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Old 21st October 2010, 01:38 PM   #91
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70 degrees from UG point ??? Oscillator ... maybe not always , but into a sufficient capacitive load while doing a standard transient analysis ..Yup. (attachment 1/2). I under-compensated the standard "blameless" (AX1 - 3'rd pix BODE) with 1 and 10 uf loads @ 70/65dg. margin. I go for 80-82dg. at UG , never a warm zoble - good 20KHZ square wave.
At 70dg. I have observed random oscillation , not enough to "blow" , but enough to feel (zoble warm) and see on my 20mhz CRO.
4th attachment is what I "shoot for" and is what a house full of working amps rely on.
BTW , this is how andy C. /GK taught me .. and has served VERY well.

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File Type: gif 70dg.gif (8.2 KB, 148 views)
File Type: gif undercomp.gif (15.8 KB, 147 views)
File Type: gif right.gif (25.1 KB, 148 views)
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Last edited by ostripper; 21st October 2010 at 01:45 PM.
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Old 21st October 2010, 03:09 PM   #92
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OS, you seem to understand the point.
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Old 21st October 2010, 07:58 PM   #93
maouna is offline maouna  Greece
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Ostripper can you help me with my amplifier? i am having troubles in compensating and making it stable.i considered to take feedback both from output stage and from vas but it resulted in greater THD with better stability margins.
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Old 22nd October 2010, 12:02 AM   #94
djoffe is offline djoffe  United States
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Default stability

I ran your circuit and looked at loop still have 80 degrees of phase margin if you change C5 from 1n to 0 and reduce C1 from 100 pF to 47 pF. It makes your 10 kHz distortion drop so that all harmonics are 100 dB below fundamental with 26 volts peak to peak output. With your original values, the even harmonics were 100 dB down, but odd harmonics were only down about 87 dB.

Now, I haven't run some of the torture tests for stability, but it seems to be a pretty conservative compensation.
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Old 22nd October 2010, 08:21 AM   #95
maouna is offline maouna  Greece
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i had already made the circuit in the past on pcb with no C5-R26 components and C1=100p ,R14=100K.C11=10p and it was oscillating in real life.maybe i have to apply some other way of compensation.
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Old 22nd October 2010, 08:06 PM   #96
djoffe is offline djoffe  United States
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Default what frequency is the oscillation? How strong? we're talking about real hardware, not simulation...

What frequency is the oscillation? How strong? With a load? Without a load? What kind of load makes it oscillate worse? When driven hard? When hardly driven? With input shorted? With input open?

The question is, is it a loop (e.g. around the whole amp loop), or is it more localized? Sometimes just the output pairs will oscillate by themselves...e.g. Q13-Q15, or Q12-Q14.

Here's a way to separate the issues...If it's a loop thing, then increasing the gain should make it oscillate less. Change R15 from 1K to 500 Ohms...The gain will about double, but the oscillation, if based on loop stability issues, should calm down.
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Old 22nd October 2010, 09:38 PM   #97
maouna is offline maouna  Greece
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I dont have an oscilloscope or a waveform generator to test the amp.with a speaker connected at the output and when there is signal at the input from an mp3 player the current consumption is normal i replaced the output speaker with a resistor of 8 ohm in parallel with a 2.2μF capacitor and input from an mp3 player. the amplifier was oscillating for sure because there was 1A of current at the ammeter (i use a transformer which gives max 1 ampere.)
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