John Curl's Blowtorch preamplifier part II - Page 3960 - diyAudio
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Old 20th May 2013, 02:47 PM   #39591
Bonsai is offline Bonsai  Taiwan
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It's either

5534 for lowest cost and some good all round tradeoffs
797 for superior noise performance
Discrete parallel single ended JFET input for lowest noise
LM49xx for lowest distortion and noise performance somewhat less that 5534

Take your pick. All require tradeoffs and none deliver #1 in everything.

1,2& 4 are sensible engineering solutions. 3 and others like it interesting design challenges and wha audiophiles apparently want and are willing to pay for.
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Last edited by Bonsai; 20th May 2013 at 02:52 PM.
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Old 20th May 2013, 02:52 PM   #39592
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Quote:
Originally Posted by vacuphile View Post
SY, JCX,

On JCX's point about the "heavy bias bjt input current noise flowing in the MM Z creates more Vnoise than the 797 vn" let me first straighten out a semantic point. The noise produced by current flowing through the MM Z is called excess noise, not current noise. From analogy with what is measured in real life resistors, I would expect a wire wound resistor made with defect free copper wire to have exceedingly small excess noise in the uA region we are talking about.
I'm afraid this is incorrect, 2pA/rt-Hz of current noise will make 2nV/rt-Hz of voltage noise in a 1K resistance no matter what the resitance is made of. Excess noise is the excess current noise resulting from a DC voltage applied to the resistor. Measurement of excess noise in a cartridge coil would probably destroy it.

https://dcc.ligo.org/public/0002/T09...rent_noise.pdf
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Old 20th May 2013, 02:55 PM   #39593
SY is offline SY  United States
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Scott, for this specific case, is current noise correlated or uncorrelated with voltage noise?
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Old 20th May 2013, 02:58 PM   #39594
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I prefer: 2pA/rt-Hz of current noise will make 2nV/rt-Hz of voltage noise in a 1K Z no matter what the impedance is made of
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Old 20th May 2013, 03:05 PM   #39595
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I stand corrected. It would be interesting to know how much the noise figure is changed with the "equivalent." Again, a few dB of noise floor is not earth-shattering, but why not make the very best if you can?
Because 'the best' is defined by more than one dimension.

Just focussing on noise is either an oversight or an attempt to tilt the discussion.

So what defines the very best? OPA211 or, with even better specs on paper, the LME49990? You will only be able to find out by using these opamps in actual circuits and measuring them and, if so inclined, listening to the results.
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Old 20th May 2013, 03:13 PM   #39596
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Quote:
Originally Posted by scott wurcer View Post
I'm afraid this is incorrect, 2pA/rt-Hz of current noise will make 2nV/rt-Hz of voltage noise in a 1K resistance no matter what the resitance is made of. Excess noise is the excess current noise resulting from a DC voltage applied to the resistor. Measurement of excess noise in a cartridge coil would probably destroy it.

https://dcc.ligo.org/public/0002/T09...rent_noise.pdf
Scott, you misunderstood what I posted.

I had already dealt with the current noise issue and a 1K Rs being slightly beyond optimum for the AD 797. A 1KHz resistor has about 4nV/rt-Hz thermal noise, dominating the 2nV opamp current noise, which adds just a bit over 10 percent to total noise here. I call that ball park right, good enough to let other design considerations prevail.

JCX was pointing to the excess noise created by the bias current in the cardridge, which you agree with me, is nonsense.
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Last edited by vacuphile; 20th May 2013 at 03:22 PM.
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Old 20th May 2013, 03:16 PM   #39597
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I understand that it is not the ultimate solution to cartridge loading but why not a variable capacitance switchable loading as I have on an older HK integrated amp? It has a normal position whatever that is and plus 100, 200, or 300 pf of switchable capacitance. Couldn't this option be used to add an adjust-ability to many phono preamps, perhaps with a low initial value and smaller steps for fine tuning?
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Old 20th May 2013, 03:22 PM   #39598
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Quote:
Originally Posted by SY View Post
Scott, for this specific case, is current noise correlated or uncorrelated with voltage noise?
Usually the correllation is weak at best, Van der Zeil covers this stuff in his books.

Yes jcx, that is understood.
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Old 20th May 2013, 03:41 PM   #39599
jcx is offline jcx  United States
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Quote:
JCX was pointing to the excess noise created by the bias current in the cardridge, which you agree with me, is nonsense.
your misinterpertation of my comment is where the nonsense came in - while I didn't spell out the bias current issue - I thought most would recall distortion is an issue with coil current biasing ferromagnetic cores - not that I really think it is a biggy when the MM transducer magnetics must have relatively large air gap compared to transformers and "large" input bias may be 1 uA

I completely distrust comments that uA bias currents in MC can have noticible magnetic effects - there is lots of air, usually fully saturated pole pieces, hi coercivity magnets giving near free space incremental permeability for any material within many times the diameter of the tiny moving air coils
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Old 20th May 2013, 03:43 PM   #39600
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Quote:
Originally Posted by vacuphile View Post
. From analogy with what is measured in real life resistors, I would expect a wire wound resistor made with defect free copper wire to have exceedingly small excess noise in the uA region we are talking about.
Only in the 0 to 50 Kelvin temperature range. Down there, defect free will lower the collision rate significantly.

Room temp, no.

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