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A case of direct coupled cathode follower
A case of direct coupled cathode follower
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Old 28th May 2014, 07:12 PM   #1
Mosquito is offline Mosquito  Argentina
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Default A case of direct coupled cathode follower

Hi,
Lately there was too much fuss about the 12AU7 tube , so out of curiosity and because they are cheap and there are a plenty around , I decided to give the tube a chance and try some sim on the software. Now, it comes out this DC cathode follower circuit, but, I'm unsure if the fairly low level of THD computed are a simulation artifact, or can be expected in the real thing. At first glance the operating points seems to be reasonable, but, you know, when a thing seems to be too easy or too good...
Well, there is included the .asc file if someone likes to check. The tubes are Ayumi models.
Cheers
J.
Attached Images
File Type: gif DC CF.gif (26.5 KB, 708 views)
Attached Files
File Type: txt 12AU7_cathode follower.asc.txt (3.0 KB, 67 views)
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Last edited by Mosquito; 28th May 2014 at 07:16 PM.
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Old 28th May 2014, 08:48 PM   #2
DF96 is offline DF96  England
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Most cathode followers are capable of low distortion. You have to be knowledgeable or ignorant to make a bad one.
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Old 29th May 2014, 09:24 AM   #3
disco is offline disco  Netherlands
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What's the anode voltage of U2?
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Old 29th May 2014, 10:37 AM   #4
ruffrecords is offline ruffrecords  United Kingdom
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What you have is a variation on the mu follower. The CF bootstraps the anode load of the first triode which gets its distortion down to close to the intrinsic distortion of the tube. The CF has 100% NFB so its distortion will also be low. With 0.1V input level the output will only be about 2V and since distortion in triodes is directly proportional to level it is not surprising the distortion is relatively low. Try with a 1V input.

Cheers

Ian
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Old 29th May 2014, 11:13 AM   #5
nl3prc is offline nl3prc  Netherlands
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Quote:
Originally Posted by Mosquito View Post
Hi,
Lately there was too much fuss about the 12AU7 tube , so out of curiosity and because they are cheap and there are a plenty around , I decided to give the tube a chance and try some sim on the software. Now, it comes out this DC cathode follower circuit, but, I'm unsure if the fairly low level of THD computed are a simulation artifact, or can be expected in the real thing. At first glance the operating points seems to be reasonable, but, you know, when a thing seems to be too easy or too good...
Well, there is included the .asc file if someone likes to check. The tubes are Ayumi models.
Cheers
J.
Hi

Next time you post an ASC files try a zip so you can include the 12AU7.INC

Thanks
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Old 29th May 2014, 11:40 AM   #6
45 is offline 45  Italy
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Quote:
Originally Posted by ruffrecords View Post
What you have is a variation on the mu follower. The CF bootstraps the anode load of the first triode which gets its distortion down to close to the intrinsic distortion of the tube. The CF has 100% NFB so its distortion will also be low. With 0.1V input level the output will only be about 2V and since distortion in triodes is directly proportional to level it is not surprising the distortion is relatively low. Try with a 1V input.

Cheers

Ian
A line stage with a 12AU7 configured as common cathode + DC coupled cathode follower can easily give THD <1% for 18-20V output. It is surely a better choice than SRPP from this point of view. No question about this. The actual gain will never be 20. 12AU7 might not be the best linear triode but it is not as bad as many people tell. Maybe it is so common that most of those people don't get good results because of the poor quality of the actual AU7's they have.
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Old 29th May 2014, 11:54 AM   #7
AndrewT is offline AndrewT  Scotland
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If one wants/needs an output impedance of around 100ohms, what valve/s should be used in the cathode follower and what circuit gets to that target?
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Old 29th May 2014, 12:11 PM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
If one wants/needs an output impedance of around 100ohms, what valve/s should be used in the cathode follower and what circuit gets to that target?
For a simple cathode follower the output impedance is about 1/gm, so you'd be looking for a gm of 1/100 = 10mA/V. An ECC88 would manage, or a pair of lesser valves in parallel.
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Old 29th May 2014, 12:27 PM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by AndrewT View Post
If one wants/needs an output impedance of around 100ohms, what valve/s should be used in the cathode follower and what circuit gets to that target?
The circuit simulated by Mosquito with bootstrapped 12AU7 CF is a very good one and I think it is as good as it gets! If you also apply few dB's of loop feedback distortion will be practically at -80 dB (0.01%) or lower at 2-3V RMS output driving some 20-50K load. The output impedance will be in the 250-350 R range.
If you really want 100 ohms to drive some 10K you can use higher gm triodes but that will not necessarily mean that distortion will be lower than above or vanishingly small. You still need to optimize the actual circuit and probably apply some feedback.
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Old 29th May 2014, 01:12 PM   #10
Mosquito is offline Mosquito  Argentina
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Quote:
Originally Posted by DF96 View Post
Most cathode followers are capable of low distortion. You have to be knowledgeable or ignorant to make a bad one.
or like in this case, a lucky ignorant that comes out with a good one..

Disco and nl3prc, I will post the data this afternoon, now Im not at home. and can not access my files. Nevertheless Jazbo has already provided the complete Ayumi models, including the 12AU7. Anode voltage of U2 is 300V after R14, U2 is running at about 2mA, U2 grid is conveniently negative respect to the cathode.

The simulation is still good with 1V , 2V, and even beyond this level. Drives a 10K load with some low end restriction, but fitting a 10uf for C1 alleviates the situation.
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Last edited by Mosquito; 29th May 2014 at 01:19 PM.
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