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Old 3rd February 2010, 03:29 PM   #51
Eva is offline Eva  Spain
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In loudspeakers, 1% (-40dB) is a typical figure both for H2 and H3 at moderate SPL. Crank it up a bit more and H2 and H3 will rise to 3% (-30dB).

Horns may be 10dB better easily.

Most amplifiers are another 30dB better (too good to care )

Warning: Performing speaker THD measurements may result in permanent changes in your point of view about amplifier THD
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Last edited by Eva; 3rd February 2010 at 03:37 PM.
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Old 3rd February 2010, 03:33 PM   #52
h_a is offline h_a  Europe
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Hi Eva,

what about a ClassD amp with the classic distortion profile (H2>H3>H4...)? Doable? Now that would be even green...

Have fun, Hannes
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Old 3rd February 2010, 03:51 PM   #53
Eva is offline Eva  Spain
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The H3 will be already present. H2 and H4 would have to be intentionally added since the usual ideal modulators have little chances to produce them. A slight mismatch between high and low side switching timings and layouts may be enough for even harmonic production (by output impedance mismatch between sinking and sourcing current, as in class AB).

One thing that I have in mind is a self biasing n-channel MOSFET output stage based in this rush cascode approach.
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Old 3rd February 2010, 07:16 PM   #54
hahfran is offline hahfran  Germany
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I think the Vish circuit is a derivative of Walker's triple cascade design ( QUAD 303) only that the frist lacks the superior thermal stability - dynamic - of the latter.
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Old 3rd February 2010, 07:39 PM   #55
balaboo is offline balaboo  United States
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Hugh;

very nice circuit you have come up with - I built a similar design that was described on the 4Square website many years ago and found it to be a most suitable guitar amplifier.
It is the only design I have tried that clips gracefully and shows no "sticking" tendency.
Mine used bog standard BD139/BD140 2N3055/MJ2955 and I cannot remember what happened to it, or why I have completely forgotten about the Rush cascode ...
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Old 3rd February 2010, 08:10 PM   #56
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Eva View Post
In loudspeakers, 1% (-40dB) is a typical figure both for H2 and H3 at moderate SPL. Crank it up a bit more and H2 and H3 will rise to 3% (-30dB).

Horns may be 10dB better easily.

Most amplifiers are another 30dB better (too good to care )

Warning: Performing speaker THD measurements may result in permanent changes in your point of view about amplifier THD
Sounds convincing but is wrong. The objective measurement does not allow to predict the subjective impression. For example, no one - except musicians - hear the difference tones ( i.e. a Doppler effect) that is produced when a
bass-midrange reproduces a 1 khz signal and a 50 hz signal, the latter with max cone excursion. The 1 khz is frequency modulated by the 50 hz cone motion towards or away from the listener and that is by analysis a non harmonic distortion however composed of harmonics.
What one actually auditively perceives as distorted is not determined by the absolute amplitude of harmonics.
For example:
PhysOrg.com) -- New research shows our brains are a lot more chaotic than previously thought, and that this might be a good thing. Neurobiologists at the University of Maryland have discovered information about how the brain processes sound that challenges previous understandings of the auditory cortex, which had suggested an organization based on precise neuronal maps.

Informally speaking the brain computes from the signals from the auditive sense nerve what you prefer to hear. There is a second mechanism which is very sensitive to fast transients. It is this genetical heritage of predators
that makes transient distortions , switching distortions ( i.e. also time errors ) very unpleasant to us contemporary listeners although their relative amplitude could be in the -80 dB range.
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Old 3rd February 2010, 08:23 PM   #57
AKSA is offline AKSA  Australia
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Distortion is apparently the only thing we can go by, yet it is problematic since two amps with identical THD are not guaranteed to sound the same. There is something else to it; either the single tone tests routinely used do not correlate well with music, or the psychoacoustics are more complex than thought (as Hahfran alludes above), or both. THD is useful, perhaps, but not definitive, topology still seems to hold the key to the X-factor. The amp should not be considered simply an engineering problem; together with the source and the speakers it is a man-machine interface and the perceptual issues of human hearing likely have more influence than we think.

Most people tacitly agree with this, since the final question is always, 'How does it sound?' Even the rusted on THD proponents are concerned about 'how it sounds'. Therefore, we must build as well as theorize and try to keep an open mind.

There are very few push pull output stages which will output more than 97% (without feedback) of the input amplitude. This rapidly worsens if we use large emitter resistors to gain bias stability. Thus a 30Vp input signal seldom rises above 29Vp at the output due to Vbe creep and voltage drops in the emitter resistors, and while this is largely corrected by the nfb loop, the output suffers large signal distortion. This circuit is pretty good at 99.7%, because it uses nfb within itself courtesy of the single ended RC. Moreover it has very low Zout (simulated at 18 milliohms, consistent with the large nfb used, but likely about double this in practice) which obviates any need for global feedback encompassing the input voltage gain section.

Balaboo, thanks for your observations. Like most circuits, this is a hotch potch of different ideas thrown together, but it is unusual and it peforms well. It does seem to have a lot of phase shift, which may not be good, however. Neither does it handle loads less than 8R well because of negative half cycle current limiting, but that could be fixed with a pre-driver. I measure loop gain at almost 67dB; this explains why it is good. It is NOT a conventional feedforward circuit, although it does enjoy a lot of negative feedback.

I have not built it yet. I'm hoping someone else here may do that.

Hugh
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Last edited by AKSA; 3rd February 2010 at 08:48 PM.
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Old 3rd February 2010, 09:03 PM   #58
hahfran is offline hahfran  Germany
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I agree it is topology. I will build the quasi version but am worried about its thermal stability. The NFB will not control thermal runaway. This is an issue because in an active speaker the size of the heatsink is determined by the overall design of the speaker.
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Old 3rd February 2010, 09:17 PM   #59
AKSA is offline AKSA  Australia
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Hahfran,

Replace the three diodes and resistor with a simple npn BD139 Vbe multiplier, mounted atop one of the output devices. Use 1k5 from collector to base, and around 1k trimpot from base to emitter. Use a 22uF cap across collector emitter; believe me, this will stop thermal runaway.

If you wish, you can also reduce loop gain by inserting a 22R resistor between the two emitters of the RC. This could also be useful.

Cheers,

Hugh
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Old 3rd February 2010, 11:17 PM   #60
wahab is offline wahab  Algeria
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strange amp...
it has none of the advantage of current feed back topology,
as well as none of those of differential inputs,
despite mimicking the two...
in respect to hugh s schematic, Q1/Q2 can be seen as a
differential, no matter that the two transistors
are complementary...
feedback is returned to the base of the second stage,
making this amp a voltage feedback one...
the difference with a classic differential is that Q2
has no other current limiting that the one provided by the
collector s resistor s network...
anyway, Q2 s current is highly variable, thus creating lots
of distorsion if the available feeback is not high enough..
i think that it s an inferior design compared to the classical
current feedback design using the same number of active devices..
this latter was largely documented in some motorola s ap notes,
and is capable of -100 db distorsion at 10KHZ, with monotonic
deacrease of harmonic products..
for useful lectures, here again a link to rod s excellent description
of the thing....

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