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Old 17th July 2003, 11:53 AM   #101
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Default question and suggestion

Andy,

how do you deterimine the dBcs? Do you use the cursor to measure the level of the fundamantal and the harmonics, or is there a more elegant way?

I am not sure about numerical problems in LTSpice, but how about doing away with the feedback loop, decreasing the input signal to a few nV and looking at the output? This would allow you to assess open loop linearity directly. You could also look at individual stages.

Regards,

Eric
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Old 17th July 2003, 12:51 PM   #102
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Quote:
Originally posted by janneman
I'm still sceptic, but so far I only have my gut feeling to back it up.

Jan Didden

I am in the same camp. It just doesn't make sense that odd harmonic distortion goes up on a symmetric design. I just couldn't mentally envision the picture Pabo was trying to paint for us. Maybe a picture or two will help?
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Old 17th July 2003, 12:58 PM   #103
mikelm is offline mikelm  England
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Default A new way to look at it

So, back to my original question: how about the subjective effects of asymmetrical vs symmetrical (generalised opinions are perfectly acceptable)?

I have not posted in this area before so...Hi

Iv'e been following this thread with interest - thanks to all - I feel like I have learnt a lot

I do not have a formal electronics training and certainly not a mathtamatical understanding of complex circuits but over the years having designed and built a few amps I have tried to build up an understanding of what is going on in cct's and what features may cause what effects.

many years ago I read an article by John Linsley hood where he stated that for reasons he could not understand he had found that s/e cct's sound more 'natural' that fully symmetrical ones, which he said somehow seemed to emphasise detail in a way that made them less appealing. Since then I have been trying to get my mind around why this could be.

I often enjoy trying to find principals in nature that apply across different disciplines

Now in advance I hearby appologise to the strictly objective science crowd that don't like subjective esoteria. We are now entering an area you may not appreciate

I can't help noticing the paralells between this subject and the ancient eastern principles of Yin & Yang.

among many other qualities, Yang is the active principle, Yin is the passive principle

generally for balance and harmony to exist is any living system the two principals have to be in balance.

In symetrical designs, particularly in the driver cct in is quite clear ( at least to me ) that this balance is missing - both elements are active. However in s/e designs, wether it be an inductor, a resistor, a current source or even a bootstrap this part of the cct is passive in as much as it simply responds to the what the signal is doing elsewhere

perhaps this balance created between the active & passive emements of s/e cct's somehow corresponds to this principal of balance in nature and consequently we find the sound to be SUBJECTIVELY pleasing.

wether this corresponds to any electrical values that we can measure I don't know but personally I like the analogy. It seems elegant.

I hope there my be perhaps one or two of you with whom this strikes a chord. I certainly do not intend to get into a huge discussion about wether this is right/wrong, relavent/irrelavent etc.

cheers

mike
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Old 17th July 2003, 01:31 PM   #104
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Hi Mike

It is not fully known yet what makes an amplifier sound pleasing or not.
Some points many (i.e. still not everyone) accept as valid are:

It is not only the amount (i.e. quantity) of distortion that counts, it is also the kind of distortion (i.e. quality).

The distortion spectrum should be falling with frequency as this is the case for most natural sounds.

Even order distortion is less disturbing than uneven order distortion since it is more musically related.

The distortion should be lowest at low volumes and not vice versa, since the overtone spectra of most musical instuments behave like this as well (the instrument where this is most obvious is a Saxophone, where the sond gets more aggressive the louder it is played).

I by myself like symmetrical designs. But one basic rule has to be taken into account: Circuits that generate symmetrical distortion mainly generate odd order harmonics while circuits that distort a signal asymmetrically mainly generate even order harmonics. Thats why some asymmetrical circuits possibly might have advantages over symmetrical ones.

Furthermore any nonlinearities generate IMD, i.e. "signals" that have not been there before and which are a dependant upon input signal AND the kind of nonlinearity involved. While the usually low distortion itself might barely change a single instrument's timbre, the IMD products can disturb clarity (i.e. "air" ?) as soon as multiple instruments are played. The higher order the nonlinearity the more complex the "signal-carpet" created by IMD.

We might not forget that it is impossible to build THE perfect amplifier, so properties have to be balanced to be most pleasing, whatever this means.

Regards

Charles
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Old 17th July 2003, 01:54 PM   #105
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Quote:
Originally posted by phase_accurate
Circuits that generate symmetrical distortion mainly generate odd order harmonics while circuits that distort a signal asymmetrically mainly generate even order harmonics.

Charles

any insight into why that's the case?
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Old 17th July 2003, 02:13 PM   #106
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Simply because Mr. Fourier wants it like that !

