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Old 6th January 2013, 12:02 PM   #841
RCruz is online now RCruz  Switzerland
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When you speak about oscilation I guess you mean output offset wondering.... as long as it is on the mV range, I guess it might be due to amplifying noise from first stage.

this build must be enclosed to minimize out put wondering.... once it gets under 10mV there are no audible penalties.

I believe this is the price to pay for high gain dc coupling.
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Old 6th January 2013, 12:21 PM   #842
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this build must be enclosed to minimize out put wondering.... once it gets under 10mV there are no audible penalties.


None of us are talking merely dc drift here. It is a fast and wild fluctuation, cannot imagine enclosing to have any effect -it's in the hertz region.

Don't know about others but i also see something which may be oscillation in the MHz region. In any case the hiss level is unacceptable and my guess is that it does indeed oscillate.

If i get to play with the board again i would remove all the decoupling lytics from the input stage, then reintroduce them one by one and add decoupling across. Alas, more interesting projects at the moment.

My interest in the issue is mainly academic - i am indeed curious why some builds work only marginally. This can never become my main phono pre due to the input current disbalance. I don't know about others but i have 3 wands for my arm with different carts, ranging widely in impedance. If i put the wrong wand in the arm a 2k cart may prematurely go to Paradise. It is an accident waiting to happen and sooner or later it will. Such a device cannot be sold to a paying customer at any price. Being a diyer does not also make me completely mad.

As for the sound, it is indeed quite pleasant keeping in mind the modest component quality. It certainly does not obliterate my other phono stages soundwise.
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Old 6th January 2013, 12:28 PM   #843
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I would be interested to know what you are comparing it with.
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Old 6th January 2013, 12:37 PM   #844
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Ananlog_Sa could you outline the cartridge risk due to the current imbalance, I have not seen this mentioned before.
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Old 6th January 2013, 01:45 PM   #845
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Such a device cannot be sold to a paying customer at any price. Being a diyer does not also make me completely mad.
Hard words for a non profit DIY project, many people spend there private time. I think you are old enough to decide build it up or not.

I left the DIY scene about for two years and saw the group buy late by random. Thus i read less about the concept only heard a predecessor in 2010 at a DIY festival. But never it was a secret for me about the risk of the design, for example oscillation.

We are DIY people and no plug and play customer, so we should solve the problems together.

To damage the cartridge it needs higher DC at the input. After trimming the input with a dummy, the offset on my board are zero with pluged cartridge.

Chris

Last edited by Take5; 6th January 2013 at 02:08 PM.
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Old 6th January 2013, 03:01 PM   #846
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Ananlog_Sa could you outline the cartridge risk due to the current imbalance, I have not seen this mentioned before.
No current should flow from preamp input into the cartridge coils. In other words, input bias current is not permitted. For this reason, JFETs at the input are much better than BJTs, like in SCP Vendetta Research.
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Old 6th January 2013, 03:23 PM   #847
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To damage the cartridge it needs higher DC at the input. After trimming the input with a dummy, the offset on my board are zero with pluged cartridge.

True. The problem is that once you change the cartridge to another with a different coil resistance you will have to repeat the offset trimming.

Failing to do this is very unlikely to cause damage to the coils but it will quite possibly cause permanent magnetization - a real pain in the backside. It is even considered a bad idea to measure the coil resistance with an ohmmeter.
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Old 6th January 2013, 03:25 PM   #848
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It is even considered a bad idea to measure the coil resistance with an ohmmeter.
Right and well, but who told you to do so ?
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Old 6th January 2013, 03:32 PM   #849
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But never it was a secret for me about the risk of the design, for example oscillation.

Then you probably had access to information which wasn't public.
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Old 6th January 2013, 03:42 PM   #850
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Right and well, but who told you to do so ?
Huh? A current many times exceeding the ohmmeter's current will pass through the coils if you connect the wrong cartridge. You may still ask who told me to do so, but i and probably many others don't listen to a single cartridge.

Sorry if i am a teeny bit surprised with this design decision. And judging by the posts above not all builders are even aware of such a danger. Maybe two years down the line a third party decides to plug their turntable into this...
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