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Old 14th February 2010, 06:39 PM   #1
hahfran is offline hahfran  Germany
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Default Recovery from clipping, soft clipping

Clipping cannot be avoided. In clipping any reasonable relation between input and output, i.e. gain, proverbially disappears as in the time domain, an increase in input is not responded with increase in output. Hence negative feedback does not work. If no SOAR protection is provided and no overcurrent protection
clipping can destroy the power BJTs . The attachment shows a schematic of an amp ( not with the original BJTs except the SOAR protection) that has been diy'ed plenty often. It features a carefully designed lead/lag compensation, and a "lavishly" designed SOAR protection. Since the VAS has close to zero CMRR a second PSU is necessary. In spite of the protection a couple of these amps have failed with burnout as a consequence of clipping. It should not happen but it did. In the original schematic Q1 throu Q4 are Sanken LAPTs with fts of 100 Mhz and 40 MHz resp. The burnout appears to have initiated in the drivers Q2,Q4.
One of the most promising ideas is to have clipping occur in a soft manner -soft clipping- "before" the power stage to ensure NFB is always working.
So far however I did not come up with a solution which does not inject
errors to the signal far below clipping such as spikes.
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Old 14th February 2010, 06:55 PM   #2
farjon is offline farjon  Brazil
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There is a soft clipping in a Randy Slone's design. See "The Audiophile´s Project Sourcebook", design Fig. 6-21, from page 172 to 178.

Amazon.com: The Audiophile's Project Sourcebook: 80 High-Performance Audio Electronics Projects (0639785330820): G. Randy Slone, G. Randy Slone: Books
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Old 14th February 2010, 07:15 PM   #3
jcx is offline jcx  United States
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in general clipping itself shouldn't stress output devices - nice low Vce sat, load limited current give low power loss in the device

limiting base/gate drive to safe levels is required but not difficult

more dangerous for the output stage is leaving clipping, turning off the saturated device while possibly having high frequency oscillation bursts as the the earlier feedback stages "unwind" from saturation - slow turn-off will give high "shoot-thru" currents and stress the device on the the other rail
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Old 14th February 2010, 07:16 PM   #4
hahfran is offline hahfran  Germany
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Thanx. I am not yet throu testing - in reality not simulated- a couple of ideas.
Btw. I wonder why all those simulations never attempt to drive a real - that is, a complex -load? They would be amazed what nasty stuff happens to those thd figures....
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Old 14th February 2010, 07:28 PM   #5
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by jcx View Post
in general clipping itself shouldn't stress output devices - nice low Vce sat, load limited current give low power loss in the device

limiting base/gate drive to safe levels is required but not difficult

more dangerous for the output stage is leaving clipping, turning off the saturated device while possibly having high frequency oscillation bursts as the the earlier feedback stages "unwind" from saturation - slow turn-off will give high "shoot-thru" currents and stress the device on the the other rail
Yes certainly if ie max is not exceeded the clipping device should not be harmed.
It is the effects of saturation in the pre-drivers that result from NFB failing that can kill the device that should be off ( VCe max ) but isn't. Preventing saturation with diode clamps (preferably germanium diodes ) has been tested.
I have once designed a (supposedly) saturation-free predriver but never built it. Typically such predrivers have very low gain per stage
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Old 14th February 2010, 07:44 PM   #6
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Quote:
Originally Posted by hahfran View Post
Clipping cannot be avoided. In clipping any reasonable relation between input and output, i.e. gain, proverbially disappears as in the time domain, an increase in input is not responded with increase in output. Hence negative feedback does not work. If no SOAR protection is provided and no overcurrent protection
clipping can destroy the power BJTs . The attachment shows a schematic of an amp ( not with the original BJTs except the SOAR protection) that has been diy'ed plenty often. It features a carefully designed lead/lag compensation, and a "lavishly" designed SOAR protection. Since the VAS has close to zero CMRR a second PSU is necessary. In spite of the protection a couple of these amps have failed with burnout as a consequence of clipping. It should not happen but it did. In the original schematic Q1 throu Q4 are Sanken LAPTs with fts of 100 Mhz and 40 MHz resp. The burnout appears to have initiated in the drivers Q2,Q4.
One of the most promising ideas is to have clipping occur in a soft manner -soft clipping- "before" the power stage to ensure NFB is always working.
So far however I did not come up with a solution which does not inject
errors to the signal far below clipping such as spikes.
There are two soft clipping approaches I have tried. One is what I call the Klever Klipper, which is described on my website at Cordell Audio: Home Page. That circuit provides an adaptive soft clipping threshold ahead of the entire power amplifier, outside its feedback loop. It prevents the power amplifier itself from ever clipping. Its clipping level adapts to the available power supply voltage so no dynamic headroom is lost. The tradeoff with circuits like that is that the distortion caused by the clipping diodes begins well below the full power output of the amplifier. This is by definition a necessity if indeed the clipping behavior is to be soft. Such distortion rising well below full power output is quite benign and somewhat tube-like, but it will disappoint those who are only into the numbers game.

