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Old 2nd November 2003, 04:10 AM   #1
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Default Single or dual differential?

I observed (maybe wrong), that a power amp with single differential gives more focus in sound reproduction. It is not about nice to listen to, but more focus.
I bought Douglas Self's book and asked him why he didnt write at all about power amp with dual differential (transistor pairs, 1pair npn and 1 pair pnp). He answered that he suspected that dual differential power amp is not as linear as single differential amp.
Is it true that single differential amp (like aleph) is better than dual differential amp (most of the pro-audio amp, like crown, crest, uses this dual differential).
What is really the different between building power amp with single differential and dual differential? Maybe both can produce music that is nice to listen to, but is single differential more focus (more linear, like Douglas Self said?)
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Old 2nd November 2003, 05:09 AM   #2
jcarr is offline jcarr  United States
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lumanauw:

Which do you mean, dual differential, or complementary differential?

Example of dual differential circuit:

http://k-amps.8m.com/cgi-bin/i/Power...eda_mosfet.jpg

Example of complementary differential circuit:

http://www.klausmobile.narod.ru/indu..._schdetail.gif

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Old 3rd November 2003, 07:25 AM   #3
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The first schematic (Kaneda) is single differential (TR1,2), with upper cascode (TR3,4). The second schematic (schdetail.gif) is dual differential (maybe I should say complementary differential) the upper is NPN (TR1,2), and lower is PNP(TR3,4).
Thanks for the example. This is exactly what my question is all about. Single differential (like kaneda) VS complementary differential (like schdetail.gif).
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Old 5th November 2003, 09:05 PM   #4
PRR is offline PRR  United States
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> single differential gives more focus in sound reproduction.

This has to be an over-generalization. Either type can be good or bad.

The choice is mainly about the fact that a NPN-PNP design can give outputs at both rails, which affects design of the second stage. It may be better, it can be worse.

Anyway: The difference you describe probably has a lot to do with driver and output stage linearity, more than input topology.

> Douglas Self... he didnt write at all about power amp with dual differential (transistor pairs, 1pair npn and 1 pair pnp). He answered that he suspected that dual differential power amp is not as linear as single differential amp.

I'm not sure I agree that it is worse, but for the levels used in audio input stages it should be about the same.

If the effective input voltage, under feedback, rises from a milliVolt to over 20mV, some types of NPN-PNP complementary diff-pair can hold "linearity" a little further than a single pair. But operation in that range isn't the "very-good linearity" we expect in audio. It may be useful in comparators and other mainly non-linear devices which need a semi-linear range for smooth settling.
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Old 6th November 2003, 10:07 AM   #5
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It is true that complementary differential (pair of npn for +voltage and pair of pnp for -voltage) makes more sense if we see them. It makes the whole circuit more reasonable, and more symmetrical. Upper differential is working for all upper transistors, and lower differential is working for the negative half. Especially if we look at high power amplifier (pro-audio) where the rail can reach more than +/-100VDC, maybe this complementary differential is the only answer, to divide the working voltage (Vce) only from +rail to ground, not to -rail. This is due to the dissipation limitation with certain current we must handle (and also limitation from maximum Vce for transistors)

But when I notice the comment from Mr.Douglas Self, and see the design of Mr.Nelson Pass, this question rises in my mind. Mr Pass once told me to read the article he wrote about how mosfets works. In his paper, I learned that N mosfet and P mosfets have very different Vgs thereshold. And when I read the history of bipolar transistors, I also learned that PNP and NPN are very different. NPN is what we found first. It is the nature of transistors. PNP is found long time after NPN, and using very different moulding from NPN. We can see from datasheets, especially mosfets (like IRF540 and IRF9540), the N mosfets always superior in datasheet than P mosfets (with the same shape).

In power amp, differential stage is what I think the most important stage in shaping the output. It works with small signal, and everything began here. So every little difference or mismatch will be sighted as all the signal amplifies. We can cover some of this mismatch by putting feedback, forcing to fix the errors. But the question is "which is better?"

