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Old 20th February 2009, 02:30 PM   #1
rtill is offline rtill  United States
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Default distortion...mosfet

what would cause distortion in this circuit? is the bias not adjusted correctly?

It sounds powerful and great, but there is faint distortion.
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Old 20th February 2009, 03:02 PM   #2
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Default your schematic

has absolutelly no info .....

....to a schematic like that ( meaning that simple ) everything or almost everything may cause an ammount of distortion

the first stage will easilly work better with a ccs
c6 vas capacitor if not calculated properly depending on layout transistor selection rail voltage and many others could be a "sonic " killer and so on and on and on ....

it could be everything ..... so give us a proper schematic and others will give you far more sophisticated comments
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Old 20th February 2009, 04:23 PM   #3
rtill is offline rtill  United States
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let's try this...
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Old 20th February 2009, 04:37 PM   #4
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Default first a simple question

what did you use for ltp stage ????? 2sa827 or something else ???

then ....after constructing did you scope this ????? like do you have crossover distortion or other nature of distortion ????

or this is just another simulation ???
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Old 20th February 2009, 05:43 PM   #5
jaycee is offline jaycee  United Kingdom
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To be honest this circuit isn't particularly good. No emitter degeneration in the LTP, no proper current source for the LTP.

To be honest if you want a simple, good Lateral MOSFET amp, Rod Elliot's P101 is a good place to start. Personally I would replace the bootstrap with a current source, but that is just me.

If you're trying to use vertical MOSFETS (IRF types) with this circuit though, forget it... it won't work properly and eventually the outputs will overheat. If that's the sort you want to build, start by looking at Quasi's NMOS amps.
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Old 20th February 2009, 07:15 PM   #6
rtill is offline rtill  United States
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Yes, I know this design isn't the best, but it works.

Transistors used are...

TR1/2 - mpsa56's
TR3/4/5 - bd139/140's
TR6/7 - 2sj162/2sk1058

I do not have a scope.
Thanks
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Old 20th February 2009, 07:26 PM   #7
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Oscillation is possible.

Does R15 get hot? (would indicate oscillation)
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Old 20th February 2009, 07:52 PM   #8
rtill is offline rtill  United States
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no it does not get hot.
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Old 20th February 2009, 08:48 PM   #9
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Default Re: distortion...mosfet

Quote:
Originally posted by rtill
what would cause distortion in this circuit? is the bias not adjusted correctly?

It sounds powerful and great, but there is faint distortion.

The short answer is that it is TIM waiting to happen and it's 20 kHz THD is probably awful. It also likely has rather low slew rate.

To begin, the use of an un-degenerated BJT input LTP is a big no-no that should have vanished from design practice in the early 1970's if not sooner.

Secondly, I strongly believe that it is foolish to try to drive output MOSFETs directly from the VAS. An EF driver/buffer stage should be put in between. Some people will disagree with me on this point, however. Just because MOSFETs have "infinite" DC beta does not give the designer license to drive them directly from the VAS.

Sadly, this is the kind of simpleton design that some well-intentioned audiophile designer will fill with very expensive capacitors and think it then sounds better. Oh, I forgot the fancy milled aluminum front panel, the blue LED and the $5000 price tag.

Cheers,
Bob
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Old 20th February 2009, 10:52 PM   #10
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You might find a few pointer s in my amp cct.
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