DIY Projects

Archives last updated: 03/17/2018

Sections:

  • Headphone Amplifiers – Discrete Solid State
  • Headphone Amplifiers – Vacuum Tubes
  • Headphone Amplifiers – Hybrids (Solid State/Vacuum Tube)
  • Headphone Amplifiers – Opamp Solid State
  • Headphone Systems
  • Musicians and Audio Professionals
  • Signal Processors
  • DIY Tutorials and Showcase

Headphone Amplifiers – Discrete Solid-State

Title: Class A MOSFET Headphone Driver
Author: Greg J. Szekeres
You can’t get much simpler than this classic class-A MOSFET driver. It’s a single-ended design that is ideal for the electronics beginner, as well as for the audiophile who wants class-A MOSFET sound without the fuss. Build it in an evening and enjoy!

Title: A Current-Domain Electrostatic Amplifier for Stax Omega II Headphones
Author: Kevin Gilmore
Designed and built for the author’s Stax Omega II headphones, this amplifier can be used with other Stax models. It is fully DC-coupled, uses inexpensive semiconductors and operates primarily in the current domain for best speed and extended frequency response.

Title: A JFET-MOSFET Headphone Driver
Author: Richard Murdey
This amplifier is based on the MOSFET headphone driver by Greg Szekeres. It has a JFET impedance buffer front-end and a choke-input power supply. The sound characteristic of the amplifier can be tuned by adjusting the low pass filter formed with the input capacitance of the MOSFET.

Title: A Pure Class A Dynamic Headphone Amplifier
Author: Kevin Gilmore
A high-current output amplifier for low-impedance dynamic headphones such as the Grados. The amplifier features an new servo biasing circuit which operates entirely outside the audio feedback loop. The author includes a balanced-bridge version of the amp for headphones that are wired in dual-mono as well as instructions for converting a Grado SR80 to dual-mono operation. The bridged amp will output 4 times the power of the standard amp.

Title: The Kumisa III Headphone Amplifier 
Author: Benny Jørgensen
A discrete all-bipolar, zero feedback class A amplifier with a new opamp-based servo. The servo operates outside the audio feedback loop by controlling a pair of current mirrors at the input stage.

Title: The Zen Headphone Amplifier
Author: Marcello Pellerano
A single-ended, class A MOSFET headphone amplifier with an active current source that results in increased efficiency (50%) and lower distortion. The active current source is based on a patented design featured in the Aleph power amplifiers from Pass Laboratories.

Title: A 50mW Class-A Headphone Amplifier
Author: Rohit Balkishan
The gain of this voltage-feedback design can be set by changing the values of two resistors. The authors presents versions optimized for low and high impedance headphones as well as one that performs well with a wide range of loads.

Headphone Amplifiers – Vacuum Tubes

Title: Output Transformerless Tube Headphone Amplifier
Author: Kurt Strain
This OTL mu-follower design uses paralleled tubes (2 per channel) to minimize noise. It also makes an excellent preamplifier.

Title: Brute Force In A Line Stage (100% Tube!)
Author: Eric Barbour
An OTL mu follower design. “One, often forgotten, purpose of the preamp is to drive headphones. I listen with a pair of AKG K240s, which is a relatively easy load to drive. This all-tube line stage and driver can power them, without resorting to output transformers.”

Title: All-Triode Direct-Drive Tube Amps for Electrostatic and Electret Headphones 
Author: Kevin Gilmore
Two electrostatic headphone driver designs: the second is DC-coupled and uses high voltage Apex op-amps as DC regulators. They will drive Stax and Sennheiser electrostatic phones, and the author includes directions for adapting the designs to other models. These are “the best electrostatic headphone amplifiers currently available, anywhere in the world.” Will also drive electret headphones. Recommended only for intermediate-advanced builders familiar with high-voltage safety!

Title: Designing an OpAmp Headphone Amplifier (Solid State and Tubes)
Author: HeadWize
Design principles and circuits for building opamp-based headphone amplifiers. This article reviews selection criteria, feedback configurations, capacitor coupling, RF shielding, output stage topology, short circuit protection, equalization, acoustic simulation, headphone distribution amplifiers and power supply options. Also discusses design with tube amp-blocks (AC feedback amplifiers and tube and tube-MOSFET hybrid opamps) as well techniques for interfacing tube amp-blocks with solid state output stages.

