Help! Anyone in Seattle area willing to take on a challenging F5T issue? - Page 2 - diyAudio
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Old 14th November 2013, 06:17 AM   #11
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by permaneder View Post
Yes
I'll look into how to best implement that tomorrow.
Thanks!
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Old 14th November 2013, 06:24 AM   #12
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I'll look into how to best implement that tomorrow.
Thanks!
Nothing easier than that. Remove them from the PCBs and solder them to the gate pins.
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Old 14th November 2013, 06:24 AM   #13
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by permaneder View Post
Nothing easier than that. Remove them from the PCBs and solder them to the gate pins.
Not sure if the leads are long enough to do that, that's what I'm going to check. I don't have the amp in front of me until tomorrow when I go to work.
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Old 14th November 2013, 08:27 AM   #14
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Originally Posted by 6L6 View Post
The wiring is actually very beautiful!
No, it is actually very terrible and could easily cause oscillation with a high-bandwidth design as this.
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Old 14th November 2013, 08:29 AM   #15
SirByrd is offline SirByrd  United States
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Originally Posted by powerflux View Post
No, it is actually very terrible and could easily cause oscillation with a high-bandwidth design as this.
Could you explain why it can cause oscillation? I'm not contesting your statement, just don't know much about this.
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Old 14th November 2013, 08:44 AM   #16
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<simplified>

Every mm of wire lenght is ~1nH. There is also some capacitance by example in the mosfet itself. A LC-network has a resonant frequency. increase either L or C or both and you'll lower that frequency.

So, by default, ringing will start to occur. If you move the resonant frequency up, there won't be enough bandwidth to amplify it to problematic proportions. Also, if the damping properties of the circuit (such as gate stoppers) are sufficiently high, each starting oscillation will stop. But some point the resonant frequency is low enough to be amplified by the semiconductors and damping will get seriously difficult. From there-on the probability of oscillation is too high to ignore.

Your used wire length is by 'rule of the thumb' and 'gut feeling' and 'experience' way too much. Ofcourse you can calculate how much is 'too long', but you could also take the safe route and prevent time wasted debugging.

more reading material http://en.wikipedia.org/wiki/Parasitic_oscillation

Last edited by powerflux; 14th November 2013 at 08:51 AM.
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Old 14th November 2013, 08:47 AM   #17
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by powerflux View Post
Every mm of wire lenght is ~1nH. There is also some capacitance by example in the mosfet itself. A LC-network has a resonant frequency. increase either L or C or both and you'll lower that frequency.

So, by default, ringing will start to occur. If you move the resonant frequency up, there won't be enough bandwidth to amplify it to problematic proportions. Also, if the damping properties of the circuit (such as gate stoppers) are sufficiently high, each starting oscillation will stop. But some point the resonant frequency is low enough to be amplified by the semiconductors and damping will get seriously difficult. From there-on the probability of oscillation is too high to ignore.

Your used wire length is by 'rule of the thumb' and 'gut feeling' and 'experience' way too much. Ofcourse you can calculate how much is 'too long', but you could also take the safe route and prevent time wasted debugging.
What would this manifest itself as? Will it prevent the amplifier from working? What do I use to measure if it is oscillating? I have an oscilloscope but not too much experience on it. I want to get the amp working first, then go from there.
Worst case I have to move the transistors all to one heatsink, but then I won't get as much power out of it in class A.
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Old 14th November 2013, 09:00 AM   #18
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HF Oscillation could be recognised in a higher-then-expected power draw with increased heat but no discernable other malfunction. LF osclilation can manifest as 'motorboating'.. Increased distortion is also a logical effect for both.

If you have an oscilloscope, connect it to the amplifier output. If what you see on the screen is not a steady, sharp line, there's noise and/or oscillation. If you can see a sinusoid waveshape on screen, it's most definitly oscillating. There are plenty osciloscope tutorials on the net to familiarize yourself with the sensible range to do your measurements in.

But first, get the amp working.
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Old 14th November 2013, 09:02 AM   #19
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by powerflux View Post
HF Oscillation could be recognised in a higher-then-expected power draw with increased heat but no discernable other malfunction. LF osclilation can manifest as 'motorboating'.. Increased distortion is also a logical effect for both.

If you have an oscilloscope, connect it to the amplifier output. If what you see on the screen is not a steady, sharp line, there's noise and/or oscillation. If you can see a sinusoid waveshape on screen, it's most definitly oscillating. There are plenty osciloscope tutorials on the net to familiarize yourself with the sensible range to do your measurements in.

But first, get the amp working.
Thank you for your input!
I built my last F5 (normal, stereo but dual mono, one chassis) with the output transistors on wires, and it worked, with no oscillation that I saw on the scope (had help checking that when I brought it up) so I'll be crossing my fingers that I don't have issues with these.
Should have more info tomorrow.
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Old 14th November 2013, 09:36 PM   #20
SirByrd is offline SirByrd  United States
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I pulled the transistors and measured them, the ones I tested seemed to be good, so I went ahead and swapped them to the right places. Now the trimpots are working as they should, just need to get the guts to bring it up past 20V and start biasing.
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