2 Watt Amplifier #2:  (added 6/03)

3 Band Equalizer:  (added 9/02)

3 Level Audio Power Indicator:  Battery-operated Three LED display that connect it to loudspeaker output   (Circuit / schematic design added 6/06)

3 Tone Gong:  Suitable for use as a door bell (Circuit / schematic design added 6/06)

30 Ddm to +30 Ddm Audio Level Meter:  circuit with balanced input and adjustable gain to drive needle type VU meter, uses special amplifier module from Op-Amp Labs (Circuit / schematic design added 6/06)

300 Watt Subwoofer Power Amplifier:  (Electronic circuit added 7/03)

Active Subwoofer and Controller:  (Electronic circuit added 7/03)

AN543: Tone Generation:  Microchip Application Note - Published 26-Aug-97   (added 2/06)

Audio Graphic Equalizer:  simple 7 band equalizer (Electronic Schematic / circuit added 4/02)

Audio Level Indicator #2:  (Circuit added 9/04)

Audio Level Indicator:  Audio Amplifier output relay delay (Circuit / schematic design added 6/06)

Audio level meter (VU Meter):  This circuit uses just one IC and a very few number of external components. It displays the audio level in terms of 10 LEDs. The input voltage can vary from 12V to 20V, but suggested voltage is 12V. The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs providing a logarithmic 3 dB/step analog display. LED current drive is regulated and programmable, eliminating the need for current limiting resistors. The IC contains an adjustable voltage reference and an accurate ten-step voltage divider. The high-impedance input buffer accepts signals down to ground and up to within 1.5V of the positive supply. Further, it needs no protection against inputs of ±35V. The input buffer drives 10 individual comparators referenced to the precision divider. Accuracy is typically better than 1 dB.  (Circuit / schematic design added 6/06)

Audio Level Meter:  (added 6/03)

Audio Tone Control:  (circuit added 7/02)

Automatic Loudness Control:  Simple add-on module Switchable "Control-flat" option  (circuit design added 3/05)

Better Volume Control:  how to create a log pot that is better than the "real" thing (Electronic Schematic / circuit added 4/02)

Build a Simple Soft Action Muting Switch:  10/14/04   EDN-Design Ideas / (Schematic / circuit added 10/05)  The circuit in Figure 1 adds a soft muting switch with power-up/power-down muting to a line-level audio circuit. R4, C1, and JFET Q1 quietly ground a signal in 100 to 200 msec when you close S1 or release it when you open S1. Potentiometer R2, set to twice Q1's cutoff voltage, makes the on/off transition times roughly equal. R2 and D3 quickly discharge C1 and mute the signal during power-down. For this process to work, the signal path should remain stable to below roughly one-third the normal supply voltages—below ±4V in this example. Q1 can then finish muting. Making Q1 a more tightly defined PN4392 can soften this requirement and allow muting of lower impedance signals. R3 unloads S1 from R2, so that D3 does not shorten the earlier transition times. S1's normally closed contact, resistor R1, and dual-LED D2 add an indicator light. D1 raises the red LED's on-state threshold to indicate green when muting is off. Replacing D1 with a short circuit causes the red LED to light. This scheme makes a more expensive DPDT (double-pole, double-throw) switch unnecessary, provides uninterrupted light as S1 switches, and reduces the LED-current change for less noise (references 1 and 2).

Designing with the LMC835 Digital Controlled Graphic Equalizer:  National Semiconductor Application Note 04-Nov-1995 (Electronic Schematic / circuit added 4/02)

Digital Delay Unit for Surround Sound:  (added 9/02)

Digital Volume Control has Log Taper:  04/11/02 EDN Design Ideas  /  (added 12/04)  Digital potentiometers provide a compact and convenient way to attenuate audio-amplifier signals. However, most such potentiometers suffer from at least one flaw: a nonlogarithmic step increment. To avoid this problem, a user must usually step the potentiometer in a nonlinear sequence to simulate a logarithmic taper. ...

Digital Volume Control:  based on DS1669 Digital Pot IC (Electronic circuit added 5/02)

Dual Channel Digital Volume Control:  (Electronic Circuit / Schematic added 10/04)

Easy Parametric and Graphic Eq's Plus Peaks and Notches:  (added 8/02)

Generating Tone using Pdo:  Toshiba 870X Series Application Note  (added 2/06)

Guitar Control:  CaStand-alone, 9V battery powered unit .  Three-level input selector, three-band tone control  (circuit design added 3/05)