Title: Top-Level OTL Tube Headphone Amplifier
Author: Andrea Ciuffoli
This OTL amp features a single-ended class A input stage and a paralleled-tube cathode follower output stage with 33 ohms output impedance. It can drive any headphone from 300 to 600 ohms. Now includes two “high-end” versions based on the 6SN7 and the 3A5 direct-heated triode.

Title: No-Compromise OTL Tube Headphone Amplifier
Author: Andrea Ciuffoli
The author achieves his quest for the ultimate headphone amplifier that offers the best possible sound without any compromises. Based on the same circuit topology as his “Top-Level” amplifiers, this version uses a Raytheon 5842 at the input and a new power supply with tube rectifiers. The article includes photographs of the completed project.

Title: A SRPP-Input Tube Amplifier For Headphones And Loudspeakers
Author: Tony Frazer
This transformer-coupled design uses a series regulated push-pull input stage and a parallel-triode cathode bias output stage. The audio output transformer is hand-wound from a kit. The amp will drive both headphones and speakers.

Title: A Zero-Feedback SRPP-Input Headphone Amplifier
Author: Simon Busbridge
Features a series regulated push-pull input stage and transformer-coupled output stage with an output impedance of about 2.5 ohms. It has zero global feedback for open, natural sound without any harshness.

Title: A Single-Ended OTL Amplifier for Dynamic Headphones
Author: Aren van Waarde
A simple OTL design that uses just two tubes (dual triodes) for stereo. It has a common cathode input stage direct-coupled to a cathode follower output stage.

Title: Hi-End Transformer-Coupled Headphone Amplifier With Digital Input
Author: Andrea Ciuffoli
A 2-stage transformer-coupled tube headphone amplifier with a DAC front-end. The amplifier can drive headphones of 300 – 600 ohms impedance. The DAC is from a Crystal Semiconductor CS4390 evaluation board and has been modified to use a transformer-based output stage instead of the on-board active filter.

Title: The SESS Tube Headphone Amplifier 
Author: Andrea Ciuffoli
The author describes the construction of a very simple single-ended, single-stage, single-tube (SESS) tube headphone amplifier with a reference power supply that uses a tube rectifier in a hybrid Graetz bridge configuration.

Title: The Morgan Jones Mini Tube Headphone Amplifier
Author: Chu Moy
This reverse-engineered version of the famous EarMax headphone amplifier has just 3 tubes, uses no global feedback, and is powered from an 19VAC wall-wart. Includes two additional versions of this amplifier optimized for driving low impedance headphones and simulation files for use with OrCAD Lite software.

Title: The 6N1P OTL Headphone Amplifier
Author: Bruce Bender
This version of the optimized Morgan Jones headphone amplifier uses 6N1P dual triode tubes and a standard power supply. The author provides extensive construction details. Includes simulation files for use with OrCAD Lite software.

Title: A Tube Headphone Amplifier/Preamp with Relay-Based Input and Power Switching
Author: Helmut Ahammer
A tube headphone amplifier inspired by the “Top-Level” and “No-Compromise” amps of Andrea Ciuffoli, except that the output stage uses a triode-connected pentode. Because this amp also serves as a preamp, the author has put in a sophisticated relay-based switching system (both input source selection and power supply). The power supplies also feature slow-on and stepped delayed turn-on. The power-on LED doubles as a warm-up status indicator.

Title: A Fender-Tone Tube Headphone AmplifierA Fender-Tone Tube Headphone Amplifier
Author: Alex Cavalli
This tube amplifier with sonic controls has a paralleled White cathode follower output stage for driving low impedance headphones. The sonic controls consist of a Fender tone stack (bass, mid and treble), and ambience and blend controls. Includes simulation files for use with OrCAD Lite software.

Title: A DC-Coupled Tube Amplifier With Futterman Output Stage for Dynamic Headphones
Author: Rudy van Stratum
The Futterman output stage in this tube amp offers the choice of DC- and AC-coupled outputs. The class AB design can output up to 24mW into a 30-Ohm load or nearly 500mW into a 600-Ohm load.

Title: A Low-Voltage Class-A Tube Headphone Amplifier
Author: Helmut Ahammer
This OTL design features a high-voltage power supply that is less than half of the one used in the author’s first amp and most other tube amps, but without compromising performance.

