WNA Class AB 125W Mosfet Amp Schematic Discussion - diyAudio
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Old 19th November 2013, 06:18 AM   #1
Raj1 is offline Raj1  United Kingdom
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Default WNA Class AB 125W Mosfet Amp Schematic Discussion

A design by Dr. David White who used to run a kit amplifier business (now defunct).

Its a good sounding design that can be modded in many ways. Thought it would make the start of a nice discussion here and maybe even inspire a few builds

Click the image to open in full size.

Last edited by Raj1; 19th November 2013 at 06:25 AM.
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Old 19th November 2013, 09:01 AM   #2
Raj1 is offline Raj1  United Kingdom
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Mods from David White:

A couple of other things you can experiment with on the audiophile mos125 and mos250. The 2.2kohm resistors feeding the shunt regulators for the opamp can be replaced with a constant current source, something around 15mA should be fine. This will greatly increase the power supply rejection. The shunt regulators can also be improved if you bypass the resistor feeding the adj terminal with a 22uF capacitor. Its also possible to improve on the constant current sources loading the opamp by using a transistor/transistor rather than a led/transistor source. I haven't tried any of these suggestions yet but if anyone is interested I can provide more detail.


If you've experimented with the mos125/250 you'll have found that they don't like very fast opamps ( SR>80V/uS ) in the ic1 position. This is mainly due to a lack of drive current from the bipolar transistors feeding the output mosfets. If you increase the drive to the mosfets you'll find that you can use just about any fast (<200V/us or slow if you like ) voltage feedback opamp for ic1, and also the increased drive current allows you to use more pairs of output devices if you want to.

Here's how you do it for a mos125. Change R17 and R20 from 470R to 180R; change R23 from 1k to 390R (3W at least ); and change R24 from 220R to 82R ( 1W )

If you make these changes, use a faster op amp than the OPA134 or OPA627, and leave out the 680pf capacitor at the input ( which I always do anyway ) you'll be rewarded with a much faster and cleaner sounding amp with slightly lower intermodulation distortion.



You can experiment with faster versions of the MOS125 by lowering the value of the 22pF capacitor, C14, across the 2k2 feedback resistor. In my experience the power amp is still unconditionally stable with a 10pF capacitor; and I have run a MOS125 with no capacitor at all, but I wouldn't recommend this. If you lower the value of the capacitor too far the amplifier will begin to oscillate. This isn't usually a major problem, the amp will typically draw 0.5A and doesn't seem to fry the zobel network, but you should always try these experiments with a dummy load rather than a loudspeaker connected to the power amp. Remember too that a power amp with no input and no output load may be stable but can become unstable when a load is connected and/or an input signal is present.
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Old 19th November 2013, 09:24 AM   #3
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Hi Raj1,

an Amp with common source is not easy to compensate. It is also difficult to reach a good performance.

This opamp based circuit is much smaller and well known. It has a fantastic performance
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File Type: txt Cordell-Models.txt (19.6 KB, 40 views)
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Old 19th November 2013, 09:35 AM   #4
Raj1 is offline Raj1  United Kingdom
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Thanks for the circuit Moschfet.

What are your other impressions of the circuit posted? It sounds pretty good to me, but I have not heard the amp you posted so can't say for sure which I would go for. I guess te simplr circuit makes more sense, but I'm not sure if it measures better etc.

Last edited by Raj1; 19th November 2013 at 09:39 AM.
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Old 19th November 2013, 09:52 AM   #5
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I can remember that I simulated such a circuit ( opamp/common source) and the results where not bad but it is easier to reach better values with less components.
The only advantage is that common source has a rail to rail output. For a good performance the circuit gets complexer.
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Old 31st May 2014, 10:11 PM   #6
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Quote:
Originally Posted by moschfet View Post
Hi Raj1,

an Amp with common source is not easy to compensate. It is also difficult to reach a good performance.

This opamp based circuit is much smaller and well known. It has a fantastic performance
Hello,

Do you have made this amplifier ?

Thanks

http://www.diyaudio.com/forums/attac...ussion-amp.jpg
http://www.diyaudio.com/forums/attac...mosfet-amp.asc
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Old 31st May 2014, 10:34 PM   #7
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Hi UltimateX86,
Quote:
Do you have made this amplifier ?
No, not this especial, but something similar. Unfortunately I don't have the schematic.
The Albs-modules uses this idea (with mosfets) and also the nice SAC-Amps SA-50(bipolar).

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