Tweaker's Asylum

• Re: Quad 405 without current limiting? - moray campbell 23:19:17 03/08/03 (3)
  In Reply to: Quad 405 without current limiting? posted by RBrezins on March 08, 2003 at 15:43:47:

  If you remove the current limiting be prepared to replace output devices, drivers and diodes regularly. I have removed the protection from my own 405 and not had a problem until I loaned the amp out. The way Mr. Ludwig has conigured the protection is probably the least invasive method I've seen. If you are modifying 405s for others, it would be a good idea to do this.
  
  

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  • Re: Quad 405 without current limiting? - RBrezins 12:26:36 03/09/03 (2)
    In Reply to: Re: Quad 405 without current limiting? posted by moray campbell on March 08, 2003 at 23:19:17:

    Thanks for the answer! It's my own amp. I opened it yesterday to check the circuit boards and noticed that some resistors were severely burned.
    I started the modification last year, but only did the input circuitry (new op-amp, higher sensitivity). I have to replace the burned parts anyway so I'll go further with the modifications.
    I won't remove the current limiting now, according to your observations it seems safer to leave it in place and modify it according to Ludwig's article (step 1).
    I won't go with the double output devices however, that's a little bit too difficult for me.

    RB
    
    

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    • Re: Quad 405 without current limiting? - MarcelvdG@Haarlem105.nl 03:09:43 03/14/03 (1)
      In Reply to: Re: Quad 405 without current limiting? posted by RBrezins on March 09, 2003 at 12:26:36:

      If the two 1kohm feedback resistors are burned, I'd recommend checking the output transistors, DC protection triacs and DC feedback loop electrolytic capacitor before turning on power! I know from experience that failure of the output transistors can damage the feedback resistors, triacs and DC feedback loop electrolytic capacitor (the 100uF one, called C2 if I remember correctly).

      Switching on the amplifier with disabled current limiting and without an op-amp in the socket can also cause the destruction of all the devices mentioned above.

      By the way, if you don't like electrolytics, possible modifications of the DC feedback loop can be found at the audio circuit:

      www.audiocircuit.com/9121-solidsamplifier-circuit/ Commercial/Quad-QUA/9121CMQUA-MOD-405-MG.htm
      
      

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      • Re: Quad 405 without current limiting? - RBrezins 02:49:59 03/17/03 (0)
        In Reply to: Re: Quad 405 without current limiting? posted by MarcelvdG@Haarlem105.nl on March 14, 2003 at 03:09:43:

        Thanks for the info. Burned resistors are R2, R41 and R39, not the most critical parts in the circuit (except for R41 maybe). I will replace the output and driver transistors anyway by more recent devices. I received all parts last Friday, but the weather was too nice to work on an amp.....

        Thanks,
        RB
        
        

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