrato Unit	A reasonably simple circuit, with results similar to the Vox AC30 guitar amp	2000
50	Mic Circuit Tester	Small oscillator to check mic & line circuits during sound setup	2000
51	Balanced Line Drivers	Use these to eliminate hum for long signal leads	2000
52	Distortion Analyser	A simple distortion meter you can use with an oscilloscope or millivoltmeter	2000
53	Output Power Limiter	Just the thing if you want to limit amplifier power to something 'sensible'	2000
54	Low Power FM Transmitter	Useful to retransmit the hi-fi to another FM receiver nearby, or as a wireless mic	2000
55	PPM and VU Meter	A versatile and useful VU meter circuit, can also operate as a Peak Program Meter	2000
56	Variable Impedance	Modify the output impedance of an amplifier with these simple ideas (Updated)	2012
57	SIM - The Simple Version	The smallest variation in amplifier performance is shown with this simple circuit	2000
58	Tone Burst Speaker Measurement Set	Based on work done by Siegfried Linkwitz (reproduced with his permission).  See Project 143 for a conventional Tone Burst generator	2000
59	Self Oscillating Amplifier	Allows you to make a power amp oscillate at a defined frequency for testing	2000
60	LED VU Meter	Standard LED VU meter, includes simple rectifier circuit for full wave detection	2000	PCB
61	Withdrawn	Details no longer available
62	LX-800 Lighting Controller	Light is always needed for theatre and live music, and this is just the ticket	2000
63	Multiple Feedback Bandpass Filter	This is the basis of an expandable equaliser and analyser or vocoder	2000
64	Instrument Graphic Equaliser	Designed especially as a guitar / bass equaliser, this unit is expandable	2000
65	Strobe Light	Designed as a companion to the LX800, can also be used by itself	2000
66	Low Noise Balanced Mic Preamp	A discrete front end makes this balanced microphone preamp very quiet	2000	PCB
67	Fast Audio Peak Limiter	This peak limiter is simple and very effective.  Uses discrete FET gain control	2000
68	300W Subwoofer Amplifier	This amp is designed especially for subwoofers	2000	PCB
69	Switchmode Car Power Supply	Low power +/-12V supply for car preamps and equalisers (etc.)	2000
70	DoZ Headphone Amplifier	The DoZ is ideally suited to headphone use.  Revision-A boards now shipping	2000	PCB
71	Linkwitz Transform Circuit	The Linkwitz Transform circuit - an equaliser to provide extended bass response	2000	PCB
72	20W/Ch  Stereo IC amplifier	Based on the versatile LM1875 from National Semiconductor	2001	PCB
73	Hi-Fi PC Sound System	You have never heard your MP3 collection, CD or games sound so good	2001	PCB
74	Simple RF Probe	Indispensable for any RF work.  Uses just 4 passive components	2001
75	Constant Q Graphic Equaliser	Constant Q design.  Can be made using as many or as few sections as needed.	2001
76	Opamp Based Power Amplifier	This is a contributed project, and has some interest as a learning exercise	2001
77	13.8V Power Supply / Charger	A supply for testing and working on car amplifiers - up to about 500 Amps!	2001
78	3-Way 12dB/ Octave Crossover	Contributed project, simple 12dB/ Octave crossover network.	2001
79	Current Sense Auto Power Switch	A current sensing switch turns on multiple devices from one master	2001
80	Reverse RIAA Equaliser	Test phono preamps for correct equalisation, or convert unused phono inputs.	2001
81	12dB/Octave 2-Way Xover	Linkwitz-Riley alignment and phase coherent (uses the P09 Rev-B PCB)	2001	PCB
82	Loudspeaker Test Box	Quickly and accurately determine the optimal impedance correction network	2001
83	MOSFET Follower Power Amplifier	Another contributed project that may be of great interest	2001
84	Subwoofer Graphic Equaliser	This is a constant Q design, eight 1/3 octave bands covering 20Hz to 100Hz.	2001	PCB
85	Simple S/PDIF DAC	Quite possibly the simplest possible S/PDIF receiver and DAC you will ever find	2001
86	Miniature Audio Oscillator	The Miniosc - low distortion battery powered audio oscillator	2002	PCB
87	Balanced Transmitter / Receiver	Updated and upmarket versions of the Project 51 balanced circuits	2002	PCB
88	High Quality Audio Preamp	An audiophile quality preamp, with full switching and programmable gain	2002	PCB
89	Switchmode Power Amp Supply	A high power (350W typical) car power amplifier supply	2002
90	Dimmer Control Polarity changer	Convert old dimmer units using a 0 to -10V control signal	2002
91	78 RPM and RIAA Phono Equaliser	Multi Standard 78 RPM and RIAA Phono Equaliser handles all 'standards'	2002	PCB
92	Guitar and Bass Sustain	A simple, low distortion limiter for musical instruments	2002