The other approach involves Baker clamps like those I used in my MOSFET EC amplifier paper. These clamp the output of the VAS before it or the output stage can saturate. With ordinary global feedback, the resulting clipping is still quite sharp and some stages in the amplifier go into cutoff. In that design, the current diverted from the VAS by the Baker clamp diodes is returned to ground through the collectors of a pair of transistors that provide the bias for the Baker clamp diodes. If instead those collectors are returned to ground through a resistor, an "output voltage" is produced. This voltage can be returned to the input stage to keep the feedback loop closed during clipping. This results in a softer clip wherein the amplifier comes out of clipping more cleanly because most of the stages never went into cutoff.

Cheers,
Bob
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Old 14th February 2010, 10:28 PM   #7
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there's anoyher method if you don't mind some compression. some pro-audio amps have opto compression that limits the amplifier output to just below clipping. it doesn't introduce any distortion to the signal itself, just reduces the overall gain of the amplifier. if you test the amp with a sine wave source, you get the amp just below clipping and the output amplitude doesn't go any higher, but a front panel LED informs you that you are overdriving the amp. i have rarely seen amps of this type come in for shorted outputs, except when the amp has been run with the limiter turned off. some models have a setting for fast or slow response of the limiter to allow for short transients without activating the limiter.
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Old 15th February 2010, 07:05 AM   #8
hahfran is offline hahfran  Germany
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Thanx for these ideas. Most possibly professional amps intended for on stage usage in live performances must be equipped with perfectly working soft clipping because there 100% reliability is mandatory. I think Bob's proposal Baker clamps returning a clipping-level proportional feedback which yields "soft compression" in combination with keeping the critical predriver VAS stages out of saturation promises to be a almost professional solution.
However testing a sim with a complex load such as a 2 way passive crossover with
baffle step equalisation and such, simulating the speakers as inductor with serial R of voice coil ( that is very simple and incorrect but..) yields a couple of surprises. For ex amps featuring thd as low as 0.0something % thd now read a solid 3% because odd harmonics go up. Such a sim demonstrates quite well one cannot go without SOAR protection as the phase shifts "demand" current from the device already stressed by max VCE.

Last edited by hahfran; 15th February 2010 at 07:17 AM.
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Old 15th February 2010, 11:02 PM   #9
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I believe a DSP could identify the frequency of the largest non-transient waves,
and add or subtract "soft clipping" harmonics of the offending notes alone. Thus
sidestep the hetrodyne IMD problem for other simultaneously passing frequencies.
Perhaps the effect would be akin to each musician having their own MI amplifier?

For sharp transients of no particular frequency, I'm less sure what to do.

Last edited by kenpeter; 15th February 2010 at 11:08 PM.
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Old 17th February 2010, 12:48 AM   #10
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i'm not so sure about that, the DSP processing delay would have to be undetectable by the musician. that's not easy with standard audio DSP devices.. maybe something with higher horsepower, like a blackfin could do it.
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