RMI-FC100, a single stage audio power amplifier
- Thread starter roender
- Start date
I stumbled across this thread today. I feel good and bad about it. I have a schematic I've been working on for about a month that is uncomfortably similar to roender's.
I have a background as a designer of IC amplifiers, including class D power amps and class AB headphone amps. So, my goal was to attempt to translate the very successful folded cascode topology from some of my IC designs to a discrete power amp.
I feel bad because mine no longer seems very original. I feel good because the RMI-FC100 amplifier has been universally acclaimed with countless diy builds over the past 13 years. Two main differences in my design are TL431 instead of LEDs for bias setting, and SSM2212 matched NPNs instead of JFETs for the input pair.
I am still working on optimum bias currents for everything, but I'm in the ballpark of the FC100. My final stage currents are lower, but my other currents are very close. I'm not using the finals with integrated diodes. I have another idea for temperature independent biasing.
I have been thinking about something like Q10 to help with DC offset, but have not yet. A lot of the discussion about compensation, bypass of R5 with C1 has been very informative to me.
I investigated CFP versus triple EF, and decided to do triple EF for bandwidth reasons. Now I am motivated to re-open my investigation of the CFP output stage. All in all, I am very impressed with roender's design & build, and the many builds that people have posted in this thread.
The only comment I have about the design is about the relative bias of the input pair relative to the folded cascode. Roender set the input pair at 6.25 mA each, and the folded cascodes at 2 mA each. This is an effective technique for maximizing gain of the amplifier. The transconductance of the input pair is higher, and the output impedance of the Q15 and Q16 is also higher. (the gain of the amplifier is gm*ro).
However, if the amp ever clips, the folded cascode transistors will run dry, resulting in some unsavory behavior. Basically, Q14 & Q16 and everything below them and to their right will have no current. Roender says he can never clip his amplifier in his system, so he has no worries. Others, however, will clip theirs. The solution is to increase the bias of Q13 and Q11 to a little above 13.5mA each. This will lower the OL gain by a factor of about 3, and I do not know what affect it will have on the sound. Probably several transistors will need heat sinks attached due to 3x dissipation. I learned this one on an IC design I was involved with once upon a time.
I have a background as a designer of IC amplifiers, including class D power amps and class AB headphone amps. So, my goal was to attempt to translate the very successful folded cascode topology from some of my IC designs to a discrete power amp.
I feel bad because mine no longer seems very original. I feel good because the RMI-FC100 amplifier has been universally acclaimed with countless diy builds over the past 13 years. Two main differences in my design are TL431 instead of LEDs for bias setting, and SSM2212 matched NPNs instead of JFETs for the input pair.
I am still working on optimum bias currents for everything, but I'm in the ballpark of the FC100. My final stage currents are lower, but my other currents are very close. I'm not using the finals with integrated diodes. I have another idea for temperature independent biasing.
I have been thinking about something like Q10 to help with DC offset, but have not yet. A lot of the discussion about compensation, bypass of R5 with C1 has been very informative to me.
I investigated CFP versus triple EF, and decided to do triple EF for bandwidth reasons. Now I am motivated to re-open my investigation of the CFP output stage. All in all, I am very impressed with roender's design & build, and the many builds that people have posted in this thread.
The only comment I have about the design is about the relative bias of the input pair relative to the folded cascode. Roender set the input pair at 6.25 mA each, and the folded cascodes at 2 mA each. This is an effective technique for maximizing gain of the amplifier. The transconductance of the input pair is higher, and the output impedance of the Q15 and Q16 is also higher. (the gain of the amplifier is gm*ro).
However, if the amp ever clips, the folded cascode transistors will run dry, resulting in some unsavory behavior. Basically, Q14 & Q16 and everything below them and to their right will have no current. Roender says he can never clip his amplifier in his system, so he has no worries. Others, however, will clip theirs. The solution is to increase the bias of Q13 and Q11 to a little above 13.5mA each. This will lower the OL gain by a factor of about 3, and I do not know what affect it will have on the sound. Probably several transistors will need heat sinks attached due to 3x dissipation. I learned this one on an IC design I was involved with once upon a time.
Welcome to the Club, Russell!
And I don't see a reason to feel bad. Opposite, by my POV. You have a so big and strong background and experience in electronic design. So, may be is time for FC-200R?!
(name is for example)I'm the one of, maybe hundred, happy builders, not educated in 1% like you, but my amp start and run from 1-st trial without any problems, mistakes, glitches and etc. And I loved his sound signature!
Also, FC-100 grab my attention, because I'm huge fan of folded cascode topology and still love my old Luxman L-530 too.
Anyway, I have no doubt, a new FC-200R will be warm welcomed in a forum!
Good luck with your development!
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Stay tuned. I'm already starting to post bits and pieces in new threads, like the one on William Chater's self-bias circuit, or another one on high BW and stability in an EF-triple output.
Yes, Russell, agree with Lordoff. It's a wonderful amplifier.
I recognised Roender's design from the start as exceptional, and have moved on with various folded cascodes since, though nothing published here. I use something similar in a commercial product and its performance is superlative and owes some of Mihai Rauta's design from 2007.
We would all love to see your cafe doodlings of a 200W version........
HD
I recognised Roender's design from the start as exceptional, and have moved on with various folded cascodes since, though nothing published here. I use something similar in a commercial product and its performance is superlative and owes some of Mihai Rauta's design from 2007.
We would all love to see your cafe doodlings of a 200W version........
