[DIY] HEXFET AMP total disaster?

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#66
Hi. I have built this amp and noticed it has a problem of thermal drift. If you assemble the amp with no mistakes, incorperating the few mods added at a later date by Elektor it sounds great, but over time a DC voltage is meauresable at the output to the speaker. I studied the cause of this thermal drift and designed a pcb that thermally couples T1 T2 and T3 T4 together as suggested by Elektor, plus I coupled D1 D2 T5 T6 together. This appears to have cured the problem of maintaining the connection between T12 and T13 to within a few mV of zero. Great care was taken to keep the layout of the pcb as symetrical as the circuit. The result is a very stable and tidy amp that is well worth building.
 
#68
I think meanman 1964 should read and understand the content of my letter befor passing an opinion. I am aware that the T5 D1 are thermally coupled as are T6 D2. My letter is meant to imply that the revised PCB layout thermally couples all 4 devices together. This conclusion was reached after watching the DC voltage at the output drift when applying the warmth of a finger to either T5 or T6.
 
#69
Braidwood Audio said:
I think meanman 1964 should read and understand the content of my letter befor passing an opinion. I am aware that the T5 D1 are thermally coupled as are T6 D2. My letter is meant to imply that the revised PCB layout thermally couples all 4 devices together. This conclusion was reached after watching the DC voltage at the output drift when applying the warmth of a finger to either T5 or T6.
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Sorry not my itention.Can you give me the layout of your board.I've a few amps with probs?Are they still working well?
 
#70
Braidwood Audio said:
I think meanman 1964 should read and understand the content of my letter befor passing an opinion. I am aware that the T5 D1 are thermally coupled as are T6 D2. My letter is meant to imply that the revised PCB layout thermally couples all 4 devices together. This conclusion was reached after watching the DC voltage at the output drift when applying the warmth of a finger to either T5 or T6.
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can you post the new pcb layout to reconstruct it.
 
#73
egberttheone said:
So I typed "Giesberts HEXFET AMP" into google and guess what !? I got lots of hits with the same problem. The weird thing is that some blame that it does work; and other say it needs modification(s), then some one says that I need to place 100N in parallel with R30 & R31 to prevent it from oscillating. Because the resistors will work like a coil.
Sorry for my bad English.
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On your original circuit diagram T10 and T11 need a 120pf capacitor from collector to base.

I designed a very similar amp and that oscillated badly until I fitted those two capacitors.
 
#74

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#75
Judging by what I see there are some changes such as lack of 2 fuses but then there are four elements that seem SMT plus some resistors in series or parallel series perhaps to reach the correct value. the other is not in fact I even when I built this amplifier one of two plates worked great while.
 
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