Headphones Amplifiers – Hybrids (Solid State/Vacuum Tube)

Title: Blue Hawaii Hybrid Electrostatic Amplifier for Stax Omega II Headphones
Author: Kevin Gilmore
Fully-balanced, discrete transistor front end and tube output stage using EL34s in grounded grid configuration. This amp is the author’s answer to the legendary Stax T2 amplifier.

Title: A Simple Tube/Opamp Hybrid Amplifier
Author: Alex Cavalli, Mark Lovell and Bill Pasculle
This amplifier uses one tube and has low voltages throughout, thereby reducing the shock hazard present in high voltage tube chasses. A grounded cathode front-end is loaded with a constant current source and drives a unity-gain opamp backend. The authors give instructions for accommodating both FET-input and BJT-input opamps.

Headphone Amplifiers – Opamp Solid State

Title: Designing an OpAmp Headphone Amplifier (Solid State and Tubes)
Author: HeadWize
Design principles and circuits for building opamp-based headphone amplifiers. This article reviews selection criteria, feedback configurations, capacitor coupling, RF shielding, output stage topology, short circuit protection, equalization, acoustic simulation, headphone distribution amplifiers and power supply options. Also discusses design with tube amp-blocks (AC feedback amplifiers and tube and tube-MOSFET hybrid opamps) as well techniques for interfacing tube amp-blocks with solid state output stages.

Title: Buffered Headphone Driver/Preamplifier
Author: Eldon Pawliw
With a fast Motorola buffer as its only active circuitry, this design specifies the highest quality parts and careful construction to avoid signal coloration. The author includes instructions for substituting other buffer ICs.

Title: Build This High Quality Headphone Amplifier
Author: Earle Eaton
Especially good for listening to digital recordings, this headphone amplifier features an opamp front-end with complementary bipolar outputs for excellent linearity and high-drive capability. It exhibits low noise, can drive headphones of any impedance and protects against shorts.

Title: SDS Labs Headphone Amplifier
Author: Sheldon D. Stokes
This headphone amplifier can output 4W – enough to drive some loudspeakers. It has a FET opamp front-end with complementary MOSFET outputs and a toroidal transformer-based regulated power supply. This version “biases” the opamp into class A operation by tying its output to one of the MOSFET gates for best sound with headphones, but the article includes an update for balanced biasing to obtain the highest output when driving loudspeakers.

Title: A Pocket Headphone Amplifier
Author: Chu Moy
A simple yet high performance design using two Burr-Brown OPA132 FET input opamps. This project is ideal as a booster for power-conserving stereo sources such as portable CD players and for interfacing with passive EQ networks such as tone controls or a headphone acoustic simulator. It is powered by a single 9V battery configured as a virtually-grounded dual supply and is small enough to fit in a shirt pocket. Now includes a balanced input and audio limiter option for turning the pocket amp into a personal monitor.

Title: A DC-Coupled, Selectable-Gain Headphone Amplifier
Author: Chester Simpson
This design uses two opamp gain stages per channel for superior input buffering and a discrete bipolar output stage with thermal tracking current mirrors. The amplifier’s overall gain can be adjusted to match the efficiency and impedance characteristic of any headphone. The author provides detailed instructions for optimizing the performance of the output stage.

Title: A DIY Headphone Amplifier With Natural Crossfeed
Author: Jan Meier
Some crossfeed acoustic simulators suffer from frequency-response anomalies (called Comb-filter effects) because they use a fixed, frequency-independent delay in the crossfeed signal. The author has developed a crossfeed filter/headphone amplifier with frequency-dependent delay that eliminates the Comb-filter effect. It is constructed with National Semiconductor LM6171 opamps, which are capable of delivering high currents into low impedance loads. In the latest update, the author has added an enhanced-bass version of this filter.

Title: The Kumisa II Headphone Amplifier
Author: Benny P. Jørgensen
A DC-coupled headphone amplifier based on the Analog Devices AD844 opamp. This design employs a temperature-sensitive biasing system on the bipolar output stage to prevent thermal runaway. Both the opamp and the output stage are biased to run predominantly in class A.

Title: A Precision Preamplifier-Power Amplifier System with Natural Crossfeed Processing
Author: Jan Meier
This article presents a preamplifier design with natural crossfeed processing for headphone listening and a novel ALL-OPAMP power amplifier design. The preamplifier is a major revision of the original DIY Headphone Amplifier With Natural Crossfeed. Both the preamp and power amp are fully regulated and feature opamps that are biased to operate in semi-class A mode. The power amplifier can output 32W per channel into a 4 ohm load or can be converted into a mono amp with 4x the output power. It uses 46 opamps per channel and does not require any heatsinking!