HD
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Luxman's A902 is a power amplifier kit from the early 1980s with such goodies as folded cascode input / VAS, DC servo, triple Darlington outputs, switch-selectable output-inclusive GNFB operation and output exclusive NFB operation, switch-selectable Class A or AB operation, stereo or monaural (BTL) operation, optional battery power supply, and optional front panel meters.
LUXKIT A902-Mの仕様 ラックスキット/ラックスマン
Worth acquiring if anyone encounters one.
LUXKIT A902-Mの仕様 ラックスキット/ラックスマン
Worth acquiring if anyone encounters one.
Wow, thank you MR. Carr for recommendation!
Looks new to me and very intriguing design. Here, in Europe Luxkit are so rare, unfortunately.
Just new version of FC-xxx.
Cheers!
Looks new to me and very intriguing design. Here, in Europe Luxkit are so rare, unfortunately.
To be honest, with the name FC-200R I don't have in mind 200W.
Just new version of FC-xxx.
Cheers!
See
https://www.diyaudio.com/forums/sol...-stage-audio-power-amplifier.html#post1349211
and short discussion after.
There are different ways of interpreting how many stages an amplifier has.
A simple long-tailed diff pair, with the output taken from the opposite side as the input, can be thought of as an emitter-follower driving a common-base. Yet no one ever calls an LTP anything but a single stage.
If you stack a cascode pair above the diff pair, is it now a 1-stage, 2-stage, or even a 3-stage amp? Most of us still call that a single stage.
What if it's a folded cascode? From a small-signal perspective, this is not different than a stacked cascode. What the folding allows is a large-signal swing on the output. It also applies the gm of the input pair to the output impedance seen at the cascode transistor's collector, same as a stacked cascode. This is typically a gain comparable to a diff pair 1st stage driving a common-emitter 2nd stage.
Yet, a lot of people, when the cascode is folded, no longer want to think of the amplifier as a single stage. Now, they want to call it a 2-stage amplifier: a diff pair driving a common-base amplifier.
I think it's a one-stage amplifier.
@RussellKinder Could you shortly explain why you have choosen TL431 over the LED biasing? IMHO LED is very elegant lownoise tec.
Thx in advance.
If I remember correctly it (here somewhere) was discussed if the fc-100 could perform better with a dc servo but afaik Mihai said that DC offset is already very low and an DC Servo would contribute much. Since you compared your amp with this one - I want to ask how is the situation for your amp? (You already wrote that you have thought about improving it)
Your RK-Auto200W is a beast of engineering. Unfortunately my knowledge is not deep enough to understand thoroughly - but I already can sense the engineering effort went into the circuit...
After a long pause this raises urge to go back into this hobby
Btw nice hidding of the second beast in it the autobias circuit ...new stuff to learn for me. Anyway thank you for your input and "connecting" both amp projects.
Thx in advance.
If I remember correctly it (here somewhere) was discussed if the fc-100 could perform better with a dc servo but afaik Mihai said that DC offset is already very low and an DC Servo would contribute much. Since you compared your amp with this one - I want to ask how is the situation for your amp? (You already wrote that you have thought about improving it)
Your RK-Auto200W is a beast of engineering. Unfortunately my knowledge is not deep enough to understand thoroughly - but I already can sense the engineering effort went into the circuit...
After a long pause this raises urge to go back into this hobby
Btw nice hidding of the second beast in it
the autobias circuit ...new stuff to learn for me. Anyway thank you for your input and "connecting" both amp projects.
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Hi,
Good question about the TL431 vs LED. TL431 is very low impedance and temperature stable, but noisy. For biasing cascode bases, the noise doesn't really matter. Also for biasing the 1st follower in the triple EF output, the noise is pretty irrelevant. It is eaten by the output impedance of the follower, and you are post gain at that point.
I'm usually not too concerned about tail current noise, because I mirror the current later on. That should cancel the tail current noise, mostly. I might consider something like Walt Jung's GLED-431 for the tail current. I don't really like glowing LEDs on my amp circuit boards, but I suppose I could paint them black.
Good question about the TL431 vs LED. TL431 is very low impedance and temperature stable, but noisy. For biasing cascode bases, the noise doesn't really matter. Also for biasing the 1st follower in the triple EF output, the noise is pretty irrelevant. It is eaten by the output impedance of the follower, and you are post gain at that point.
I'm usually not too concerned about tail current noise, because I mirror the current later on. That should cancel the tail current noise, mostly. I might consider something like Walt Jung's GLED-431 for the tail current. I don't really like glowing LEDs on my amp circuit boards, but I suppose I could paint them black.
Thx for your explaination. Accroding to leds. You might be ablte to use IR Leds. They sometimes slightly bleed into the lower visible red spectrum but not comparable to a full green bright one.
I have seen Walt Jungs GLED431 a while ago but visited the thread today again.
Mr. Jung's ultra-low noise VREF - the GLED431
IR noise refering to :
Some noise measurements for LEDs and zener diodes
Well forward Voltage is slightly lower then the Red-Green ones.
I was although once thinking to make use of ldo'S - but havent been through the whole idea.
I have seen Walt Jungs GLED431 a while ago but visited the thread today again.
Mr. Jung's ultra-low noise VREF - the GLED431
IR noise refering to :
Some noise measurements for LEDs and zener diodes
Well forward Voltage is slightly lower then the Red-Green ones.
I was although once thinking to make use of ldo'S - but havent been through the whole idea.