93	Recording & Measurement Microphone	A selection of projects for making your own mics for recording and/ or measurement (includes PCB Version of P93)	2002	PCB
94	Universal Preamp/ Mixer	A small and versatile mixer and preamp, suitable for many uses.	2002	PCB
94A	Universal Preamp/ Mixer	Alternative wiring for the versatile mixer/ preamp, suitable for many uses.	2002	PCB
95	Low Power -Ve Supply	Low power -ve supply for car audio preamps and crossovers	2002
96	48V Phantom Supply	Low noise 48V regulator and phantom mic supply circuits.	2002	PCB
97	Hi-Fi Preamp	Another preamp, but this one has tone controls, and all pots are PCB mounted	2002	PCB
98	Battery Charger	Automatic Charger for Battery Operated Hi-Fi Preamps	2003
99	Infrasonic/ Rumble Filter	A 36dB/octave 17 Hz high pass filter to remove ultra low frequencies (Published 2003)  Rev-B PCB	2009	PCB
100	Headphone Adaptor	A simple adaptor to provide a headphone output to an existing power amplifier	2003
101	MOSFET Power Amp	200W MOSFET Hi-Fi Power amplifier (Published 2003)	2012	PCB
102	Simple Pre-Regulator	Use this to adapt P05 to a power amp supply for your preamp   (Published 2003) - added 'enhanced' version	2016
103	Subwoofer Phase Control	A standard phase control for use with subwoofers.	2004
104	Preamp/ Crossover Muting Circuit	Muting circuit suitable for all line-level applications.	2004
105	Withdrawn	Details no longer available
106	hFE Tester	An hFE tester with switched collector currents for the Device Under Test	2004
107	Polarity Reversal Switch	Switch the absolute polarity (180° phase shift) of signals with this simple circuit	2004
108	Switchmode PSU Protection	This contributed circuit can help prevent failure of your switching power supply.	2004
109	Portable Headphone Amp	Another contributed circuit, this is a complete portable headphone amp that features crossfeed.	2004
110	IR Remote Control	Volume and mute are provided by this simple but functional circuit.  Suitable for all preamps.	2004	PCB
111	PIC Speaker Protection	This doesn't replace P33, but it is possibly the ultimate speaker protection.	2005
112	Dummy Head Recording Microphone	Surprise yourself with one of the most realistic recording processes known.	2005	PCB
113	Hi-Fi Headphone Amp	Yet another headphone amplifier, but this one has PCBs available	2005	PCB
114	PWM Power Amplifier	Based on the ColdAmp BP4078 400W module, Class-D is here.  Definitely worth looking at!	2005	PCB
115	GainClone Amplifier	Complete GainClone amplifier, including full chassis details (2 part article)	2006	PCB
116	PWM Subwoofer Amplifier	Using a ColdAmp BP4078 module, this article has complete construction details	2006	PCB
117	1.5kW Power Amplifier	Just the thing for those for whom no amount of power is enough	2006
118	PC Power Switch	Simplest ever PC switching unit - turn on peripherals with the PC	2006
119	Signature Analyser	A component signature analyser that allows in-circuit parts tests	2006
120	Crowbar Speaker Protection	The final answer to speaker protection.  Comes with many cautions and warnings!	2007
121	Reading Inductance	Use your DC voltmeter or frequency counter to measure crossover coils.	2008
122	Ultra-Simple Mic Preamp	A single low cost IC and tiny PCB make a great utility microphone preamp.	2008	PCB
123	18dB/Octave Crossover	A selection of 18dB/octave (3rd order) crossover filters.	2009
124	Dummy Load	High power dummy load for amplifier testing.	2009
125	4W-LRX	4-Way Linkwitz Riley Crossover	2009	PCB
126	PWM Dimmer	PWM LED Dimmer/ Motor Speed Controller.	2009	PCB
127	TDA7293 Amplifier	Dual power amplifier, using the TDA7293 power opamp IC.	2009	PCB
128	VU Meter Bridge	Build a pair of VU meters to monitor your recording or PA level.	2010	PCB
129	Matrix Mixer	Now you can build a matrix mixer to suit your exact requirements.	2010	PCB
130	Inverse A-Weighting	Undo the rolloff caused by the A-Weighting filter	2010
131	Light 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
132	Air Bearing Tonearm	More of a proliferation of ideas and techniques than a "real" project, the vinyl lovers should look at this	2010
133	PA-PC Audio Interfaces	Interfaces from the headphone output of a PC to PA mic input, and PA speaker line to PC recorder	2011
134	4mA Current Loop Microphone	Common for professional measurement microphones, but not well known outside the noise measurement field.	2011
135	Phase Correlation Meter	More of an experimental circuit than anything else, it should help anyone trying to build a phase meter. (Published 2011)	2020
136	Hardware Based RTA	A real-time audio analyser - contributed project, based on the multiple feedback bandpass filter described in Project 63.	2011
