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Old 4th October 2010, 03:42 AM   #501
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1000W is only twice as loud as 100W. Someone who has 1kW is going to want to play louder than someone with just 100W, otherwise what use has all that extra money gone towards? So more watts will never be the answer to clipping.

Did you get normal or expedited shipping for the book? Still waiting for mine here...
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Old 4th October 2010, 03:55 AM   #502
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normal shipping to save cost. The book contains so many valuable information. It may be classified by the mainland custom.

some commercial amps may have their protection partially acting too early. If we can disable the circuit, the amp may sound differently.

My amp clips frequently. It is fun to listen the difference between clip or not.
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Old 4th October 2010, 04:06 AM   #503
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Yeah, me too - 4 other books in the same order which were in stock arrived ahead of schedule in around 2 weeks. Can't have been on a ship Yours must have travelled by air too to arrive so quickly.

My first amplifier was a NAD3020, I still have it even though I recall selling it on to my younger sister at some point. She gave it back to me to fix as its volume control had gone noisy - never did get around to sorting that. This amp had as one of its USPs 'soft clipping' - I haven't examined the circuit but I believe it operates prior to the output stage. There's a slide switch on the rear to defeat it.
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Old 4th October 2010, 04:13 AM   #504
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Originally Posted by Lynn Olson View Post
Some loudspeakers are more sensitive to variations in source impedance (damping factor) than others. Any speaker using passive 4th-order crossover filters, or the acoustical version, which is a vented box, is going to be more sensitive to source impedance than a speaker using low-Q alignments.

For the raw driver, damping factor doesn't matter that much, since voice coil resistance (Re) is in series with the inductors in the crossover, speaker wiring resistance, and the source impedance of the power amplifier. With a typical voice-coil DCR of 6 ohms, inductor resistance anywhere from 0.1 to 0.5 ohms, and speaker cable resistance anywhere from 0.05 (very unusual) to 0.3 ohms (for long runs), an amplifier source resistance much lower than 0.2 ohms (which is a damping factor of 80) is basically wasted. From the perspective of the driver, it is the ratio between DCR and all the source impedances that matter - and once this ratio is much beyond 40 or so, there's not much difference to the response or distortion of the driver.

4th-order crossovers (electrical) or 4th-order (vented) cabinets are a little more touchy, simply because they are 4th-order filters, and are more sensitive to small parameter variations. Nearly all the filter damping is coming from the power amplifier, not resistive terms in the filter. Unfortunately, in loudspeaker drivers, the Q is not that well controlled.

The cone mass is very predictable; 1% repeatability is common and expected. Likewise, the number of turns in the gap is well-controlled, since this is wound on a machine with a turns counter. Compliance is not well-controlled, with a 20~30% shift during the break-in process. But compliance shifts fortunately have little effect on cabinet alignment or crossover behavior.

It's the BL product that is troublesome. BL product are the turns multiplied by the Teslas of field strength in the gap; L stands for length of wire, which is well controlled, but the effective B field that the VC sees is quite another matter. Overhung voice coils, by far the most common, have a significant portion outside the gap, and the magnetic field lines are not well-controlled in the region (in the gap the field lines are parallel and reasonably equally spaced). Move the voice coil a bit (excursion) and the different portions of the VC are exposed to the nonlinear field structure above and below the gap (which are asymmetric). Various attempts have been made to linearize the out-of-gap field, but even the complex systems are far less linear than what's in the gap - and overhung VC always have mixture of turns exposed to the fields inside and outside the gap. Not only that, the VC is continually moving, so low frequency excursions intermodulate higher frequencies.

The old-school Altec and Lowther approach is an underhung VC, which later used by TAD for their Alnico-magnet woofers. This is inherently far more linear, basically the loudspeaker equivalent to Class A operation. But once the VC starts to leave the gap to get that boom-boom bass everyone loves, then linearity goes downhill quickly. Overhung VC speakers are cheaper to build, which is another reason they dominate 99% or more of the market. Guitar speakers are equal-hung, which gives the most efficiency, but also the most distortion, since any movement at all makes the VC leave the gap.

So unfortunately BL, and therefore Qts, is not a well-controlled, linear parameter. It is not an accident that the specified Qts is what driver manufacturers call a "small-signal" parameter, which is a cute way of saying it is level-dependent. What you get at 40 dB is not the same as what you get at 90 dB. This is inherent to the technology, sorry.

