mini-mosfet

PeteMcK · 15th May 2009, 12:26 AM

any comments on this amp, bearing in mind the designer's goals?

http://www.redcircuits.com/Page123.htm

"a minimum parts count, without sacrificing quality"

jaycee · 15th May 2009, 09:49 AM

As basic as you can get I think. SIngle supply, minimum parts count.

Thing is, you can get good parts for pennies which would let you build a much better amp for only a little more expense. The simplicity can only really be justified for absolute beginners.

Similarly, the preamp part, these days there is no point in doing that with discrete transistors - good opamps are cheap.

CBS240 · 15th May 2009, 01:26 PM

Hi

I think this is the type of circuit that would discourage a beginner. Unless the output DC bias is set to about 4A, there will be bias shift. But then, they would melt.

The comments note the pots are very sensitive to begin with. (where are the source resistors?) You could go with a lightly bias class B, but you sure would be hard pressed to get 0.02%. IOW, use laterals or thermal compensation is required.

woody · 15th May 2009, 07:19 PM

First the mosfets are an old part at least 30 years old. Being
in the T0220 case they will not handel much power. The high
thermal resistance of this case and no thermal compensation
in the bias circuite( that pot) makes for an accident ready to
happen. I sure wouldn't try it but if you realy want to I have
a pair of these mosfets that you can have for the price of postage
( offer only good to you if you live in the USA)

PeteMcK · 15th May 2009, 11:55 PM

thanks for the comments guys; 'absolute beginner' - that's me where amps are concerned. I was attracted by the simplicity.
I have a pair of 2sk1058/2SJ162 I want to try in a simple amp.
I thought that the way that bias pot was wired a bit dodgy.
Would some diodes in the bias chain make it a bit less touchy?

jaycee · 16th May 2009, 06:09 AM

If you have that pair then there are good solid circuits you can build that are only a little more complex in terms of parts count, but a hell of a lot easier to set up and use.

Take a look at the Ampslab lm60 circuit. That kind of topology works well for such mosfets.

If you want something with part types etc, then Dr Bora's "Sigma" amp is a good design. It shows two pairs of outputs but you can easily use one pair, and a lower voltage if neccesary.

http://bas.elitesecurity.org/sigmasema.pdf

15th May 2009, 12:26 AM	#1
PeteMcK diyAudio Member Join Date: Feb 2002 Location: Sydney	mini-mosfet any comments on this amp, bearing in mind the designer's goals? http://www.redcircuits.com/Page123.htm "a minimum parts count, without sacrificing quality" __________________ ‘today… there lives alongside the twentieth century the tenth or thirteenth. A hundred million people use electricity and still believe in the magic power of signs and exorcisms” Trotsky

15th May 2009, 11:55 PM	#5
PeteMcK diyAudio Member Join Date: Feb 2002 Location: Sydney	thanks for the comments guys; 'absolute beginner' - that's me where amps are concerned. I was attracted by the simplicity. I have a pair of 2sk1058/2SJ162 I want to try in a simple amp. I thought that the way that bias pot was wired a bit dodgy. Would some diodes in the bias chain make it a bit less touchy? __________________ ‘today… there lives alongside the twentieth century the tenth or thirteenth. A hundred million people use electricity and still believe in the magic power of signs and exorcisms” Trotsky

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15th May 2009, 09:49 AM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	As basic as you can get I think. SIngle supply, minimum parts count. Thing is, you can get good parts for pennies which would let you build a much better amp for only a little more expense. The simplicity can only really be justified for absolute beginners. Similarly, the preamp part, these days there is no point in doing that with discrete transistors - good opamps are cheap.

15th May 2009, 01:26 PM	#3
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	Hi I think this is the type of circuit that would discourage a beginner. Unless the output DC bias is set to about 4A, there will be bias shift. But then, they would melt. The comments note the pots are very sensitive to begin with. (where are the source resistors?) You could go with a lightly bias class B, but you sure would be hard pressed to get 0.02%. IOW, use laterals or thermal compensation is required.

15th May 2009, 07:19 PM	#4
woody diyAudio Member Join Date: Jan 2002 Location: Tyrone Ga. U.S.A.	First the mosfets are an old part at least 30 years old. Being in the T0220 case they will not handel much power. The high thermal resistance of this case and no thermal compensation in the bias circuite( that pot) makes for an accident ready to happen. I sure wouldn't try it but if you realy want to I have a pair of these mosfets that you can have for the price of postage ( offer only good to you if you live in the USA)

16th May 2009, 06:09 AM	#6
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	If you have that pair then there are good solid circuits you can build that are only a little more complex in terms of parts count, but a hell of a lot easier to set up and use. Take a look at the Ampslab lm60 circuit. That kind of topology works well for such mosfets. If you want something with part types etc, then Dr Bora's "Sigma" amp is a good design. It shows two pairs of outputs but you can easily use one pair, and a lower voltage if neccesary. http://bas.elitesecurity.org/sigmasema.pdf

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