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Thread: DC offset question.

  1. #21
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,700
    I'm Adrian.

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    Quote Originally Posted by farflungstar View Post
    This is my worry. I really can't say for absolutely sure what caused it (the techie said it was DC at the input) - but what other equipment did it? The problem came up (or was very apparent) when I bought that ill fated CDP and connected it direct to the amp using its inbuilt volume. That was returned but I cannot be 100% sure it was the cause.

    I checked the DC at the outputs of the truth pre after the techie told me about the DC - one channel (at the 200m setting) was 00.2 the other 01.7 - but had I checked it at the 2000 setting I don't know what that would have been. For this reason I contacted the maker (who said it was negligible and not likely to cause any issue) and adjusted the input buffers and output buffers to 0 on both channels using the 200m setting on the meter. In theory there is no DC in the circuit except perhaps the anomoly of bad probe contacts on the outputs when measuring.

    The truth is never switched off and I never connect or disconnect stuff when things are switched on. The only components are the cart, Vida, hashimoto, truth, ref 75, Tannoys.

    Is there a possibility that the small input tubes in the 75 could have an effect? It was soak tested using them at the centre without a problem but I have a brand new set of power and signal tubes.

    It's silly but I still haven't switched the amp on - I just dare not.

    The truth has caused no problems with the Hiraga amp but of course it's a different animal to the ARC.
    One more doubt - I bought the 75 second hand, and the CDP shortly afterwards - there is the possibility that the ref 75 was faulty when I bought it - this is the problem - so manyy variables I feel like dumping the lot.

    I just remeasured the pre outputs having rigged up an RCA plug to the meter probes. Right 000 left -000 - set at 2000m. At 200m I get right 00.4 left -00.2.

    I have replaced ALL tubes with new.

    Should I try it?
    Last edited by farflungstar; 04-05-2018 at 14:04.
    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: Apr 2012

    Location: London

    Posts: 369
    I'm james.

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    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?

  3. #23
    Join Date: Feb 2008

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

    Posts: 6,109

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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?
    If you have a digital multimeter, put it on the lowest value of the AC range And measure the AC voltage. Some multimeters can display the value of a frequency in terms on an AC voltage.

  4. #24
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,700
    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

  5. #25
    Join Date: Jan 2009

    Location: Essex

    Posts: 18,158
    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?
    Have you listened to this month's choice in the Album Club?

    Barry

  6. #26
    Join Date: Dec 2015

    Location: Alicante. Spain.

    Posts: 1,700
    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

  7. #27
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 45,999
    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 ....

    I've said it before and I'll say it again: democracy simply-doesn't-work
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  8. #28
    Join Date: Jan 2009

    Location: Essex

    Posts: 18,158
    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.
    Have you listened to this month's choice in the Album Club?

    Barry

  9. #29
    Join Date: Apr 2012

    Location: London

    Posts: 369
    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

  10. #30
    Join Date: Feb 2008

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

    Posts: 6,109

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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.

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