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Old 2nd July 2012, 04:35 AM   #2681
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Quote:
Originally Posted by Lazy Cat View Post
The only thing VFB is better are DC conditions, but that we do not hear through our speakers. You should make one medium complex SSA and you'll get an impression of what we talk about here. LTP's are sounding dull compared to CFB.
Oh, thanks L.C. i was afraid to have been mistaken during 40 years of my life.

Wahab, please, play with my simulations here, Crescendo revisited

You will see the differences between CFB and VFB *on the same amp*.
VFB ---> NFB
Bandwich: 200Khz ---> 3Mhz
Slew rate: 220v/µs ---> 1200v/µs
hd: 0.0023% ---> 0. 0002%
IM: x ---> x /10

As you can see, and as said L.C, improvement is in all the domains (except DC stability). The major improvement is listening pleasure.
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Last edited by Esperado; 2nd July 2012 at 04:53 AM.
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Old 2nd July 2012, 06:04 AM   #2682
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As a 67 years old electronic and sound engineer, which had dedicated near all his life in music reproduction/creation i would like to add the followings:

This SSA (or CSA) amp, proposed by Lazy Cat is the best DIY amp i ever seen during all my life, and at least equal to the best produced amps, all categories.

Lazy Cat do not play the "Guru" like some on this forum, do not takes poses, do not play with poetic words to promote itself in the light.
He just had designed a fantastic Amp in a wonderful manner.

First there is nothing open to criticism in the schematic. Simple, accurate, using the most clever technology at each stage *with no compromise*, and no 'religion', or 'esoterism'.

Including the so rare but so good sounding Current feedback topology.

We can feel that each part of his amp has been considered during long hours of work with attention, modesty, deep and objective reflexion between 'pro' and 'cons'. Both as individual stages and part in the whole. It is the very first time, looking at a schematic that i had nothing i would try to change ( Apart the protection stages . It is just 'Harmony'.

Printed circuit design is just perfect. Care had been taken everywhere to shorten the path lengths and reduce parasitic caps and inductances: A Master piece that can be taken as a reference.
With something more, the beauty of the design, beauty of the realization, as a cherry on the cake.
And all that with always this simple goal in mind: To reproduce music in the most transparent way.

Well, there is some well known names of electronic designers in the hifi hall of fame . Some are present on this forum. Some are very passioated people, innovative and honest, (Nelson Pass is one of them) others are just fake VIPs, experts in the art of 'make believe'.

On my admiration scale, Lazy Cat is at the top. Because he offer us both his great and *deep* knowledge and somethind more, witch is in the domain of the art, beauty and love, with a so modest attitude i can only admire... and love.

At a very disappointing moment of our history, where all the objects proposed by the industry smeel bad, because there are only marketing pieces of junk, with no love inside, neither for the product, neither for the consumer, this work is more than a gift.

Looking at this work, it was one of the rare times in my life i had this kind of emotion you can feel listening to a master piece record. Music is beautiful (Schematic), interpretation full of sensibility and virtuosity (Design) , production just perfect (Assembly).

And this is a single man work ? Wow !
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Last edited by Esperado; 2nd July 2012 at 06:13 AM.
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Old 2nd July 2012, 06:07 AM   #2683
sonnya is offline sonnya  Denmark
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I can only agree with Christophe.

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Old 2nd July 2012, 07:52 AM   #2684
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Quote:
Originally Posted by wahab View Post
Hi , Edmond

I was talking effectively of the impossibility for a GNFB loop
comprising several stages to reduce distorsion of a given stage
using available loop gain provided by the following stages.

To simplify , the IPS gain used for GNFB can reduce distorsion of the VAS
as well as the one produced by the OS , while eventual loop gain quantity
that originate from the VAS can reduce distorsion for the VAS and the OS.

If the OS has also some gain that is used for GNFB , this loop gain
quantity could only reduce the distorsion of the OS.
Wahab how can you say that in earnest? Don't you see that the distortion of whatever stage is never changed by feedback? You can only see that the distortion of the whole chain enclosed by the feedback is lowered.
The distortion of the stages within the feedback loop do not change at all!
As I said, each stage inside the loop always works open loop, because there is nothing that changes in that stage. The only thing feedback changes is the effective signal level at the input.

If you have an amp with say three stages with gains 10x, 20x and 0.9 times, for a total open loop gain of 180. With 1V in that would give 180V out (if the amp could do that).

Now you close a feedback loop around it for a loop gain of 18x. How does that work? It works because the feedback puts 1/10 of the output signal on the inverting input. The effective input signal is the difference between the non-inverting and the inverting input, of course.
So now the effective input signal becomes not 1V that you put in, but only 0.1V! Since the amp itself has not been modified it still amplifies 180x, so now the Vout becomes 0.1V (effective Vin) x 180 = 18V...

