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Old 22nd June 2009, 10:01 AM   #1
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Default Trouble with LM4702 amplifier

Hi everyone ,i have build amp based on AN1645 schematics ,i use IRFP9240/IRFP240 for out stage , bias is simple only a resistor and a condenser .
Everything works well sound was good and i like it, but after my mistake(i conect only Gnd and Vee with working 3VA transformer) everything works but apear a strange sound , may i broke in LM4702 or i do something with transistors
P.S. nothing were conected to out of amp ,and sources are conected between 0.22R 5W resistors.
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Old 24th June 2009, 10:56 AM   #2
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A strange noise is a very fuzzy description. When does it appear? How long is it? Intermittent or steady? How does it sound? Hum, buzz, hiss, changing in pitch?
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Old 24th June 2009, 12:25 PM   #3
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pacificblue - thanks a lot for reply!!!

strange sound it's a distorsion(fuzz) on high frequences ,that apears on mid of power .
strange is this that it apear when is mid power used.
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Old 24th June 2009, 12:34 PM   #4
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I wonder if the bias is changing as the FETs heat up during the loud bits?

Vfets require temperature compensation.
Resistor biasing cannot give temperature compensation. Resistor biasing suits Lfets.
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Old 24th June 2009, 01:24 PM   #5
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FETs are cold .
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Old 24th June 2009, 05:25 PM   #6
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Maybe you could also give a bit more information about your application. In the first post you wrote about a 3 VA transformer. What does that have to do with the amplifier? Are you shure that this noise was not present, before you connected that transformer? I. e. did you use the amplifier at that medium power setting before that accident?

Is the noise only present at medium power and disappears at higher power? Or does it appear from medium power upward? Does it rise in intensity with the power level?

Could there be a damaged smoothing capacitor in your power supply?
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Old 24th June 2009, 05:51 PM   #7
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OK.
i'm 100% sure that the noise apears after i do my mistake, because i listened to it fo 3 days.

Capacitors are ok because i conect it to ma LM3886 gainclone and all it's ok no noise.
And Yes this noise its only present at the medium power, and it present not in all kind of music when i'm listening to Rock or POP music or other dynamic musis it's not present but in Jazz or Clasical it very awful to listen to it. about tranformer i say because it's very powerful and i don't have ayn fuses or resistors to protect my amp.

In fact i can use it as poweful amp for disco.

it's very strange for me
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