John Broskie's Guide to Tube Circuit Analysis & Design

30 Jun 2012

CCDA Noval Rev A.
Last year, I laid out a new version of the noval CCDA PCB, bringing it into Rev. A. The PCB is now 7 by 6 inches (1 inch longer than the old PCB), which allowed me to add an improved B+ power supply and to add a second set of output coupling capacitors.

I so believe in this feature that I plan on incorporating it into all my future PCB designs. Of course, you can use a single pair of coupling capacitors, but two offers your the possibility to tailor the sound to the recording or the music or your mood. Listen differently.

In case you forgot, CCDA stands for Constant-Current-Draw Amplifier. The CCDA is a compound circuit consisting of a grounded-cathode amplifier input stage that DC connects to a cathode follower output stage. Each triode sees the same cathode to plate voltage and the same load resistance and same idle current draw. The grounded-cathode amplifier and the cathode follower are in voltage phase, but in anti-current phase, which results in the constant current draw feature of this topology.

The constant-current-draw feature means that amplifying the input signal will not vary the amount of current drawn from the power supply and consequently not perturb the power supply, greatly simplifying the design of the power supply. In addition, the cathode follower output stage offers a low output impedance and isolates the input stage from the external electronic world, which is a good thing. Before moving on, note the two jumpers, J4 & J5. What do they do?

 

Positive Feedback
We can all use some positive feedback. Not a lot, just some. Jumper J4 introduces some positive feedback into the circuit. With jumper J4 in place and capacitors C3 & C6 left off the board, the CCDA benefits from a small helping of positive feedback, which slightly increases the gain and slightly lowers the output impedance. I used a JJ ECC802, a long-plate 12AU7 type and a lovely little tube. With positive feedback, the gain increased by 3dB and the Zo went up by about 100 ohms, from 480 ohms to 570 ohms. In other words, nothing too scary and no chance of oscillation.


CCDA with Positive Feedback

Jumper J5, on the other hand, creates a conventional CCDA arrangement.


Straight CCDA without Positive Feedback

So, what does positive feedback sound like? That was the question on my mind, as I had such excellent results with my Tetra Sans PS phono stage. (In fact, the Tetra Sans PS is one reason that I had this new PCB made, so that it might complement it nicely sonically and provide the phono stage its needed heater and B+ power supply connections.) The first thing my ears noticed was how very quiet this CCDA was. Usually, the first thing I notice when I replace my Aikido line stage with another topology is noise, but not this time. I am sure that the measured noise is a tad greater, but I couldn't hear it from my speakers.

Well, it is very easy easy to fool one's self. My favorite quote from Richard Feynman is:

"The first principle is that you must not fool yourself—and you are the easiest person to fool."

I wanted to hear the same extra sprinkle of excitement that I had heard with the positive-feedback-laden Tetra Sans PS phono stage—and I did. After about four hours of listening, I still heard it. (Usually, I can only fool my ears for ten minutes or so.) I have been rediscovering John Lee Hooker, the old bluesman extraordinaire. And dang if his recordings didn't sound hot. There was a marginal increase in excitement, liveliness, vivacity, ebullience, vitality, zest, exhilaration.. The key word in the last sentence was "marginal." Yet, in human affairs, the marginal can be decisive.

Here is a long quote from the letter I wrote to the editor of an electronic magazine over a decade ago:

First of all, I believe that anyone who sells $2,000 interconnects should not be allowed to die a natural death, failing that, horsewhipped with the cords. But at the same time, I have no quarrel with the audiophiles who tell me that those same cables transformed his system or, at the other extreme of expense, that four sheets of toilet paper in between his loudspeaker and their stands made his system sound ten times better.

In fact, I believe he is understating the difference. For the difference between recognizing and not recognizing is infinity. As Vanna White turns just one more letter, which is after all maybe only 2% of the available letters, the contestant recognizes the quote: “These are the times that try men’s souls...” Prior to turning that letter, there was no recognition; afterwards, total recognition. The result was not linearly related to effect. Marginal differences do not necessarily have only marginal results.

In Jude Wanniski’s great book, The Way the World Works, he develops his theory of marginality: marginal differences can, in human affairs, can have hugely disproportional effects. The camel's back is fine until the extra piece of straw, which is only a 0.00000001% increase in weight, breaks its back. The fastest runner in the world is famous and receives fat promotional deals; the second fastest runner, the one who is 99.993% as fast, is largely ignored and is offered no Nike endorsement jobs. The model who works for your local department store makes $200 per photo shoot, not the $200,000 that Cindy Crawford makes; yet she is only marginally less attractive than Cindy Crawford. If she were marginally more attractive, she would make $2,000,000 per photo shoot and we would worship her as a freshly born deity.

The whole of high-end audio lives in the realm of marginality: the $30,000 vacuum-tube amplifier's output is only marginally different from the average $200 Chinese receiver's output; as seen on the scope, no difference can be discerned. But, like the extra letter revealed by Vanna, small audible differences can have large effects.

