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Old 18th January 2003, 10:35 AM   #1
plp is offline plp
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Default Minimalist A/B fet amplifier

Has anyone a link to a good quality all fet/mosfet minimalist class A/B amp? Something around 60watt into 8ohm.

Also, has anyone had any experience with the original Trans Nova amps designed by Jim Strickland some years back? Not the later Hafler amps. I was wondering if anyone has actually heard one of these amps in comparison with more conventional designs.
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Old 18th January 2003, 11:35 AM   #2
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I described one in detail here on diyaudio you may have already seen. My first ever Class A amp.
It is minimalist in that it has only 2 fets in the signal chain per channel (50W class A). I'm reluctant to mention it though because everybody around here is probably sick of me going on about it by now. It sounds ok though.
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Old 20th January 2003, 04:41 AM   #3
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First off, study this follower circuit:

http://www.audiodesignguide.com/my/Cool_Follower1.GIF

The outputs in the original design were three pair of the small plastic TIP41/42 type, replace these with one pair of Semelab BUZ900/905. The one pair will be more than enough for 60W/8R or 100W/4R. The input pair may be any small signal type suitable for 10mA or so, the MPSA06/56 would be fine. Use 15V for the supply for the input pair and 1200 ohms for the bias resistors (instead of the 390 ohm), use a pair of 100 ohm trim pots (instead of the 0.18 ohm). The BUZ need no emitter/source resistors. The capacitor need only be a 1F (or larger) film type across the gates of the outputs.

Hook the output of this follower straight to ground, since the sources of the BUZ are the case this is easy, just use no insulator and ground the heatsinks!

To convert to a TransNova output stage all we need is a 5K input resistor to the bases of the input pair and a 100K feedback resistor from the same point to the center tap of the power transformer. We will need a regulated 15V supply for the input pairs, one supply to drive as many channels as needed. We will need a 40V supply (30-0-30V 150VA or larger), one per channel is needed.

Set the 100 ohm pots to about 25 ohms, then turn on and adjust for the idling current desired in the output stage. After the temperature and bias stabilize (just turn up the bias until the heatsink is as warm as you want it to be, the actual value is unimportant) then trim these again for DC off-set.

This circuit will only work for lateral MOS like the BUZ or the J50/K135 type. IRF type will blow up.

The amount of power in class A is only limited by the size of the heatsinks, and moves into class AB for loud peaks only.

If BUZ 900D/905D are used with minimum bias (100mA) and suitable heatsinking and supply voltages are used, 250W in class AB is possible out of only one pair of outputs.
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Old 20th January 2003, 06:43 AM   #4
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djk,

How I wish you could post a schematic of this circuit you are talking about as well as for many of your other insightful posts.
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Old 20th January 2003, 07:16 AM   #5
halojoy is offline halojoy  Sweden
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Arrow Some links - Designed by: Andrea Ciuffoli

Quote:
Originally posted by Samuel Jayaraj
djk,

How I wish you could post a schematic of this circuit you are talking about as well as for many of your other insightful posts.
Designed by: Andrea Ciuffoli

On this page you have the "Cool Follower 99"

And several other of Andrea Ciuffoli interesting Amp Ideas.
Both tubes & solid state

This is probably one of WWW most interesting Sites For Amplifiers

Thanks, djk
for giving us the hint - and remind us
------------------------------------------------------------------

- Also remember that a Class A PUSH-PULL is TWICE as effective
as a Class A with Current Source.
- Or half the heat - for same output WATTS. = half as big Heatsink
- Or +3dB, double output power - for same heat.

- for whatever those 3 extra dB is worth
Can be just as well to find a Speaker with extra 3dB Effiency.
Or why not both!

/halo
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Old 21st January 2003, 10:23 AM   #6
djk is online now djk
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Samuel Jayaraj :
How I wish you could post a schematic of this circuit you are talking about as well as for many of your other insightful posts.

I can really use a new computer. I bought a scanner with the idea of posting hand drawn schematics, but I need a newer computer to run it. But my 16 year old car is also demanding attention.

halojoy:
Andrea Ciuffoli

I really enjoy Andrea's site. Converting the follower to lateral MOSFETs allows for a higher input impedance and makes the design stable over temperature. Making it a TransNova output stage with voltage gain without adding any extra semiconductors is kind of trick!
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Old 21st January 2003, 11:03 PM   #7
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Folks,

while browsing the web I came acros this qute FET amp. I've build it and it works fine when you can control oscillations.
I now use it for my small studio monitors while mixing recordings.

You HAVE to build this as described on Aren's page.
Aren's FET amp

Let me know what you think...
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Old 12th February 2003, 04:39 PM   #8
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Default minimalist a/b amp

Here is my minimalist class a/b amp. Does a bit over 50W. Distortion @ 1w is 0.25% second order. The bias ciruit and power supplies are not shown. Insert your favorite temperature compensated bais circuit. Needs +/- 40V and +95V (not much current @95v) . Input impedance is rather low, could add a input buffer like the one used on the Zen4, but then it would not be minimalist. I have the output bias set @ 20mA. Could go higher and run more class A but then I would need bigger heat sinks and I really can't hear the difference anyways.
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Old 14th February 2003, 10:27 AM   #9
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SumTingsBurning,

sorry for being sceptical, but have you measured those data on a working equipment implemetning the shematic you have enclosed to your letter?

It does not utilize any global feedback, neither a Sziklai output configuration wich means that the last stage would do a lot of distortion. (It is neither class A...) If things were so easy...

Please correct me if I am wrong, but if this amplifier is high fidelity then I have to start everything from scratch.
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Old 14th February 2003, 02:47 PM   #10
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>It does not utilize any global feedback, neither a Sziklai output configuration wich means that the last stage would do a lot of distortion. (It is neither class A...) If things were so easy...

Sorry, what's a Sziklai output configuration?
I gotta look that one up.
I measured the distortion on 2 different pieces of test equipment and got the same result. No it does not use global feedback.
I was trying to see how good an amp I could make without global feedback using a minimal number of parts. There is nothing unusual here, a voltage amp followed by Mosfet followers.
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