This variation in BL and Q under dynamic conditions has implications for cabinet and crossover design. Since BL and Q are not well-controlled, it calls into question the wisdom of high-order alignments that rely on precise control of the filter elements. Driver mass, cabinet volume, and the passive components in a crossover are well-defined, but cone compliance, BL, and Qts are not, and shift under dynamic conditions in a complex way, with significant stored energy in the spring-return of the spider and high-Q resonances and standing waves of the diaphragm and interior cabinet volume.

There are a lot of high-end speakers these days that have high-order crossovers in vented boxes; they will be much more sensitive to source Z of the amplifier than speakers that use closed-boxes, or resistive vents, and also have low-Q crossovers (2nd-order or lower). But relying on the amplifier to magically "straighten out" what's happening in the VC gap isn't going to happen; the entire amplifier-speaker interface rests on the field-line structure in the gap, and how linearly it moves the voice coil. If the field lines are curved and distorted, then the amp-speaker interface has to follow along, and behave the same way. The entire amplifier is always seen through the prism of the field lines that drive the voice coil. This is true of all magnetically driven loudspeakers, regardless whether they are planar, ribbon, or conventional.

This is where the great Class AB debate about bias stability under dynamic conditions has relevance for loudspeakers. A transistor amplifier is making its transition out of Class A (around a watt or so) at about the same level that a loudspeaker is leaving what is considered the "small-signal" region - in other words, anywhere from 85 to 92 dB SPL at one meter, depending on typical efficiencies. It also tells us that high-efficiency speakers in the 96+ dB region may be a lot less forgiving of dynamic bias shift than less-efficient speakers.
You've also forgotten the effects of the voice coil inductance which is a major source of high frequency distortion. It varies both with the diaphragm displacement and the current in the voice coil
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Old 4th October 2010, 04:28 AM   #505
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Originally Posted by Bob Cordell View Post
Hi Pete,

That A/B test was one of those tests where many people were surprized in several ways, including myself. The tube amp in question was one that I had designed myself solely for the purpose of hearing the difference between a tube amplifier and a SS amplifier where the listening conidtions were well-controlled AND I was familiar and trusting of both designs. Up to that point, I had not designed a tube amplifier since I was in high school. So I designed the tube amp with more modern knowledge and some updated techniques. For example the tube amp used transistor current sources for the tube differential pairs and screen voltage supplies that were regulated with MOSFET pass transistors. Some may call this cheating :-). The tube amp is briefly described on my web site (CordellAudio.com - Home).

Anyway, I was surprized at how good the 35 wpc tube amp sounded.

I subsequently took the amp to a tube-head meeting where it was compared with other tube amps in the usual uncontrolled way. It was well-liked, so I guess it had at least some legitimate tube pedigree despite some of the modern circuit techniques. (those push-pull KT88's, loafing at 35 wpc, really stepped on the SETs).

At the HE2007 workshops, my recollection is that a decent number of people could hear a difference, but about half were wrong in identifying which was the tube and which was the solid state. Similarly, I think that about half preferred the one that was solid state and about half preferred the one that was tubed.

Cheers,
Bob
One reason why any difference was heard was that the peak level was well over 100W (or was it 200W, 230W comes to mind) as I recall from the SS amp. It was level matched so the tube amp (35W) was well into clipping. It was amazing that they sounded as close as we observed since this does not in any way meet the requirement for no clipping. This demonstrates that the tube amp is an excellent compressor!

I've been thinking for years that I'd like to rebuild a ST-70 with 6550s or KT88s seems tubes need to be derated also.

Richard Modafferi likes the idea of using emitter followers in tube power amps:
Audio frequency power amplifier with ... - Google Patent Search
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Old 4th October 2010, 02:11 PM   #506
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Originally Posted by PB2 View Post
One reason why any difference was heard was that the peak level was well over 100W (or was it 200W, 230W comes to mind) as I recall from the SS amp. It was level matched so the tube amp (35W) was well into clipping. It was amazing that they sounded as close as we observed since this does not in any way meet the requirement for no clipping. This demonstrates that the tube amp is an excellent compressor!

I've been thinking for years that I'd like to rebuild a ST-70 with 6550s or KT88s seems tubes need to be derated also.

