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Parametric & Sub-Woofer Equaliser - A simplified version, which performs surprisingly well  (electronic schematic added 1/07)

Peak Level Indicator - Dot-mode three-LED display.  9V battery powered portable unit (added 3/05)

Peak Reading Audio Level Meter - indicates peak audio response on an analogue meter, similar to a tape recorder recording level meter (added 4/02)

Personal Media Player PMP - Intersil Application Block Diagram  (app note added 6/06)

Phono Preamplifier - Simple circuitry suitable for moving-magnet cartridges.  Passive high-frequency equalization.   (electronic circuit added 06/07/08)

Phono Preamps For All - Various circuits for moving coil and moving magnet pickups  (schematic added 6/07)

Piezo Pickup Amplifier Circuit - AS098 Jenson Transformer Application Note  (app note 6/07)

Poor Man's Surround Headphones - 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."  Designed by Steve Connors  (circuit design added 01/08)

Portable E-field Receiver - Here is a tiny receiver built into an Altoids tin suitable for use with a laptop computer or portable tape recorder. The prototype antenna is only 1 foot long when fully extended and the receiver fits into a shirt pocket... (electronic design added (6/07)

Portable Headphone Amp - Another contributed circuit, this is a complete portable headphone amp that features crossfeed.  (schematic added 6/07)

Portable Headphone Amplifier - 3V Battery powered High Performance unit (added 9/04)

Pre Distortion Techniques - Tape Linearizer and a Distortion Analyzer (added 4/02)

Precision Audio MilliVoltmeter - Measures10mV to50Volt RMS in eight ranges Simply connect to your Avo-meter set @50µA range (added 9/04)

Projector - Intersil Application Block Diagram  (app note added 6/06)

Rctheremin Circuit - (schematic / circuit design added 9/02)

Reader's Xover - This is well worth reading if you can't decide on biamping (added 6/07)

Removing the DC Thump from Audio Circuits - (schematic / circuit design added 9/02)

Revisiting Koss "The Plug" Headphones - 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.....  Designed by Chu Moy  (circuit design added 01/08)

Room Noise Detector - One LED monitors three levels: 50, 70 & 85 dB.  Useful to detect too noisy environments.  (electronic circuit added 06/07/08)

Satellite Radio - Intersil Application Block Diagram  (app note added 6/06)

Satellite Set-Top Box NTSC/ PAL - Intersil Application Block Diagram  (app note added 6/06)

Scanmate Audio Activated Recorded - (Tony van Roon's schematic / circuit added 10/02)

Sound Detector - (electronic circuit added 7/03)

Sound Operated Switch zen 22142 - This sensitive sound operated switch can be used with a dynamic microphone insert as above, or be used with an electret (ECM) microphone. If an ECM is used then R1 (shown dotted) will need to be included. A suitable value would be between 2.2k and 10kohms. The two BC109C transitors form an audio preamp, the gain of ....  (added 2/06)

Spatial Distortion Reduction Headphone Amp - (circuit / schematic design added 6/06)

Speaker Box Audio Amp - ….(design added 08/08)

Speaker Measurement Set - Based on work done by Siegfried Linkwitz (reproduced with his permission) (added 6/07)

Speaker Microphone Circuit - (diagram added 2/07)

Speaker Protector - (circuit added 7/02)

Speaker Signals to Line Level Inputs - (electronic schematic added 8/02)

Spyder an Eight Output Pedalboard Power Supply - Outputs regulated 9V DC (added 4/02)

Stereo BroADCast Limiter - Circuit for the Ramsey FM10a/FM25 Transmitter (added 4/02)

Stereo LED VU meter using discrete components BC547 transistors - (diagram added 2/07)

Stereo Width Controllers - 2 to choose from. Expand or contract the stereo sound stage(electronic circuit added 7/03)

Subwoofer Equaliser - The Linkwitz transform circuit is a hugely flexible way to equalize the bottom end of a sealed loudspeaker enclosure. A speaker that is corrected using this method is flat from below resonance to the upper limit of the selected driver. The low frequency roll off point is determined by the parameters of the transform circuit.....(added 4/02)