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    SP-14 to Class D amplifier

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    Fitero

    Posts : 21
    Join date : 2015-05-10

    SP-14 to Class D amplifier

    Post by Fitero on Thu Feb 22, 2018 7:06 pm

    I've had this preamp connected to several solid state amplifiers since building it a few years ago, but this week I tried something new for me;

    I hooked it to a balanced input only, class D amplifier. It was not happy! The speakers give out a fairly loud, high pitched squeal. I tried several wiring and grounding variations to no avail. OK. Next I tried placing a buffer that I made (Kuartlotron Buffer) between the preamp and the amp. This brought the squeal amplitude down quite a bit, but it's still there.

    The music sounds marvelous, despite the squeal, so I would like to resolve the issue.

    I've searched for appropriate buffers, but I don't know enough to pick the right one. I'd be grateful for any suggestions.

    Thank you.

    pigface

    Posts : 83
    Join date : 2011-04-10
    Location : pittsburgh

    Re: SP-14 to Class D amplifier

    Post by pigface on Thu Feb 22, 2018 9:18 pm

    I had  a similar experiance . When I built my SP14  I tried using it with  my Krell  FPB
    (Class A) through the balanced  inputs , nightmare !  I tried  different cables  then
    RCA to XLR jumper adapters . Both resulted in only obnoxious very loud
    noises , buzzing and screeching , from the speakers for a few seconds and then the amp
    shutting down and going into protection mode !
          I then tried a generic electronic balanced to unbalanced device  that was a only little
    better , at least I could hear music through  one speaker now then the other , one at a time
    and “less” hum and screeching  .  I think they all were creating a very bad ground loop
    resulting in the noise ?
         I finally had Roy build me a balanced to unbalanced adapter box that uses lundahl
    LL1527 ?  transformers in it .   Finally it works right , and sounds incredible , perfectly
    quiet . I couldn’t be happier . It sounds so much better than with my old solid state Preamp  .
         I don’t know  if this would work with a class D amp for you or not ?  Give Roy a
    shout  and ask him .
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    Fitero

    Posts : 21
    Join date : 2015-05-10

    Re: SP-14 to Class D amplifier

    Post by Fitero on Thu Feb 22, 2018 10:38 pm

    Thank you for the suggestion PF!!

    That rings true, as I've seen a similar scenario in a couple of Don Sachs' versions when he has used XLR inputs.

    I'll bug Roy now.
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    Peter W.

    Posts : 894
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: SP-14 to Class D amplifier

    Post by Peter W. on Fri Feb 23, 2018 10:02 am

    Fitero wrote:Thank you for the suggestion PF!!

    That rings true, as I've seen a similar scenario in a couple of Don Sachs' versions when he has used XLR inputs.  

    I'll bug Roy now.

    Back in the day, when solid-state was 'new' a number of amplifier manufacturers used interstage transformers to prevent just what you are describing. So, the suggestion is both apt and effective.
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    Roy

    Posts : 178
    Join date : 2018-01-14
    Location : Netherlands, Europe

    Re: SP-14 to Class D amplifier

    Post by Roy on Fri Feb 23, 2018 10:17 am

    Could it be your Dynaco pre amp is oscillating? Or perhaps that Class D power stage..
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    Fitero

    Posts : 21
    Join date : 2015-05-10

    Re: SP-14 to Class D amplifier

    Post by Fitero on Fri Feb 23, 2018 8:30 pm

    Interesting. There are cheap solutions available at the local Guitar Centers apparently, but I hate to waste money. A pair of good isolation transformers isn't to expensive if it takes away the noise.

    It's a VTA SP-14 preamp. The JBL/Crown amp has no trouble connected directly via XLR cable to my Logitech Transporter. I'll know soon enough!

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