Richard Modafferi likes the idea of using emitter followers in tube power amps:
Audio frequency power amplifier with ... - Google Patent Search
As a solid-state centric designer for many years, my first reaction on hearing the tube amp was that it doesn't have a right to sound that good.

BTW, I always try to keep in mind the distinctions among:

amplifier is musical
amplifier sounds good
amplifier is neutral

Ideally, a well-behaved neutral amplifier always sounds good, but some program material and loudspeaker combinations sound better with an amplifier that is synergistically not neutral.

It seems that tube amplifiers have always had a reputation of being more musically tolerant to clipping than SS amps. We often hear something like "A 35-watt tube amp can play louder than a 35-watt SS amp".

I included optional dynamic soft clip (I called it the Klever Klipper) into my Super Gain Clone (SGC) amplifier and also described it in a chapter of my book. It does indeed give the SGC a more tube-like tolerance for signals that will clip it. It sounds good, even though THD begins to rise significantly at power levels above about 1/3 power in the SGC, which is rated at 40-50 watts. I show a curve of THD vs power for the SGC in the book with the Klever Klipper turned on and off.

Cheers,
Bob
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Old 4th October 2010, 07:04 PM   #507
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As applied to the "all amps sound the same" people, witless is a kind word. It implies that they are not thinking about how indefensible their absolute statement is. It is very likely that they are saying it to bait and insult amp designers; in which case, a much darker adjective would apply.
not everyone is capable of hearing a difference.
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Old 4th October 2010, 11:57 PM   #508
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Originally Posted by abraxalito View Post
Yeah, me too - 4 other books in the same order which were in stock arrived ahead of schedule in around 2 weeks. Can't have been on a ship Yours must have travelled by air too to arrive so quickly.

My first amplifier was a NAD3020, I still have it even though I recall selling it on to my younger sister at some point. She gave it back to me to fix as its volume control had gone noisy - never did get around to sorting that. This amp had as one of its USPs 'soft clipping' - I haven't examined the circuit but I believe it operates prior to the output stage. There's a slide switch on the rear to defeat it.
Hi abraxalito,

One of the key things of interest in soft clipping circuits is whether they are inside a feedback loop or not. They work much better if they are ahead of the amplifier-proper and not inside the feedback loop, since feedback tends to sharpen up the clipping too much if the clipping is inside the feedback loop.

Cheers,
Bob
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Old 5th October 2010, 12:59 AM   #509
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Quote:
Originally Posted by Bob Cordell View Post
One of the key things of interest in soft clipping circuits is whether they are inside a feedback loop or not. They work much better if they are ahead of the amplifier-proper and not inside the feedback loop, since feedback tends to sharpen up the clipping too much if the clipping is inside the feedback loop.
Hi Bob

You say 'a feedback loop' and then 'the feedback loop'. Are these the same feedback loop? I ask because I've designed soft clipping circuits before that have what might be called an envelope feedback loop. This is in addition to the normal NFB which is applied to the power amp. So they operate in the same way as a compressor - sense the output envelope and feed that back to an AGC stage to lower the overall gain.

In a former job I used to design active subwoofer amps. One of the big challenges there is to handle overloads gracefully - the sound of the compressor played one of the biggest parts in the overall sound of the device.

With a subwoofer, the power supply is one of the most expensive parts so an engineering optimisation is called for to get the most out of it. Supplies sag when under load, and the drive unit's load varies with frequency so a clipper based on predicting what the power supply is going to do is almost impossible. I played with a feedback system from the output rails - I did this by using a rail-to-rail opamp, driven from a resistively divided version of the output rails. It can be arranged that this opamp clips at 90% of the maximum output voltage of the power amp - its clipping generates an error signal which feeds back to lower the gain at the input. There's absolutely no added distortion then below 90% of output, below the clip level of the modelling opamp.
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Old 5th October 2010, 06:53 AM   #510
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errata: page 257 "for this reason, trimmer R22" referring to Figure 12.8 on page 256. There is no R22 in the schematic, so I guess this was referring to R12.

(I'm cheating a bit by reading ahead, but I was thinking to apply the error correction concept to a Leach amplifier circuit I'm using as a test bed, which will also be used to try out the On Semi Thermaltrak parts.)

I'm glad to see some introductory coverage of basic circuits and concepts, which was assumed knowledge in Self's and Slone's books. Not all of us are starting out with EE degrees.
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