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Old 18th August 2011, 01:30 PM   #141
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Quote:
Originally Posted by kenpeter View Post
C3 (DC servo cap) is the main one that worries me.
And I got this exact same problem in most of own simulated designs.
Sim won't complain when DC offset reverse biases this polarized cap.
Even if it leaks, might never know, just won't roll off the DC anymore.
I use a 1000uF 35V for C3. There is <1mV of DC offset. I'm not sure what you're worried about.

The cap that blew for me was a 35V bypass capacitor (C9). And it was because I had forgot to replace them with higher voltage types when I installed the proper power supply that I am now using (+-40V). Stupid mistakes are made when you're more focused on bending leads of Q12 and Q13 to rearrange the pins for the PCB (KSC1845F - Higher Vceo, different pinout than BC546).

I have monitored DC offset continuously throughout cold and warm temperatures, a wide range of output volume, and it stays around 1mV or less. I do use a matched input pair. I have been running it for a while now.
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Old 18th August 2011, 05:40 PM   #142
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Quote:
Originally Posted by kenpeter View Post
C3 (DC servo cap) is the main one that worries me.
And I got this exact same problem in most of own simulated designs.
Sim won't complain when DC offset reverse biases this polarized cap.
Even if it leaks, might never know, just won't roll off the DC anymore.

I suggest a series of two 1000uF polarized caps, back to back.
With the midpoint pulled down to the negative rail though 47K.
Bypass this stack of polarized elko's with a good 1uF film cap....
The subject certainly deserves some attention.

Here, this capacitor is protected by back to back diodes: one limits the reverse bias to a maximum of 0.5~0.6V, and the other prevents the eventuality of a freak event triggering a latch-up.

Regarding the polarity, the bias current from the transistors clearly shifts the odds in the good direction, and it is unlikely a mismatch in the input transistors could reach more than 25mV, even with unmatched devices.

At temperatures of ~25°C, aluminium electrolytics can tolerate 0.5V of reverse bias indefinitely. Not something I would reccommend, but in any case we are very from this value here.

Even if the capacitor leaks, its leakage resistance would need to go below ~10K to begin to have adverse effects, and anyway, if the worst happens and the cap becomes shorted, the offset voltage will be multiplied by the gain of the amplifier, which should normally not damage the speaker.

I am not sure applying the "correct" bias to back to back capacitors is a good idea: such large capacitors have a non-zero leakage current, in the nanoamps range, and this current tends to fluctuate randomly.
Applied to the 10K feedback this current could inject some VLF noise into the amplifier, making the cure worst than the problem it is supposed to address.

As for the film cap in parallel: actual measurements tend to show adverse effects rather than improvements: see here for actual data:
paralleling film caps with electrolytic caps
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Old 19th August 2011, 04:08 AM   #143
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Even stOOpid'r way to bias them caps...
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Old 19th August 2011, 04:15 AM   #144
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What we got in common here, are two feedback loops in quadrature.
Don't much mess with each other because they work at 90 degrees.
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Old 19th August 2011, 05:43 AM   #145
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Originally Posted by kenpeter View Post
Even stOOpid'r way to bias them caps...
no,
clever,
but heavy class A = and $$$
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Old 19th August 2011, 09:18 AM   #146
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Heavy Class A, where? This crosses only 1A, for 3A peaks.
Granted, never allowed turn off, but only 150mA so wasted.
Cooking 25W per 2n3055 at idle. 50W a $$$ burning fireball?

Might look JLH at first glance, but a much tamer AB variant.
Maybe you didn't spot shunt regulated current source Q8?
Classic overbias in the center is solved here...

Q8 is my "second control loop in quadrature" we spoke of earlier.
While the main loop is hopefully holding linear control over voltage,
Q8 is holding Schottky curved control over output stage currents.
The objective to enforce quasicomplimentary square laws...

Last edited by kenpeter; 19th August 2011 at 09:47 AM.
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Old 19th August 2011, 09:24 AM   #147
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Originally Posted by kenpeter View Post
150mA so wasted.
Cooking about 25W per 2n3055 at idle, is that $$$?
mine simulator b2spice showed 1.4A/1.7A through upper/lower 3055 at idle...
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Old 19th August 2011, 11:17 AM   #148
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
What we got in common here, are two feedback loops in quadrature.
Mmm... quadrature with a limp then: for Q8 to act on Q7's current, it first has to change Q6's current, which is sensed by the FB because it alters the output level, and makes the FB loop restore the equlibrium via the (noble) horizontal path.

Quote:
Even stOOpid'r way to bias them caps...
Quite the opposite: it is very clever and elegant.

But IMHO, it looks like a beautiful solution to a non-existent problem....
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Old 19th August 2011, 12:51 PM   #149
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I simulate 0.86A upper, and 1.07A lower, idle...

Base current helps the upper work the load, so
collector current asymmetry is to be expected.
True crossing isn't exactly where the collectors
in the first screenshot appear to cross equal.

But Schottky currents are perfectly symmetrical.
Nothing deceptive about the second screenshot.
Attached Images
File Type: gif Clipboard01.gif (20.9 KB, 476 views)
File Type: gif Clipboard02.gif (21.9 KB, 462 views)

Last edited by kenpeter; 19th August 2011 at 12:59 PM.
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Old 19th August 2011, 02:28 PM   #150
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What is the purpose of Q4?
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