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Thread: DC offset question.

  1. #21
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    Quote Originally Posted by Jazid View Post
    If you know of anyone with an oscilloscope then it might be worth looking for ultrasonics which might be there as Andrew has suggested? It seems a long shot but safety is always sensible. If not and you are worried then perhaps get a cheap charity shop type amp and use that as a burner to see if any issues arise?
    I wish I did. I don't understand oscillation, what could cause it?

    I'm going to open up the truth again tomorrow as fine adjust so I get absolute zero DC book matter is negligible.
    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

  2. #22
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,853
    I'm openingabottleofwine.

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    Quote Originally Posted by farflungstar View Post
    It's simple - Vida phono stage into hashimoto sut into Truth ldr pre into amp - no transformers. Though now I wish I still had the old slagle autoformers....
    Any DC on the output of the Vida will be blocked by the Hashimoto SUT. The Truth line-level attenuator would pass DC as it is a simple resistor L-pad; as would the Slagle autoformer.

    You have already measured the output of the chain to be free of DC (to within the accuracy of your multimeter), so as others have suggested check for any VHF oscillation.

    With the inputs to your valve amp shorted, what is the DC reading across the output to your speakers?
    Barry

  3. #23
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    Quote Originally Posted by Barry View Post
    Any DC on the output of the Vida will be blocked by the Hashimoto SUT. The Truth line-level attenuator would pass DC as it is a simple resistor L-pad; as would the Slagle autoformer.

    You have already measured the output of the chain to be free of DC (to within the accuracy of your multimeter), so as others have suggested check for any VHF oscillation.

    With the inputs to your valve amp shorted, what is the DC reading across the output to your speakers?
    The Truth is not like another pre - having seen inside it I truly do not understand how it works, I do know that it has a very wide bandwidth and is buffered both at input and output. This I could also see in the box, and is where adjustment pots for DC are located. There is nothing but light/or not between the two boards. I could see no circuitry for this though it has an extraordinary power supply. I won't put pictures without permission - and I don't want to go off topic - oscillation... Shorting the input at amps? Holy cow.

    There are 3 possible culprits for the damage, the Truth, The amp (maybe it was broken when I got it, or the CDP I bought and returned. All three are equally possible.

    Now oscillation.... Cry, slash wrists

    If I found someone with an oscilloscope what should be do/look for?
    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

  4. #24
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 99,005
    I'm Grant.

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    Think you won't be happy until you get another pre. And I don't blame you
    Regards,
    Grant .... ؠ ......Don't be such a big girl's blouse

    I've said it before and I'll say it again: democracy simply-doesn't-work
    .... ..... ...... ...... ................... ..... ..... ..... ..... .....
    FIIO K7 BT, M11 PLUS, BTR7, KA5 - OPPO BDP-103D - PANASONIC UB450 - PANASONIC 4K ULTRA HD TV - PIXEL 6 - AVANTREE LR BLUETOOTH - 2* X600 SOUNDCORE - HEADPHONES INCLUDE, FIIO, NURAPHONES', FOCAL, OPPO, BOSE, CAMBRIDGE, BOWER & WILKINS, DEVIALET, MARSHALL, SONY, MITCHELL & JOHNSTON - 2*ZBOOK'S- MERCURY BD ROM, ROON, QOBUZ, TIDAL, PLEX, CYBERLINK, JRIVER - MULTI HDD'S -

    Oh my god! There's nothing wrong with the bidet is there?

    “Nothing discloses real character like the use of power. It is easy for the weak to be gentle. Most people can bear adversity. But if you wish to know what a man really is, give him power. This is the supreme test. It is the glory of Lincoln that, having almost absolute power, he never abused it, except on the side of mercy".

    “You see these dictators on their pedestals, surrounded by the bayonets of their soldiers and the truncheons of their police ... yet in their hearts there is unspoken fear. They are afraid of words and thoughts: words spoken abroad, thoughts stirring at home -- all the more powerful because forbidden -- terrify them. A little mouse of thought appears in the room, and even the mightiest potentates are thrown into panic.”

