with two SITs per channel is - as I said , just one of 3 possible iterations; true MOS, Schaded MOSs
in upper quadrants, SITs in upper quadrantsif it doesn't work with TL07x/08x, it's lousy made
Thx.. will take a look at both.
I am having a blast over here, working on the pre-amplifier and keeping your power-amp in mind, I wrote an email earlier today, to another diya fellow and highlighted something of importance.
I don't want to sideways your tread, just sharing some of the joy I feel, sitting here an planning round 2 of my Scion project. I also must admit, as time passes, my admiration of the XA25/252 output stage and how things are done is basically a genius move and we have two fellas to thanks here, Zen and Pa. This is the best time to be alive
I am having a blast over here, working on the pre-amplifier and keeping your power-amp in mind, I wrote an email earlier today, to another diya fellow and highlighted something of importance.
There's been some development on my side. I managed to get a hold of the PASS Xs-300 gain and output stage schematic, (but the gain stage have a fatal flaw which tells me it is incorrect), which is the same as in the Xs-150. What I discovered was that all the way back in 2013 when the Xs was launched, they got rid of the degeneration resistors found in the XA.5 series and XP preamplifiers. Meaning: Xs, XA.8 and XA25 does not feature degeneration. That the XA25 does not use degeneration, is known since some time. This greatly enhances the dynamics and overall clarity.
We can also see that something else which changed between the .5 series and XP to the Xs and .8 series was the implementation or infusion of harmonic content. This and the no degeneration is the secret sauce of why .8 sounds better. This circuit addition is basically the same as the H2 Harmonic Generator found here on diya. Gotta love Pa for sharing so much of his work. All hail the diya community
If you look at the attached simplified XA25 schematic and read Nelson's statement, the XA25 is closer to Nelson's ideal approach of "as few amplification stages and components as possible". The output stage job is final voltage and adding current and since each OS-MOSFET has a limited current output, there is the need to use multiple devices in parallel, not only that, the total package power of IRFP240/IRFP9240 is limited to 150W. All this forces PL to use many output devices. Something which is avoided by using the IXYS high power "puck" found in the XA25 (and the GamuT D200i which I was going to build).
The IXYS can go to over 650W and over 100A per device, but since we are into audio, the DC region of SOA limits the current to around 10A per device. So say that the final voltage is 35V, you end up with 350W per device (maximum). So, theoretically, the XA25 could reach 700W/Chn with adequate cooling. And if you make a balanced version, one could reach 1400W or 1.4kW... LOL
But wait, there is more... The XA25 is using a Thyristor as safety device, reassuring that the OS-MOSFET never see's more than 10A and they have CCS and no source pin degeneration. All of this is pointing towards XA25 being a better amplifier platform than the XA.8 ... at the output stage.
While the XA25 is superior at the last stage, the XA.8 is sort of superior at the input stage with its SuSy structure.
Subjectively, if one think about it, the Xs preamplifier with its modified UGS gain stage, using CCS and dc-servo for ultra low noise floor and stability, should be a perfect platform for the perfect PASS power-amplifier, aka, balanced XA25.
Maybe a balanced XA25 with SuSy gain stage is the perfect storm"
I don't want to sideways your tread, just sharing some of the joy I feel, sitting here an planning round 2 of my Scion project. I also must admit, as time passes, my admiration of the XA25/252 output stage and how things are done is basically a genius move and we have two fellas to thanks here, Zen and Pa. This is the best time to be alive
what you put above in quote is ...... well, not precise enough
like everything in life, when speaking of technical things, one must be precise with technical terms; so, I'm just going to draw important question: "where exactly is degeneration not featured?"
I know the answer
Anyhow, most important thing which Pa is teaching us in all these years is- go and build and learn and have fun; this is bloody DIY Forum, not Debating one
like everything in life, when speaking of technical things, one must be precise with technical terms; so, I'm just going to draw important question: "where exactly is degeneration not featured?"
I know the answer
Anyhow, most important thing which Pa is teaching us in all these years is- go and build and learn and have fun; this is bloody DIY Forum, not Debating one
Sure, I can give it a try.
The basic structure of the gain stage is A differential Common Source amplifier.
"When drain current (Id) flows through the channel and source, there is a voltage generated across the source degeneration resistor (Rs). This voltage raises the source potential by Id*Rs, so for a given gate voltage Vg, the resistance causes a decrease in Vgs since Vgs=Vg-Vs. Since Id for a given Vgs will be larger for a FET with higher transconductance (𝑔𝑠gs), the degeneration will also be higher, source degeneration can be very useful in making amplifiers less susceptible to variations in 𝑔𝑠gs."
"My late business partner Joe Sammut was adamant about the qualitative difference, and could spot it in blind testing, so I took it seriously. The difficulty comes the fact that there is a reason why people use degeneration (the "other form of feedback") in gain stages—it stabilizes the characteristics of the parts so that you don't have to do precise matching and compensation to keep circuits stable. At the same time, it acts like the feedback it is. Routinely, your "no feedback" solid-state amplifiers depend on degeneration in the gain stages to control the performance, and so it does not achieve all the goals that make SET tube amps attractive.
