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.
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.
Attachments
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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
Cheers
Ian
HiHi,
Lately there was too much fuss about the 12AU7 tube
Well, there is included the .asc file if someone likes to check. The tubes are Ayumi models.
Cheers
J.
Next time you post an ASC files try a zip so you can include the 12AU7.INC
Thanks
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.
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.
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.
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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Works OK for me, on par with the former SRPP.
I worked with values at hand, some deviating.
I worked with values at hand, some deviating.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Didn't spent it on interconnects
I was thinking in this kind of input to drive:
http://www.albarrymusic.com/
go to products/M608 monoblocks/specs
http://www.albarrymusic.com/
go to products/M608 monoblocks/specs
The 12AU7 intrinsic distortion is bad compared to most other audio tubes. Morgan Jones demonstrated this beyond doubt and I have verified it myself. Put a 6CG7 in the same topology and it will beat the 12AU7 hands down.
Whether it is 'better' than an SRPP depends entirely on what you want to do with it. Whatever the standing current, the SRPP will output twice the current that you can get from a CF. That's what the SRPP was designed for - an output stage.
Cheers
Ian
You also need to specify the load you are driving and the level you want to drive into it.
Cheers
Ian
Hard load to drive...
Specifications
Output ……………… 60Watts into 8 Ohms (18dBW)
Frequency response ………............….2Hz-110 KHz
Damping factor ……………........…. greater than 500
Input impedance ………….................……5K1 Ohms
Input sensitivity ……………...............…..550mV 0dB
I'm enjoying this build for good detail / improved channel separation / in your face splashing dynamics. Unfortunately the 12AH7 went microphonic in this setup, probably because of the higher amplification of V1. I'll check for oscillation as I left the grid stoppers out.
Specifications
Output ……………… 60Watts into 8 Ohms (18dBW)
Frequency response ………............….2Hz-110 KHz
Damping factor ……………........…. greater than 500
Input impedance ………….................……5K1 Ohms
Input sensitivity ……………...............…..550mV 0dB
I'm enjoying this build for good detail / improved channel separation / in your face splashing dynamics. Unfortunately the 12AH7 went microphonic in this setup, probably because of the higher amplification of V1. I'll check for oscillation as I left the grid stoppers out.
ken-rad 12ah7 is closer to the 12sn7 than the 12au7 as to distortion . v2 is way over on the voltage across it should be only 180 volts with about 300 volt supply max. v1 is fine . pin out is different than sn7 with 16 vs 20 on the mu . You can run the AH7 harder at say 6 to 7ma but lower voltage a bit . What year are your ken-rads ? I have about 12 made in the 40's .
Whoops, indeed. These are JAN 1943.
How is PSRR of the cathode follower? I have just two reservoir caps underneath, combining L&R channels on one cap might give bleeding...
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