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Thread: PL400 help

  1. #1
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    PL400 help

    I was given a PL400 series 1 few weeks ago, it's in absolutely perfect condition and after plugging it in and hooking to my test speakers it seemed to work great. It still has all the original outputs, drivers, and caps so I want to do some work to it and install the DC protection before I install this amp into my setup. I'm hoping to eventually turn this into the white oak PL400 but finances are a little tight right now so I figure I'm going to start with the cheaper upgrades like the power supply caps and DC protection and I think I read something about installing a 470uf cap and 2 diodes to prevent latching up. I just want to do what I can to prevent damaging my speakers. Any advice and info or links to good useful info would be greatly appreciated. I did a DC measurement on my speaker terminals and I did get a reading of 0.06 v on the left and 0.21 v on the right. Is this within spec?? Also I was thinking of increasing the value of the power supply caps but 1 website said doing this will cause problems, it's this true?? And for my last question, when I install the DC protection board do I put it before or after the zobel network?? (I apologize for not spelling zobel correctly, lol. Again thank you for your time and help.

  2. #2
    Administrator Gepetto's Avatar
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    Old 'Arn Enthusiast
    Quote Originally Posted by fastassford@gmail.com View Post
    I was given a PL400 series 1 few weeks ago, it's in absolutely perfect condition and after plugging it in and hooking to my test speakers it seemed to work great. It still has all the original outputs, drivers, and caps so I want to do some work to it and install the DC protection before I install this amp into my setup. I'm hoping to eventually turn this into the white oak PL400 but finances are a little tight right now so I figure I'm going to start with the cheaper upgrades like the power supply caps and DC protection and I think I read something about installing a 470uf cap and 2 diodes to prevent latching up. I just want to do what I can to prevent damaging my speakers. Any advice and info or links to good useful info would be greatly appreciated. I did a DC measurement on my speaker terminals and I did get a reading of 0.06 v on the left and 0.21 v on the right. Is this within spec?? Also I was thinking of increasing the value of the power supply caps but 1 website said doing this will cause problems, it's this true?? And for my last question, when I install the DC protection board do I put it before or after the zobel network?? (I apologize for not spelling zobel correctly, lol. Again thank you for your time and help.
    Your offset that you measured is quite high. It should measure in the range of -0.010 to -0.015V on each channel.

    You can increase the size of the PS caps with no issue.

    The DC protect board includes a Zoebel network which replaces the one installed in the amp.

  3. #3
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    Quote Originally Posted by Gepetto View Post
    Your offset that you measured is quite high. It should measure in the range of -0.010 to -0.015V on each channel.

    You can increase the size of the PS caps with no issue.

    The DC protect board includes a Zoebel network which replaces the one installed in the amp.
    To further clarify, the DC protect board mounts on the binding posts. New binding posts are provided as it can be difficult to reuse the originals.

  4. #4
    Forum Veteran Gibsonian's Avatar
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    You should be able to improve DC offset with adjustment of pots on the amp PCB.

    Do you have the manual?

  5. #5
    Early PL400C series pc boards used in 400 series 1 amps had dc offset pots. Later PL14's did not.

    Likewise, the PL0171 pc board used in 700 series I also had dc offset pots. PL14A, and PL20's used in 700B's did not.

    The PL36's pc boards used in 400 and 700 series II amps had no dc offset pots.

    Trial-and-error matching of the 1st or 2nd diff. pair transistors was used by Phase Linear on PL14's and PL20's in order to improve dc offset. NOTE: There is a simple technique of closely matching junction resistances between transistors whereby you can lower dc offset.

    Trial-and-error replacement of the LF356 or LF351's IC's on PL36 pc boards was used by Phase Linear in order to improve DC offset.

    Ed
    Last edited by oldphaser; 10-10-2018 at 05:22 PM.

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