So you can't say that the gain of one stage only decreases distortion of that stage. The feedback doesn't do anything on that stage, feedback doesn't do anything on any stage, it just fudges Vin to make the output what you want it to be.

jan
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Old 2nd July 2012, 09:58 AM   #2685
brianco is offline brianco  Ireland
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Originally Posted by FdW View Post
............. I will not discuss this any further. (these are questions for myth-busters).......................................... .

Totally agree; let's hope that the subject is now (more) than adequately dealt with!

This red-herring has played out for far too long when included as part of a thread regarding a specific amplifier design. It has become the tail wagging the dog. If it is considered - by some - of great importance, then I would suggest it deserves a thread of it's own.
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Old 2nd July 2012, 10:34 AM   #2686
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Quote:
Originally Posted by janneman View Post
..... feedback doesn't do anything on any stage, "it just fudges Vin to make the output what you want it to be.".........

jan
Poifect !
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Old 2nd July 2012, 11:07 AM   #2687
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OK point taken. Sorry guys.
BTW Bigun your post was right on the mark, don't be modest

jan
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Old 2nd July 2012, 12:49 PM   #2688
dadod is online now dadod  Croatia
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Quote:
Originally Posted by Esperado View Post
Oh, thanks L.C. i was afraid to have been mistaken during 40 years of my life.

Wahab, please, play with my simulations here, Crescendo revisited

You will see the differences between CFB and VFB *on the same amp*.
VFB ---> NFB
Bandwich: 200Khz ---> 3Mhz
Slew rate: 220v/µs ---> 1200v/µs
hd: 0.0023% ---> 0. 0002%
IM: x ---> x /10

As you can see, and as said L.C, improvement is in all the domains (except DC stability). The major improvement is listening pleasure.

It is clear that you are in love with this amp, and I can accept that. I just can't accept that this type of amp(current feedback) is the Holy Grail of all amps. You need it for other kind of amplification, but for audio nothing is wrong with voltage feedback.
I do not think that an audio amp needs so wide bandwidth, and so fast slewrate, 220 V/usec is more than enough.
I simulated a bit last LTsice schematic you have sent and here is the result. I changed D1 and D3 only from zener 3.9 V to 4.7 V as I do not have models for your originals.
At output level close to clipping THD20k is still quite low:

Fourier components of V(vin)
DC component:8.60974e-011

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 2.191e+00 1.000e+00 0.00° 0.00°
2 4.000e+04 1.325e-10 6.048e-11 73.22° 73.22°
3 6.000e+04 2.382e-10 1.087e-10 2.02° 2.02°
4 8.000e+04 7.669e-11 3.500e-11 -1.00° -1.00°
5 1.000e+05 3.325e-10 1.517e-10 -1.38° -1.38°
6 1.200e+05 2.219e-10 1.013e-10 -58.95° -58.95°
7 1.400e+05 3.414e-10 1.558e-10 -6.02° -6.02°
8 1.600e+05 4.316e-10 1.970e-10 -69.82° -69.82°
9 1.800e+05 2.542e-10 1.160e-10 -17.76° -17.76°
Total Harmonic Distortion: 0.000000%

Fourier components of V(vout)
DC component:-0.00715549

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 5.888e+01 1.000e+00 -0.38° 0.00°
2 4.000e+04 1.153e-03 1.958e-05 -13.89° -13.51°
3 6.000e+04 5.273e-04 8.956e-06 -76.37° -75.99°
4 8.000e+04 2.683e-04 4.557e-06 -175.14° -174.77°
5 1.000e+05 7.603e-04 1.291e-05 -83.66° -83.28°
6 1.200e+05 1.828e-04 3.105e-06 -167.69° -167.31°
7 1.400e+05 2.934e-04 4.982e-06 -77.47° -77.09°
8 1.600e+05 1.352e-04 2.297e-06 -164.91° -164.53°
9 1.800e+05 1.656e-04 2.812e-06 -77.33° -76.95°
Total Harmonic Distortion: 0.002643%

I my humble opinion this amp is in the B. Cordells EC MOSFET amp class and I doubed that any golden ears could distinguish between them.
dado
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Old 2nd July 2012, 01:28 PM   #2689
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by dadod View Post
I changed D1 and D3 only from zener 3.9 V to 4.7 V as I do not have models for your originals.
At output level close to clipping THD20k is still quite low:
Total Harmonic Distortion: 0.002643%
I think using voltage source of equivalent voltage rating is more accurate than changing zener voltage from 3v9 to 4v7. This topology has issues with stability. You cannot change a value and expect it will be fine. Problem is, there is trade off here between THD and stability. If you reach such a low distortion in simulation (0.0026% seems to be too good to be true), I have a feeling that it is not a stable circuit anymore.

