"The Electron" a Cathode Follower Headamp Microphone Project

[rock soderstrom]

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.

Edit: C3=10uF as drawn is too small for full range bandwidth.



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.

 

[Khron]

Version 3

What's the difference between Version 2 and 3?

 

[rock soderstrom]

What's the difference between Version 2 and 3?

Ha ha, now there is a difference. I made a pic upload mistake...thanks!

 

[pasarski]

V1 is simplest (the embedded picture, the attached pictures are different). A noobie question:

- Isn't it a problem to connect the grid to the output that way with the signal present on top of the 60v?

 

[rock soderstrom]

V1 is simplest (the embedded picture, the attached pictures are different).

Thanks! I have corrected the attachments, everything should be correct now.

A noobie question:

- Isn't it a problem to connect the grid to the output that way with the signal present on top of the 60v?

I don't think so. The (DC) situation is pretty similar in an AKG C60, isn't it? What do you think?



Edit: John Broskie uses a similar arrangement here:

 

[pasarski]

That C60 schematic makes my smooth brain hurt, especially how the top of the transformer primary is connected straight to the cathode, which is not at the ground potential. Must be a special transformer? (EDIT: Explained to this smoovebrain below by @Khron )

But yes, even I can see how the signal is similary present where the 180 meg resistor is connected. Aikido schematic is much easier to read. It's a bit different situation with no capsule and polarization voltage though.

Very interested to see how this turns out!

Another question:

- Are you sure the circuit has enough headroom for drums? What I've learned playing with fet circuits in spice is that low distortion at lower levels and high headroom doesn't always go hand in hand.

EDIT: Ok it seems that cathode followers are particularly good headroom wise

 

[Khron]

Must be a special transformer?

Why? Its ground leg is still AC-coupled, so there's no DC going through it, hence no need for reinforced insulation or magnetic gap.

 

[RuudNL]

Isn't it a problem to connect the grid to the output that way with the signal present on top of the 60v?

No, it is not. Because the cathode of V1.2 is more positive than the grid, so in fact the tube gets a negative grid voltage. (The voltage over R2).

 

[Delta Sigma]

Love this @rock soderstrom . I've been thinking of tackling a similar project this winter. Keep meaning to go back through these CF threads; but what is the reason for the E88CC? What makes a good CF tube selection anyway? I assume there are different candidates than plate followers.

Exactly HOW much headroom would a CF like this have? Would it still be smart to employ a pad?

 

[rock soderstrom]

but what is the reason for the E88CC?

This is because I also use E88CC in the cathode follower position in my reference Aikido amp and it works exzellent. So I will start with that, I will also try other types.

What makes a good CF tube selection anyway? I assume there are different candidates than plate followers.

I believe that many tube types are generally suitable, but since CF circuits are often used as driver stages, more current-potent tubes are better suited. Whether this is important for a microphone I don't know, we will see.

Exactly HOW much headroom would a CF like this have? Would it still be smart to employ a pad?

These are good questions, I hope I can answer them at the end of the project. I'm curious...

Edit: At the moment I'm not quite sure what the bottleneck is with regard to the headroom. The capsule, the impedance converter or the transformer? What effect does the level of B+ and the polarisation voltage have in this context?

 

[Delta Sigma]

Edit: At the moment I'm not quite sure what the bottleneck is with regard to the headroom. The capsule, the impedance converter or the transformer? What effect does the level of B+ and the polarisation voltage have in this context?

This is exactly why I'm thinking a capsule bias voltage pad would be perfect for this project. Should cover all your bases. You could do this with a switch on the mic body, internal jumper, or up it to a 7 pin XLR

 

[KrIVIUM2323]

Hi,
Good idea! I like cf headamps i used and played with! I will read with interest!

About tube type: there is a real big difference between your plan and the c60: the tube and there might be a big difference between E88CC and ac701k in some registers.
First is current capacity obviously. And ac701 could never dream of the current output the E88cc will drive easily. In the headamp it means eventually a lower output Z ( do you need something circa 50ohms out Z?) and largely bigger drive capability ( maybe use a 1:2 transformer for 200ohms out Z and a bit of 'free' voltage gain?) but this will come at a cost. Which is the second difference: grid current.

