Amp/Preamp Asylum

• Re: It can sound like increased noise or "hash" in the signal all - BrassTeacher 14:45:31 04/29/07 (2)
  In Reply to: It can sound like increased noise or "hash" in the signal all posted by HenryH on April 29, 2007 at 09:46:02:

  To add to HenryH's suggestions:

  1. Along with the pops Henry mentioned, supply caps drying out can be heard as "hum" through the speakers. Henry is right, if they are to the popping stage, do not walk, RUN to have someone replace them, or run to get the caps yourself if you know how.

  2. Any other electrolytics in the circuit, especially any that perform coupling duties, once they start drying out and losing capacitance, you will hear a general "dulling" of the sound, and perhaps more noises that sound "static-y".

  3. Resistor value drift. Resistor noise is a MUCH bigger factor in the way an amp sounds than a lot of people think, especially signal path resistors. The ones that will drift the most though, are the ones that have to carry heavy current. The worst case, however, of resistor value drift that will occur over time is in the trimpots that adjust DC offset and bias current for the output transistors. The cheaper the construction of the trimpot, the worse it can be. DC offset will probably be a much more audible effect than bad bias, depending on how far off one or the other may be.

  If you check DC offset or bias, and try to adjust a really old, really cheap trimpot, my advice would be to verify that either needs adjustment, then turn the amp OFF, and try turning the pot, and cleaning it if possible, checking for smooth travel before turning the map back on to adjust. Also, mark you starting point before doing anything with a permanent marker, and reset the pot to that point before firing it up and adjusting. Yes, I learned that one the hard way. I was lucky in this instance though. I was going to correct the DC offset in an old NAD 3020, had it powered up, went to turn the pot, it resisted just a tiny bit, then it completely crumbled! And I mean completely!. The wiper came up out of the amp on my screwdriver tip, and the remainder cracked.

  BT
  
  
  

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  • general "dulling" of the sound - jensw 20:34:05 04/29/07 (1)
    In Reply to: Re: It can sound like increased noise or "hash" in the signal all posted by BrassTeacher on April 29, 2007 at 14:45:31:

    Thanks, to both of you.
    I think that is what I got (Yamaha M60) but also less resolution, if that is not the same. I will change the "signal" lyt caps, in L and do nothing in R. As it well could be that the amp just that sounds like that, I will then find out by changing things in only L.
    
    

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    • Well, if that fixes it, go ahead and do the R channel as - HenryH 21:39:45 04/29/07 (0)
      In Reply to: general "dulling" of the sound posted by jensw on April 29, 2007 at 20:34:05:

      well, because it's not far behind.

      
      Henry

      Mushrooms are the opiate of the mooses
      
      

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