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GB for Salas Folded Simplistic Phono PCB

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Bksabath

Bksabath

Banned
Joined 2007
Second day of fault finding on my boards

Mea culpa not getting regulator working having missed the bit of the instructions regarding R3X and R6X
now that part is ok so one start to take out each component and double cek.

But voltage at node R14 R4 is no where near where it should be so got the lot out again and .... (big roll of drums like )...

What do you mean bias for 2SK369BL should be round 9mA?
I got less than 7 whit R2 and R3 = 1.1 Homs
Duff 2SK369 ? X4 ???????

Tried IDSS got 7ish mA (yes my meter also cost more than general purpose Oscilloscope)
(Current trough the drain whit gate and source shorted I am measuring 7 mA whit 1k resistor in series and 15 V supply is this correct way ?)
Link on how to measure go sumwhere else nice SCH for slightly retarded would be great help
No change by tweaking VR1 voltage at node R14/R4 7.6ish V Q3 not doing much at all
Yes changed a few Led around (had same 1.7 V in the draw) but still no change
Thing is working in a way as I can ear same music whit volume right up
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
7mA IDSS is suspicious for K369BL. 8-16 mA for BL says Toshiba. Use 10-15mA IDSS samples with 3-11 Ohm source pin resistors so to get 9mA through each in circuit operation says the FSP construction PDF. When having that, vary the rail voltage with VR2x until you have proper TP readings says in the biasing section. VR1 is for vernier and RC filtering. You may as well let it be halfway.

Better use 11V PSU and 100 Ohm Drain resistor in your IDSS testing because you drop too much voltage now and that maybe restricts full IDSS.
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
Not the same if try K170s. DC current bias you may achieve right with proper IDSS, but gain will go lower and noise will go higher. Not a tiny bit out, really off spec. Boards are thick and best quality there is. No sense to skimp there, then the branded audio caps and macho RCA connectors people choose will snob the boards and they will never cooperate.:D
 
Bksabath

Bksabath

Banned
Joined 2007
369 are ok at 10.8 to11.2 mA
More fault finding.... I am sure I have done same stupid thing
Got RCA at the output 5 pin plug for floating cart connections and decent braided screen over the top on inputs

IMO RCA PHONO on PHONO is bad and should be banned

Then again no plugs is even better ...weld cable straight on legs of Q1 Q2

Then spend loads on fancy caps

But then again me stupid
 
Bksabath

Bksabath

Banned
Joined 2007
One board working

R2 and R3 where wrong so was testing method

Those 2 are IMO most important resistors on the board just a few Oms out was enough to stop the circuit from working

I wonder what the effects of trimming those to perfection could be and how sound may change as a result
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
Bksabath said:
One board working

R2 and R3 where wrong so was testing method

Those 2 are IMO most important resistors on the board just a few Oms out was enough to stop the circuit from working

I wonder what the effects of trimming those to perfection could be and how sound may change as a result
Click to expand...
That is why there is a look up table to match IDDS & R2 R3 resistors values in the PDF guide.
 
Salas

Salas

diyAudio Chief Moderator
Joined 2002
Paid Member
Nothing much. Its a DC bias control vs resistors added noise compromise to run those FETs at about 80% IDSS. The Yfs is already high there. They tend to thermally drift quite a lot anyway before any seriously high resistor value enters the source pin circuit to control them better with more local feedback. That is added noise affordable in the MM configuration only nonetheless. The other option was to use active CCS load instead of R13 so to fix them but that would shoot its current noise right in the heart of them. Not a compromise I would like in the MC configuration. It proved in many copies that it works acceptably with passively sourced current bias as is with least input noise and complexity.
 
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