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Crackling channel on newly built Air Pre

M

midcm1

pfm Member
Can anyone suggest what the problem could be on my newly constructed Air Guitariste pre ?

Problem - low level crackling sound on one channel. Independant of volume i.e it doesn't get louder unless the volume is wound right up and even then it doesn't get that much louder. Problem is definitely in one side of the pre amp.

Used with a single ALWSR, powered from a Naim 140 power amp running at 21.2volts (at the moment) to both channels. Not sure the noise was there before I added the SR and I'm not going to disconnect anything just yet - enjoying it too much ! Running into passive SBLs.
I've checked all joints and they look ok, although used lead free solder and dry joints are a little more difficult to recognise. Could it be a faulty cap or really a dodgy connection ?
Using Simics instead of Evox SMRs at the moment. (Anyone got any spare ??)
Any help/comments would be appreciated.
 
first check the interconnect. Start simple , then check the socket that the interconnect is plugged into.
 
Check and recheck the 47uF cap used in the feedback loop on the crackly channel - 'leaky' electrolytics can do this (whether tantalum, silmic or others). Swapping it out for anything else will effect a quick diagnosis.
 
Martin

Is this C6 ? If it is I've possibly stressed the legs on this component and maybe damaged it (footprint on pcb is for a larger component), could this produce the constant crackling sound I'm experiencing ?
I've other 47uf electrolytic caps I could try to prove this, however no more 47uf simics to replace if it is faulty. Reading the blurb on the design this should really be a polyester, could you or anyone else suggest an alternative cap, a good polyester that is easily sourced if this is the problem component ?
 
Yes, it's C6, which is supposed to be an Evox part; there's no problem using a decent electrolytic cap though, and it's a damn sight more convenent since film caps of this capacity are few and far between (and never cheap!)

If your silmic does turn out to be the problem (i.e. a bit leaky), drop me a PM with an address - I can probably scrounge up another one out of the bits box for you.

One teaching-to-suck-eggs-afterthought - have you checked that the electrolytic caps are both installed with the -ve leg connected to the 0v (star)? In this circuit connecting an electrolytic C6 in reverse wouldn't damage it, but might create enough leakage current to produce the problem described.
 
Followed your suggestion Martin and no change. Also swapped out the other two silmics and no change (polarity ok). Resoldered a couple of joints, again no change. Powered the problem channel from a bench PSU instead of the SR'd supply from the 140, again no change. Cleaned the pcb with IPA, still no change. The problem existed on my first proto build in a cardboard box, having now remade all connections again with better cable etc I can discount connections. Any other ideas ? Short of swapping out everything on the board I'm a bit lost now. Any other ideas ????
 
Try comparing dc voltages in each channel, around the potentiometer, and at each terminal of each transistor, say. If no dc in the pot then if it occurs in the circuit proper you know where to start looking.

On getting your silmics to reach the pads meant for the MMK caps, drill a hole at 5mm spacing from one of the correct holes, where there is no track, insert the leads, and bend the one through this new hole under the board, in the direction of the desired pad, sleeved if required. You might need to solder an extension if it doesn't reach the pad.

David
 
Finally got round to replacing all the transistors........

Problem solved.

I'm not sure if this was a faulty component or poor joints (I'd resoldered them already though), orientation was correct in all cases.

Thanks to all who contributed to this one.
 
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