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Old 30th September 2007, 09:26 PM   #1
Thoru is offline Thoru  Netherlands
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Default Weird oscillation problem appeared

Hi,

I've built a Slone fig 11.4 amp. I built two channels which are almost identical. I stored them in a drawer to finish the protection PCB before I wired everything up. When I stored them in my drawer I made sure they both worked perfect.
When I finished the protection PCB I wanted to wire everything together. So I hooked up the first channel and it worked flawlessly. Then I hooked up the second amp and it gave a slowly rising DC offset and some smoke. Back at the test-bench it appeared to have started oscillating at +- 80kHz. I still works, as in, it amplifies everything at it's input, but with a nice 80kHz signal riding on top of it. The smoke probably came from the resistor damping the output inductor. There was no speakerload connected, so there was not much place to dump enough power to get something smoking (with transistors still operational). It also visually appeared to come from there, but the time it smoked was very short.

Now I wonder, why did one channel start oscillating, while the other is fine (I double checked that)? I attached a schematic with in green the peak-peak voltage of the oscillation referenced to ground. The input was shorted during this measurement and only my scope connected to the output. It seems to be somewhere in the output stage, since the input and VA stage are virtually free of oscillation. Only between C9 and C10 we see some of the oscillation, but that seems to come from the collector of Q6. It's also slightly shifted in phase compared to the other measurements.

Does anybody have some clue? I'm really desperate on this one.

Thanks in advance,

Remco Poelstra
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File Type: pdf slone-amp-oscillation.pdf (27.2 KB, 87 views)
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Old 1st October 2007, 08:08 AM   #2
Elvee is offline Elvee  Belgium
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Hi Thoru,

There must be something wrong with your schematic: as it is shown, the VAS is rather an attenuator. Q17 will try to keep the current through Q6 constant, and the signal will only cause minor perturbations.
Now, there is probably enough loop gain elsewhere to make it appear to work, but it's certainly not the way it is supposed to function.
I suggest you double check your schematic and your physical prototype to determine the actual configuration.
When you come back with the information, we will be able to make meaningful comments.
LV
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Old 1st October 2007, 08:30 AM   #3
Thoru is offline Thoru  Netherlands
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Hi,

Thanks for your reply.
That's very strange. Q17 shouldn't be conducting at all, it's there to protect the VAS stage for overcurrent. Q8 sends 7mA thru the collector of Q6. With 47Ohm, that gives 0.3V Vbe for Q17, so it should be off. I'll check whether it's conducting or not.

Regards,

Remco Poelstra
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Old 1st October 2007, 12:33 PM   #4
Thoru is offline Thoru  Netherlands
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I verified the VAS and Q17. Q17 is indeed turned off and the VAS stage seems to be able to amplify the signal at it's input, so that seems to be correct also.
Does that give any clues?

Regards,

Remco Poelstra
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Old 1st October 2007, 01:39 PM   #5
Omicron is offline Omicron  Belgium
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Looking at the measurements in your schematic I wouldn't say that the VAS is free from oscillation. Also, the VAS is current driven so you wouldn't see much of a voltage swing on the collector of Q1 anyway (hence the 0 mVpp). I'm pretty certain your amplifier as a whole is oscillating and not just the output stage. The oscillation frequency seems to support this. For an oscillating output stage (as for example in a complimentary feedback design) one would expect to see much higher frequencies (tens or hundreds of MHz).

Maybe first check for bad solder connections around the compensation capacitors C9 and C10 and R13. And also around R11 and C6. If all seems ok then maybe the design just is unstable. But in that case one would expect the other channel to oscillate as well. Is it truly identical? PCB layout may play a significant role here. Also any capacitive load at the output before L1 could easily trigger oscillations (as e.g. in a coax test lead attached to the junction of R33 and R34).

Try to stabilize it by experimenting with the value of C9 (increase it). You can simplify the compensation scheme by removing C10 and R13 and placing C9 directly from the base of Q5 to the collector of Q6. Once you've found a value that works you can refine the compensation scheme by again adding C10 and R13 and tweaking it further. Don't be satisfied when the amplifier stops oscillating when there is no input signal, test it with a low level square wave input and check the output for ringing.

PS: looking at figure 11.4 in Slone's book I see a complimentary feedback pair while yours is a darlington configuration! Also the transistor types differ, which could also play a role.
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Old 1st October 2007, 02:14 PM   #6
Thoru is offline Thoru  Netherlands
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Thanks for your reply.

Yes, I modified the original design of fig 11.4. The EF stage is more stable and has a more constant output impedance in class B mode, so that's why I chose that one.

I'll try to get it stable with different capacitor values. The problem is that dismantling the PCB is a lot of work, since I sandwiched the output transtors between the solderside of the PCB and the heatsink. But well, it seems I can't avoid taking it all apart. I'm only afraid of turning the amplifier on without a heatsink. I turned the potentiometer in such a position that Vbias is minimal, but will it than still have thermal runaway without heatsink? It normally runs cold in this situation.

Will changing C6 help? Ot is it better left at 5pF?

Both PCB's are made from the same design. The oscillating one from a single-sided PCB and the not-oscillating one from a double sided PCB. I didn't remove all coper from the top side since that consumed to much etching liquid. But since the now-oscillating amp was once stable I supposed it wasn't that important.

I will let know how it all ends, it can take a while, due to the mechanical construction.

Regards,

Remco Poelstra
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Old 1st October 2007, 03:07 PM   #7
Omicron is offline Omicron  Belgium
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I don't think that changing C6 is going to solve your oscillation problem. It's purpose is to limit the bandwith of the amplifier. With the values shown it only affects much higher frequencies (> 1MHz).

C9/C10 and R13 form the amplifier's compensation scheme so those are the ones you should play with.

Best to leave the transitors attached to the heatsink as they can get hot when the amplifier breaks out into full blown oscillation. When the frequency of oscillation is high enough it can happen that the output transistors can't turn off in time. When that happens they both conduct for a moment during every cycle, resulting in a very large current and some VERY hot transistors. The amplifier can die in a hurry in such a case. Now, I don't think you need to worry about that at 80KHz, but still if you're playing around with the compensation caps it's wise to have the heatsinks in place.

Maybe you can rig somethig up with some (short) pieces of wire in place of C9 so you can try different values from the component side?
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Old 1st October 2007, 03:36 PM   #8
jaycee is offline jaycee  United Kingdom
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Try changing to "regular" Miller compensation - remove c9, r13 and put C10 around 100pF.

At a guess, in the double sided one that is not oscillating, the blank layer is acting as a shield which prevents stray RF causing the oscillation. Is this surface connected to ground? Is there clearance around all the through-holes?
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Old 1st October 2007, 03:38 PM   #9
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Agreed.
Experiment with increasing R13 first.

Adam
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Old 1st October 2007, 03:47 PM   #10
Thoru is offline Thoru  Netherlands
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Yes, the top layer is connected to GND and AGND. Corresponding to which components are on top of it. I divided the layer to reflect the two GND's.

I will see what I can do with headers to change the components.

So, first increase R13, then try single Miller compensation and if that works, try to get back to two pole compensation. Well, I don't think my evening will be long enough to try all that in one run .

Regards,

Remco Poelstra
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