.DF96 is right, it has with the feedback loop to do. I sold quite a few of these kits back then and Erik soon found out that the "blue" plastic cap in the the feedback loop had a tendency to break, giving the symptoms mentioned in the first post. We routinely changed them and the problems where gone.
Hi, just a quick thank you for all your advice given. I have the amp back in my possession and the fault has been cured...no nasties on switch off. Revintage had it right I think and the amp had an intertmittent fault "blue capacitor" in the feed forward circuit...the values in my amp were .68mf/400v, different to the schematic I believe. My engineer did a couple of other things including moving a resistor that was very close to the rectifier valve and placing it directly under the circuit board itself and putting a screen around the valve as it is in very close proximity to the input leads...a dab of fresh solder on the valve bases and where they was shown an R value in series with a C value, my amp only had the R so he kindly fitted the required capacitor which apparently rolls the bass response gentle and reduces saturation of the output transformers? Anyway, getting out of my depth here!!
As it is connected as the hf amp in a pro balanced set up, of course it hummed mercilessly when connected..a Behringer "hum destroyer"..ie, isolation transformers balanced/unbalanced earth loop breaker came to the rescue. Even with this on the inputs and the 200mv input gain problem dealt with by turning down the output from the active DBX crossover, this amp had that something special. More than just sound stage, clout, hf shimmer..it just sounded that one step closer to the real thing when compared to the class AB transistor device that usually performs this function.
But there is some residual hum, which although settles back once warmed up, is always just present. Not too annoying with the NXT panels that have an efficiency around 89dB/watt/metre and no problem of course if it was a bi amped passive system..the amp is very quiet in the hiss department..in fact just about inaudible.
Now if I could just drop that hum by another 6dB or so would make this a very fine amp indeed..
Any ideas!!!!
As it is connected as the hf amp in a pro balanced set up, of course it hummed mercilessly when connected..a Behringer "hum destroyer"..ie, isolation transformers balanced/unbalanced earth loop breaker came to the rescue. Even with this on the inputs and the 200mv input gain problem dealt with by turning down the output from the active DBX crossover, this amp had that something special. More than just sound stage, clout, hf shimmer..it just sounded that one step closer to the real thing when compared to the class AB transistor device that usually performs this function.
But there is some residual hum, which although settles back once warmed up, is always just present. Not too annoying with the NXT panels that have an efficiency around 89dB/watt/metre and no problem of course if it was a bi amped passive system..the amp is very quiet in the hiss department..in fact just about inaudible.
Now if I could just drop that hum by another 6dB or so would make this a very fine amp indeed..
Any ideas!!!!
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