137	Powered Box Amplifier	Complete Preamp, crossover & power amps, designed for powered PA speakers, Leslie cabinets, 'party' systems, etc.  Set of 3 PCBs.	2019	PCB
138	Mains 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
139	Mains Current Monitor	A versatile, safe & accurate way to measure (and view with a 'scope) the current drawn by mains powered equipment	2012
139a	Simple Current Monitor	Much like P139 (versatile & safe), but a much simpler way to measure the current drawn by mains equipment	2012
140	True 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
141	VCA 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.	2013	PCB
142	Simple 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
143	Tone 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
144	Mains 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
145	Silent 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
146	Overload/ Clipping Indicator	Overload indicator for mixers, preamps or power amps.  Simple opamp comparator circuit gives fast response.	2013
147	BJT Muting Switch	A little known technique that doesn't look like it could ever work - using bipolar transistors.	2013
148	State Variable Crossover	Perfect for loudspeaker system development, or can be used as part of a biamped or triamped system.  12dB/octave continuously variable filters.	2014	PCB
149	Musical Instrument Graphic EQ	Guitar, Bass or Keyboard Equaliser.  A greatly improved version of Project 64.	2014
150	Wien Bridge based equaliser	A functional building block that can be used in mixers, preamps, guitar and bass amps, etc.	2014
151	High 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
152-1	Bass 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
152-2	Bass Guitar Amplifier (Part 2)	Part Two of the bass amp project describes the compressor, soft-clip and crossovers, and discusses power amp needs.	2015
153	Frequency '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
154	PC 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
155	Variable 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
156	12V 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
157	3-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
158	Low 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.	2015	PCB
159	3-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
160	LM386, 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
161	High 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
162	Voltage Controlled Oscillator	A voltage controlled oscillator (VCO) isn't something you need every day, and you may not have thought you'd need one.  You may be right, but some things are just too interesting to ignore.	2015
163	Preamp Input Switching Using Relays	How to use relays for input switching, including several designs for logic controls to allow push-button input selection	2016
164	Signal 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
165	Valve 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
166	Push-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
167	MOSFET 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
168	Low 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
169	Battery Powered Amplifier	There seems to be some mystique about amplifiers that don't connect to the mains, and may therefore considered to be more 'pure'.  However, you don't need to shell out a fortune	2016
170	6dB/ 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
171	Infrasound 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
172	Wattmeter 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.	2019
173	Constant 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
174	Ultra-Low Distortion Oscillator	Ultra-Low Distortion Sinewave Oscillator, a contributed project with both exceptionally low distortion and lightning-fast settling time	2017
175	BTL 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
176	Fully 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
177	Constant Current Transistor Tester	Test transistors using constant collector (actually emitter) current.  Ideal for matching small signal and power transistors (Bipolar types only)	2018
178	Low 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
179	Sinewave Oscillator	A Filament Lamp Stabilised Wien Bridge Oscillator	2018
180	Amplifier '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
181	Accelerometer	Audio Accelerometer For Speaker Box Testing (amongst other things)	2018
182	Pseudo-Random Noise Generator	A maximum length sequence (MLS) noise generator with much better noise than a reverse-biased transistor junction (includes pink noise filter)	2019
