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Old 21st April 2013, 08:33 PM   #2701
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I agree. I'd really like to see a SPICE analysis that takes the SOA curve, it's time constants, and runs this against an output stage with a test signal to show exactly what frequencies are involved in what failures, at what loading, and so on.
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Old 21st April 2013, 08:54 PM   #2702
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Quote:
Originally Posted by michaelkiwanuka View Post
Hi Bob,

You might consider including calculations for your SOA protection circuits in your second edition. This would aid folk in designing their own protection circuits.

Secondly, you could show how designers may confirm and tweak their calculated SOA protection component values in SPICE, as well as simulating the effect of ambient temperature on the protection locus.

All this information is available in an article you can have by sending me mail.
Hi Mike,

This is an excellent idea. Thanks for sending me that article. Simulating protection circuits in SPICE makes sense. I'm also tempted to discuss some real-world lab tests that can safely exercise protection circuits. A long time ago I created a nasty load that included an inductor, and I was able to provoke the really nasty protection spikes that often occur (we used to call those "tweeter eaters"). This same sort of thing could easily be demonstrated in a SPICE simulation.

We always need protection, but fortunately nowadays we have more SOA and can set the triggering thresholds higher so the protection circuits come into play much less often.

Cheers,
Bob
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Old 21st April 2013, 09:42 PM   #2703
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> fortunately nowadays we have more SOA
True, boys as well as girls. But fortunately? No!
(in Dutch SOA means: Seksueel Overdraagbare Aandoening. Translated: STD, sexually transmissible disease)

Cheers,
E.
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Old 22nd April 2013, 02:19 AM   #2704
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Quote:
Originally Posted by Edmond Stuart View Post
> fortunately nowadays we have more SOA
True, boys as well as girls. But fortunately? No!
(in Dutch SOA means: Seksueel Overdraagbare Aandoening. Translated: STD, sexually transmissible disease)

Cheers,
E.
I never was very good with foreign language :-).

Those TLA's get me every time (Three Letter Acronym).

Cheers,
Bob
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Old 22nd April 2013, 02:49 AM   #2705
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Default SOA etc

Quote:
Originally Posted by Bob Cordell View Post
I'm also tempted to discuss some real-world lab tests that can safely exercise protection circuits. A long time ago I created a nasty load that included an inductor, and I was able to provoke the really nasty protection spikes that often occur (we used to call those "tweeter eaters"). This same sort of thing could easily be demonstrated in a SPICE simulation.

We always need protection, but fortunately nowadays we have more SOA and can set the triggering thresholds higher so the protection circuits come into play much less often.
Yes please Bob.

I'm really a speaker man and would like to say something about speaker loads.

Firstly, the Otala test signal is absolute rubbish and from fairy-tale land .. like his zillion V/us slew rates.
__________________

I've only found one real life case where the current demand of a speaker exceeds what you'd expect from its impedance curve.

A bass unit usually has a large voice coil with many turns and a hunk of steel (the polepiec) in the middle. This gives it a large Inductance and hence impedance. However, as the coil moves, the hunk of steel may also move out of the coil and the Inductance (and Impedance) drops dramatically. The current through the coil then increases which tends to push the coil further out of the gap.

This is an unstable phenomena and the coil jumps out of the gap with nasty consequences.

There are various 'cures' but a simple dodge is to let the suspension limit as the coil reaches its 'linear' Xmax limits. Rather non-intuitively, distortion near the limit is reduced when you do this.

The bass units that jump out of the gap are from makers who believe a static 'linear' compliance gives dynamic linearity.

I think Don Barlow did an AES paper on this but it might have been an internal Engineering Memo.

Apart from this, the current / SOA demands can be predicted from the speakers complex impedance.
_______________

From the SOA protection point of view, double slope or even high single slope protection was prone to what we used to call 'snap crackle pop' protection cos this was what it sounded like when triggered ... either by reactive loads or the above. (Your 'nasty protection spikes' are part of this)

Dem we de days of serious double slope SOA protection in big (and small) amps.

Simple current limiting was the 'best sounding' protection when triggered.

But as you say, since the late 80's we have much better devices available. IME, a 50W @8R amp can get away with simple fuses in the PSU leads.
______________________

PTC 'fuses' (eg RXE075 PTC Fuses - Speaker Protection - Jaycar Electronics) can be very closely matched to speakers (and amps?). Unfortunately, when triggered and automatically reset, they take some time to regain their Low resistance.

In most cases, this leads to less than a fraction of a dB of power loss but the marketing department always complains.