    "You don't have free will. You have the appearance of free will.”

    “There's a war out there, old friend. A world war. And it's not about who's got the most bullets. It's about who controls the information. What we see and hear, how we work, what we think... it's all about the information!”


    ***SMILE, BE HAPPY***

  5. #25
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,853
    I'm openingabottleofwine.

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    Quote Originally Posted by farflungstar View Post
    The Truth is not like another pre - having seen inside it I truly do not understand how it works, I do know that it has a very wide bandwidth and is buffered both at input and output. This I could also see in the box, and is where adjustment pots for DC are located. There is nothing but light/or not between the two boards. I could see no circuitry for this though it has an extraordinary power supply. I won't put pictures without permission - and I don't want to go off topic - oscillation... Shorting the input at amps? Holy cow.

    There are 3 possible culprits for the damage, the Truth, The amp (maybe it was broken when I got it, or the CDP I bought and returned. All three are equally possible.

    Now oscillation.... Cry, slash wrists

    If I found someone with an oscilloscope what should be do/look for?
    If the Truth has buffer stages at both the input and the output and if they are DC coupled throughout (and the presence of DC zero adjustment pots suggests that it is), then mis-adjustment of these pots could easily cause DC to be present on the output. But your measurements have already shown that this is not the problem.

    Shorting the inputs of your power amplifier will not cause it any harm at all; it is no different to using a 'passive' pre-amp with the volume set to zero. It is unlikely your valve power amp is producing DC on the output, since the output is transformer coupled to the speakers, but a small amount of insulation breakdown between primary and secondary windings could occur. Unlikely, but worth checking.
    Barry

  6. #26
    Join Date: Apr 2012

    Location: London

    Posts: 685
    I'm James.

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    I wouldn't get too exercised Adrian, it's most likely that summat went/was wonky in the amp which has now been fixed, and nothing more. The amp has a clean bill of health, I presume from a reputable service agent?

    Re. Oscillation as I understand it it's a PITA that may pop up for all sorts of surprising reasons. Unity gain op amp circuits are certainly one possiblity as the OP amps are often unstable without gain (poor spec or faked ICs), another is unusual capacitive coupling effects at the input stage, and the interface between pre output and power amp input is a likely source of such potential mismatches. They are easy enough to find with an oscilloscope and a probe set up with a small sensing coil moved around the two boxes.

    One thing that should kill oscillation is bandwidth limiting, but you need to know where it is occurring and the approximate frequency. As an example you could then apply an appropriate inductor to dampen the excitation.

    Sent from my GT-I9195 using Tapatalk

  7. #27
    Join Date: Feb 2008

    Location: http://www.homehifi.co.uk

    Posts: 6,288

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    Two things that grabbed my attention are: the Truth is never switched off, and the engineer said it was DC at the input.
    If an amp or preamp is never switched off, one or more components will eventually fail. Very often it is due to the soldering on one or more components starting to break down. That in turn will lead to things like thermal run away, and components failure due to excessive voltages from any circuit part malfunction. Zener diodes and transistors are typical examples of that.
    There is no way any engineer can say that the output stage of an amp has failed because of DC at the input, unless that engineer has access to the equipment that connects to the amp and has tested them together to show that the DC at the input is producing such a large swing that the output fails. But even then, a steady state DC voltage of even 100mV or more would be well within the capability of the output stage to handle that offset. It would be the speakers that would more likely go faulty before the amp since speakers cannot handle a large DC offset for too long.
    Any qualified audio engineer who wishes to challenge me on that, please step forward.

  8. #28
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 99,005
    I'm Grant.

