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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 23rd February 2018, 01:11 PM   #11
maxlorenz is offline maxlorenz  Chile
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Finally, your favorite eternal beginner and humble narrator has succeeded.
It was hard. Like "awakening the Dragon"...

The faulty amp, believe it or not, had the 100V bootstrap cap damaged! Or so it appears...My cap tester kaput (kids, discharge caps before testing) so I cannot confirm leakage but replacing it for a good one made the amp work flawlessly...and I seem to remember Rod Elliot, perhaps (great site) mentioning something about that those caps never fail...or was it my imagination?

I had the chance of a short listening test in stereo until the other amp kaput!

I now always think that these catastrophic events are a blessing in disguise because we learn so much during the trouble-shooting.

I replaced the corresponding bootstrap cap thinking that the was the fault and it ended being a PNP driver gone.

While I was there I couldn't help thinking that using a big cap in the signal chain must not be good for memory, right? So, apart thinking about running a version without the said cap, I started looking for information about current source plus bootstrap. I stumbled across a very interesting thread about CCS-bootstrap by Dadod, which I am now reading. Must be a worthwhile experiment. He also used the Blame as model.

Please bear in mind that I am not proceeding in a strict scientific way to evaluate my mods: I simply try them and decide, from memory (hehe) if I like it or not. I have not the time to build another pair of amps and do the ABA test for each mod, so you are warned.

So when trying to fix the amp, I decided to make the simple cascoded VAS mod and it worked. I think the cascoded VAS gives the more velvety midrange so now I have it in stereo. One VAS asks for 12,7mA and the other asks for 10,8mA. Offset is steady around 10mV. Voltages for the bases of the outputs are +/-550mV and amps run cool.

So, how does it sound, my cascoded-CFP plus cascoded-VAS plus cascaded output modified blame?
Stunning. First, one notices reduced highs which is intriguing but then one realizes that this is how live music sounds, and the high pitched instruments just sound right. Mids are a bliss. Velvet is the word that comes to mind. Instrument's textures and timbres are very well defined and distinct and this a salient quality of this amp!
Bass is all there. Depth of images is great and so is layering. Musical lines of every instrument are well designed and I can hear a lot of previously hidden details...an anecdote: in my first listening I worried because I started to hear loud clicks and pops and the scope gave a jittery trace before music started, until I remembered that I was listening to a vinyl upload on youtube, "Stone Flute" by Herby Mann. I have listened to this before but never with such loud clicks and pops!


Anyway, you know always a child is handsome for his father, so I am probably not being objective, but anyway, I listen to music in an subjective way, hehe.

The good things are that inexpensive, low bias, class AB amps can sound grreat and that I am learning a lot to improve my Sony TA5650.

I cannot upload a decent diagram but I promise I will in the future.

Cheers,
M.
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Old 23rd February 2018, 02:10 PM   #12
bimo is offline bimo  Indonesia
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I use same input but simple (no CFP).
I get THD = 0.000275% at 31W/8Ohm, 20kHz.

If I uses CFP, THD20 rise but THD1 fall.
Attached Images
File Type: png input.png (5.8 KB, 1148 views)
File Type: png thd.png (63.3 KB, 1129 views)
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Old 23rd February 2018, 02:15 PM   #13
jaycee is offline jaycee  United Kingdom
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Cascoded VAS and input are worth doing. Not sure about the more esoteric things Peufeu tried.
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Old 24th February 2018, 12:53 PM   #14
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by bimo View Post
I use same input but simple (no CFP).
I get THD = 0.000275% at 31W/8Ohm, 20kHz.

If I uses CFP, THD20 rise but THD1 fall.
Hi guys. Thanks for your inputs.

Bimo, are you sure you need that resistor to JFET gate?
Did you tried or just simulated cascoded-CFP?

I wish I knew how to use simulators...

Anyway, the mod is simple and transistors are cheap, so why don't try it and see/hear what does it bring?

You only need a pair of PNP and a pair of 1K resistors that go between emitter of the new PNP and source of JFET which is also the output node of Szicklai...or at least I wired that way to get even currents (2.09mA) between legs.

Another easy mod is cascodying the tail. But later as now I will continue my listening tests.

Cheers,
M.

PS: now I am mentally listing all the circuits that include differential pair in my arsenal to proceed to modification.
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Old 25th February 2018, 06:36 AM   #15
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by maxlorenz View Post
Hi guys. Thanks for your inputs.

Bimo, are you sure you need that resistor to JFET gate?
Did you tried or just simulated cascoded-CFP?

I wish I knew how to use simulators...

