[time-nuts] Phase noise measurement (was - no subject)

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Aug 23 05:38:32 UTC 2010


The noise measurements for the HPSs 5.1 preamp:
http://www.synaesthesia.ca/LNmeasurements.html

indicate that while the high frequency noise is about 2.2x lower than an 
optimised single ended 2SK369 preamp its flicker noise is far higher.
If one uses 5 2SK369's connected in parallel the flicker noise should be 
even lower whilst the high frequency noise will be comparable/
If the feedback resistor values are reduced perhaps 3 @SK369BLs will 
suffice.
Even lower flicker noise should be achievable if IF9030s are substituted 
for the 2SK369s.

Bruce

Bruce Griffiths wrote:
> http://www.synaesthesia.ca/LNschematics.html
>
> Is a better link, in that one can actually view the circuit schematics.
>
> There are a few simple refinements that will dramatically improve the 
> low frequency PSRR of the single ended JFET circuits in the HPS5.1:
>
> 1) split the 3k3 resistor feeding the green LEDs into into 2 series 
> 1k6 resistors and bypass the common node of these 2 resistors to ground.
> This low pass filters the noise current flowing in the LEDs due to 
> power supply noise.
>
> 2) It would probably be even more effective if the base of the cascode 
> transistor were driven by a voltage equal to the JFET source voltage 
> plus about 3.7V.
> It should, for example, be possible to use a selected JFET to do this.
>
> 3) The output servo should drive the noninverting input of the opamp 
> via a CBCS cascode (or equivalent) with a load resistor connected to 
> the input stage positive supply rail.
> This should improve the PSRR dramatically. I use something similar in 
> one of my low noise preamps albeit with a few LEDs in series with the 
> resistor to provide most of the voltage drop as in my case the 
> required voltage drop is reasonably predictable. This reduces the 
> noise contribution from the servo integrator.
>
>
> Bruce
>
> Bruce Griffiths wrote:
>> The link isnt particularly useful as guests cant view the attachments 
>> and registration is disabled!!!!
>>
>> Bruce
>>
>> dk4xp at arcor.de wrote:
>>>> Wenzel Audio Amp referred to in this email. Perfect! I drive with it a
>>>> 3561A and  a 7L5!  Works for me.  The only problem is getting any more
>>>> 2SK369.
>>>> Any recommendations?
>>> NXP   BF862, available from digi-key.
>>>
>>> I have used it in a similar hookup with good success. Its virtue is the
>>> low noise voltage AND low input capacitance at the same time.
>>> You could deploy MANY of them in parallel until you get
>>> into the capacitance range of a single Interfet device.
>>>
>>> One heroic effort for audio is here: 
>>> http://www.diy-audio-engineering.org/index.php?board=2.0  HPS5.1
>>>
>>> I currently use 3 pairs of SSM2210 in front of a AD797.
>>>
>>> regards, Gerhard
>>>
>>
>>
>>





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