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Old 23rd December 2006, 12:04 PM   #11
PMik is offline PMik  United States
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Wynand

Are you still interested to get the original article ?

I can send it for your email

Ps. I have today bought a "functional" Cresendo 180W for a symbolic amount of cash compared to the real cost of the parts/project, the owner informed me was not up to the task of getting the project finalized to perfection
The issues he had incounted was the ones bjornagain has posted the solution of in this tread ...
Thx bjornagain
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Old 23rd December 2006, 08:57 PM   #12
Dusk is offline Dusk  United States
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Smile Crecendo Article

Hi Pmik
Yes i wouldlove to get thatarticle on the Crecendo.....if you could e-mail it.

THANKS
Dusk
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Old 23rd December 2006, 09:41 PM   #13
PMik is offline PMik  United States
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Sure thing, Just tell me where to email
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Old 24th December 2006, 12:41 AM   #14
Dusk is offline Dusk  United States
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Default Crecendo

Hi Pmik
sorry.........here is my e-mail musicmanblu@prodigy.net
Thanks again
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Old 24th December 2006, 09:25 AM   #15
PMik is offline PMik  United States
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Article on its way for your inbox
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Old 25th December 2006, 05:37 AM   #16
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Hi, pMik,

Kind enough to send me a copy too? Although I am having some other projects, I would certainly like to make a cresendo in the future (don't know when that's going to be) for I had heard so much about it.

My mail : tony_chan_cf@yahoo.com.hk

Tks a lot
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Old 25th December 2006, 10:53 AM   #17
PMik is offline PMik  United States
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Old 25th December 2006, 11:46 AM   #18
Bonsai is offline Bonsai  Taiwan
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I'm not surprised there are oscillation problems with this amp - where is the compensation? I read the original amplifier article where they discussed using the input capacitance of the output mosfets as part of the compensation scheme. Besides the fact that this IMO is a highly dubious engineering approach, one of the things mosefet manufacturers do is regularly shrink their mosfet die in order to improve the yield in their fabs - this also leads to changes in device characteristics - normally not enough to cause major issues, but if you have a design that's on the edge, then it will.

This amp probably has parasitic problems in the output stage and needs some gate stopper resistors (100 Ohm to 470 Ohm) and it needs some dominant pole compensation - either across the colletor load resistors on the input stage along the lines of Bob Cordell's design, or a small value cap (20-30pF) across from the collectors of the VAS cascodes to the front end diff amp load resistors. The out put inductor is also important and needs to be in the region of 2-3uH minimum with a 1-2 ohm resistor in parallel. Lets not also forget the Zoebel network. In the ME version, resistors were added from the cascode collectors to ground, presumably with tghe mistaken notion that this would set or stabilize the loop gain and prevent instability.

I think the designer of this amp tried to maximise loop gain to get the lowest possible distortion without looking at the phase marging on his amp and doing what ALL designers have to do: Make a trade off between phase margin and loop gain. The good designers get the best of both worlds, and the not so good ones end up with inadeqaute feedback or inadequate phase margin - both are unacceptable and unfortunately, the Crescendo falls into the latter category. A great pity, because with a little tradeoff, this could have been a trouble free amp, no matter who built it.

Maybe someone on the forum should model this amp and lets look at what needs to be tweaked to clean it up for once and for all.

Merry Christmas

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Old 25th December 2006, 12:16 PM   #19
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I believe you need to take one more glance at the schematics, the Cresendo is eqv. with 220 Ohm gate stopper resistors
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Old 25th December 2006, 12:44 PM   #20
Bonsai is offline Bonsai  Taiwan
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PMik,
yes it does, but it still does not have any dominant pole compensation. Also, the gate stopper resistors have gate source capacitors on the gate side, so they put a pole on the amp response which probably doesn't help the oscillation problem.
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