Anyway, the mod is simple and transistors are cheap, so why don't try it and see/hear what does it bring?

You only need a pair of PNP and a pair of 1K resistors that go between emitter of the new PNP and source of JFET which is also the output node of Szicklai...or at least I wired that way to get even currents (2.09mA) between legs.

Another easy mod is cascodying the tail. But later as now I will continue my listening tests.

Cheers,
M.

PS: now I am mentally listing all the circuits that include differential pair in my arsenal to proceed to modification.

I just simulated it. I do not have time to make a prototype.
I sent my design to a friend, he will make a prototype. But it take a time.
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Old 27th February 2018, 11:53 AM   #16
maxlorenz is offline maxlorenz  Chile
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OK. I know have the input section of my Sony TA-5650 properly modified with Peufeu's Cascoded CFP trick. The effect was easily detected as being the same caused on the Blame. Less aggressive highs, more midrange presence (the bodies of the instruments are very pronounced), better delineation of melodic lines, in all, if I knew how to speak Latin, I would say the sound is now rotundum

Fortunately, the DC offset was low and I had not to mod the circuit: 10mV and 1,8mV.

It is now decided: I will swap all my differential pairs for the cascoded CFP unit. See attached picture of the units in preparation. I think I reached the limit of reduction. More than this, it would have to be SMD...
I am also studying mixed CFP (JFET + BJT) to replace JFET inputs...

With a little of effort, the units can be placed in existing space.

Guys. try it. At least for experimentation...remember, all mods must be evaluated not less than 48hrs before verdict.

Cheers,
M.

PS: an "all cascoded-CFP" amp would be interesting...

PS2: I attached the diagram of the mod. The amp has now 100R degeneration on the emitters. The 1K CFP resistor is fitted on the emitter of the NPN BJT instead. This way it worked better.
Attached Images
File Type: jpg CASCODED CFP unit.jpg (633.6 KB, 771 views)
File Type: jpg SONY TA5650 AMPLI CASCODO-CASCODO2 CFP SCHEMA.jpg (278.2 KB, 804 views)

Last edited by maxlorenz; 27th February 2018 at 12:10 PM.
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Old 28th February 2018, 01:11 PM   #17
maxlorenz is offline maxlorenz  Chile
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Sorry, the first post has the wrong Peufeu diagram.
This one is the final, which has the cascoded-cascoded VAS:
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File Type: gif AMNESIS before-after.gif (7.6 KB, 787 views)
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Old 2nd March 2018, 12:33 PM   #18
maxlorenz is offline maxlorenz  Chile
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Continuing with the experiments, yesterday I successfully "improved" the differential amplifier input section of my single ended Tokin 2SK180 VFET amp

Since it has JFET diffamp, I decided to make a "cascoded-compound Sziklai" arrangement. For that I studied some academic articles but they were too much for my limited understanding so I made it the brute force with SK170 N JFET both as the cascodying element and as the "driver" of the Sziklai pair and a BC560 PNP as the "follower" of the same, with a 1K emitter resistance. It worked. And first listening tests are very rewarding.

Guys, this mod always improve attacks and detail retrieving. Pizziccatti e stacatti, plucking, smashing of the keyboard, or any ambient noise are greatly enhanced, which, summed to the extra wide soundstage, beyond speakers, makes the experience like a "live" event and you keep wandering -is that the recording or the neighbor???

Photo of the small unit and diagram of the target amp.

Cheers,
M.
Attached Images
File Type: jpg diffamp TOKIN 2SK180.jpg (60.6 KB, 422 views)
File Type: jpg diffamp cascodo CFP.jpg (328.2 KB, 372 views)
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Old 2nd March 2018, 04:22 PM   #19
padamiecki is offline padamiecki  Poland
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hi Max!
these are my old plays with Peufeu's memdist, the idle is ~90mA
cheers!
Attached Images
File Type: png acsmall.png (27.4 KB, 169 views)
File Type: png fft.png (32.8 KB, 125 views)
File Type: png sch.png (77.1 KB, 357 views)
Attached Files
File Type: asc memdist.asc (11.5 KB, 33 views)
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Old 2nd March 2018, 08:49 PM   #20
stocktrader200 is online now stocktrader200  Canada
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Hahfran, you are likely correct as I built a class D operating at 2.2 mhz @ 42 v rails
the sound is very class A. the 2 problems are creation of low level noise on FM when in the same case and reliable over current protection speed.
distortion is in the area of 0.003 as the corrective op amp is a opa2132

Last edited by stocktrader200; 2nd March 2018 at 08:52 PM.
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