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    I would agree. Also if it's never off then it's connected and disconnected while on which isn't good practice
    Regards,
    Grant .... ؠ ......Don't be such a big girl's blouse

    I've said it before and I'll say it again: democracy simply-doesn't-work
    .... ..... ...... ...... ................... ..... ..... ..... ..... .....
    FIIO K7 BT, M11 PLUS, BTR7, KA5 - OPPO BDP-103D - PANASONIC UB450 - PANASONIC 4K ULTRA HD TV - PIXEL 6 - AVANTREE LR BLUETOOTH - 2* X600 SOUNDCORE - HEADPHONES INCLUDE, FIIO, NURAPHONES', FOCAL, OPPO, BOSE, CAMBRIDGE, BOWER & WILKINS, DEVIALET, MARSHALL, SONY, MITCHELL & JOHNSTON - 2*ZBOOK'S- MERCURY BD ROM, ROON, QOBUZ, TIDAL, PLEX, CYBERLINK, JRIVER - MULTI HDD'S -

    Oh my god! There's nothing wrong with the bidet is there?

    “Nothing discloses real character like the use of power. It is easy for the weak to be gentle. Most people can bear adversity. But if you wish to know what a man really is, give him power. This is the supreme test. It is the glory of Lincoln that, having almost absolute power, he never abused it, except on the side of mercy".

    “You see these dictators on their pedestals, surrounded by the bayonets of their soldiers and the truncheons of their police ... yet in their hearts there is unspoken fear. They are afraid of words and thoughts: words spoken abroad, thoughts stirring at home -- all the more powerful because forbidden -- terrify them. A little mouse of thought appears in the room, and even the mightiest potentates are thrown into panic.”

    "You don't have free will. You have the appearance of free will.”

    “There's a war out there, old friend. A world war. And it's not about who's got the most bullets. It's about who controls the information. What we see and hear, how we work, what we think... it's all about the information!”


    ***SMILE, BE HAPPY***

  9. #29
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    Quote Originally Posted by StanleyB View Post
    Two things that grabbed my attention are: the Truth is never switched off, and the engineer said it was DC at the input.
    If an amp or preamp is never switched off, one or more components will eventually fail. Very often it is due to the soldering on one or more components starting to break down. That in turn will lead to things like thermal run away, and components failure due to excessive voltages from any circuit part malfunction. Zener diodes and transistors are typical examples of that.
    There is no way any engineer can say that the output stage of an amp has failed because of DC at the input, unless that engineer has access to the equipment that connects to the amp and has tested them together to show that the DC at the input is producing such a large swing that the output fails. But even then, a steady state DC voltage of even 100mV or more would be well within the capability of the output stage to handle that offset. It would be the speakers that would more likely go faulty before the amp since speakers cannot handle a large DC offset for too long.
    Any qualified audio engineer who wishes to challenge me on that, please step forward.
    To clarify. The Truth should have zero DC at input and output - when initially tested there was a DC reading on the offending channel output AND a reading on both channels at the input buffers. This is now resolved. No DC.

    Yes it's kept on all the time, as per the makers advice - but nothing is ever disconnected unless it is switched off. (The truth has no on/off switch).

    There are very very specific rules for using the Truth which have to be followed. It cannot be operated without a source connection as this can cause a huge 10v DC surge through the output. I have obeyed this rule.

    There are less than half a dozen components and nothing like resistors or diodes. I'm waiting to hear from the maker about the oscillation possibility.

    The official ARC service dept said that DC on the input was the most likely cause, and that due to the amps balanced nature I wouldn't have heard anything to indicate it (a hum).

    I use CARDAS unbalanced to balanced convertors/connectors to attach the truth to the 75 - could they cause a problem?

    Also, the hashimoto sut is the X version which trades a few bottom Hz for a far more extended top end - I've heard SUTs can ring at high frequencies, could this be the oscillation?

    I still haven't switched the amp on. I've swapped out all tubes for new ones. I just stare at it. Even if I buy a cheap passive to keep me going I may be depriving myself of a stunning, and blameless preamp.
    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

  10. #30
    Join Date: Feb 2010

    Location: Maidenhead, Berks

    Posts: 288
    I'm Lee.

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    Why don't you connect up the Slagle to the ARC and some speakers and connect an mp3 player/phone to the Slagle and play some music. That takes everything else out of the equation. You could also open up the cdp and check if there are caps on the output (there usually are) and then you could try that as a source.
    Cheers Lee

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