Here is another example: long ago, I bought pizzas from a local pizzeria, which were frustratingly sometimes fantastic, sometimes mediocre. It drove me crazy, so I took to tasting the pizza before driving home. When it came up delicious beyond the dreams of gluttony, I ran back into the restaurant and demanded to see the pizza maker. "This pizza is delicious! How do you do it?" I asked. He thanked me for noticing and went on to explain his methods and techniques—all of which were small, only marginally better than what his coworkers performed. Yet, they all added up to superb pizza. By the way, when you enjoy a particularly fine meal, do not give the waiter a bigger tip, give the chef a tip. If more customers did this, the restaurants would serve tastier food over time.

I have always been disappointed by the apparent lifelessness of CDs, regardless of the cost of the player. It was a marginal difference compared to LPs, but an important difference. And what is only a marginal improvement can nonetheless make a huge difference in my listening pleasure. If it's not for the sake of pleasure, why bother listening to music?


My New Project
Okay, back to tube electronics proper. I like the new CCDA a great deal and I want to combine it with Tetra Sans PS phono stage in a single box. I bought a black-painted, steel chassis box from Hammond, 17 by 3 by 10 inches big. In my mind the box was huge, but in reality it is much smaller than I expected.


Hammond Chassis Box from Antique Electronics Sales

Steel is a pain to work with, but it does magnetically shield much better than an aluminum chassis can, which can be important with a phono stage. Still, if had it to do over again, I would buy the aluminum chassis instead. Here is the front layout:

My plan is to use a three-position selector switch, a A5 stepped attenuator, a two-capacitor selector switch, and an AC power switch that offers three positions: off, line only, line and phono.

The AC-PH power switch switches the AC voltage to the power transformer and the rectified DC voltages for the heater and B+ for the phono stage. In other words, when you are listening to a CD, the tubes in the phono stage are not being wasted.

Here is how I plan on laying out the inside of the chassis.

I might place the two toroid transformers upright against the right side of the box. Speaking of transformers, I will use two Triad brand toroidal transformers, the VPT230-110 and VPT12-1080.

The 230-240Vac transformer will yield about 330Vdc of raw DC voltage after the rectifiers; the 12Vac transformer, about 15.5Vdc of raw DC voltage, which will be regulated down to 12Vdc. These transformers are 25Va designs, which will just make it. For example, the six 12V @150mA heaters will draw a total of 0.9A and the transformer can deliver 1.1A of DC current. The high-voltage B+ transformer can put out a tad over 50mA, which should be plenty for the four 12AT7 and two 12AU7 tubes, which will draw a total of about 40mA.

The CCDA Noval kit comes with the choice of power-supply capacitor rating, 200V or 400V. If a the VPT230-110 transformer is wired up to put out 120Vac, then over 100mA of B+ voltage (about 160Vdc) would be available. In fact, if you do not plan on powering a phono stage with the CCDA Noval PCB and plan on using it as a stand alone line stage amplifier, then the lower B+ voltage is the way to go, as the 200V capacitors are much higher in capacitance and a tube such as the 6N6P or ECC99 or 6H30 could be run hard (lots of idle current).


Click on image to see closeup

The back of the chassis will hold 10 RCA jacks, a phono ground jack, and an AC power connector (maybe a fuse holder).

So, far, I only drilled the five front panel holes, so I have a lot more to do. I plan on using the chassis upside down, with a top plate, separated by 1/4th inch hex spacers, so as to let the hot air out (I will also drill several small holes on the bottom panel).

I must mention that I only have one Tetra Sans PS PCB left for sale and only 8 CCDA Noval PCBs. Yes, more will be made, but not until after I finnish moving.

The Rev. A CCDA Noval PCB and kits are available at the GlassWare/Yahoo store now.

 

Next Time
I hope to show my finished project and give some details on other new PCBs.

 

//JRB

     
Kit User Guide PDFs
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Download 5687 Aikido USer Guide PDF

BCF User Guide

Download PS-3 User Guide

SRPP+ User Guide

Aikido All in One user guide
Janus regulator user guide

Aikido Phon Preamp user guide
Aikido 9-Pin Stereo User Guide

 

E-mail from GlassWare Customers

Hi John,

I received the Aikido PCB today - thank you for the first rate shipping speed.
    Wanted to let you know that this is simply the best PCB I have had in my hands, bar none. The quality is fabulous, and your documentation is superb. I know you do this because you love audio, but I think your price of $39 is a bit of a giveaway! I'm sure you could charge double and still have happy customers.
     Looking forward to building the Aikido, will send some comments when I'm done!
   Thank you, regards
Gary

 

Mr Broskie,

I bought an Aikido stereo linestage kit from you some days ago, and I received it just this Monday. I have a few things to say about it. Firstly, I'm extremely impressed at the quality of what I've been sent. In fact, this is the highest quality kit I've seen anywhere, of anything. I have no idea how you managed to fit all this stuff in under what I paid for it. Second, your shipping was lightning-quick. Just more satisfaction in the bag, there. I wish everyone did business like you.

Sean H.


9-Pin & Octal PCBs

High-quality, double-sided, extra thick, 2-oz traces, plated-through holes, dual sets of resistor pads and pads for two coupling capacitors. Stereo and mono, octal and 9-pin printed circuit boards available.

Designed by John Broskie & Made in USA

Aikido PCBs for as little as $24

http://glass-ware.stores.yahoo.net/

 


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