Regards

Charles
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Old 17th July 2003, 02:31 PM   #107
MBK is offline MBK  Singapore
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Hi Mike,

I also like to draw parallels between fields. I'm not sure about the yin and yang thing, but the comparison I always see is between audio and photography. Same problem, different realms.

Some observations on the significance of different aspects of "distortion".

Example: scanned photos. Like in Audio, at this point the recording process is out of reach, we can only hope the "reproduction" of stored info to be rich in information, and pleasing.

Now for instance, picture defects such as out of focus on the edges, or slight hue problems, usually don't detract from a picture. But a single piece of dust in an important bright area is a major annoyance. This represents a tiny fraction of the information of the picture "lost" - but it is highly visible. I have no problems imagining a similar effect in Audio: that 0.5% of distortion A matters far less than 0.001% of distortion B.

Incidentally other parallels come to mind: say, that a highly compressed JPG scan of a photo often actually accentuates the defects of the lens. I have pics taken with high quality Zeiss lenses, and others with lesser lenses. Some lens effects (vignetting, very low gradients of shade and intensity), and differences between lenses, can best be seen on highly compressed pictures. Translated to Audio this explains how the distortionor loss of the most "insignificant" (the least significant bit) can be the most objectionable, and the easiest to hear. Other obvious distortions may be easy to measure but less important to perception.
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Old 17th July 2003, 02:44 PM   #108
jcx is offline jcx  United States
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Quote:
Originally posted by andy_c
The distortion of the complementary amp with cascoding is much lower now.
...
I'm at a loss to explain this. I'm running out of time, so I'll just post the schematic of the single-ended design and comment more later.

Of course you don’t understand, you haven’t read Cherry and Hawksford!

Cherry’s “ESTIMATES OF NONLINEAR DISTORTION IN FEEDBACK AMPLIFIERS” JAES V48#4 2000 p299-313 provides a method of calculating individual component distortion mechanisms contribution to overall distortion of a amplifier, more importantly it gives a intellectual framework for reasoning about distortion and feedback in a feedback amplifier. The example calculations for a simple audio amplifier circuit in the article also would give you some perspective that will make much of this wandering about in simulation land more directed and informative.

On the cascode VAS results, Hawksford’s REDUCTION OF TRANSISTOR SLOPE DISTORTION IN LARGE SIGNAL AMPLIFIERS, M.O.J. Hawksford, JAES, vol.36, no.4, pp.213-222, April 1988 explains that the major distortion in the VAS isn’t a “simple” exponential series from gm/Vbe but the more difficult to model nonlinear CB and CE conductances. Hawksford goes on to give one of the few explanations of the “correct” version of the cascode connection I have seen (of course Pass gets this right too) and Hawksford shows the Baxendall “Super Pair” (without attribution, but no one else I’ve read seems aware of its origin either)

Both are well worth buying from the AES ($5 ea for pdf) if you can’t find them in the library (think how much of your time you’ve spent simulating; I guarantee the learning rate reading these papers is much higher)

When you linearize your VAS with the Super Pair as I've shown in the Class A headphone amplifier thread, ( < -140 dB 2nd harmonic @ 2KHz) then you might be able to see input Vbe distortions that follow the even harmonic cancellation you've spent so much time looking for
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Old 17th July 2003, 02:47 PM   #109
andy_c is offline andy_c  United States
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Default Re: Question to ANDY_C

Quote:
Originally posted by janneman
Other thing, if I calculate roughly the influence of the comp caps, assuming a 20x CLG, the 50pF comes to about 1k impedance around 15kHz, no? In parallel with the Rc in the assym case lowers the open loop gain there with 6dB wrt the symm case, because in the symm case the 1k in // with the CS load would still come out around 1k, no? So, I'm not sure how you tweaked the OLG to be the same, but I feel there is a 6dB diff between the two.
Jan,

The capacitance seen by the collector of the input stage should be, by Miller's theorem, Ccomp * (1 - Av), where Ccomp is the compensation cap and Av is the voltage gain of the VAS. We knowl that Av is large and negative, but we don't know exactly what its value is without simulating it - because of the very high collector load impedance of the VAS. So the capacitance seen by the input stage should be quite high, much larger than Ccomp.

I'm at work now so I can only post short replies at long intervals.
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Old 17th July 2003, 03:03 PM   #110
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Quote:
Originally posted by jcx
Hawksford goes on to give one of the few explanations of the “correct” version of the cascode connection I have seen (of course Pass gets this right too) and Hawksford shows the Baxendall “Super Pair”
Thanks for pointing out those articles. What is the "correct" version of the cascode circuit? I thought at least that was pretty well defined- it means using a driver and a cascode transitor of the same polarity, connecting collector and emitter together and connecting the base of the cascode transistor to a voltage source that referenced to the rail and very stiff.

Is correct cascode the same thing as super pair?

Regards,

Eric
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