So if the NPN and PNP are not the exact oposite (with exact behavior) will my original question here makes sense?
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Old 6th November 2003, 11:48 PM   #6
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> high power amplifier (pro-audio) where the rail can reach more than +/-100VDC, maybe this complementary differential is the only answer, to divide the working voltage (Vce) only from +rail to ground, not to -rail.

Nope. Same voltage either way.

> the history of bipolar transistors, I also learned that PNP and NPN are very different. NPN is what we found first. It is the nature of transistors. PNP is found long time after NPN

Sounds like a highly simplified history. From 1954 to 1964, THE most common transistor was the Germanium, and 99% of those were PNP, from computers to telephone switchgear to car and pocket radios. The reason "all" cars (not just Cadillac) went to 12V Negative Ground was so we could bolt the collector of a monster Ge PNP to the radio case for cooling. Then came Silicon, which is slightly easier to make in NPN. But both polarities of both materials were available from the beginning, and there is an exhaustive survey of complementary topologies in a book I have that was published just a few years after the transistor was invented.

But history does not matter. Yes, a PNP BPJ has slightly higher ohmic loss and Vbe than an NPN BJT of the same size, but the difference is very small and you can find pairs of devices where the difference from NPN to PNP is less than the difference from one transistor to the next of the same type. In MOSFETs the difference is larger, enough so you "never" use a P-type for switching if you can avaoid it. But in linear or AB amplifiers, you always add some fixed resistance (or other feedback) to set bias current and reduce nonlinearity. This has to be larger than the devices' own internal resistance, so the difference between the P-side and the N-side becomes very small.

> differential stage is what I think the most important stage in shaping the output. It works with small signal

There are many ways to look at amplifiers, and you do have to look at the design from all directions.

But in one big way, the input stage is the "easiest". The output devices have to swing current from 0 Amps to 5 Amps, and large voltage swing too. The input devices can be designed (by having plenty of current gain in the whole amp) to swing only from 99µA to 101µA with nearly constant voltage (and low voltage if you want). While they have a very critical job, they do not have to carry the strain of pushing big current or even large current changes. In principle you can make the signal across the input pair(s) as small as you like, and push distortion down to the vanishing point and accuracy up as high as you could ever hope for.
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Old 7th November 2003, 02:45 AM   #7
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You folks are worrying too much. The complementary differential has lower distortion, all else being equal. If you build a design like the comp symmetry example shown, just put a large cap across the second stage base to the nearest supply. Either side will do. I found an increase of distortion of 5 times in an example that I measured more than 30 years ago.
Think it through. First you lose GAIN (6db) because you have only one working input stage. Second, you INCREASE even harmonic production, because you are not equally driving the second stage transistors. Try it and see. Don't worry about the intrinsic mismatch in N vs P transistors or fets, it still is better to use both together.
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Old 8th November 2003, 12:54 AM   #8
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> You folks are worrying too much.

Who is worrying? lumanauw has an idea that one topology is better than another. In the hands of a skilled experienced designer, this may be true (though all-else is never equal). But at lumanauw's apparent level of learning, it is important to realize that no topology is "unavoidably" better than another. We can easily design a "dual differential" that is bad (heck, I've done that), and a "single differential" that is better. Many sing-diff inputs work very well. Complementary diff has some "obvious" advantages, but they can be botched with careless detailing. lumanauw's theory may be right. But he needs to understand that the devil is in the details.

That's where Doug Self is useful. He has worried the Classic Topology to the bone, and pointed out many "small" details where most designs could be significantly better. The concept and many of the details can be applied to other topologies. But you have to learn them before you can apply them. Self teaches this well. He gets fabulous performance out of something that looks like a 1972 Fisher. We can criticize that he works mostly by measurements, not by ear. But his details look like things that "should" be better to the ear, and others with golden-ears can judge his work for themselves. There are details where I disagree with his theory, but he is honestly investigating and sharing which is all anybody can do.

> you lose GAIN (6db)

Too true. Though gain in BJT is cheap. And with overall feedback and slow output devices, gain has to droop inside the audio band to keep the loop stable. That -tends- to lead to a design where almost all gain is made in one stage, usually the second stage, so the total gain can be compensated to one-pole response. With the huge potential gain of BJTs, this usually means deliberate reduction of input stage gain with emitter resistors.

