Simplistic NJFET RIAA

How you decided R2, R3 will be 10R without knowing the 369s IDSS? Came paired with resistors in some minikit?

Yes it came paired but no resistors. I do not remember why I bought the 10 ohm. Was a coincidence I think.

I think the 369 was 13.16 to 13,20 but I am not sure. Do I have to buy knew 369 measure and decide resistors ? Better lower R 2.3 by paralleling with 20 ohm ?

Gives 6,66 ohm Can I measure the voltage over the 10 ohm to see the current and modify the resistors if the current is to low ?
 
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diyAudio Chief Moderator
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Yes it came paired but no resistors. I do not remember why I bought the 10 ohm. Was a coincidence I think.

I think the 369 was 13.16 to 13,20 but I am not sure. Do I have to buy knew 369 measure and decide resistors ? Better lower R 2.3 by paralleling with 20 ohm ?

Gives 6,66 ohm Can I measure the voltage over the 10 ohm to see the current and modify the resistors if the current is to low ?

I was just wondering why you ended up with B+ below 33V. Its the resistors are higher vs. Idss than in the table.

If you will measure drop across them after the system gets warmed up it should indicate less than 9mA each?

There is one good thing with them 10R, the input Jfets will stay better put for bias with ambient delta in a small box especially.

A loss can be a dB or two of gain (will tell exactly when I will know IR2,3). That is balancing out them R2,3 bit more Johnson for || values than in the table also.

My suggestion is don't change it if your carts will be sounding strong enough. If you will ever feel you need a +1 to 2dB, parallel each with 27R since your Jfets are 13mA+ and readjust VR2x for the known procedure.
 
Hi alex, where did you get your 2SA970's, and did you measure the HFE? I'm wondering at what point the better linearity ceases to be a good trade-off vs lower HFE. If I could get real 2SA970's with HFE as high as a typical BC560 I might try it.

Hi nezbleu,
I purchesed 20 from here:
2SA970BL PNP Audio Transistor [180-1070] - $0.35 : Surplus-Electronics-Sales.com, Your source for hard to find parts!

About my measurements: unfortunately, it's not a straight forward answer.
I have two identical cheap Chinese DVM testers with hFE. One was showing 310-360 on these 20 trans. Second came to 420-480.
To witch one to believe - I do not know...
So, I choose the strongest sets for my Folded (abut 470 on strong reading DVM).
BS560C (from GB kit) came to 600-620 on my high reading DVM.
I did not see any reason why not to try these 2SA970's since Salas advised for 350 and higher...
 
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Hi Salas

Today I had a misfortune and need your input to figure it out. For simplicity I will refer to the components in the folded simplistic pcb guide.

I recently recovered an old psu from previous (non folded 28v builds) and connected it to one of my latest folded builds.... At the time I was not aware it was just outputting 40vdc and that is not enough for a good regulation using 35v out from the shunts.... anyway it played ok for some days but this morning one of the channels had a severelly reduced output level. (note that I left it on during one week and everything was ok)

I opened the case and measured 29v drop over R11 so I had only 6v over the buffer fet.

I believe this fet is gone bad and needs replacement, but what I do not understand is why did it go bad..... this build was done over one year ago and this only happened after I experimented with the low voltage psu...... can a low input voltage into the shunt produce anything bad that might burn the amp output buffer ?
 
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IMO either the buffer fet is leaking through drain gate (do not know if that can increase drain current) or the ccs fet went bunkers and exceeds it's original 6mA ccs current or the drain resistor abruptly changed it.s value... strange.

Even if the second stage amp is working correctly and has 10v on the drain, if the buffer Vdd is 6v, it's Vout (source) is limited to under 6v.
 
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diyAudio Chief Moderator
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I have seen 2SK170s failing for no apparent reason before. There are some weak ones it seems but its not often at all to see them fail randomly when used for under 100mW self dissipation. Still I remember one or two in a friend's builds. He was using a rather heavy soldering iron characteristically, l don't know if that can weaken some for long run reliability or not after they start working OK nonetheless. Your suspect FETs should not show less than 35R across else they have failed shorted or partly shorted.
 
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Ok... but IMO the bad fet should be the ccs one because I have too much current on the buffer (double normal) ... what do you think ?

So this is fate.... nothing to do with low voltage on the psu....

Strangely I was preparing myself to demo it to a friend and during the night s... happens.

Luckily I had another build (with better specs :)) and already swapped the boards :)