183	Signal Detecting Audio Ducking Unit	Ducking is a common application for PA systems, video production or anywhere you need to reduce the level of a background signal in the presence of speech	2019
184	Li-Ion Battery Cutoff	Li-Ion batteries are ideal for many projects (especially test equipment), and this under voltage cutoff lets you ensure that the battery is not over-discharged if/ when you forget to turn it off	2019
185	Polarity Tester	Speaker, Microphone & Circuit Polarity Tester. Ideal for checking that everything in a system is properly phased to prevent sound anomalies.	2019
186	Workbench Amplifier	Single Chip 25 Watt/ 8 Ohm Workbench Power Amplifier.  Ideal for testing speakers, signal tracing, testing preamplifiers and a host of other uses.	2019	PCB
187	Moving Coil Head Amp	Finally , a pair of moving coil head amplifier designs for MC phono cartridges.  Includes a discussion of noise and low noise circuitry.	2019
188	Surround Sound Decoder (Mk. II)	While Project 18 shows a surround sound decoder, this one is far more complete, and uses readily available PCBs supplied by ESP.  It's in operation, and works very well indeed.	2019	PCB
189	Audio Wattmeter	Measure true power from an amplifier into a dummy load, or from an amplifier into a speaker.  Handles reactive loudspeaker loads and shows the actual power delivered.	2019
190	Microphone Muting Circuit	This simple project can be used to mute any microphone by the performer, including mics that are phantom powered.	2019
191	Peak Voltage & Current Detector	If you are unsure if your amplifier is way under or over-powered for your loudspeakers, this simple project can be used to monitor the peak voltage and current demanded while listening.	2019
192	12V to ±12V Switchmode Supply	If you use a single 12V DC wall supply, that's often not enough to run many projects.  This supply gives you ±12V from the single supply from the 'wall wart'	2019
193	12V to P48 Phantom Power Supply	Getting a suitable transformer for a P48 phantom supply isn't always easy, but this switchmode boost regulator can provide +48V from a single DC supply of 12-36V	2019
194	Withdrawn	N/A
195	Guitar 'Talk Box'	The guitar 'talk box' has been around for a long time, and it was made famous by many musicians in the 1970s.  It's still popular, and you can build your own.	2019
196	12V Float-Charge Battery Supply	This project is basically a 12V version of Project 98, and a 12V battery backup system is useful for electronic clock drives, or surveillance equipment.	2019
197	Low Frequency Boost & High Pass Filter	If you need to equalise a vented speaker enclosure, this low frequency boost and high pass filter circuit may be just what you need.	2019
198	MOSFET Relay	MOSFET relay using Si8751/2 MOSFET driver ICs.  Suitable for mains switching (with caveats) or for high voltage speaker protection where a relay will arc.	2019	PCB
199	ABC NYE EQ	ABC New Years Eve Concert Equaliser (Specific To Australia, but ...)  End the muffled sound broadcast by the ABC!	2020
200	DIY LDR Optocoupler	Build your own 'Vactrol™' using a LED and LDR (light dependent resistor).  This has been 'transplanted' from an article where it was shown as part of construction project.	2020
201	Multi-Channel Trailing-Edge Dimmer	This project came about from the MOSFET relay (P198) and is suitable for use in the Project 62 'LX-800' dimmer, or used as a stand-alone system.	2020
202	Piezo Preamplifiers	Piezo guitar/ violin/ double bass etc. pickups are common, and here are a few options.  Includes a charge amplifier (also ceramic phono cartridges).	2020
203	Guitar/ Studio Spring Reverb Unit	A complete spring reverb sub-system for guitar, keyboards or studio use.  Possibly the most complete reverb system currently available.	2020	Pending
204	Frequency Shifter	Used for acoustic feedback reduction.  There's a choice of two circuits, one being an updated version of the first published frequency shifter, plus a high-performance version.	2020	Pending
205	4-Channel Mixer	For Microphones Or Instruments.  It's built using existing ESP boards (other than the clipping indicator which will be available at a later date).	2020	PCB
206	Guitar Vibrato Unit	An update on the original Project 49 unit, but using LED/ LDR optocouplers to allow high level audio without distortion.	2020
207	High-Current AC Source	If you need to run tests on very low resistances, this is ideal.  With up to 100A output current (intermittent), you can test things that are otherwise impossible.	2020
208	Speaker box DC Protection	Stand-alone DC protection circuit for speaker enclosures.  Prevents random amplifier failure from killing your expensive speakers.	2020
