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Old 12th September 2012, 12:34 PM   #1091
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by sonnya View Post
Stop stop....

TI has through newnes published a book called "OP AMPS for EVERYONE" by CARTER and MANCINI. ISBN 978-1-85617-505-0-90000

Read chapter 1 - 13.!! Please!!!!
Or get it here http://www.ti.com/lit/an/slod006b/slod006b.pdf
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Old 12th September 2012, 12:50 PM   #1092
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Originally Posted by homemodder View Post
I think youre very confused, go read the book as Sonnya suggested.
In the early 70th, (1971) i was working as one of the two engineers in the research and development office of the most important French hifi manufacturer. We where so aware of those slewrate questions that we where the first to use Planar power transistors (new at this time on the market) for their higher speed.
And guess what ? Our amp was ... current feedback with no compensation cap in the feedback line.!!!

A French guy has recently made a restoration of this amp:
https://plus.google.com/photos/10929...217?banner=pwa
Measured performance by this guy:
2X50W RMS in 8 Ohms. (200w peak)
0.05% THD,
flat at 20 000 hz.
Rare at this time, specialty for a consumer level equipment low medium priced with a SSS (so simple schematic).

You will notice the protection circuit and the volume made of a 24 pos switch (resistances array), making a constant source impedance without the noise of a potentiometer.
And the "physiological filter" following at each volume position the evolution of the Fletcher and Musdon curves.
You will notice the star design of the hp tracks on the board, feedback strait from the HP output point, which had generated many remarks and questions from concurrent engineers at this time (why those two parallel tracks coming from the same point ?), prove we where aware of what happens in a feedback loop and what is the impedance of a track with current ) .

This guy made praise about the sound of this amp, 40 years later !!!

May-be i'm not confusing things so much, result of my own experiments and experiences and not things collected in books?
So many hours measuring, experimenting, trying to discover and understand the phenomenas :-)
May-be i'm simplifying too much my explanations, but, as we are not on a scientific forum, i try to make the things understandable to everybody, even non electronically experienced.
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Old 12th September 2012, 01:00 PM   #1093
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I would like to add, that, arguing again audiophiles (= in the desert) from so many years about the benefit of feedback and influence of slew rate, i was so delighted, reading Bob Cordell's articles years later, to see that it had reached exactly the same conclusions than us.

Quote:
Originally Posted by Joachim Gerhard View Post
Incredible extention with oszillation is not good
And even overshoot on square waves :-)

