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Old 11th December 2018, 09:06 PM   #591
shaan is offline shaan  India
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Quote:
Originally Posted by Medisinmannen View Post
Will add caps on my boards when i recive new pcb...
You can use 1206 series SMD caps on your existing boards for minimum footprint. 270pF 50V will suffice.
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Old 14th December 2018, 03:50 AM   #592
drgnanam is offline drgnanam  India
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Originally Posted by shaan View Post
Hello Members.

Since mid August I have been receiving a few reports of the amplifier setting up normally but running into oscillation and overheating as soon as the music volume is slightly increased. The sound is crackling and noisy with high current draw from power supply. No report of catastrophic failure, but it has been annoying the builders. Generally the suggested solution was to replace the 5W resistors with 2W ones, but even this didn't help a couple cases.

While I didn't face the same problem beforehand in my builds, recently after my last bunch of 5W resistors was exhausted I ordered another bunch. These were same value but from different manufacturer. And during tests was faced by the very same problem of oscillation with signal input.

I isolated the oscillation to the output section and tried whatever I could to make it go away. After a month of experiments, today evening it went away.

The culprit is the unequal gate-source capacitance of the N and P channel MOSFETs - the 2SK1058 has 600pF between gate and source pins while the 2SJ162 has 900pF.

Installed a 270pF 500V ceramic capacitor between the gate and source pins of each of the N MOSFETs (on the positive side of the board), and that's all. Now the amp is fully tame and calm. I cranked up the volume and gradually ran it right into clipping with music and no hint of oscillation in the scope. No crackle, no noise, no heat, just music, with the same 5W resistors that are very inductive.

I'm a bit embarassed because it's a very commonplace and effective trick. I wonder why it didn't struck me earlier . I understand that in an otherwise well sounding amplifier this kind of problems are quite unwelcome and I apologize to everyone who faced it during their build. In the next bunch of V4H boards there will be accomodation for the two capacitors.

Thanks for your time.
shaan
Hi shaan,
Does this problem occurs with V2 also, l had one board failure after set up , and another one looks OK , can you confirm this
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Old 14th December 2018, 05:41 AM   #593
shaan is offline shaan  India
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No I did not face the problem in V2. Unfortunately I don't have any V2 boards to test. The effect of these capacitors on sonics is zero and if anything they won't make the amp unstable. So if you are thinking of adding them to your boards then feel free to do so.
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Old 14th December 2018, 02:25 PM   #594
drgnanam is offline drgnanam  India
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thankyou
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Old Today, 06:07 AM   #595
Vunce is offline Vunce  United States
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I finally got a bit of time to start the V4H amplifier boards

Shaan,
From the start, I installed the 270pF caps to guard against potential oscillation like you had discussed.
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