It is definitely the future to explore.
Well, when Mark Alexander (from Analog Design), more than 20 years ago, published his schematic of a current feedback amplifier (so called),
http://sjostromaudio.com/hifi_files/misc/AN211_better_text.pdf
i asked an evaluation board from A.D. and tried-it.
I was so impressed by the sound that i dedicated all my analog work on CFB since this time.
Despite this idea is, indeed, not so new (often used in tube amps) we can wonder why solid state power amps designers do not use current feddback more often.I believe several reasons.
First of all, both the large bandwidth and poor PSU rejection can cause some problems in mass production.
Second, most of the time, they just reproduce the same idea they where learned at school, without trying real new or "exotic" ways. Did they even listen to music?
Third, some so called "gurus" prefer to make believe in magic, promote themselves with wonderful words and extraordinary prices, or play with audiophile "selected" parts like a cook with products. The hugest thread here illustrate this perfectly: http://www.diyaudio.com/forums/anal...ch-preamplifier-part-ii-1340.html#post1867706.
Last, any correlation between real quality and reputation in the hifi business?
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Mark Alexander came very late to the party. There are many CFB designs even from the 80s, Pioneer made big success with A400X and some models from their exclusive range which were CFB. A400X was a low cost entry level amp but sounded like an amp worth 10 times more. Nowadays Marantz (Top of the range) are using a design which is 98 % the exact design from the pioneer A400X (just checked this last night due to another thread topic). So far the most advanced CFB designs Ive seen come from pioneer in the late 80s early 90s.
I don't know if A400 was a current-feedback amp, but A 400 was introduced in 1990 ... after the paper of mark Alexander in AES ?
Did Pioneer uses IGBT or FET power transistors like Alexander design ?
Im not sure but there were many other CFB amps on the market. If you look up any marantz high ender like S11 or S9 youve got the schematic bar one little detail. Pioneer called it super linear amp and better versions can be seen in A09, M6 and other models. See A09 its excellent amp.
Elantec and comlinear were the frontrunners on CFB technology and have 100s of patents regarding CFB including the feedback circuit marantz is using.
Pioneer nearly always used triple ef with BJTs but these top range models used excellent diamond buffer triples which is best outputstage Ive seen on a amp, this is without error correction and even works very well without feedback. Barring low end amps I dont use any other outputstage although I introduce a improvement on it. See Bob Cordells book for low performance circuit of the circuit.
Elantec and comlinear were the frontrunners on CFB technology and have 100s of patents regarding CFB including the feedback circuit marantz is using.
Pioneer nearly always used triple ef with BJTs but these top range models used excellent diamond buffer triples which is best outputstage Ive seen on a amp, this is without error correction and even works very well without feedback. Barring low end amps I dont use any other outputstage although I introduce a improvement on it. See Bob Cordells book for low performance circuit of the circuit.
Alex i will take CFA for a 1000.00 ....
"The CFA was invented by David Nelson at Comlinear Corporation, and first sold in 1982 "
Just saying .......
Not exactly, JLH introduced the circuit as seen on this thread in late 1960s which is CFB. They did introduce the diamond buffer inputstage thats true.
I think JLH didn't have any fancy name in mind. I think he wasn't the 'patent guy' type.
There s no patent for JLH circuit, see Class A audio site for the circuit. Its CFB without the input transistors forming the diamond. Better in some ways as it doesnt suffer from input transistors causing error current due to their emitter capacitance but extra circuitry needed for thermal stability. Comlinear have patent to address the issue. A better way is bootstrapping but I think national got patent for it this year and was used on lme479 series opamps.
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Congratulations, LZ, and thank you for sharing! 
I have a question about regulated PS for the SSA.
Since this amp has extended frequency response and has enormous slew rate, I suppose the regulated PS will be challenged...I usually use Teddy Pardo's "PowerReg" for this purpose (the only one that I know) with success.
Introducing the PowerReg - pink fish media
It has a plain LM317/337 monolythic reg to set the voltage and a D44/45H11 as output transistor, which I understand are fast but I don't know if they are fast enough. Could you help?