FETs is different. Gain is harder to come by. In a sense, they already have the "emitter resistors" built-in that we add to many BJT stages. If you can find semi-matched complements (I sure agree that small mismatching is no big deal) then all-complementary is one preferred topology.

> you INCREASE even harmonic production, because you are not equally driving the second stage transistors.

Less if you detail so the standing current is very-large compared to the signal current (base current in the output stage). This is one place where many BJT designs come up a dollar short.

I'm also, at my current level of understanding, becoming less concerned about THD and more about distortion spectrum. A spectrum with no even-order is un-natural. If the slope of the spectrum is steep and/or all products are far below system noise, that won't matter. And I'm not sure I can equate lumanauw's observation "more focus" with distortion spectrum.
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Old 8th November 2003, 11:45 PM   #9
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I would like to make the case for complementary differential topology:
It isn't the INPUT stage that is lowered in distortion, it is the 2'nd stage which is usually single ended and has to develop almost all the gain for the amp, which improves. This problem was first addressed with 'bootstrapping' using a cap connected to the output of the amp to give positive feedback and increase the driver load impedance. The next approach was to use a constant current source as a load, favored even today by Doug Self. Finally, the equal driving of both driver transistors, either with a current mirror, or with a complementary differential input.
I have used each of these approaches over the last 35 years, and personally, I prefer the complementary differential fet input. Don't tell Doug Self, but fets actually work darn well as input stages, and have many advantages, such as no need for an input capacitor, and very high slew rate operation, without any noise tradeoff. Also, they tend to be more RFI resistant, because their input diode is off, rather than conducting.
While I have the greatest respect for what Doug Self has published, please don't box yourself in a corner by thinking that that his input is the only or necessarily the best approach to circuit design.
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Old 9th November 2003, 04:29 AM   #10
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This is an extremely well-informed thread. Comments from Jonathan, PRR and John are outstanding.

I agree strongly with PRR's comment about distortion spectrum and its affect on musical perception. Distortion spectrum is, in my opinion, the crucial factor; you want something with maximal H2, progressively decreasing with increasing harmonics, and least, if not zero, from H6 onwards. The total distortion figure is not too important, though intermodulation performance is VERY important. The significant thing is to have a distortion spectrum which is not too dissimilar from that found in the natural world. High levels of odd order, with evens missing, or very small, is not natural sounding.

A complementary dual diff will greatly minimize H2, H4, H6 - even order - while not much increasing H3, H5, H7 - odd order. A single diff will have quite high H2, some H3, some H4, and some H5. This observation is counter-intuitive because a complementary dual diff 'looks' right. The point concerning driving a fully symmetrical voltage amplifier (more correctly described as a transresistance amp as it converts current input to voltage output) is well taken. There are linearity advantages to a fully complementary VAS, but, as before, even order harmonic generation is nulled.

I agree with John that pretty much any topology can be made to sound good. The schematic is just the beginning; it takes careful dimensioning, component choice and layout to produce good sonics, but it is possible voice almost anything to sound good. Good sonics are probably equal part topology, dimensioning, component choice and layout, and a huge amount of work is required to make it happen. You will NEVER know by simply examining the schematic, any more than you can definitively gauge the emotional impact of a symphony by studying the score. My personal take is for a progressively reducing harmonic spectrum, adjusted so H2 predominates, but with very careful 'voicing' which is really only achievable with lots of listening. It is also true that some people like the SE sound, while others the PP sound. This is even true of Class A versus AB; for myself, I prefer Class AB as it seems somehow more dynamic and lifelike, at least in my designs. You can't please 'em all, and you'll die trying!

We tend to concentrate on the technology in straight electrical terms, but much work remains to be done on the psycho-acoustic phenomenon, the subjective aspects of why we like what we like, and why we tend to become religious zealots about it. More work is also required in the area of harmonic spectrum, particularly as it relates to musical scales, and profiling musical instrument tones. Much of this work is actually known, but for some reason it does not seem to be found in the audiophile and designer communities.

Cheers,

Hugh
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