I had built a set of powered speakers but I was having problems with mains hum and noise. Because powered speakers can be plugged into any electrical outlets in the house, it is not possible to know the layout of the earthing system to ensure that mains hum is not introduced. Normally an audio system will be set up in such a way that all earthing comes back to a common star point. The normal solution to eliminating hum if the earthing cannot be arrange in a suitable way would be to place audio isolating transformers in the line level signals. I wanted to avoid using transformers to achieve a more flat frequency response.

I started looking at alternate ways to electrically isolate line level audio signals, and I came across the IL300 by Vishay. The IL300 is an opto-coupler that has an additional feedback photodiode that allows a sample of the internal LED flux to be measured and used to compensate for the non linear characteristics if the opto-coupler.

The device consists of an dual wound toroidal transformer providing 2x15Vac. The windings are isolated from each other. The circuit is divided into three sections. The first section is the input side of the opto-isolator, and includes a 7805 voltage regulator and and op-amp OPA1641. The gain of the op-amp is influenced by the signal fed back from inside the opto-coupler, and adjust the drive to the opto-coupler accordingly. This arrangement effectively linearizes the signal.

The second stage of the circuit includes the 7808 voltage regulator and the op-amp OPA1641. The signal from the output of the opto-coupler is buffered by the op-amp and fed into the third section.

The third section of the circuit includes the 7812 voltage regulator, a 55 timer and double pole relay. The purpose of the timer circuit and relay is to prevent power on thump from occurring in other circuits downstream such as the active crossover and main amplifier within the powered speakers. The 7812 voltage regulator derives its supply from the second bridge rectifier BR2, which also powers the output stage via the 7808 voltage regulator. When powered up, the 555 timer begins to time out. The time delay is set using the trimpot. When the timeout occurs, the relay is activated, connecting the audio output stage to the actual output of the device. When powered off, it is necessary to cause the relay to de-energize immediately with no time delay to interrupt the audio signal path before any thump is passed through. This is achieved using the resistor divider consisting of R8 and R9, the capacitor C19, and diode D4. D4 is used as a blocking diode to prevent the stored charge from capacitor C2 from feeding back into the resistor divider network when power is removed. The intention is that the voltage measured at the intersection of R8 and R9 will drop almost instantly to zero when power is removed. The signal at this point is used by the reset pin of the 555 timer, and causes its output to turn off when below a certain threshold. The purpose if the jumper is to remove this function by tying the reset put to VCC causing the output of the 555 timer to turn off at it's own natural rate. It is not intended that it will ever be used in this configuration and was more for testing purposes. In normal operation, the pin 4 of the 55 timer will be connected to the intersection of R8 and R9 (pins 2 and 3 of the jumper)

This project is used as a pair, one for each audio channel.

CAUTION:

This project was designed for use with 230Vac.