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Whats a maximum acceptable DC Offset for a budget amp?

Discussion in 'd.i.y.' started by colasblue, Oct 13, 2010.

  1. colasblue

    colasblue pfm Member

    Right chaps

    I've got my previously dead NAD 7150 up and running though still slightly ill and in need of a recap in all probability (caps on way but I wasn't buying them until it was a runner).

    The issue is that although I've replaced a couple of small signal transistors that seemed to be the source of the main problem it still has quite a high DC offset on the affected channel.

    The repaired one is -0.5v to other one is a much healthier -0.1V.

    The protection circuit and relay doesn't trip unless it gets over about 4.8v which is what it was doing before, but none the less -0.5v seems quite a lot in the context of the higher quality equipment I'm more used to.

    There doesn't seem to be any way to actually adjust the DC offset by means of trimmers or anything. The circuit has two trimmers but neither has any effect on this.

    The biasing is done by an active circuit which I'm not fully familiar with so I may need to do a bit of research, but it doesn't appear to have any adjustment mechanism.

    My question is really am I trying to fix somethig that isn't actually broken?
  2. martin clark

    martin clark pinko bodger

    That's about 10x the amount that is conventionally accepted as reasonable for a DC-offset (ie +/-50mV max)

    0.5v is 30mW into an 8ohm load, by no means disasterous, but not good either. Worth getting to the bottom of IMO.
  3. neiljadman

    neiljadman Senior Member

    I just fixed up and old Nait2 that was somewhat compromised to say the least...DC offset is at 0.4 and 1.1mV ...
  4. colasblue

    colasblue pfm Member

    Thanks guys - thats what I thought.

    I'll see what it does after the full recap since its entirely likely that some caps have gone slightly leaky.

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