Useful hum and signal path information

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Neil C
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Useful hum and signal path information

Post by Neil C » Wed Nov 17, 2004 12:12 pm

I posted a question about hum on one of the channels on my Spacexpander on the Sound on Sound magazine forum, and I got this very good reply from the technical editor, Hugh Robjohns. I thought it might be useful to reproduce it here: <BR> <BR>'Looking at the circuit diagram, if you've only got hum on one channel then the chances are it's a dried up capacitor in the output valve cathode of the hummy channel. <BR> <BR>Hum in valve gear can be caused by duff/tired/knackered valves, but is more likely to be dried up decoupling capacitors and smoothing capacitors. In any vintage gear like this it is sensible to replace all the electrolytic capacitors as they dry out in the heat from the valves and they can explode in a very spectacular and extremely messy way <BR> <BR>The basic signal path is that the two inputs are mixed together and buffered by the first half of the first valve. The signal is then passed to the second half of the valve which drives the spring line through a coupling transformer &#40;to match impedances&#41;. <BR> <BR>The output of the spring line is amplified by both halves of the second valve. The reverb level is controlled remotely through the jack plug connection. <BR> <BR>The reverb signal is then split in two and each side is mixed with the relevant channel of the direct input signal. These mixes are then buffered by the two halves of the third valve to feed the output sockets. <BR> <BR>The power supply is very crude with just a solid-state bridge rectifier and a bunch of smoothing capacitors, but I see there is also a 'hum adjust' control to try to balanced out mains hum through the transformer. <BR> <BR>It might be worth tweaking this to see if it helps your problem. BUT IF YOU ARE NOT CONFIDENT WITH ELECTRONICS DONT OPEN THE UNIT AT ALL. THERE ARE LETHAL VOLTAGES EVERYWHERE!!!' <BR> <BR>14/10/04

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interruptor
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Useful hum and signal path information

Post by interruptor » Wed Nov 17, 2004 12:47 pm

hey neil, thanks for posting it here too. this will be useful when i need to revise my own spacexpander.

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