The Constant Current Draw Amplifier (CCDA)
(AKA the APEX "Cathode Follower")
There is a lot of talk about the Asian microphones and their "Cathode Follower" (CF) circuits. What usually follows in the blogs is the endless discussions about the merits of or the terrible sound of the CF circuit.
But there is much more to this than is usually discussed. In fact the way the ALCTRON engineers, who build all the APEX/Nady and the like microphones, used this circuit is pretty innovative. Considering that the same option was available to AKG in the 1960s with the C12 and the ELA m251, the Alctron design shows a creative use of tube resources that didn't occurr to top flight engineers at that time in Germany.
Editors Update Sept 7, 2010:
We have been informed by David Bock of Bock Audio that this amplifier configuration was in fact used in 1995 in the Soundelux U95 microphone that he designed and what's more, it was used before that by a German manufacturer (name unknown at this time) in the 1930s. Thanks to David Bock for helping us keep the facts straight.
See David's World Class microphone products at www.bockaudio.com
As the title indicates this configuration shown below is not a "Cathode Follower" but actually a composite amplifier, consisting of two triodes, designed to do some very specific things. Those things end up being very valuable when implemented as the front end of a Large Diaphragm Condenser (LDC) microphone.
The basic Constant Current Draw Amplifier (CCDA)
So the correct name is the CCDA. What does it mean? It is called constant current draw because no matter what signal is going though the amplifier the current draw is always the same. Why is this a good thing? Well when audio is pumping the current up and down inside a single tube, it has the effect of dragging the power supply voltage up and down in exactly the opposite direction of the signal.
When you use the CCDA circuit the second tube is working exactly the opposite to the first because the output of the first stage section is out of phase with the input. So the second tube is reducing current when the first tube is drawing more current. The result is that as far as the power supply is concerned, the current flowing is CONSTANT.
Why does current stability matter?
When the power supply current does not vary, the voltage to the tube amplifier also does not vary. This results in lower distortion because the power supply is not affecting the audio which is not affecting the power supply which is not affecting the audio which is not ...
well you get what I mean. Everthing is much more stable.
And One More Thing ...
One of the weaknesses of the conventional "plate follower" tube amplifier is that is has a relatively high output impedance. Early tube engineers used transformers to take the high impedance output and convert it down so it could drive a low impedance "load" like a loudspeaker or a long microphone cable for example. But connecting a transformer to this high impedance tube output still meant roll-off in high frequency and low frequency response. The transformer had to be very carefully matched to the tube circuit.
What a tube microphone really needs is a tube amplifier with very high input impedance and a low impedance output! Can that be done? You betcha!
This is one of the reasons we like the 6922 tube. It provides these characteristics in one bottle!
On top of the constant current operation we described above, the CCDA circuit has another advantage. The second triode can be used to correct that nasty high output impedance problem! Simply run the second triode as a cathode follower circuit. This takes the output impedance of a plate follower 6072 down from 20,000 ohms to a mere 600 ohms when using the cathode output!
Now normally that would take some extra components to couple the first tube to the second tube. Those extra resistors and capacitors would have an impact on the sound. But what if you could DIRECTLY connect the first stage amplifier to the second stage?
With this "direct coupling" there is virtually no signal degradation and we get all the advantages we spoke about for the cost of few extra micro-amperes of current and the addition of one resistor!
So is the Alctron micrphone design sub-standard? Not in our opinion. The early tube microphone designers in the 1940's could not get their hands on small dual triodes in one envelope. George Neumann worked with the technology of the day so his only choice was to match the tube and transformer as best he could. The AKG people did not innovate with their designs that used the 6072 tube but rather stuck to common practice. In the C12 they simply grounded the second triode and turned off the filament.
Our feeling is that the Engineers at Altron did something innovative by using the CCDA circuit in their microphones. You can see the difference it makes to frequency response and distortion in our article on Transformers.