HackoBoy Posted September 17 Report Share Posted September 17 Hello I have two amplifiers, one of which had a problem with the output relay and I changed it. But I noticed that one channel hums weakly, and the other more loudly. After swapping the terminals to the preamp, the hum moved to the other channel. When I tested with an audio signal, the hum comes from a component mounted in the feedback of the input TL082 (opto-resistor) and the closer my finger is to the housing of this element - the hum intensifies, if I touch it even louder it hums. It serves if the input level is too high - to limit the signal (indicated by an LED) How to remove this buzzing (if there is no applied voltage to the optocoupler - the output shows infinity, if there is an applied voltage - the output shows 300k ohms) The other phenomenon when there is an audio source connected - and the potentiometers are at a minimum - the outputs of the amplifiers play weak music. Quote Link to comment Share on other sites More sharing options...
HarryA Posted September 17 Report Share Posted September 17 "After swapping the terminals to the preamp, the hum moved to the other channel." Would not this suggest the problem is in the preamp? Quote Link to comment Share on other sites More sharing options...
HackoBoy Posted September 19 Author Report Share Posted September 19 I tried the following today. It is difficult to test the speakers and I collected test cables (audio headphones connected in series through a small capacitor and resistor) and I was able to hear that the sound from the preamplifier was clear. And when I adjust the output level of the preamp - there is no noise at minimum, and no sound at all. But when I connect headphones to the output and have reduced the amplifier to a minimum - a slight music + buzzing is heard, which intensifies when I bring my finger to an element of the feedback of the TL082 (through which the amplification is controlled) Quote Link to comment Share on other sites More sharing options...
HarryA Posted September 19 Report Share Posted September 19 I once had an amplifier that played music with no inputs. It turned out that it was a local am radio station; the results of poor grounding. If you have a wee soldering iron try resoldering the ground on the TL082 op amp. Perhaps that will help. Quote Link to comment Share on other sites More sharing options...
HackoBoy Posted September 21 Author Report Share Posted September 21 I got nervous initially, one channel of the speakers hummed very weakly and was almost inaudible, but directly through headphones at the output, a hum is heard the problem turned out to be in the amplifier. I found where the input signal of the amplifier is. Even if I short the input of the amplifier it still hums. Quote Link to comment Share on other sites More sharing options...
HackoBoy Posted September 22 Author Report Share Posted September 22 Hello. Where can I buy a shielded audio cable? The problem is that two Oberton B1200 amplifiers - at minimum amplification have Brum in the outputs + very quiet music. When I gave up after all the measurements - I slightly moved part of the cables to a different position (now to the toroid, then to the radiator of the final amplifier) the noise started to mix in the left or right or both channels. I test this with plugged in headphones at the output instead of running to the speakers every time I did not expect that with serial production and many years of work - they did not understand it. They used a ribbon cable to the different boards. 1: Preamplifier to the front panel with the potentiometers. Ribbon 5 core cable for each channel 3 wires in a braid 2: Pre-amplifier to Power amplifier board: ribbon cable with 4 cores (the two are +15 V, -15 V stabilized by zeners from the Power amplifier board with which the preamplifier is powered) the other two are (the signal to the Power amplifier board ) here I will use 2 cores in a braid separated by the choke Am I headed right? Quote Link to comment Share on other sites More sharing options...
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