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Old 23rd September 2007, 04:07 PM   #1
hihopes is offline hihopes  South Africa
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Default Mosfet mods

I have really tried to sort this out without asking for advice, but I donít know where to turn any more. I am doing everything I can think of to linearize/stabilize the mosfets in my hybrid amp. So far I have done the following:-1) Fiddled with gate stopper values. 2) Put caps right on the drain pins where the power supply is connected. 3) Output inductor 4) Zobel There has been a lot of improvement from these mods, but there is still a slight distortion apparent in certain passages with complex waveforms (FWIW, the biggest improvement I noticed was due to the PS bypass caps).
Can anyone suggest anything else I can try?
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Old 23rd September 2007, 08:16 PM   #2
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what kind of MOSFETS are they?
what are you driving them with?
class A or AB?
gotta schematic?
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Old 24th September 2007, 03:09 AM   #3
Leolabs is offline Leolabs  Malaysia
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Lateral or Hex???
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Old 24th September 2007, 08:09 PM   #4
hihopes is offline hihopes  South Africa
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Exicon 10n16 and 10p16 lateral mosfets. Single supply rail, class AB I think.
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Old 24th September 2007, 08:51 PM   #5
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Many possibilities.
Quote:
there is still a slight distortion apparent in certain passages with complex waveforms
Can you describe the distortion?
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Old 24th September 2007, 09:52 PM   #6
hihopes is offline hihopes  South Africa
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The distortion is like a kind of haze/buzz around piano notes. It varies in intensity according to the frequency of the note. It is not a loud/obvious distortion. You have to put your ear near the speaker to hear it. It is also not noticeable most of the time, but there is a particular passage of piano with a lot of reverb that I am using as a reference to test my mods. (It isn't the recording. My Hart Linsley Hood amp which uses the same mosfets doesn't produce the haze in this passage)
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Old 25th September 2007, 01:10 AM   #7
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This is crossover distortion caused by a mis-biased AB output stage.
Piano brings this out more than anything.
can you post a schematic? Voltage readings would help too.
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Old 25th September 2007, 07:29 AM   #8
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As diy4 suggests, need to see the circuit.
I've also seen oscillation cause haze/fuss on certain types of sound. Piano notes have fast edges and this can set it off, especially into capacitive loads. Don't put a cap across the output as a test unless you've got some way to protect the output stage from melt-down.
If it is instability then there are many potential causes, some of which have nothing to do with the schematic. Ground paths are a notorious cause.
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Old 25th September 2007, 08:48 PM   #9
hihopes is offline hihopes  South Africa
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I will see what i can do for a drawing with voltages. In the meantime I can tell you it is a single supply rail, so has DC blocking output caps. (Phillips 4700uF bypassed with 0.47 polyprops) I have put a zobel, 10r and .15uF polyprop, after the caps and before the inductor. I may remove the zobel but i am just giving it a chance to settle first before making a decision on that.
I am also thinking that it may be some sort of ringing that i am hearing which is being triggered by complex waveforms. If it is crossover distortion, then why is it so seldom audible?
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Old 27th September 2007, 06:57 PM   #10
hihopes is offline hihopes  South Africa
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Thanks for the input so far. My clumsy fingers and unfocussed mind have struck. I have blown something (probably a transistor). I forgot to drain the caps before working on it and shorted a cap into earth. Will post again when I can fix it and measure it.
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