JLH,
your profile settings do not allow anyone to email you. Please email me thru the forum instead.
-- josé k.
your profile settings do not allow anyone to email you. Please email me thru the forum instead.
-- josé k.
Having spent a few days in bed due to the flue I have had the time to read through JR MacDonalds three patents 2795654, (2796468) and 2965853.
As I see it he started with a cascode CF in the first patent and ended up with a zero gain OP amp in the third. As the dominant contribution to Zout is RDCprim and RDCsec my question is if anything is gained adding even more NFB to get the CFs Zout lower.
In the third patent the first one is refered to as "PACF". Ie what he calls a "Parallell Augmented Cathode Follower" is nothing but a cascode follower and can as well be built by using a pentode CF with less effort. This is of course to simplify things a little but I like the KISS-principle.
I have presented a pentode follower idea in the parallell thread "20W.... " . http://www.diyaudio.com/forums/showthread.php?s=&threadid=105859&pagenumber=5 and must build one to see if it works as good as the sim indicates.
As I see it he started with a cascode CF in the first patent and ended up with a zero gain OP amp in the third. As the dominant contribution to Zout is RDCprim and RDCsec my question is if anything is gained adding even more NFB to get the CFs Zout lower.
In the third patent the first one is refered to as "PACF". Ie what he calls a "Parallell Augmented Cathode Follower" is nothing but a cascode follower and can as well be built by using a pentode CF with less effort. This is of course to simplify things a little but I like the KISS-principle.
I have presented a pentode follower idea in the parallell thread "20W.... " . http://www.diyaudio.com/forums/showthread.php?s=&threadid=105859&pagenumber=5 and must build one to see if it works as good as the sim indicates.
It is too soon to tell. I will probably have some kind of CF amp available. I like to put my designs through some serious testing before they get turned into products. Right now I have three different CF amplifiers running with a few more planned. There will be lots of testing and eventually one or two designs will prove themselves out. The final designs get a bunch more testing before they get turned loose on the world.
It is the third of several prototype test amps. I had about 8 different prototype PCB's when I designed the TubelabSE and 3 to do the SimpleSE. The first CF amp PCB has already been robbed for parts. It was seen in the 20W CF thread, post #54 and 55. There will be several more before I am happy with the design.
I started experimenting with bootstrapped CF output stages early last year. I developed a power amplifier that put out 20 watts with very low distortion for the design contest mentioned in post #41. It was a very complicated design using DSP technology. I learned about Macdonalds work in the first post of this thread, so I set out to build a power amplifier using one of his circuits. I decided that the ACF-2 circuit shown in post #25 would be the best choice for an output stage. It is similar to the circuit shown in the patent #2965853 (included). There are several advantages to this design over a simple CF:
V4 is a CF fed by the same signal voltage that the output stage (V3) sees. When carefully adjusted, V3 operates with a constant plate to cathode voltage. This lowers the device dissipation allowing serious efficiency improvements if V4 is replaced by a modulated SMPS. I heve been experimenting with this technology. The constant voltage removes the effects related to parameter shift VS voltage, lowering the distortion. The same voltage is applied to V1. This allows it to function in a similar manner. Since the voltages across V1 all move in phase, the input capacitances tend to cancel, so the stage input impedance is very high up to the MHz range. The feedback can be adjusted (24), so that the stage has overall gain. This feature can be used to reduce the drive voltage requirements, allowing for a better driver design.
In my implementation I replaced V5 with a solid state CCS. I made (24) variable as shown in this schematic. A stage gain of two seems to be the best balance between distortion, drive requirements and stability. V1 - V2 is a 12AX7 and V3 - V4 is a 6AS7, 7236, 5998A or a 6336A. The zener voltage is adjusted to set e3 such that the voltage (and dissipation) is evenly split between V3 and V4. This amp operates with a 600 ohm OPT. The cheap Chinese OPT that I am using now has RDCprim of 12 ohms. I am sure that there are better ones out there.
I am testing versions with V4 replaced with a mosfet, and a version with V4 replaced by a variable SMPS. I plan to test versions with pentodes for V1 and V2.
Attachments
Hi Tubelab,
Any progress on your Augmented CF power amplifier? I just bought a pair of Eastern Audio 600 ohm output x'formers too.
Thanks.
Regards,
TC MA
Any progress on your Augmented CF power amplifier? I just bought a pair of Eastern Audio 600 ohm output x'formers too.
Thanks.
Regards,
TC MA
I haven't had any time to work on it. I posted the schematic in another thread, but it should be considered a work in progress.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118289
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118289
tubelab.com said:
Hi Tubelab,
Thank you for sharing your schematic in another thread with us. I have two questions in mind:
(1) What are the approximate +ve and -ve high voltages that you use in your schematic?