For those sonic reasons there have been several efforts at operating bipolar transistors undegenerated, and while the benefits have been noted, they have been accompanied by reliability issues. No need to mention names...
I set removal of degeneration from power amps as a design goal and over the course of several years came up with a couple of reliable techniques that are employed for the first time in the XA25. The result is better dynamics and more "life" to the sound, and with power FETs it turns out that there are a couple more advantages that you don't see with bipolar transistors.
Your basic FET character is "square-law," where the current through the device is a square function of the voltage across the control pins, the gate and the source. In this regard, the FET is very much like a tube. It turns out that push-pull Class A operation of square law devices can result in intrinsic cancellation of distortion to literally mathematical zero. Unfortunately, degeneration in the circuit introduces distortion to that arrangement.
After all, your amplifier should be very stable and a joy to listen to
The basic structure of the gain stage is A differential Common Source amplifier.
"When drain current (Id) flows through the channel and source, there is a voltage generated across the source degeneration resistor (Rs). This voltage raises the source potential by Id*Rs, so for a given gate voltage Vg, the resistance causes a decrease in Vgs since Vgs=Vg-Vs. Since Id for a given Vgs will be larger for a FET with higher transconductance (𝑔𝑠gs), the degeneration will also be higher, source degeneration can be very useful in making amplifiers less susceptible to variations in 𝑔𝑠gs."
***
From N. Pass himself."My late business partner Joe Sammut was adamant about the qualitative difference, and could spot it in blind testing, so I took it seriously. The difficulty comes the fact that there is a reason why people use degeneration (the "other form of feedback") in gain stages—it stabilizes the characteristics of the parts so that you don't have to do precise matching and compensation to keep circuits stable. At the same time, it acts like the feedback it is. Routinely, your "no feedback" solid-state amplifiers depend on degeneration in the gain stages to control the performance, and so it does not achieve all the goals that make SET tube amps attractive.
For those sonic reasons there have been several efforts at operating bipolar transistors undegenerated, and while the benefits have been noted, they have been accompanied by reliability issues. No need to mention names...
I set removal of degeneration from power amps as a design goal and over the course of several years came up with a couple of reliable techniques that are employed for the first time in the XA25. The result is better dynamics and more "life" to the sound, and with power FETs it turns out that there are a couple more advantages that you don't see with bipolar transistors.
Your basic FET character is "square-law," where the current through the device is a square function of the voltage across the control pins, the gate and the source. In this regard, the FET is very much like a tube. It turns out that push-pull Class A operation of square law devices can result in intrinsic cancellation of distortion to literally mathematical zero. Unfortunately, degeneration in the circuit introduces distortion to that arrangement.
***
Back to the "where exactly is degeneration not featured?" .. It is no longer used in the JFet source pins in the gain stage and it is not used in the MOSFET source pins at the output. I believe that if you are matching Idss to better than 5% and the use of CCS, using no degeneration should yield a very good result. After all, your amplifier should be very stable and a joy to listen to
proper state of affairs is:
XA25 is having avoided degeneration in FE and non-existent in OS ; I'm 158% sure of that
Xs and XA.8 are having avoided degeneration in FE and existing degeneration in OS; I'm 99.85% sure of that; while it's easy (once when you know how) to remove degeneration in FE, paralleling several devices in OS is practically impossible without source resistors
there are the ways to do it with just few of them (mosfets, bjts, whatever) in parallel, though not exactly simple, and with dubious positive outcome and most likely no other reason than just to do it - meaning - I can parallel pucks that way (but why) while paralleling just few TO247 is not good enough when you need more of them .......... and pics of XS and XA.8 amps are clearly showing bunch of Pana 3W resistors
Pa is practical man ......
anyhow, that game still (and always be) reserved for Big Boyz, actually constructing things; we Peasants will stay dealing with lesser things
that's it, I'm done replying to your long contemplation posts; main reason was (up to now) to clarify and rectify things you wrote, just to prevent someone actually eager to learn something, to take errors as facts
I have some entertaining books to read, much more fun than chasing paper rainbows
Yes and yes, this is evident from Pa himself in the positive feedback article.
Without sharing too much of original stuff, the Xs-150/300 and XA.8 use source degeneration resistor (0.47Ohm 3W for XA60.8) in the OS as evident in the picture attached which is from the XA60.8 gain stage, but they all use the same platform. Also, another change between .5 to .8 is the use of CCS in the output stage.
Now is the time to move on and study, plan and eventually build
Attachments
I have a AN-15432 - 1500VA 4x32V sad in shoe box, sits, parts and extra enough aluminum. 40v rails possible? Cons?
well, some of Greedy Boyz were just slightly more successful in twisting my hand to get them
I'm more than late/not actual with several Trade threads, and Babelfish XA252 is just one of them
in mid of solving some decent quantity of LT3092 (still going to be sole option for this amp, no LM334) then I can ring da Bell
I'm more than late/not actual with several Trade threads, and Babelfish XA252 is just one of them
in mid of solving some decent quantity of LT3092 (still going to be sole option for this amp, no LM334) then I can ring da Bell