Ac701 being optimised for mic dutyes it doesn't display a lot of grid current by design. E88CC on the contrary will probably exhibit more. Even if used with low current or heater starved (lower voltage circa 5,7v).

In your first case ( v1) it could 'cook' the capsule as PRR once answered to one of my planed experiment in same kind of circuits as your V1 schem.
I still did the experiment whith a chinese k67 clone. Caspule did'nt seemeed to suffer but i had other issues with this circuit so disregarded it quickly, so can't say longterm effects. That said PRR rarely made such statements without reason. Maybe someone with better understanding could explain? That said i like the simplicity of V1 schematic.

About headroom: in my experience(s) this is the capsule the weak link. Headamp will rarely see more than 2/3volts, correctly sized transformers would not be an issue either.

Edit: i don't see any difference between V2 AND V3!

 

[rock soderstrom]

Hi,
Good idea! I like cf headamps i used and played with! I will read with interest!

Thanks for your feedback!

About tube type: there is a real big difference between your plan and the c60: the tube and there might be a big difference between E88CC and ac701k in some registers.
First is current capacity obviously. And ac701 could never dream of the current output the E88cc will drive easily.

I am aware of that, the C60 was just an example for the basic circuit. I also analyzed the C12a, the Sony C800 and C37a, the Royer Mics and so on. The AC701 tube is really special, the other tubes like the 6AU6/EF94, EF95 (all as triode wired) etc. are closer to the E88CC in terms of technical data. The AT4060 even uses an E88CC in a (White?) cathode follower circuit.

In the headamp it means eventually a lower output Z ( do you need something circa 50ohms out Z?) and largely bigger drive capability ( maybe use a 1:2 transformer for 200ohms out Z and a bit of 'free' voltage gain?) but this will come at a cost.

That is a good point. In my first draft I assume an output impedance of the ACF circuit of approx. 300 Ohm (100 Ohm plus one Rk resistor of 200R). But I would like to achieve at least an output impedance of 200Ohm at the output of the transformer, a 1:1 type is not enough, a 1.5:1 would be ideal, but I only have 2:1 types at the moment, so I am calculating with this one. I guess a 1:1 type would also work. The very brave ones leave out the OPT for the most direct path...with short cable runs this is no problem.

The output capacitor must be chosen relatively large, because the impedance is so low (the usually large step down factor of the OPT in tube mics is missing!), for a 2 to 1 OPT I calculate with f=1/(2*Pi*R*C) or C=1/(2*Pi*R*f) then for a lower cut-off frequency (-3dB) of 20Hz a capacitor with 106uF(!), if my math is correct...

Here it would make sense to install a switchable HPF with different capacitors. OH mics do not always need the full bass and low mids range. Which frequencies would make sense here from the mixer's point of view?

Which is the second difference: grid current.

Ac701 being optimised for mic dutyes it doesn't display a lot of grid current by design. E88CC on the contrary will probably exhibit more. Even if used with low current or heater starved (lower voltage circa 5,7v).

Yes, that's the big white elephant in the room. It remains to be shown how well the E88CC copes with the high impedance situation. If it doesn't work, I'll use a different type of tube with two triode systems.

In your first case ( v1) it could 'cook' the capsule as PRR once answered to one of my planed experiment in same kind of circuits as your V1 schem.
I still did the experiment whith a chinese k67 clone. Caspule did'nt seemeed to suffer but i had other issues with this circuit so disregarded it quickly, so can't say longterm effects. That said PRR rarely made such statements without reason. Maybe someone with better understanding could explain? That said i like the simplicity of V1 schematic.

Okay, PRR knows his stuff, no doubt but I can not see in the moment what could be the problem. I would be very interested to hear other opinions. That's why I mentioned the C60 circuit, which works very similarly, where the capsule is at the cathode potential of the CF, which is technically (DC wise) one or two volts off the potential of the 81K resistor, which shouldn't make much difference, right?

About headroom: in my experience(s) this is the capsule the weak link. Headamp will rarely see more than 2/3volts, correctly sized transformers would not be an issue either.

Thanks for info!

Edit: i don't see any difference between V2 AND V3!

But there should be a difference now, maybe your browser still shows you the wrong picture (from the cache), which I uploaded first.