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Old 15th May 2012, 06:27 PM   #31
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Quote:
Originally Posted by DF96 View Post
Within a feedback loop it all gets more complicated than that.
Hello DF96, i imagined it's not as easy ...


i ordered some parts so i can try some of the suggestions, i dont expect them to be here before friday.
i am testing the right channel of the amp. the left channel is connected to the supply, with a load and with the input grounded. I am wondering if it would be best to disconnect left, so it can't interfere with the one i am testing? is it enough to just disconnect the b+ rail from the CCS V1/V2?
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Old 15th May 2012, 06:42 PM   #32
DF96 is offline DF96  England
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Probably, yes.

You can remove the resistor on the zener string without waiting for parts. That might be enough to cure it.
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Old 15th May 2012, 09:03 PM   #33
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some testing done,
first i removed B+ from the left channel to make sure i only measure whats happening on the right. i conected the pc to the input terminal, feeding 40Hz 0.2V. NFB loop is open, and increase the volume.

-i removed R13 from the loop, LED is off, but thats the only change. the bias balance circuit (attached) reacts, i see the LED flashing at LF.

-as suggested by MelB i decreased C23, first to 250uF and then to 150uF with no change. same as above.

looks like i have to wait for a few zeners and caps to try some other suggestions
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Old 15th May 2012, 09:36 PM   #34
DF96 is offline DF96  England
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By 'remove R13' do you mean just cut it out or replace it with a short? The latter is correct.
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Old 15th May 2012, 09:55 PM   #35
MelB is offline MelB  Canada
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Your LM7812's should have a small cap on each side!
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Old 15th May 2012, 10:25 PM   #36
MelB is offline MelB  Canada
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Wow...this is not a simple design...over my head certainly. Do we have all of the "parts" posted now? I wish I could help.
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Old 16th May 2012, 10:00 PM   #37
tomchr is offline tomchr  United States
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Here's what I think is going on:

I think the reason you're having issues with motor boating is that the "reference" for your CCS is derived by a resistive divider from the power supply. Take the "CCS" on the input pair: The "reference" is formed by R30, R31. I use the quotes (" ") to indicate that it really isn't much of a reference, it's just a fraction of the 320 V supply. The 320 V isn't exactly 320 V either. It's whatever B+ happens to be minus the voltage drop across R7+R8 in the supply. On start-up, there is very little drop across R7+R8. As the input tube starts conducting, the supply voltage to the input stage will drop as the voltage across R7+R8 increases. This causes the voltage across R30 to decrease. This causes the current of the "CCS" to decrease. This causes the supply voltage to increase (less drop across R7+R8). Now the voltage across R30 increases, hence, increasing the current in the "CCS". This is motor boating. I bet the frequency is roughly 1/(2pi*R30*C25). If you double C25, I bet the motor boating frequency will be half.

Similarly, the stability of the "CCS" on the phase splitter is dependent on the stability of the 24 V bias voltage.

The motor boating is caused by the current of the "CCS" being strongly dependent on the power supply voltage. Replacing R30 with a zener diode would be one way to fix this issue. It would result in a somewhat temperature dependent CCS current but at least the motor boating issue should be fixed. With the current circuit values, I suspect a 47 V zener would work. Or use one of the 51 V zeners in series with the "ON" LED for a quick test... This test would reveal if it's worthwhile building a different CCS or if the motor boating is caused by something else.

~Tom
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Old 16th May 2012, 10:34 PM   #38
DF96 is offline DF96  England
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It is a bit more complicated than that. A single RC can't cause LF oscillation as it can only give 90 degrees of phase shift. The extra phase shift comes from the coupling caps as they have a time constant in the same ballpark as the 'CCS'.
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Old 17th May 2012, 12:25 AM   #39
MelB is offline MelB  Canada
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Here's a thought. Get rid of R1. 1 Hz is an insanely low frequency and to me has to involve a large cap and a relatively small imbalance. Like an imbalance between L and E on your power transformer.
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Old 17th May 2012, 12:35 AM   #40
tomchr is offline tomchr  United States
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Quote:
Originally Posted by DF96 View Post
It is a bit more complicated than that. A single RC can't cause LF oscillation as it can only give 90 degrees of phase shift. The extra phase shift comes from the coupling caps as they have a time constant in the same ballpark as the 'CCS'.
OP said the motor boating was there even with the feedback disconnected, so I doubt it's a stability issue within the main feedback loop.

You're right about the 90 degrees phase shift of an RC, however. I just ran a sim of the input stage. My sim doesn't show signs of motor boating... Perhaps my thinking of earlier was incorrect. I've been proven wrong before...

I just happened to notice that just about every supply in the amp comes from a voltage doubler -- including the main B+ supply. I suggest monitoring all the supply voltages on an o'scope. I'm a little skeptical about pulling hundreds of mA from a voltage doubler, but I haven't worked that much with those. Of course, if all supplies are solid, the issue isn't with the supplies.

~Tom
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