Various Circuits

Serial Asynchronous Receiver

This is an RS-232 compatible receiver, implemented by using 74xx circuits.
Operation: the start bit switches the SR latch (74HC00) on, and the counters (74HC393) start counting up. The shift register samples each bit (from the Start bit until the 7th bit) just in the middle. At the end of the 7th bit (actually at the beginning of the Stop bit), the data of the shift register is loaded into the output latch. The circuit is reset just in the middle of the stop bit, so a new Start bit can be detected immediately.

---+       +-------+-------+-------+-------+-------+-------+-------+-------+-------+---
   | Start |   0   |   1   |   2   |   3   |   4   |   5   |   6   |   7   |  Stop 

2 x 30W HiFi Stereo Amplifier

The power amplifier is mostly based on the Videoton EA6383, though it contains some improvements. The biggest difference in my design is the use of Sziklai-pair instead of Darlington in the output stages, mainly because of better thermal stability. Other improvements include emitter degeneration and current mirror in the long-tailed pair section.

Assembled and mounted in its metal enclosure:

The preamplifier is a regular 2-stage opamp design. The first stage does the real voltage amplification and the second stage is the active tone control circuit, utilizing a Baxandall tone stack. Here is a very good source of audio related topics.

Exciter Guitar Effect

The exciter effect generates non-existing harmonics and adds it to the original sound of an instrument. This is particularly useful for enhancing the clean tone of the electric guitar. The upper harmonics can be generated either by multiplier circuits, or by distorting a specific upper frequency band of the original signal.
The below exciter effect circuit works this way: A high input impedance buffer amp (T1, T2) receives the signal, which is then split in two ways: it is fed directly to one of the inputs of the mixer amp (IC1-A), and is fed into a high-pass Sallen-Key filter (T3), which provides high-frequency signal to the distortion circuit (IC1-B, D1, D2). The filter is tunable between approximately 800Hz-8kHz with a dual-gang potentiometer (R21/1, R21/2), and two more passive filters (C4-R7 and C5-R8) ensure that all the frequencies below 500Hz are being firmly cut. The output of the distortion circuit is further filtered with another tunable high-pass filter (T4, R22/1, R22/2) in order to eliminate the lower base frequencies; the generated upper harmonics are finally mixed with the original signal by the mixer amp (IC1-A).

The above circuit is based on the ideas from the Harmonic Sweetener, but it uses only a single opamp package, and furthermore all the filters are tunable.

Built inside its 1590BB enclosure. The controls from left to right are: 1st filter (R21), Drive (R23), 2nd filter (R22) and Mix (R24).

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