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Many hi-fi amplifiers and professional power amps (and loudspeaker systems) provide some of protection, either to protect the speakers from an amp fault, and/or vice versa. Some of these are implemented at a very basic level - for example the use of a 'poly-switch'. The poly-switch is a non-linear resistor, having a low resistance at normal temperatures and a much higher resistance at some designated temperature. Unlike 'ordinary' thermistors whose characteristics are more or less linear, the poly switch has a rapid transition once the limit has been reached. The circuit also includes a mute function, which leaves the speakers disconnected until the amplifier has settled, and disconnects the speakers as quickly as possible after power is removed to prevent the turn-off noises that some amps generate. These can range from a low level thump 5 to 10 seconds after power is turned off, to whistles, squeaks and other strange noises that I have heard from amps over the years.

This is a small protection circuit from loudspeakers, from DC voltage that likely to exist after some damage in the power amplifier. If a DC voltage is presented in the exit of amplifier, RL1 it interrupts immediately the line of loudspeakers preventing thus to reach in he. Parallel it provides a delay time of 3 seconds from the moment where the power supply will be applied. This delay protects the loudspeakers from undesirable bangs that are observed when open the supply switch.

Here's low power stereo amplifier built around TDA2822 chip. Many people may have heard of the TDA2822 before, but for those who haven't, it is a small power amplifier that will drive two channels. It is usually in an 8-pin DIL package, but older versions I have seen are 14-pin or similar (there are datasheets for both variants). For simplicity though, my circuits show schematics for the 8-pin DIL package. The datasheet is here, provided by ST. This article is based along the usage of the TDA2822M variant of the chip series as it is commonly available. The TDA2822 is similar, but has slightly more pins so is less used.

This device is fully functional sound card for PC. The main advantage of using PCM2704 against PCM2702 is much easier construction. As you can see on the block diagram it has built-in 5V and 3.3 voltage regulator, HID interface (MUTE, VOL+, VOL-), S/PDIF output. The circuit can be powered directly from USB port. Next advantage is that the output DAC is able to drive directly 32ohms headphones, but the output power is only 12mW. For all details please refer to the PCM2704 datasheet.

This is probably one of the most legendary fuzz pedals that has ever existed, and for good reason too! This thing can produce some completely saturated fuzz that is capable of slipping into feedback easily if you crank it up loud enough. I just can't say enough about this pedal. This is, without a doubt, my all-time favorite fuzzer. It's a genuine kick in the teeth that will have a permanent spot in my setup. Supposedly, this is the fuzz pedal that Jimmy Page used on the first couple of Led Zeppelin albums, which makes it even more famous. Below is a picture of the circuit board in a ToneBender MKII.

This amplifier evolved from the previous 6 transistor mosfet amplifier. That design was already so good there seemed no reason to look for improvements, but the high open-loop distortion at 20kHz and the resulting triangular 'error voltage' extracted suggested that the non-linear mosfet capacitance was responsible for almost all the distortion, and although the closed-loop distortion remains below 0.01% this could easily be reduced further by driving the mosfets from a lower impedance just by adding an emitter-follower.

This 200W MOSFET Power amplifier is suitable for many applications such as Guitar Ampplifier, Mic or Home theater. As many people prefer because of its legendary robustness of MOSFET transistors. MOSFET amplifier is rated at 200W power with 4Ω speakers. It has good frequency range of 1 dB 20Hz up to 80kHz. THD is less than 0.1% at full power and signal to noise ratio when compared to 200W is better than -100 dB unweighted.

This is a very simple, low cost, Hi-Fi quality power amplifier. You can build it 5 ways, like it's shown in the table (from 20 W to 80 W RMS).

This is my MP3/AAC player project. The difference to most other players is that decoding is not done on a specialized IC (like VS1001), but directly on the microcontroller. It can play MP3 (all bit rates) and AAC (up to ~256 kbps) from SD card in real time. The project is work in progress.

This MP3 player similar to iPOD shuffle that is based on the latest innovative BU9432 chip from RHOM. It features USB 1.1 / 2.0 Controller, MP3 decoder, system controller for loading MP3 files from a USB flash drive, USB hard drive, USB CD-ROM or USB DVD-ROM drive all in one chip. Once USB flash drive is connected, BU9432 automatically searches for MP3 files for playback. Audio is controlled by tactile buttons; Play, Stop, Previous Song and Next Song. BU9432 can decode VBR MP3, MP2, MP1, Layer 1, 2, 3 files with Sampling rate: 8K - 48KHz and Bit rate: 8Kbps - 448Kbps. It can also recognize FAT16 and FAT32 USB flash drive / hard drive with capacity from 32MB to 2TBytes. Audio playback is exceptionally good with 93dB signal to noise ratio and 88dB dynamic range. This MP3 player is also controlled by a small IR remote control with the following functions; Play/Pause, Previous, Next Song, Vol - and Vol +.