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Thread: DC offset question.

  1. #51
    Join Date: Apr 2012

    Location: London

    Posts: 685
    I'm James.

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    Those figures (assuming 0-2mV) aren't scary and are ordinary enough for kit at start up. I'd say you have found that the Vida isnt the obvious problem. The truth is currently behaving properly so no issue at present there. We are left with ultrasonics which are a possible cause but difficult to spot without an oscilloscope, a glitch caused by the unstable behaviour of the Truth, or a dodgy cd player which is no longer available.

    Oscillation is problematic as it is likely to be generated at the interface between two components so swapping kit may hide rather than eliminate it as a cause. If the cd player didn't behave into your amp, or your Hiraga, or another amp, then it is a possible cause, but why it was misbehaving we'll never know for sure.

    I agree with you that your best course for future safety is a capacitor coupled interface, tvc, buffer, trafo, pre, or whatever. The Truth maybe should be sold or mothballed for whenever you live in a zone without power cuts, have a cap/trafo coupled power amp.

    Sent from my GT-I9195 using Tapatalk

  2. #52
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    Quote Originally Posted by RothwellAudio View Post
    Why is the output of a phonostage being fed into a step-up transformer? Surely the output of the step-up transformer should be fed into the phonostage?

    My suspicion, though this is just speculation, is the The Truth is a bit of a wacky "minimalist/purist" design using video op-amps running at unity gain and in order to use the minimum number of components there is no DC blocking cap or series resistor at the output. A series resistor of about 100 or 200 ohms is considered good practice because it isolates the op-amp from capacitive loads which can send the op-amp into oscillation. An examination of The Truth in isolation might show no problems but when the interconnecting cable and the input to the ARC amp are added there may be enough capacitance to send the op-amp into oscillation.
    Anyway, that's just guesswork, but it is a possibility.
    Jajaja - no of course the hashimoto feeds the Vida, the Vida the truth. Sorry about that.
    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

  3. #53
    Join Date: Apr 2012

    Location: London

    Posts: 685
    I'm James.

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    I'd assumed that was what was happening, Hashimoto into Vida into Truth... Either way the dc doesn't look like coming from that end of things.

    Sent from my GT-I9195 using Tapatalk

    Oops, posts crossed

  4. #54
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,848
    I'm openingabottleofwine.

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    Quote Originally Posted by RothwellAudio View Post
    Why is the output of a phonostage being fed into a step-up transformer? Surely the output of the step-up transformer should be fed into the phonostage?
    My thoughts exactly, the SUT would probably run into saturation, and whilst it would block DC, there would be a risk of magnetising the core. But I assumed Adrian had described his system accurately.

    It's difficult to find much technical information about the Truth, but their claim that the input impedance is 'near infinite' does suggest the use of op-amps.
    Barry

  5. #55
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    Quote Originally Posted by Barry View Post
    My thoughts exactly, the SUT would probably run into saturation, and whilst it would block DC, there would be a risk of magnetising the core. But I assumed Adrian had described his system accurately.

    It's difficult to find much technical information about the Truth, but their claim that the input impedance is 'near infinite' does suggest the use of op-amps.
    There are no opamps in the box. And yup I wrote it arse about tit. Sorry.
    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

  6. #56
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    One more question, please humour my stupidity.

    I have nulled the DC at output to 00.0 but this changes depending on volume position (voltage applied to the light source LED), to -01.7 at full output, the same happens when the balance control is used full turn.

    I am assuming it is measuring a negative voltage, or that ground is actually now carrying voltage. Is this correct? If so might this be bad for the connected amp?
    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

  7. #57
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

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    1.7 mV, positive or negative is very low and should be no problem for any amp.

    The ground doesn't carry a voltage.

  8. #58
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    Quote Originally Posted by Firebottle View Post
    1.7 mV, positive or negative is very low and should be no problem for any amp.

    The ground doesn't carry a voltage.
    Thank you, I just don't get why the volume/voltage pushes the meter from null into a negative reading when increased (it powers the led).

    But I agree 1.7mv is tiny.
    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

  9. #59
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,848
    I'm openingabottleofwine.

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    Quote Originally Posted by farflungstar View Post
    Thank you, I just don't get why the volume/voltage pushes the meter from null into a negative reading when increased (it powers the led).

    But I agree 1.7mv is tiny.
    Without seeing the circuit diagram for the Truth, it is difficult to say for sure, but I suspect the LED and the buffer amps might share the same power
    supply, which is not, IMO, a good idea.
    Barry

  10. #60
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,885
    I'm Adrian.

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    I have emailed Audio Research about compatability whey the Truth and the jfets input stages of the ref 75. The Truth is measuring perfectly but ARC's topology may be an issue - let's see what they say.

    Meanwhile I've bought and am waiting for a Luxman AT3000 TVC to replace the Truth, so I can finally switch on the newly returned 75 without worry.
    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

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