209	Resistor/ Capacitor Decade Boxes	Resistance/ capacitance decade (or substitution) boxes can be handy.  There are three different circuits, so choose those you need.	2020
210	AC and DC Electronic Fuses	Electronic fuses for AC or DC, with latching when a fault is detected.  Very fast acting, but can be slowed down if necessary.  Reliable protection for delicate electronics.	2020
211	Guitar Spring Reverb Unit	Using the P113 headphone amp PCB, this spring reverb unit is for guitar amps or studio use.  Very high performance, and the PCB is available now.	2020	PCB
212	High Impedance DC Voltmeter	With an input resistance of 50MΩ or even 500MΩ you can measure voltages in very high impedance circuits.	2021
213	DIY voltage controlled amplifier (VCA)	It's not hi-fi, but it is fun to build and play around with.  Using only common parts, it's ideal for 'utilitarian' applications, guitar tremolo, etc.	2021
214	'Zero Capacitance' Guitar Lead	If lose 'tone' when the volume control on your guitar is turned down, this project will maintain full frequency response with almost any source impedance.	2021
215	P215-P27 Revisited Guitar Amp	The Project 27 guitar amp has been around since 1999, and this is a low-power version, more suited to most players today.  Nominal power is 40W, using P27 boards.	2021	PCB
216	Speaker Emulation Load	A Reactive Dummy Load For Testing Amplifiers.  Verify that your amplifier(s) don't have protection circuit 'artifacts' when subjected to a reactive load	2021
217	Low Power Amplifier	This is classified as a 'practice' amplifier, as it allows the reader to practice construction of an amplifier, and learn how amps work.  It uses low cost parts throughout.	2021
218	High Q Gyrator Filter	A number of ESP projects have used gyrators, but the one described here is different.  It can be made to have a very high Q, providing a very sharp filter response.	2021
219	Valve Amp Speaker Switch	Valve amps may be damaged unless the switching uses make-before-break circuitry.  That's what's provided here (and there's one for transistors amps as well).	2021
220	Switchmode Buck Converter	A tiny switchmode buck converter, to reduce up to +40V DC to the needed voltage.  It's only around 100mA, but will be more than enough for many simple projects.	2021
221	Tweeter Amp Regulator	If you really don't want your tweeter amp running from ±35V or more (~70W into 8Ω), use this regulator to reduce the voltage to something 'sensible', such as ±22V.	2021
222	Mains Soft-Start	Designed for (large) toroidal transformers, limit the inrush current to a sensible value.  Unlike P39, this version doesn't use a transformer.	2021
223	Bench Power Supply	A full-featured dual bench power supply.  Unlike P44, this unit uses dedicated circuitry.  It has many options, such as series/ parallel operation, your choice of meters, etc.	2022
224	External In-Line Soft Start	This is (as near as I can tell) a unique project.  It's permanently connected to the mains, and it operates when you turn on your amplifier (or other load).	2022
225	Inrush Current Tester	Testing for worst-case inrush current isn't easy, but this mains switching unit allows repeatable results for transformer and electronic loads.	2022
226	Versatile Tone Controls	A variable tone-control circuit that offers greater flexibility than most.  Includes options to use it for musical instruments (especially guitar/ bass)	2022
227	Hybrid Relay	This was developed because the IC for the P198 MOSFET relay is not available.  It uses a standard relay and a MOSFET relay, and is designed for loudspeaker protection.	2022
228	Negative Impedance Test Amp	Driving transformers with negative impedance can increase performance dramatically.  This test amp lets you experiment with negative impedance to get the best result.	2022
229	Enhanced Reverb Mute	Most reverb systems mute the output, which stops reverb noise when it's off, but it also kills the 'tail' as the reverb decays.  By using an instant input mute and a delayed output mute, you can retain the reverb's natural decay, but prevent noise after the delay times out.  Uses an LED/LDR optocoupler for a smooth decay.	2022
230	Signal Routing Panel	Your workbench functionality can be greatly improved with this simple project.  It has inputs for a tuner and CD player, and you can switch between direct and via your latest project circuit.  It includes balanced inputs and outputs if you need them.	2022
231	Fast Discrete Opamp	Using 6 transistors, this opamp can provide 40dB of gain at up to 1MHz.  There are several options, and it's ideal for experimenters.  Very low distortion too.	2022
232	Distortion Measurement System	This system lets you measure distortion, noise and other system anomalies.  The full version described is 'over the top', but you can include the bits that you need.	Dec22