Quote:
Originally Posted by Joachim Gerhard View Post
Concerning if i have a priority of CFB against VFB that is not easy to answer in an absolute way out of context
If you can design the inverting feedback input stage/ pole to do not introduce any delay or additional distortion, there will be no difference i presume :-) Reason why an Op Amp behave much better when used inverted.
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Last edited by Esperado; 12th September 2012 at 01:27 PM.
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Old 12th September 2012, 01:17 PM   #1094
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I make a lot buffers and one i make is adjustable in overshot. With the right ( damped ) overshot and put behind a CD player i got an extraordinary sound.
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Old 12th September 2012, 01:32 PM   #1095
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Joachim Gerhard View Post
Esperado, i have looked at your modification of the Crescendo and you got lower distortion
after the modification.
Concerning if i have a priority of CFB against VFB that is not easy to answer in an absolute way out of context. My power amp and my preamp use current feedback, my phono has a current conveyer input stage with 75usec transimpedance and an active second stage with VFB. So i have at least one stage of voltgae feedback in the chain and it sounds great.
The TSSA though that i build gives me the best sound so far i had in my system.
Is it the speed, is it the spartanic simplicity, is the the PSU and how i build it ?
I can not tell. What i can tell is that it sounds very clean although it has distortion in the 0.1% range at higher level. I knew already that my hearing is not particular sensitive to low order harmonic distortion since i participated in the Klippel test. I scored in the middle group at around 1% TH low order weighted. Since then i know that the hunt for ultra low distortion is fruitless, especially when the amp gets very complex and a lot of PN junctions are in the chain.
I am completely with you in this. I accept that SSA or modified Crescendo sounds exceptionally good, but techically very high Slewrate of SSA could not be reason for that.
VFB or CFB both type of the amps could sound equally good if designed correctly.
I have JLH 80W amp(modified by me as original had a hum and stability problems) and I like the sound very much, could be lateral fets reason for that? The distortion is higher then in my TT amp and still it sounds so good.
Regarding discution abouth if it is more beneficial to raise the open loop bandwidth or go for massive gain in the bass I do not have fixed oppinion abouth it. I have the amps of both type and I don't hear any differece. All I have read about it lids to non importance of wide open banwidth.
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Old 12th September 2012, 01:51 PM   #1096
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I like the TSSA because the Lateral Fets are with the drains at the output. I think that way the amp is more stable into capacitive load and no VAS is needed. I see that as a modulated current source and Lateral Fets are very good constant current sources. The output impedance is then lowered with the current feedback. I have high sensitivity speakers and use the TSSA only over 80 Hz so i do not need high damping factor. I tried it wideband too with a big transmission line speaker and got great bass. i laready reported about the "massaging effect". A transmission line of cause does also not need a high damping factor because it has a quite flat impedance curve in the bass, at last in a damped line of correct proportions. Not that my dynamic speakers have much capacitance.
I even terminate the speakers with a Zobel that has the same characteristic impedance at HF as my cables to prevent HF ingress. It works particular well with low inductance, high capacitance cables.
Maybe there is more to that interface distortion problem then we think. Non of the commercial companies i know publish measurement of the whole chain, amp-wire-speaker, but that is what we hear.
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Old 12th September 2012, 02:12 PM   #1097
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Quote:
Originally Posted by Joachim Gerhard View Post
I like the TSSA because the Lateral Fets are with the drains at the output. I think that way the amp is more stable into capacitive load and no VAS is needed. I see that as a modulated current source and Lateral Fets are very good constant current sources. The output impedance is then lowered with the current feedback. I have high sensitivity speakers and use the TSSA only over 80 Hz so i do not need high damping factor. I tried it wideband too with a big transmission line speaker and got great bass. i laready reported about the "massaging effect". A transmission line of cause does also not need a high damping factor because it has a quite flat impedance curve in the bass, at last in a damped line of correct proportions. Not that my dynamic speakers have much capacitance.
I even terminate the speakers with a Zobel that has the same characteristic impedance at HF as my cables to prevent HF ingress. It works particular well with low inductance, high capacitance cables.
Maybe there is more to that interface distortion problem then we think. Non of the commercial companies i know publish measurement of the whole chain, amp-wire-speaker, but that is what we hear.
JLH 80W MOSFET amp has 0.22 ohm at the output so there is no high damping factor and still I like bass produced by it. I have closed box loudspeaker with not so efficient bass driver(Scanspeak 18W/8545-00) and the bas is so good. I was thiking to add 0.22 ohm resistor to my TT amp(very high DF) as aditional louspeaker output and in this case I can choose low or high damping.
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Old 12th September 2012, 02:40 PM   #1098
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Yes, the English do that sometimes. A little less damping can sound better on some speakers. Especially when the sound is too dry.
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Old 12th September 2012, 03:23 PM   #1099
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Quote:
Originally Posted by dadod View Post
I accept that SSA or modified Crescendo sounds exceptionally good, but techically very high Slewrate of SSA could not be reason for that.
Never accept, never suppose. Experiment by yourself. ;-)

It is very easy to build a VFB feedback version of the symmetrical SSA (tuning feedback cap compensation on the VFB) and to compare both sound and performance.
About my Crescendo, just the feedback differ.
So you can conclude sound difference is mainly due to the slewrate difference.
Plus, but in a minor way, the suppression of the harmonic distortions generated in the inverting input carrying feedback signal in the VFB version.
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Old 12th September 2012, 03:40 PM   #1100
Salas is offline Salas  Greece
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Quote:
Originally Posted by dadod View Post
JLH 80W MOSFET amp has 0.22 ohm at the output so there is no high damping factor and still I like bass produced by it. I have closed box loudspeaker with not so efficient bass driver(Scanspeak 18W/8545-00) and the bas is so good. I was thiking to add 0.22 ohm resistor to my TT amp(very high DF) as aditional louspeaker output and in this case I can choose low or high damping.
dado
Quote:
Originally Posted by Joachim Gerhard View Post
Yes, the English do that sometimes. A little less damping can sound better on some speakers. Especially when the sound is too dry.
Meaning that amp? I got one still going strong. Using it for when everything else is under planning or cruel operations. Built on 80's L bracket modded PCBs that skip the 0.22R and speaker line fuse originals. I should add it and see what happens at a point I guess. Will reserve it for when the devil will find work for idle hands to do, and that is very British being a Morrissey lyric too.
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