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Hum from passive-pre

Discussion in 'audio' started by lordsummit, Nov 10, 2017.

  1. Darth Vader

    Darth Vader From the Dark Side

    Nope! Speakers don't hum but some amps do. How loud the hum is will depend on both the amp and the sensitivity of the speakers i.e. a sensitive speaker like horns will produce the hum at a louder volume than a sealed box design.

    My 87db sensitive speakers with my BIG amps are silent with ear pressed against them.

    However loud hum can be tricky to resolve in some cases such as earth loops.


  2. Tony L

    Tony L Administrator

    The passive pre is just a little DIY job which came free with my Stereo 20. It is one if the stepped attenuators from eBay, Valab I think, in a little plastic box with four ins and one out (i.e. has an input selector switch as well as the attenuator). It is wired up with some thin, probably silver wiring that is rather too short and seems to break very easily, but it sounds like at least one input still works. It actually sounds superb, I used it for a while with one of my 303s for a while and it was fine other than the steps being a bit too crude. Assuming the input in question is still working on both channels I can’t understand how it can be Richard’s problem as it is just a little stepped attenuator passive.

    PS Richard, if you pop round with it I’ll happily resolder any loose wires!
  3. lordsummit

    lordsummit Moderator

    Thanks Tony! I’m thinking of just removing all the unnecessary wires, and just joining one input to the volume control.
  4. Arkless Electronics

    Arkless Electronics Trade: Arkless

    It's as "Jim Audiomisc" says.... The wiper is the output of the pot and at zero vol the wiper of the pot is short circuit to earth. This means it is at a very low impedance and the high impedance mains hum impinging by mainly capacitive coupling is in effect shorted to earth. At full vol the wiper is fully connected to the source component, which will usually have a very low impedance like 100R. To the mains hum this is much the same and again little or no hum is heard. The worst case is when the vol control is electrically at mid point. Assuming a 50K pot and low impedance source this worst case means the pot has an output impedance of 12.5K at mid point and this much higher impedance is unable to short all of the hum to ground, hence you hear some. It will usually be noted that it gets worse as you move your hand towards the vol pot and often worse still if you touch the vol control, assuming it's metal. A correctly grounded metal enclosure for the pot and extending the grounding to the body of the pot will often solve it.

    It is for exactly the same reasons that a passive is theoretically at its worst at the electrical mid point (electrical mid point because with a logarithmic pot as used for a vol pot the physical mid point is not the electrical mid point) of the pot. As this is the point of highest output impedance, it is also the point where the low pass filter formed by the output impedance of the pot and the capacitance of the cable between the passive and the power amp is at its lowest frequency. Meaning? Meaning that with a poor choice of passive pot value and too long a length of interconnect you could be getting treble being rolled off at say 15KHz at mid volume but it's extended to say 50KHz at very low and very high volumes.
  5. lordsummit

    lordsummit Moderator

    Found the problem. One of the fancy bargain wires I’d purchased was faulty. Some additional swapping around led to the discovery that the old rca leads between pre and power caused it to go away, and disconnecting the new lead from the pre magnified it. I suspect there’s a grounding issue on the cable. They’re getting me a new lead, and meanwhile peace and harmony is restored. Phew. Thanks guys, this is a marvellous place.
  6. flatpopely

    flatpopely Prog Rock/Moderator

    Hope the DAC sounds good to you.
  7. lordsummit

    lordsummit Moderator

    It is. Keep playing the posh stuff I have ripped, it’s really singing. Got an XRCD of Interplay on right now. The trumpet is incredibly natural, and the cymbals have a really nice zing. It has got that naturalness that my old AAA5 had. Need to play something proggy next in your honour.
  8. flatpopely

    flatpopely Prog Rock/Moderator

    Pink Floyd WYWH please.
  9. lordsummit

    lordsummit Moderator

    Off for beer and burger to celebrate the success of my daughter, when I get back for sure!
  10. Paul R

    Paul R pfm Member

    All clear. Hard to go too wrong with a pot in a box...

    But if one of the 0v wires is cracked you could get some hum in an otherwise working system, sounds like this may have been the case but outside the box...


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