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Thread: A visit to RJS, aka mixing it with the big boys.

  1. #21
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

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    Hi Barry, fire away with the questions my good man.

    I'll answer your last one first, yes things can go horribly wrong There are instances of speakers being blown when catastrophic faults occur, including with the old Croft OTL before Glen changed to regulated HT.
    I believe there is a member on here who has experienced a Croft OTL self destructing.

    With push pull OTL circuitry the HT is in the region of + and - 120 to 180V so the energy available under fault conditions is considerable. Most (some?) circuits have HT fuses which go some way to limit any fault current, but a better approach is to include a gas discharge tube across the output as well. This clamps almost instantaneously when the voltage exceeds the breakdown so offers a much quicker protection than just a fuse.



    This is the circuit of my low power OTL, 6W per channel into 8 ohms, the HT is actually a little lower than 145V.
    You can see that the output valve grid resistors are quite large, this is done to limit the fault current if one output valve fails.

    The overriding consideration is not to over stress the output valves, a good contender for output valves is the 6KG6 or EL509 as they have a large Va rating.

    With SE OTL circuitry there is an output capacitor, frowned on by the OTL purists, but as I have heard with the above ABSOLUTOR amplifier it doesn't seem to detract from the sound. The 6KG6 valves are all in a parallel cathode follower configuration, 3 per channel, with a choke load to ground and two 3300uF parallel output capacitors.

    The low dc resistance of the choke load would be beneficial under fault conditions, the HT fuse in this amp is 1.5A.

    Cheers,
    Alan

    PS The Absolutor amp/design isn't one of mine.
    Last edited by Firebottle; 17-07-2015 at 11:56. Reason: PS added
    I love Hendrix for so many reasons. He was so much more than just a blues guitarist - he played damn well any kind of guitar he wanted. In fact I'm not sure if he even played the guitar - he played music. - Stevie Ray Vaughan

  2. #22
    Join Date: Jan 2013

    Location: Birmingham

    Posts: 6,772
    I'm James.

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    WHO builds the Absolutor?!
    Main system : VPI Scout 1.1 / JMW 9T / 2M Black / Croft 25R+ / Croft 7 / Heco Celan GT 702

    Second System : Goldring Lenco GL75 / AT95EX / Pioneer SX590 / Spendor SP2

  3. #23
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

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    It's of Polish origin, see http://www.blog.absolutor.pl/absolut...lutora-2015-03

    Available in the UK through G Point Audio.

    (I've run out of beer, oh no )
    I love Hendrix for so many reasons. He was so much more than just a blues guitarist - he played damn well any kind of guitar he wanted. In fact I'm not sure if he even played the guitar - he played music. - Stevie Ray Vaughan

  4. #24
    Join Date: Jan 2013

    Location: Birmingham

    Posts: 6,772
    I'm James.

    Default

    Quote Originally Posted by Firebottle View Post
    It's of Polish origin, see http://www.blog.absolutor.pl/absolut...lutora-2015-03

    Available in the UK through G Point Audio.

    (I've run out of beer, oh no )
    Cheers Alan
    Main system : VPI Scout 1.1 / JMW 9T / 2M Black / Croft 25R+ / Croft 7 / Heco Celan GT 702

    Second System : Goldring Lenco GL75 / AT95EX / Pioneer SX590 / Spendor SP2

  5. #25
    Join Date: Mar 2011

    Location: Preston

    Posts: 197
    I'm Paul.

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    Rob's system is cracking. I heard a previous incarnation with the DAC Level 4 (I think) and the 'baby' 845 amps. Very good. I'm struggling to guess how it sounds better with those massive amps and the KIN pre etc.... There was nothing wrong as it was.

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