233	Isolated Power DC-DC Supplies	A collection of ideas to provide a galvanically isolated power supply.  All low-power, but provide enough current for a couple of opamps or to switch the gates of MOSFETs.	Dec22
234	Resistor Substitution Box	Probably the simplest project I've published, this box uses a 10k, 10-turn pot and switched resistors to cover the range from ~100Ω to 100k (or 80k).	Dec22
235	Current Feedback Opamp	Current feedback (CFB) opamps are much faster than anything you're used to.  This is a DIY (fully discrete) circuit you can use to look at the capabilities of these devices.	Jan23
236	AC Millivoltmeter	A new design for an AC millivoltmeter, covering the range from 300µV to 30V in 10dB steps.  This is a new design that supersedes P16 (that design is 24 yeas old now).	Feb23
237	JFET Test System	JFET circuits are always a bit tricky to design.  This tester automatically sets the bias voltage to get the same drain current for each JFET, making tests much easier.	Mar23
238	Low Current HV DC	It's not every day that you need a high-voltage, low current DC source, but when you need one it's easy to build.  Several options are available.	May23
239	Signal Detecting Power-On Unit Mk II	For equipment to be turned on with the main amp.  Detects signal and applies power.  Includes circuitry to prevent some amps from re-triggering.	May23
240	10 Watt Audio Amp/ DC Supply	A single-supply bench amplifier that can be used as a source/ sink power supply at up to 500mA.  Ideal as a basic amplifier or battery charge/ discharge unit.	Jul23
241	Z-Weighting Filter	Z-Weighting is 'flat' response, covering the audio band.  All superfluous frequencies are removed, with a 12dB/ Octave filter for low frequencies, and 18dB/ Octave for HF.	Aug23
242	Cosine Burst Generator	A simplified version of the Project 58 cosine generator, designed by Siegfried Linkwitz and updated by Ray Hernan.  This uses a 10-cycle burst, which makes it easier to build.	Aug23
243	'Retro' Hi-Fi System	There are some people who like the idea of a retro hi-fi, but can't find (or afford) the genuine article.  This project is based on the Sansui AU-555A integrated amplifier.	Sep23
244	LED Level Indicator	A 3-LED level indicator for hi-fi, mixers, digitisers, etc.  Superseding the Project 60 LED meter, as the ICs are now obsolete.	Oct23
245	MOSFET Relay	A new MOSFET relay, using the TI TPSI3052-Q1 IC.  This one can be used for high-speed switching, as well as a second-source for P198.  It's a very impressive IC!	Nov23
246	Clipping Indicator	A clipping indicator that can be set for preamps or power amps.  It will also work with single supply BTL amps with one external capacitor.  PCB coming soon.	Dec23	PCB
247	Tape Head Preamp	Best described as an experimental circuit, since I don't have a tape deck and can't test it.  Uses the P06 Phono preamp board and provides NAB and IEC EQ for 7½ & 15 ips.	Dec23	PCB
248	Low Voltage Charge-Pumps	Many times you'll need a low current voltage booster or inverter (to get a -Ve supply from a single +Ve supply).  The details here should help.	Jan24
249	Guitar Booster Circuits	A collection of circuits that can be mixed and matched for guitar/ bass boosters with or without additional equalisation.  Includes JFET, transistor and opamp designs.	Feb24	NEW
250	Inductor Saturation Tester	Test the maximum current through any inductor, based on core saturation.  Use a digital scope with single sweep mode.  An analogue scope can also be used.	Mar24	NEW
251	Protected DC Load	A DC load that includes protection against the preset power level from being exceeded at any voltage or current.  Uses an analogue multiplier to derive power.	Apr24	NEW
252	6-Band Guitar EQ	A simple guitar equaliser circuit using simplified multiple feedback (MFB) bandpass filters.  There are only a few different component values, simplifying construction.	May24	NEW
253	18dB/ Octave State-Variable Xover	18dB/ Octave state-variable active crossovers are uncommon.  This one is more unusual than most.  The performance is very good, and it has some surprising benefits.	Aug24	NEW
254	24/ 18dB/ Octave Asymmetrical Xover	An asymmetrical crossover can provide a useful difference in group delay, allowing you to time align drivers without having to use a phase-shift network or stepped baffle	Sep24	PCB
ABX	ABX Comparator	Full ABX testing unit, without microcontrollers or a PC
X	A-B Switch Box	Can you really hear the difference between amps or cables?

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.

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