Title: The Lindesberg Portable Headphone Amplifier With Crossfeed
Author: Toni Kemhagen
This state-of-the-art portable headphone amplifier is based on the AD823 and AD812 opamps and employs several techniques to minimize opamp input errors such as balancing input impedances and composite amp output stages. It also features separate power supplies for the voltage gain and output stages. The crossfeed network is from a design by Ingvar Ohman.

Title: A Portable DAC with Headphone Amplifier
Author: Andrija Ifkovic
This portable DAC can improve the sound from portable devices, such as PCDPs, and other digital gear that has a TOSLINK optical or coax digital output. It features the Burr-Brown DIR1703 receiver and the Analog AD1866 DAC ICs for digital conversion and a high-quality multi-loop headphone amplifier that can drive low-impedance loads with authority. This PDAC will accept input digital streams up to 24-bit/96kHz.

Headphone Systems

Title: Build These Noise-Canceling Headphones (plus: Binaural Mike Headset, Audio Probe and Parabolic Mike)
Author: Jules Ryckebusch
Muzzle the din and relax to the sound of silence with headphones that combat the uproar with “anti-noise.” The author includes modifications for a binaural microphone headset, an audio probe and a parabolic microphone.

Title: Poor Man’s Surround Headphones
Author: Steve Connors
Make surround headphones from a pair of circumaural headphones and earbuds. The author uses these headphones with the SB Live PC soundcard, but they will also work with any 4-channel surround recording. “The surround effect is excellent… You can tell back from front easily.”

Title: How To Remagnetize Old Headphones
Author: Tom Provost and Gary D’Amico
The magnets in old-style high impedance headphones can gradually lose their potency resulting in diminished output volume. The authors describe two methods for restoring de-magnetized headphones.

Title: An Equalization Circuit for the beyerdynamic DT990 Pro Headphones
Author: LXH2
This passive EQ corrects a rising characteristic in the frequency response of the beyerdynamic DT990 Pros and turns them into one of the most natural and accurate sounding headphones on the market.

Title: Notes on DIY Electrostatic Headphones
Author: Chu Moy
The principles of electrostatic headphone design and a study of the DIY projects by J.P. Wilson, Philip Harvey and Neil Pollock from a series of Wireless World articles published between 1968 and 1979. For advanced DIYers experienced with high-voltage construction techniques only!

Title: Troubleshooting Electrostatic Headphones
Author: Mark Rehorst
Tips for troubleshooting and repairing electrostatic headphones.

Title: Revisiting Koss “The Plug” Headphones
Author: Chu Moy
Koss’ The Plugs are inexpensive canal-type headphones that suffer from a “peaky” frequency response, weak acoustic isolation and a lightweight sound due to inadequate acoustic damping and poor ear cushion design. The author describes a series of modifications for The Plugs that allow this product to reach its full potential. The techniques shown in this article may work for other canal-type headphones that have a construction similar to The Plugs.

Title: Modifying the Sennheiser HD500 Headphones
Author: Charles Miller
The author details modifications for the HD500 headphones that correct their bright and “phased” sound. The result is an extended frequency response and better dynamic range that make these headphones among the “best headphones on the planet.”

Title: DIY Electrostatic Headphones
Author: Andrew Radford
Electrostatic headphone built in the enclosure of an old pair of dynamic headphones. The author gives detailed instructions for constructing the electrostatic transducers and a high voltage amplifier from PA transformers.

Title: Measuring the Effective Impedance and Sensitivity of Headphones
Author: Ben H. Tongue
The author describes the construction and use of a FILVORA (Fixed Insertion Loss Variable Output Resistance Attenuator) and DFILVORA (Dual Fixed Insertion Loss Variable Output Resistance Attenuator) for measuring the impedance and sensitivity of headphones, including modern stereo headphones, magnetic headsets and piezo-electric ceramic headsets.

Title: The Collected Grado Headphone Mods 
Author: Skippy et al.
This collection of Grado headphone modifications comes from posts by Skippy, Beagle, TimD, Voyager, Neruda, Chych and Squirt in the HeadWize forums. The mods are grouped into 3 categories: Transducer, Earcups and Headband, Earpads, Cables. There are even instructions for how to fix the infamous Grado “grattle.” The article specifically mentions the Grado SR-60, SR-325 or the Alessandro-Grado MS-1, but the mods may be adaptable for other Grado models (and for other brands of headphones).