Last edited by kgrlee; 22nd April 2013 at 02:53 AM.
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Old 22nd April 2013, 03:22 AM   #2706
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Quote:
Originally Posted by kgrlee View Post
This is an unstable phenomena and the coil jumps out of the gap with nasty consequences.
JBL tech. notes mention this. I always assumed it was their research.
Who first published?
Had not considered the implications for the amp.

Best wishes
David
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Old 22nd April 2013, 04:02 AM   #2707
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Quote:
Originally Posted by Dave Zan View Post
JBL tech. notes mention this. I always assumed it was their research.
Who first published?
Don Barlow did this work in the 70's. I'm sad his name doesn't even appear in AES lib.

He's best known for the LEAK sandwich speaker but he was a polymath who made many contributions to the state of the art in speakers, turntables and pickups.

I was privileged to have worked with him in his last days at LEAK/Wharfedale.

[edit]Dr. Barlow appears as DA Barlow. I note a 1991 JBL paper credits him for something. So do Mills & Hawkesford in 1986.[/edit]

Last edited by kgrlee; 22nd April 2013 at 04:20 AM.
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Old 22nd April 2013, 04:05 AM   #2708
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Quote:
Originally Posted by kgrlee View Post
... since the late 80's we have much better devices available. IME, a 50W @8R amp can get away with simple fuses in the PSU leads.
.. but the amp must be designed to be nice to the speaker when it loses a rail.
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Old 23rd April 2013, 11:43 AM   #2709
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Quote:
Originally Posted by kgrlee View Post
Yes please Bob.

I'm really a speaker man and would like to say something about speaker loads.

Firstly, the Otala test signal is absolute rubbish and from fairy-tale land .. like his zillion V/us slew rates.
__________________

I've only found one real life case where the current demand of a speaker exceeds what you'd expect from its impedance curve.

A bass unit usually has a large voice coil with many turns and a hunk of steel (the polepiec) in the middle. This gives it a large Inductance and hence impedance. However, as the coil moves, the hunk of steel may also move out of the coil and the Inductance (and Impedance) drops dramatically. The current through the coil then increases which tends to push the coil further out of the gap.

This is an unstable phenomena and the coil jumps out of the gap with nasty consequences.

There are various 'cures' but a simple dodge is to let the suspension limit as the coil reaches its 'linear' Xmax limits. Rather non-intuitively, distortion near the limit is reduced when you do this.

The bass units that jump out of the gap are from makers who believe a static 'linear' compliance gives dynamic linearity.

I think Don Barlow did an AES paper on this but it might have been an internal Engineering Memo.

Apart from this, the current / SOA demands can be predicted from the speakers complex impedance.
_______________

From the SOA protection point of view, double slope or even high single slope protection was prone to what we used to call 'snap crackle pop' protection cos this was what it sounded like when triggered ... either by reactive loads or the above. (Your 'nasty protection spikes' are part of this)

Dem we de days of serious double slope SOA protection in big (and small) amps.

Simple current limiting was the 'best sounding' protection when triggered.

But as you say, since the late 80's we have much better devices available. IME, a 50W @8R amp can get away with simple fuses in the PSU leads.
______________________

PTC 'fuses' (eg RXE075 PTC Fuses - Speaker Protection - Jaycar Electronics) can be very closely matched to speakers (and amps?). Unfortunately, when triggered and automatically reset, they take some time to regain their Low resistance.

In most cases, this leads to less than a fraction of a dB of power loss but the marketing department always complains.
Hi kgrlee,

Thanks for this post. These are all excellent posts. I'm a "speaker guy" too, although I don't consider myself an expert on the electromechanical aspects of drivers. Your obervations about what happens to woofers when they are pushed to excessive excursion are very helpful.

Cheers,
Bob
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Old 23rd April 2013, 01:16 PM   #2710
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Default LT1166

Hi Bob,

Reading your excellent book, I found something that might lead to confusion. It's about Miller compensation in conjunction with a LT1166, page 547..548. You wrote: "If the Miller capacitor is just connected to one end of the LT1166 dynamic bias spreader, distortion will result because this node contains the signal plus half the non-linear spreading voltage. Instead, Miller capacitors of equal value (C1 and C2) should be connected from each end of the bias spreader back to the input of the VAS. This will force the virtual center tap of the bias spreader to be the desired linear representation of the signal."
(last sentence underlined by me)

First, this is the best you can do. The text however, suggests that all non-linearities caused by the dynamic bias spreader are eliminated. Regrettably, this is not the case. The center tap lowers the distortion, but not completely. It's still higher compared to a fixed bias. Please, see also this post.
Perhaps you could elaborate on this subject in the next edition of your book.

Cheers,
E.
__________________
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goed verliezen dan dooft het licht…(H.M. van Randwijk)
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