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Old 9th January 2011, 06:46 PM   #11
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I have problems with mine when i used wirewound resistors in the output, it disappeared when i changed them to carbonfilm
Ole
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Old 9th January 2011, 07:20 PM   #12
WuYit is offline WuYit  Sweden
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Hi,
the Crescendo amps are notorious for oscillation problems, and no wonder. I would change some things there in post #3.
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Old 9th January 2011, 07:25 PM   #13
kriss99 is offline kriss99  Greece
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Me too I have it with wirewound resistors.
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Old 10th January 2011, 07:37 AM   #14
SUZTEK is offline SUZTEK  South Africa
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wirewound resistors acts as an inductance and the mostets will oscillate.I have built these amps and and as long as the mosfets are mounted directly on the pc board ,as opposed to wiring leads, oscillation should not occur.
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Old 10th January 2011, 12:15 PM   #15
AndrewT is online now AndrewT  Scotland
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The gate stopper (r23) should be mounted very close to the gate lead of the output mosFET. To reduce the inductance of the wire and lead even more the gate stopper can be soldered direct to the gate lead very close to the body of the FET.
This can reduce the total length of connection from~10mm to ~5mm. 20mm is a non starter (potential oscillator)
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Old 3rd December 2012, 11:02 PM   #16
orrason is offline orrason  United Kingdom
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Default 180 w Oscillations

Hi

I have also a similar oscillations on my 180 W Crescendo which has paraller FETs at the output. There is an oscillation burst on the positive top when I turn upp the volume on the sine wave. At around +/- 20V at the output the HF bursts do start. The amplfier can also go easy into swinging mode without signal with around 4MHZ high output swing. Putting my finger close to the variable bias resistor can initate the oscillations. I have put 68 nf across the bias resistor and changed the source resistors to metal films without any change. Maybe I should 300 pf input capacitance to the SK135 between the G and the S ?

Any other suggestions ?
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Old 4th December 2012, 02:59 AM   #17
wahab is offline wahab  Algeria
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Quote:
Originally Posted by orrason View Post
Hi

I have also a similar oscillations on my 180 W Crescendo which has paraller FETs at the output. There is an oscillation burst on the positive top when I turn upp the volume on the sine wave. At around +/- 20V at the output the HF bursts do start. The amplfier can also go easy into swinging mode without signal with around 4MHZ high output swing. Putting my finger close to the variable bias resistor can initate the oscillations. I have put 68 nf across the bias resistor and changed the source resistors to metal films without any change. Maybe I should 300 pf input capacitance to the SK135 between the G and the S ?
Quote:
Originally Posted by wahab View Post
As explained on the original article the amp compensation rely
on the fets input capacitances to providing a shunt compensation ,
an hazardous "solution" that will leave some builders chasing ghosts...

To be stable enough without relying on the said random parameters
two compensation caps , about 22pF each , should be implemented
from inputs to outputs of the cascodes.
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Old 4th December 2012, 04:19 AM   #18
Bonsai is offline Bonsai  Taiwan
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I commented on this amp here on the forum a few years ago along the same lines as Wahab - that Crescendo is indeed a 'Tormento'.

Elektor have been notorius over the years for releasing designs that were marginal in terms of stability.

This amp is crying out (make that 'yelling') for some decent compensation - like Cdoms from the VAS cascode collectors to the bases of the VAS transistors.

I just did a quick calc and Wahab's proposal is a good one - it closes the loop gain at c. 1.5MHz (10pf) and about 750 kHz (22pF). With the current design it looks like the ULGF is very ill defined.

However, looking at your trace, if the oscillation is up at a few MHz, it could also mean you have output stage parasitic oscillation as well . . .
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Old 4th December 2012, 04:45 AM   #19
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This type of oscillation comes from the mosfet itself and the n-channel fet is the most sensistive. Best you can you is to increase R23 and R24 to 330 or 470 ohm.
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Old 4th December 2012, 04:50 AM   #20
CBS240 is offline CBS240  United States
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Forget increasing the bias to a rediculous level, or doing injustice to the effective Ft by increasing the gate stopper. Just place a small zero from the gate to drain of the output mosfets right at the device pins. ~40pf in series with ~200R and solder it from gate to drain and that oscillation burst will go away for good.
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