Signal Processors

Title: An Acoustic Simulator For Headphone Amplifiers
Author: Chu Moy
A simple acoustic simulator that uses shaded crossfeed to create a more realistic sound image in headphones. Based on a design published over 20 years ago by Siegfried Linkwitz in Audio magazine, this update has better sound and is tailored for use with headphone amplifiers. It has a “perspective” switch to adjust the level of crossfeed and can feed a headphone amplifier input or drive headphones directly. The author provides suggestions for further modifications.

Title: Designing A Pocket Equalizer for Headphone Listening (Solid State and Tubes)
Author: HeadWize
Unlike loudspeakers, headphones generally require some equalization to achieve the most balanced sound. This article examines several passive and active equalizer designs that frequently appear in circuit compendiums. All of them are portable, and the active versions can be powered from batteries. Also includes a section on tube-based equalizers (not pocket-sized though).

Title: A DIY Headphone Amplifier With Natural Crossfeed
Author: Jan Meier
Some crossfeed acoustic simulators suffer from frequency-response anomalies (called Comb-filter effects) because they use a fixed, frequency-independent delay in the crossfeed signal. The author has developed a crossfeed filter/headphone amplifier with frequency-dependent delay that eliminates the Comb-filter effect. It is constructed with National Semiconductor LM6171 opamps, which are capable of delivering high currents into low impedance loads. In the latest update, the author has added an enhanced-bass version of this filter.

Title: An Enhanced-Bass Natural Crossfeed Filter
Author: Jan Meier
This project is a follow-up to the article A DIY Headphone Amplifier With Natural Crossfeed. The author shows a natural crossfeed filter design with a bass emphasis (similar to the Linkwitz circuit – see An Acoustic Simulator For Headphone Amplifiers) to compensate for low-frequency cancellation effects.

Title: A Soundfield Simulator for Stereo Headphones
Author: Chester Simpson
An adaptation of the Linkwitz acoustic simulator (see An Acoustic Simulator for Headphone Amplifiers by Chu Moy above) as a front-end for headphone amplifiers with very high input impedance – such as the author’s own headphone amp project (see A DC-Coupled, Selectable-Gain Headphone Amplifier). This circuit features a continuously adjustable Soundfield control.

Title: The Analoguer
Author: Jan Meier
The Analoguer is an audio filter that mitigates the harshness of CD recordings using a signal processing technique called “windowing.” As in the author’s Precision Preamplifier-Power Amplifier System, the opamps in this project are also biased to operate in class A.

Title: The Psychoacoustic Bass Enhancer
Author: Jan Meier
The device described in this article makes signals below 60 Hz audible in loudspeakers and headphones that normally might not be able to reproduce these frequencies, by synthesizing harmonics of these lowest tones and by choosing the spectrum of these overtones to mimic the harmonic distortion of ear’s basillary membrane.

Title: A Portable DAC with Headphone Amplifier
Author: Andrija Ifkovic
This portable DAC can improve the sound from portable devices, such as PCDPs, and other digital gear that has a TOSLINK optical or coax digital output. It features the Burr-Brown DIR1703 receiver and the Analog AD1866 DAC ICs for digital conversion and a high-quality multi-loop headphone amplifier that can drive low-impedance loads with authority. This PDAC will accept input digital streams up to 24-bit/96kHz.

Musicians and Audio Professionals

Title: Headphone Monitor Switch
Author: Rudy Trubitt
For use in field recording. Has provisions for stereo, left-only mono, right-only mono, left+right mono and even left-right reversed.

Title: Not Just Another Headphone Amp (Guitar Practice)
Author: Alan Gary Campbell
A battery-powered musician’s practice and monitoring amplifier. Based on the classic LM386 low voltage power amplifier, it is compact and sturdy with excellent sound, and has optional equalization for use with guitars.

Title: Designing an OpAmp Headphone Amplifier (Solid State and Tubes)
Author: HeadWize
Design principles and circuits for building opamp-based headphone amplifiers. This article reviews selection criteria, feedback configurations, capacitor coupling, RF shielding, output stage topology, short circuit protection, equalization, acoustic simulation, headphone distribution amplifiers and power supply options. Also discusses design with tube amp-blocks (AC feedback amplifiers and tube and tube-MOSFET hybrid opamps) as well techniques for interfacing tube amp-blocks with solid state output stages.

