rock soderstrom
Tour de France
Hi guys, I'm actually still busy with my other microphone projects. The "EasyMod" series is almost finished, the “Goldie” microphone is currently being tested in the lab and in the studio.
Nevertheless, I'm itching to start this new project thread to discuss my next endeavor.
I want a microphone (actually two) for miking loud, percussive instruments. Drum overheads and similar tasks are in the foreground, the microphone should be able to handle high levels and produce a realistic sound. Other applications such as choir recordings and general stereo arrangements are of course also conceivable.
Due to the aforementioned objectives, I concentrated on tube cathode followers and analyzed possible candidates. Here you can find a corresponding info thread on the topic of cathode followers in mics, I have also added some schematics there.
For the actual realization of the tube impedance converter, I decided to use the so-called “Aikido” cathode follower, which was designed by the great John Broskie. You can find tons of infos about this circuit on his webpage.
Why?
I use the original Aikido circuit (with gain) in various amplifiers, including an ironless headphone amplifier which has been my reference amplifier for over 15 years. I love the sound of this amplifier and want to transfer its qualities into a microphone.
The original Aikido circuit consists of two tubes, each with two triode systems. Unfortunately, this does not fit into my planned housing (mic body)
, so I am concentrating on the rear part of the Aikido circuit, the Aikido Cathode Follower (ACF).
I hope that this will allow me to grasp the essential character of the Aikido circuit.
How is the Aikido circuit designed and what makes it so special? Let the original designer have his say, who could summarize it better?
"The ACF is basically the last half of the Aikido amplifier, a modified cathode follower buffer circuit that provides near unity gain and a low output impedance. This modified cathode follower scrubs away the power-supply noise from its output. This feat is accomplished by proactively injecting a small sample of the power-supply noise into the bottom triode's grid, which generates a countervailing current variation, which nulls the ripple from the output. In addition, the Aikido cathode follower produces lower distortion by using the triode’s own non-linearity against itself by providing a complementarily non-linear load for the top triode’s cathode."
"This unity-gain buffer, using a modified cathode follower, offers a high input impedance, a low output impedance, low distortion, and a great PSRR figure. In addition, the ACF does not invert the phase."
That sounds very promising and exactly what you want from a CF circuit in a microphone. That's why I dared to try an adaptation, which I would like to present here. There are actually three design ideas (more or less). I want to use the E88CC tube beside its bad reputation for this job.
Here is the first version, my favorite so far. The capsule polarization voltage is tapped between the two triode systems, similar to the AKG C60/C12a, except that these mics uses only a cathode resistor instead of the second triode. The diaphragm is connected to GND to make it less attractive to dust.
The second version, were a voltage divider provides the voltage for the capsule. This one is more flexible if I want to use a higher B+...and there is more filtering for the polarisation voltage.
Version 3
Okay, so far my first drafts. Does it work like this? Have I done something wrong? I welcome any constructive suggestions. What do you think and what is your favorite version?
Other tube types would also be conceivable (if the E88CC doesn't cope so well with the high impedance situation) such as 6n1p, 6CG7, 12AU7 or similar.
The component values may still have to be adjusted.
Many thanks to John “The Electron” Broskie for the inspiration, design and the countless ideas on his website.
PS: please no FETs vs tubes discussion.
Nevertheless, I'm itching to start this new project thread to discuss my next endeavor.
I want a microphone (actually two) for miking loud, percussive instruments. Drum overheads and similar tasks are in the foreground, the microphone should be able to handle high levels and produce a realistic sound. Other applications such as choir recordings and general stereo arrangements are of course also conceivable.
Due to the aforementioned objectives, I concentrated on tube cathode followers and analyzed possible candidates. Here you can find a corresponding info thread on the topic of cathode followers in mics, I have also added some schematics there.
For the actual realization of the tube impedance converter, I decided to use the so-called “Aikido” cathode follower, which was designed by the great John Broskie. You can find tons of infos about this circuit on his webpage.
Why?
I use the original Aikido circuit (with gain) in various amplifiers, including an ironless headphone amplifier which has been my reference amplifier for over 15 years. I love the sound of this amplifier and want to transfer its qualities into a microphone.
The original Aikido circuit consists of two tubes, each with two triode systems. Unfortunately, this does not fit into my planned housing (mic body)
, so I am concentrating on the rear part of the Aikido circuit, the Aikido Cathode Follower (ACF).I hope that this will allow me to grasp the essential character of the Aikido circuit.
How is the Aikido circuit designed and what makes it so special? Let the original designer have his say, who could summarize it better?
"The ACF is basically the last half of the Aikido amplifier, a modified cathode follower buffer circuit that provides near unity gain and a low output impedance. This modified cathode follower scrubs away the power-supply noise from its output. This feat is accomplished by proactively injecting a small sample of the power-supply noise into the bottom triode's grid, which generates a countervailing current variation, which nulls the ripple from the output. In addition, the Aikido cathode follower produces lower distortion by using the triode’s own non-linearity against itself by providing a complementarily non-linear load for the top triode’s cathode."
"This unity-gain buffer, using a modified cathode follower, offers a high input impedance, a low output impedance, low distortion, and a great PSRR figure. In addition, the ACF does not invert the phase."
That sounds very promising and exactly what you want from a CF circuit in a microphone. That's why I dared to try an adaptation, which I would like to present here. There are actually three design ideas (more or less). I want to use the E88CC tube beside its bad reputation for this job.
Here is the first version, my favorite so far. The capsule polarization voltage is tapped between the two triode systems, similar to the AKG C60/C12a, except that these mics uses only a cathode resistor instead of the second triode. The diaphragm is connected to GND to make it less attractive to dust.
The second version, were a voltage divider provides the voltage for the capsule. This one is more flexible if I want to use a higher B+...and there is more filtering for the polarisation voltage.
Version 3
Okay, so far my first drafts. Does it work like this? Have I done something wrong? I welcome any constructive suggestions. What do you think and what is your favorite version?
Other tube types would also be conceivable (if the E88CC doesn't cope so well with the high impedance situation) such as 6n1p, 6CG7, 12AU7 or similar.
The component values may still have to be adjusted.
Many thanks to John “The Electron” Broskie for the inspiration, design and the countless ideas on his website.
PS: please no FETs vs tubes discussion.
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(the embedded picture, the attached pictures are different). A noobie question: 
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