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The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
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Old 18th February 2018, 01:34 PM   #1
maxlorenz is offline maxlorenz  Chile
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Default The AMNESIS amp: a good amplifier, like a gentleman, has no memory.

First of all, big thanks to Hepha´stos (ascended Master), to Peufeu, to Douglas Self, to uncle Charlie, DestroyerX, and to all those that investigate on thermal memory distortion and finally to everyone who is willing to help me here.

This is not a thread about THD. It is a thread about experimenting with putative Low Thermal Memory Distortion or LTMD circuits.


Edit:

Being december the 26th 2018, I feel it is safe to say that I achieved my goals of transparency and dynamics for my experimental amplifier in the form of the "Amnesis QUAD bootstrapped" amplifier, meaning bootstraps in all sections of the amp...bootstrap being a form of positive feedback, there is a tendency to oscillation , so if you feel inclined to build it, it is recommended to have a scope. Sound quality is worth it.

See official schematics in post #379.
There are many output variants that can be explored, like simple EF, simple driver with bootstrapped outs, Bootstrapped Darlington ß la Pass, and the still untamed boostrapped Sziklai output. I am sure it will also run with VFETs as bootstrap element.



I am proposed to implement the mods that Peufeu studied for diminishing putative thermal memory distortion effects on solid state amplifiers, but adding my own experiments on the output section, which Peufeu did not (unless I am not aware of that) address. In fact, I began my research modifying the outputs (Cascode all thy outputs), that is why Peufeu's articles were so interesting to me. I took the liberty to re-post the proposed amp from Peufeu. (it may have a minor error on the cascoded CFP like my experiment proved)

I have no electronic background; only the experience and scarce knowledge I obtained here, so be patient.

The blameless amp is the prototype to modify and since I have a Blame ST from Destroyer X, the idea is modify it section by section as Peufeu proposed and evaluate if there is any improvement in sound to be achieved.

I already swapped differential amplifier BJTs (Q1 and Q2) for cascoded CFP (J103 P channel JFET cascodying BC556-BC546 Sziklai pair) and it worked. Current is 2,09mA by side, though one may have to trim mirror resistor R8 to achieve 0 offset.

The sound, and this confirms Peufeu's claims, became very smooth and relaxed, with soft highs and more prominent midrange and mid-bass (my test speakers are very bright sounding) very soothing experience...the bad news is that the other channel has a problem that I could not solve yet: -27V at drivers bases, without oscillation But, hey! That is the way we learn the most!

I let to the theorists the task of discriminating if this supposed sound improvement comes from increased linearity, from decreased thermal memory or from shared thermal task of units with different thermal behavior...

Time ago I had already tried a mod to the output section that I liked: I cascaded the output BJTs with other BJT power devices (D44H11/D45H11) in common-base. Not properly a cascode, since it is not a common emitter output (even if I CAScaded the COllector ) apparently, but I perceived an improvement in transparency and dynamics. I have also modded another amp, the VSSA, with this trick and I also noticed a worthwhile improvement. My plan is putting, like on the input section, a cascoded CFP output later. You may notice that, absolute and eternal beginners like yours truly, are willing to try mods that reasonable people would never even think of...

Then, I will try the mods to the VAS circuit (cascoded-cascoded VAS) that Peufeu recommends.

But the final goal of all this is to gain experience to attack the modification of my Sony TA5650 VFET amplifier to make a real AMNESIS amp. I already cascoded the VFET common-source output section, which not only make it usable and stable, but also made it a very good sounding amplifier, with increased dynamics and punch.