Title: Using A Headphone Console As A Balanced Line Distribution Amplifier
Author: Dennis Bohn
The author details a special input jack assembly for converting a headphone console into a balanced line distribution amplifier. Note: this article specifies the Rane HC6 Headphone Console, but is applicable to other similar products.

Title: Creating a 2nd Headphone Feed When Only One Is Available
Author: George Kourounis
Simple steps for using a power amplifier as a second cue system.

Title: An Equalization Circuit for the beyerdynamic DT990 Pro Headphones
Author: LXH2
This passive EQ corrects a rising characteristic in the frequency response of the beyerdynamic DT990 Pros and turns them into one of the most natural and accurate sounding headphones on the market.

Title: Thoughts And Processes Relating To Recording And Mixing With Headphones
Author: LXH2
How to make final mixes in headphones so that they sound accurate on a variety of playback systems. The advantages to mixing in headphones are many, but can result in mixes with poor sound when played back over loudspeakers, if not done correctly. The author gives tips and techniques for successful mixing in headphones and details the construction and use of a binaural “roving mike” for the tracking process and a tunable bass filter for recording different sources with strong bass content.

Title: A Pocket Headphone Amplifier
Author: Chu Moy
A simple yet high performance design using two Burr-Brown OPA132 FET input opamps. This project is ideal as a booster for power-conserving stereo sources such as portable CD players and for interfacing with passive EQ networks such as tone controls or a headphone acoustic simulator. It is powered by a single 9V battery configured as a virtually-grounded dual supply and is small enough to fit in a shirt pocket. Now includes a balanced input and audio limiter option for turning the pocket amp into a personal monitor.

Title: Designing A Limiter For Headphone Amplifiers
Author: HeadWize Design Paper
A review of diode-based limiter design. Includes discussion of hard and soft limiters, the differences between diode types as clipping elements, variable limiters and multiband limiters.

Title: A Portable DAC with Headphone Amplifier
Author: Andrija Ifkovic
This portable DAC can improve the sound from portable devices, such as PCDPs, and other digital gear that has a TOSLINK optical or coax digital output. It features the Burr-Brown DIR1703 receiver and the Analog AD1866 DAC ICs for digital conversion and a high-quality multi-loop headphone amplifier that can drive low-impedance loads with authority. This PDAC will accept input digital streams up to 24-bit/96kHz.

DIY Tutorials and Showcase

Title: Etching PC Boards with the Toner Transfer Method
Author: Christopher Garber
How to etch PCBs with the help of a laser printer.

Title: Designing PC Boards with ExpressPCB
Author: Christopher Garber
A tutorial for laying out printed boards with the popular ExpressPCB application. Normally, ExpressPCB files must be sent to the ExpressPCB company to produce the boards. The author shows how hobbyists can output copy from the ExpressPCB application to make their own PCBs.

Title: Apheared’s Project Scrapbook
Author: Michael Shelton
A compilation of the author’s numerous project postings in the DIY Workshop forum. The projects include portable headphone amps with current-boosted output stages, amps with VU meters, amps in mint box enclosures. “Apheared” is the author’s username in the forums.

Title: The Collected Grado Headphone Mods
Author: Skippy et al.
This collection of Grado headphone modifications comes from posts by Skippy, Beagle, TimD, Voyager, Neruda, Chych and Squirt in the HeadWize forums. The mods are grouped into 3 categories: Transducer, Earcups and Headband, Earpads, Cables. There are even instructions for how to fix the infamous Grado “grattle.” The article specifically mentions the Grado SR-60, SR-325 or the Alessandro-Grado MS-1, but the mods may be adaptable for other Grado models (and for other brands of headphones).

Title: A Compact 50W Integrated Amplifier with Meier Headphone Section
Author: Tim Harrison
This 50W per channel, stereo integrated amplifier is made from opamps. The preamp and headphone amp use Burr-Brown OPA2134 opamps. The power amplifiers are based on the National Semiconductor LM3876 power opamps. It has a facilities for 4 line-level inputs, with LED indicated switching.

Title: A Passive Preamp/Switchbox for the Desktop
Author: Chu Moy
This switchbox with volume control can select between two sets of inputs and two sets of outputs. The volume control is a shunted linear taper pot that mimics a log taper. The compact size of the switchbox lets it fit on crowded desktops, and the simplicity of the design makes it an ideal project for beginning DIYers