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DIY For Audio
Need help with a little circuit
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<blockquote data-quote="handsome" data-source="post: 1106227" data-attributes="member: 772"><p>Cap-coupled valve preamplifiers need circuits like this because at startup you get a very large voltage at the output as the coupling caps charge up. This can easily destroy whatever the preamp is connected to (in my case it was a rather expensive sound card). The transistors are probably connected as a darlington. These circuits usually work by having a resistor connected to the +ve rail and then to a capacitor that is connected to earth. The base of the transistor/darlington is connected to the junction of the resistor and capacitor. At switch on the capacitor charges up slowly through resistor and at a given level will turn the transistor on and triggering the relay. Try measure the junction of resistor and capacitor at switch on and see if the voltage there does actually ramp up. A simple addition to these circuits is to add a diode from said junction to the +ve rail at switch off this will discharge the cap very quickly and allow the real to mute the circuit quicker.</p></blockquote><p></p>
[QUOTE="handsome, post: 1106227, member: 772"] Cap-coupled valve preamplifiers need circuits like this because at startup you get a very large voltage at the output as the coupling caps charge up. This can easily destroy whatever the preamp is connected to (in my case it was a rather expensive sound card). The transistors are probably connected as a darlington. These circuits usually work by having a resistor connected to the +ve rail and then to a capacitor that is connected to earth. The base of the transistor/darlington is connected to the junction of the resistor and capacitor. At switch on the capacitor charges up slowly through resistor and at a given level will turn the transistor on and triggering the relay. Try measure the junction of resistor and capacitor at switch on and see if the voltage there does actually ramp up. A simple addition to these circuits is to add a diode from said junction to the +ve rail at switch off this will discharge the cap very quickly and allow the real to mute the circuit quicker. [/QUOTE]
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DIY For Audio
Need help with a little circuit
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