I hope this humble effort awakens renewed interest in this most important topic.
Cheers,
M.
Attached Images
File Type: gif Memory compensated annoy_full_amp.gif (23.2 KB, 4573 views)
File Type: jpg CASCODED CFP MOD.jpg (874.4 KB, 4488 views)
File Type: jpg BLAME CASCODED.jpg (987.2 KB, 4060 views)
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Last edited by maxlorenz; 26th December 2018 at 01:15 PM. Reason: New significant information.
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Old 18th February 2018, 02:08 PM   #2
angrypat is offline angrypat  United States
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The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
interested
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Old 19th February 2018, 08:23 AM   #3
hahfran is offline hahfran  Germany
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well semiconductors come with memory there is no way to avoid it is just semiconductor physics it is "semi" responsible for memory effects. Informally the carriers of electric current in semiconductors are not "free electrons" as in for.ex. a metal like gold. That transforms to capacity, after all a capacitor is memory ( see sample and hold). Thus there is not only "thermal memory" there is further signal memory . Although the model of billiard-ball electrons flying from cathode to anode in a tube is wrong in physical reality those good old vacuum tubes come almost without memory.
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Old 19th February 2018, 12:29 PM   #4
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by hahfran View Post
well semiconductors come with memory there is no way to avoid it is just semiconductor physics it is "semi" responsible for memory effects. Informally the carriers of electric current in semiconductors are not "free electrons" as in for.ex. a metal like gold. That transforms to capacity, after all a capacitor is memory ( see sample and hold). Thus there is not only "thermal memory" there is further signal memory . Although the model of billiard-ball electrons flying from cathode to anode in a tube is wrong in physical reality those good old vacuum tubes come almost without memory.
Indeed, but we can try to minimize it, to render it negligible...in terms of audio satisfaction I mean...

Cheers,
M.
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Old 19th February 2018, 01:05 PM   #5
hahfran is offline hahfran  Germany
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see thread How fast should an amp be? broadband topology
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Old 19th February 2018, 04:25 PM   #6
hahfran is offline hahfran  Germany
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I'm convinced linear amps will yield to switching amps. The plagues are reduced to accuracy of ADC ( not really a big item) and to timing errors ( a big item) of the switches. Eventually the topology will be a sort of current dump where a very linear and very fast linear amp with small power - 1 watt about- corrects errors of the switching amp.
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Old 20th February 2018, 11:53 AM   #7
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by hahfran View Post
see thread How fast should an amp be? broadband topology
I know that one. Thanks.

The higher (if stable) the better...apparently we are most sensitive to transients (attacks) and to harmonic signature of sound sources (instruments; voices) as evolution or The Designer wanted.

Maybe that is why I tend to prefer passive current sources (?)

Anyway, now I'm stuck with one of the amps. See, the first board accepted the mod luckily without problem: the VAS is asking for 5mA and I was able to set offset to mV unit level...and it sounds great.

The other board though, maybe because of a little error in the original cascoded CFP diagram (it must be 1K between second BJT (NPN) emitter's and CFP output node) the amp keeps behaving strangely, with high negative bias voltage for the drivers. The VAS apparently is asking 25mA to the passive CCS and that collapses the positive voltage available, or that is what I understand, even when I use single transistor VAS, but I am not sure where that current is going...I couldn't detect any damaged units nor oscillation. I replaced most active devices anyway. If I cannot make it work, I will try an active CCS. If everything fails, I will have to build a new board...

Thanks for your interest,
M.
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Old 20th February 2018, 02:31 PM   #8
hahfran is offline hahfran  Germany
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but the transients ...( if these could be reproduced with the resp recording system) must be finally reproduced as sound field. That has to be considered.
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Old 20th February 2018, 02:55 PM   #9
hahfran is offline hahfran  Germany
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hum I think the approach of DIY is at best inconsistent - incomplete anyway. Y ou see those guitar guys they knew the guitar the pickups the various filters the amp AND the speakers is ONE unit. That is the way to go for DIY. Leslie West's (Mountain) classic sustains had only been possible with such a one unit. Not with any other. More advanced HiFi magazines had demonstrated that it matters not so much what the data of an amp and a speaker one by one are for the sonic impression - that is what matters.
For ex. the Rock classic Get ready sounds best in a convertible car with the amp clipping. At least up to 80 mph. As philosopher Russell puts it bluntly "There is no objective Hamlet in the theater and outside neither". It is all about "mood" and that cannot be measured
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Old 20th February 2018, 05:32 PM   #10
maxlorenz is offline maxlorenz  Chile
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Perhaps you are right.

I recognize two ways to listen to music:

The first is "passive", where you let the reproduced sounds make an "impression" on your vigil consciousness.
In the second, that I call "active", your mind re-creates the music heard instantly as it flows, which requires a permanent awareness of the moment...it is hard to explain. I will try to pick clearer words next time.

Both experiences can be fulfilling, the first being soothing and the second more alike critical listening, if you wish. Some systems make you incline to the first one and some to the second.
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