Thanks again.
M.
I have a question about regulated PS for the SSA.
Since this amp has extended frequency response and has enormous slew rate, I suppose the regulated PS will be challenged...I usually use Teddy Pardo's "PowerReg" for this purpose (the only one that I know) with success.
Introducing the PowerReg - pink fish media
It has a plain LM317/337 monolythic reg to set the voltage and a D44/45H11 as output transistor, which I understand are fast but I don't know if they are fast enough. Could you help?
Thanks again.
M.
The input stage is the one to protect from PS noise. This way you can choose very fast small (TO126) transistor because you need only a few mA. Many video output transistors are very fast. C3503/A1381 for example is 150M/3pF, while D44 and D45H11 is 15M/220pF and 12M/400pF, not a good match.
For output stage of discrete amps, many of us prefer the dynamic sound of conventional filter. If I had to use a regulator, I will prefer only one transistor regulator, and as fast as possible, like the 2SC5200 (40MHz?) as used by Apex amplifier. Or mosfet one. Or CLC. I have built darlington regulator a-la SSA Crescendo by Christophe (Esperado), with different transistor, but not yet implemented.
Hi guys
Enormously enjoying reading your posts about CFB historical facts, amp models & their schematics, successful SSA builds, everything what you've been posted so far.
Please keep up the spirit alive and go on with making amplifiers.
Regards Andrej
Hello LC,
Are there ever going to be boards for the SSA ..?
Congratulations, LZ, and thank you for sharing!
I have a question about regulated PS for the SSA.
Since this amp has extended frequency response and has enormous slew rate, I suppose the regulated PS will be challenged...I usually use Teddy Pardo's "PowerReg" for this purpose (the only one that I know) with success.
Introducing the PowerReg - pink fish media
It has a plain LM317/337 monolythic reg to set the voltage and a D44/45H11 as output transistor, which I understand are fast but I don't know if they are fast enough. Could you help?
Thanks again.
M.
Hi maxlorenz
SSA is a three stage amplifier, where after first two stages (input and VAS) normally comes bias control circuit (Vbe multiplier usually for BJT/lateral mosfet output or TL431 zener regulator usually for vertical mosfet drivers/outputs). This parts all together forms front-end stage which can/should have separate regulated power supply with GND connected to a common GND star point (both input and output PSU).
Output stage doesn't really need regulated PSU, just enough capacitance (2200 uF/Aload max) and well decoupled rail points at all crossings and at the output transistors near output itself (0,1 to 0,47 uF MKP/100 V). I also recommend you to decouple DC between +/- rails directly (higher voltage MKP-s).
Regarding front-end PSU, it is important to have small ripple (suggested below 80 db regarding to ouput DC voltage) so LM-s should use adjust pin decoupling, usually 10-47 uF elco is satisfactory. Regulator's bandwidth - rails frequency modulation suppression is important up to 100 kHz, from there up decoupling capacitors play major role so your's LM regulators are just fine. For a top notch results you can add shunt regulator to LM output, followed by necessary decoupling capactors, as always.
If SSA sch includes well defined CCS-s and TL431 zeners for intermediate DC points, PSRR is so high, that regulated PSU-s will bring a little to a whole amplifier's performance.
Regards Andrej
Hi a.wayne
There are some PCB-s published in a SSA thread. Nico made PCB at begining, bigun made it in his thread (SMT parts), there were some PCB proposals from Alex here in this thread. Many derivates, depends to SSA schematics variant choosen. For a SSA BIGBT HP I made PCB, as you've probably noticed, but than there was CCS PCB installed as add on module (separate small PCB) for better performance (more like R&D design).
As I remember you're interested in something more like Zout low (many output pairs) and that kind of SSA didn't arise yet.
Look I suggest you to concentrate to one specific schematic that will suit you and we can make it work and design PCB.
As you know now my work is concentrated to CSA which is combination of SSA and TSSA, which have two output pairs only.
Regards Andrej
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