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Old 12th February 2010, 04:31 AM   #61
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So, about the mod that Elector suggested not needed?
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Old 12th February 2010, 09:42 AM   #62
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Originally Posted by meanman1964 View Post
So, about the mod that Elector suggested not needed?
Right, not needed. I think the suggestions are just for the IGTB version, which behaves a bit different than the HexFet version. However, all these suggestions are just made to keep the amp stable.

Btw, I use two of these amps for a subwoofer in bridge configuration. So I had to exchange the IRF540/9540 pair because they are just specified for 100V. The 220pF limits the slew-rate to about 9V/us. Keeping the spec value of 35V/us, one has to use 63pF instead, but I did not test the stability, yet.
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Old 28th March 2010, 09:20 AM   #63
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About the extra miller cap would it also work with the original fets?
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Old 28th April 2010, 05:20 AM   #64
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Eddelarue,

When you take a look at R31 from your modded schematic and you compaire it with R21 from the original schematic isn't R31 placed wrong?
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Old 28th April 2010, 12:47 PM   #65
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Originally Posted by meanman1964 View Post
Eddelarue,

When you take a look at R31 from your modded schematic and you compaire it with R21 from the original schematic isn't R31 placed wrong?

Hi meanman1964,

I used from this Mosfet output version

[DIY] HEXFET AMP total disaster?
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Old 28th April 2010, 03:11 PM   #66
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Than I'm right about R31?
Is your modded amp still working OK?
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Old 28th April 2010, 03:49 PM   #67
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Quote:
Originally Posted by meanman1964 View Post
Than I'm right about R31?
Is your modded amp still working OK?
Long time ago I built this amplifier as a prototype and it worked very well. This prototype does not exist anymore with me.

regards

eD
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Old 28th April 2010, 10:42 PM   #68
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Originally Posted by seppi0815 View Post
Finally, I removed the 22nF and the amp keeps still stable, again important are just these two Miller caps to move the gain of the oscillation frequency <1. Hence, the 20k resistors “damage” the design principle of this type of amp.

All other components are kept the same as proposed by Elektor in its initial Hexfet 540/9540 version. Be sure you have equal! components as described in the Elektor article.

My comments to the other modes given in the thread vs. stability:
a.) Changing the “gate stoppers” to higher values does not help a lot, just slowing down the slew-rate
b.) Changing the current feed-back on (R25,R28) has just influence on the open loop gain, but not on the gain at the oscillation frequency.
c.) Changing the bias current of the input diff-pairs from 2mA to 4mA ok, will have positive effect on the Miller caps, but not necessary.
d.) Changing the band limiting network in the input diff pairs ok, will have positive effects on the gain at the oscillation frequency, but not necessary.
Can I also use this mod with the original hexfet 540/9540 or is it only for the IRFP240/9240 ?
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Old 28th April 2010, 11:06 PM   #69
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I found the only place I needed a capacitor was on the VAS C to B on my IRFP240/9240.
I found a 220pf/100pf worked fine.
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Old 29th April 2010, 05:06 AM   #70
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Quote:
Originally Posted by nigelwright7557 View Post
I found the only place I needed a capacitor was on the VAS C to B on my IRFP240/9240.
I found a 220pf/100pf worked fine.
So did you also replaced the hexfet 540/9540 by the IRFP240/9240?
Did you add only the VAS cap or did the other mods as well?
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