Quote:
Originally Posted by Esperado View Post
improvement is in all the domains (except DC stability)
So how do you define your minimum stability (requirement) in simulation?

Quote:
Originally Posted by Esperado View Post
You will see the differences between CFB and VFB *on the same amp*.
VFB ---> NFB
hd: 0.0023% ---> 0. 0002%
Is it simulated performance? What is the real number, not simulated numbers? I believe that it is very difficult to achieve simulated performance without careful and skillful fine tuning because from the nature of this "current" feedback it is easy that the input transistor will have too low Vbe.

I saw that you have trimmed the current source (lower ccs has 3K32, upper ccs has 3K3) and also the base voltage of VAS transistor with 30 Ohm above zener. Could you give a hint how do you do the trimming process?
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Old 2nd July 2012, 02:10 PM   #2690
dadod is online now dadod  Croatia
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Quote:
Originally Posted by Jay View Post
I think using voltage source of equivalent voltage rating is more accurate than changing zener voltage from 3v9 to 4v7. This topology has issues with stability. You cannot change a value and expect it will be fine. Problem is, there is trade off here between THD and stability. If you reach such a low distortion in simulation (0.0026% seems to be too good to be true), I have a feeling that it is not a stable circuit anymore.
When I simulated with voltage source of 3.9 V I got this:

Fourier components of V(vin)
DC component:1.67198e-010

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 1.183e+00 1.000e+00 -0.00° 0.00°
2 4.000e+04 3.045e-10 2.574e-10 -116.19° -116.19°
3 6.000e+04 1.369e-08 1.157e-08 146.39° 146.39°
4 8.000e+04 2.402e-10 2.030e-10 34.05° 34.05°
5 1.000e+05 8.349e-09 7.056e-09 -54.02° -54.02°
6 1.200e+05 7.537e-11 6.370e-11 -158.86° -158.86°
7 1.400e+05 9.989e-10 8.442e-10 167.09° 167.09°
8 1.600e+05 7.847e-11 6.632e-11 125.45° 125.45°
9 1.800e+05 1.014e-08 8.567e-09 66.91° 66.91°
Total Harmonic Distortion: 0.000002%

Fourier components of V(vout)
DC component:0.00114608

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 3.176e+01 1.000e+00 -1.47° 0.00°
2 4.000e+04 1.425e-03 4.485e-05 -11.25° -9.78°
3 6.000e+04 1.658e-01 5.221e-03 73.42° 74.88°
4 8.000e+04 1.137e-03 3.581e-05 176.95° 178.42°
5 1.000e+05 8.904e-02 2.804e-03 -119.80° -118.33°
6 1.200e+05 1.617e-04 5.092e-06 -43.20° -41.73°
7 1.400e+05 5.725e-02 1.802e-03 45.59° 47.06°
8 1.600e+05 5.330e-04 1.678e-05 171.37° 172.84°
9 1.800e+05 4.010e-02 1.262e-03 -150.47° -149.00°
Total Harmonic Distortion: 0.632175%

and with voltage source of 4.7 V I got this:


Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 1.183e+00 1.000e+00 0.00° 0.00°
2 4.000e+04 5.483e-12 4.634e-12 0.04° 0.04°
3 6.000e+04 8.223e-12 6.950e-12 -0.07° -0.07°
4 8.000e+04 1.097e-11 9.268e-12 -0.16° -0.16°
5 1.000e+05 1.371e-11 1.158e-11 -0.24° -0.24°
6 1.200e+05 1.645e-11 1.390e-11 -0.31° -0.31°
7 1.400e+05 1.919e-11 1.622e-11 -0.38° -0.38°
8 1.600e+05 2.193e-11 1.854e-11 -0.45° -0.45°
9 1.800e+05 2.467e-11 2.085e-11 -0.52° -0.52°
Total Harmonic Distortion: 0.000000%

Fourier components of V(vout)
DC component:-0.00485397

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 2.000e+04 3.180e+01 1.000e+00 -0.37° 0.00°
2 4.000e+04 1.013e-04 3.184e-06 -37.91° -37.54°
3 6.000e+04 3.877e-04 1.219e-05 -84.20° -83.84°
4 8.000e+04 1.218e-04 3.831e-06 -173.05° -172.68°
5 1.000e+05 1.040e-04 3.272e-06 -82.05° -81.68°
6 1.200e+05 9.281e-05 2.919e-06 -170.45° -170.08°
7 1.400e+05 3.915e-05 1.231e-06 96.16° 96.52°
8 1.600e+05 7.232e-05 2.274e-06 -168.40° -168.03°
9 1.800e+05 9.196e-05 2.892e-06 102.03° 102.39°
Total Harmonic Distortion: 0.001441%

so maybe thia amp works better with 4.7 V zener instead of 3.9 V.
dado
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