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Old 13th August 2008, 08:26 PM   #1
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Default differences between these class A amps

hi..we build mj amps with friend..they are risky amps because of high bias and balance risk..the mosfets can burn if there is sth is wrong.for example not quite matched small transistors..

some audophiles says mj amp is a very good amp.
as Nelson says the best amp is a piece of wire with gain i decided to build jfet version.it only has 4transistors.2 less pieces than mj amp

do you have any idea of differences between these amps?which one can sound better?what are the advantages and disadvantages of these amps?
thank you
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Old 13th August 2008, 08:29 PM   #2
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Default this is i want to build

here its the circuit i want to build..its named grandmos silver design..
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Old 13th August 2008, 08:31 PM   #3
lineup is offline lineup  Sweden
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difference
between your one and what?

I need at least two (2) Class A to compare

mj amp .. what is?

Regards, Lineup
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Old 13th August 2008, 08:32 PM   #4
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i posted it ok now
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Old 13th August 2008, 08:39 PM   #5
lineup is offline lineup  Sweden
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Okay.
Now you have posted 2 circuits.

I say there is not much difference, in basic idea.
Apart from one has got bipolar and the other jfet input.
( Which can make a great difference, really, when comes to distortions harmonics spectra. )
Both are complementary with similar output stage (output from DRAINS).

Generally these drain output power amplifiers require some care
to watch the BIAS CURRENT in output.

As you are using Class A operation, this is less of a problem.
The output temperature and current will stabilize after some time after switch ON.

I recommend good & big heatsinks, as this further makes the output power transistors
run with less temerature differences at different output levels
and different room temperatures.


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Old 13th August 2008, 08:53 PM   #6
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ok i see..but is it good to feed the output transistors with only 2little jfets? someone says its not good to get much gain with only one transistor..they say its better to cascade the small transistors..but nelson says less parts is better..i dont know what to do :S
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Old 13th August 2008, 09:13 PM   #7
lineup is offline lineup  Sweden
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Quote:
Originally posted by umut1001

is it good to feed the output transistors with only 2 little jfets?

someone says its not good to get much gain with only one transistor..

they say its better to cascade the small transistors..

but nelson says less parts is better..

i dont know what to do :S

Cascoding will NOT make more gain. But can reduce distortion quite a bit, if you make the cascode in a good way. Sometimes is not much to improve by cascoding. Other times it is.

Nelson Pass has designed so many cascode amplifiers.
He is the one that knows better than ANYONE else here WHEN to cascode and WHEN NOT. Cascoding makes more parts (less simple) but changes not the number of gain stages or the gain.


I say you try to get Nelson Pass to advice you.
Hope this topic will get his attention
Because there is almost no other to better give you advice on:

1. Class A
2. MOSFET
3. Cascode



============================


About using 'only' two jfets to drive those lateral MOSFET,
2SK1058 and 2SJ62.

1. Use 2SK170BL & 2SJ74BL
The BL will have higher current operation = more gain!

2. When using less gain we get less feedback to correct the output signal.
Another way to say it, less open loop gain gives less negative feedback and a bit higher THD distortion


Now, the Grandmos Silver Design is one Low Global Feedback amplifier. ( It is not a ZERO Global Feedback (no feedback))
It is done on purpose!

Using Low Global Feedback, is something I often do. And also Nelson Pass
This can give another (more pleasant)sound and change types of distortions,
even if this distortion is at a higher level, than for a normal High Level GNFB amplifier ( Global Negative FeedBack ).


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Old 13th August 2008, 09:31 PM   #8
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thanks for interest Lineup!i am an electronics engineer and my proffesor said that cascading reduces distortion i remember nowwe listened to mj amp(cascaded one) it sounds really good..but i want to make simpler and not loosing any details i bought 12pairs of 2sk170bl and 2sj74bl matched transistors today..i hope i receive them in 2weeks..

i hope Nelson comes and reads this thread..and says advantages and disadvantages.

thanks a lot
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Old 13th August 2008, 09:48 PM   #9
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by umut1001
thanks for interest Lineup!i am an electronics engineer and my proffesor said that cascading reduces distortion i remember nowwe listened to mj amp(cascaded one) it sounds really good..but i want to make simpler and not loosing any details i bought 12pairs of 2sk170bl and 2sj74bl matched transistors today..i hope i receive them in 2weeks..

i hope Nelson comes and reads this thread..and says advantages and disadvantages.

thanks a lot

in that case just make F5 ;

it's certainly simpler

and probably better
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Old 13th August 2008, 10:01 PM   #10
lineup is offline lineup  Sweden
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Quote:
Originally posted by Zen Mod

in that case just make F5 ;
it's certainly simpler
and probably better
well worth a try, umut1001
... 100-reds and 100-reds of F5 builders can not be all wrong. Can they?

Schematic and instructions by Nelson Pass:
http://www.firstwatt.com/downloads/F5-om_sm-080527.pdf
http://www.firstwatt.com/downloads/F5-om_sm-080527.pdf

Schematic:
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