(2) Are the Eastern Audio transformer of roughly 15 ohm DCR directly connected to the L-OPT-2 and L-OPT-1 taps?
Thanks.
Regards,
TC MA
The DC voltages come from adjustable power supplies and were adjusted to meet the needs of each tube being tested. The negative supply spent most of its time around 250 volts. The positive supply was (and still is) set on 280 to 300 volts with 6336A's (depending on how much current I draw).
The OPT is connected to the OPT1 and OPT2 terminals. The tube current is set by the voltage divider at the grid of V2A (V4A). The pot across the OPT sets the amount of feedback, and thus the output stage gain. With the pot set for full feedback (wiper at the top on schematic) the stage gain is 1 and the output voltage (across the OPT) is equal to the voltage at the grid of V2A. As the gain is increased there is some DC offset introduced. Some gain lowers the drive requirements but too much gain leads to instability especially when driven to clipping. I got best results with the output stage gain set between 1.5 and 2.
The bias adjustment should come from a regulated source. It doesn't need to be a lot of voltage. I have problems with the pot connected across the two power supplies. They are both old tube regulated power supplies. The output voltage does shift with line and load variations espcially since I am drawing about 500 mA out of one of them which is rated for 300.
The zener diodes D1 and D2 (D3, D4) set the voltage distribution between the top and bottom output tubes. Try to get equal voltage across both halves. I am currently using 3 X 53 volt zeners in series.
Future experiments:
Replace the top output tube half with a mosfet. I have it all simulated but haven't had the time to build it yet.
Install a CCS from the lower output tubes cathode to the negative rail. Then adjust the bias for zero volts across the OPT so that an ungapped OPT can be used. Again simulated but not yet built.
The original microprocessor controlled SMPS version did not have the augmented follower circuit installed. I would like to add it and see what happens.
I am currently working on a smaller version that uses the American 7233 tube or the Russian 6S19P.
The OPT is connected to the OPT1 and OPT2 terminals. The tube current is set by the voltage divider at the grid of V2A (V4A). The pot across the OPT sets the amount of feedback, and thus the output stage gain. With the pot set for full feedback (wiper at the top on schematic) the stage gain is 1 and the output voltage (across the OPT) is equal to the voltage at the grid of V2A. As the gain is increased there is some DC offset introduced. Some gain lowers the drive requirements but too much gain leads to instability especially when driven to clipping. I got best results with the output stage gain set between 1.5 and 2.
The bias adjustment should come from a regulated source. It doesn't need to be a lot of voltage. I have problems with the pot connected across the two power supplies. They are both old tube regulated power supplies. The output voltage does shift with line and load variations espcially since I am drawing about 500 mA out of one of them which is rated for 300.
The zener diodes D1 and D2 (D3, D4) set the voltage distribution between the top and bottom output tubes. Try to get equal voltage across both halves. I am currently using 3 X 53 volt zeners in series.
Future experiments:
Replace the top output tube half with a mosfet. I have it all simulated but haven't had the time to build it yet.
Install a CCS from the lower output tubes cathode to the negative rail. Then adjust the bias for zero volts across the OPT so that an ungapped OPT can be used. Again simulated but not yet built.
The original microprocessor controlled SMPS version did not have the augmented follower circuit installed. I would like to add it and see what happens.
I am currently working on a smaller version that uses the American 7233 tube or the Russian 6S19P.
Hi Tubelab,
Thank you for your reply. Originally I intend to build a single-ended power amplifier based on the augmented CF using the Eastern Audio transformers, but it appears that the augmented CF circuits and the project may be too difficult for me technically.
So now I am point-to-point wiring a line preamplifier (zero gain) using the acf-2 circuit first. In the beginning, I will stick to the original circuit. I will try to biase the circuit values so that the DC offset is zero and the output voltage becomes zero as well and thus a high quality ungapped output transformer can be used here.
When the preamplifier works and I can learn more about the acf-2 circuit from building the preamplifier, I will go ahead to build the power amplifier based on the augmented CF using E130L tubes.
Regards,
T.C. MA
Thank you for your reply. Originally I intend to build a single-ended power amplifier based on the augmented CF using the Eastern Audio transformers, but it appears that the augmented CF circuits and the project may be too difficult for me technically.
So now I am point-to-point wiring a line preamplifier (zero gain) using the acf-2 circuit first. In the beginning, I will stick to the original circuit. I will try to biase the circuit values so that the DC offset is zero and the output voltage becomes zero as well and thus a high quality ungapped output transformer can be used here.
When the preamplifier works and I can learn more about the acf-2 circuit from building the preamplifier, I will go ahead to build the power amplifier based on the augmented CF using E130L tubes.
Regards,
T.C. MA
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