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xrk971 16th January 2017 06:05 AM

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Very strange is that I don't really detect anything different in the listening when the FFT supposedly goes berserk. It may just be a quirk of the UCA202. My Focusrite interface card should be coming soon, so maybe see if things improve then.

I was curious and tried using REW (speaker meas software) and looked at HD plots with volume pot in "good position" vs "bad position" and the HD plots don't look too different. There is a problem with rising 2nd harmonic peak though in the higher frequencies.

Salas 16th January 2017 06:18 AM

That's weird inconclusive stuff indeed. Try the moderate 4.7k R1 step when using Rew for now and wait for some alternative Jfet package to reference against I would say. Have you got any 2SK170s ?

xrk971 16th January 2017 06:23 AM

Yes, I have 2sk170's I can try but you would think matched monolithic sk389's would be the cat's meow here. The little solder pads for the metal can are very small and I would prefer not to have to remove that little metal can package for fear of damaging the pads. Although Teabag PCBs have extra thick copper so may be fine.

Salas 16th January 2017 06:31 AM

uPAs are monolithic again so maybe after having a couple of those. 2SK170s could give an answer when installed on uPA pads, but without de-soldering station maybe you are right to be cautious about iterations.

Salas 16th January 2017 06:43 AM

As for ever removing the LSKs better cut them at board level, they got plenty of lead length to remain usable. You can go clean the little pads with a manual de-soldering pump on one side and the iron on the other whenever, later. Because the plastic alternatives all go to the uPA pads. They have drain gate source marked for each side JFET's pins already.

RCruz 16th January 2017 11:46 AM

Hi Salas

Can I use 8mA Idss BF256B ?

Is J3 identified on the board as Q5 ?

Salas 16th January 2017 12:25 PM

Yes & yes but take it down to 4.5-5mA with R3

RCruz 16th January 2017 01:07 PM


My sim does confirm that... thank you :)

RCruz 16th January 2017 01:40 PM

can I use 10V zener for Za ?

Salas 16th January 2017 01:45 PM

I haven't tested such voltage there but it may work when the relay has enough difference across. You try it and see if in hand, but if to buy get the tested one.

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