New Octal Aikido Cathode Follower
The noval Aikido cathode follower (ACF) PCB now has a brother: the octal ACF stereo PCB. Why? Some just prefer the sound from octal tubes, such as the famous 6SN7, so its absence was felt, myself included. In other words, it wasn't just requests that prompted my making this new PCB, as I, too, love the 6SN7 and 12SX7. I am not saying that the 6SN7 is perfect, as such a statement makes little sense without stating perfect for what, but I do experience a small joy each time I return to the tube after listening to noval tubes, such as the 6DJ8, 12AU7, ECC99 and others.

My initial idea was to just remake the noval ACF PCB into an octal version, a relatively easy transformation. But upon some reflection, I realized that I could easily make an improved octal version. Improved? Yes, indeed. But first, let's review the stock Aikido cathode follower configuration.

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.

Okay, that was how it works, now we move on to what it could be used for. Passive line stages often prove inadequate, incapable of adequately driving high-capacitance cables or low-input impedances. I find this outcome quite paradoxical, as it is easier to believe that less must be more in terms of signal transmission, just as the telescope with the fewest lenses must present the clearest image. Yet, time after time, I have found adding a buffer stage, both solid-state- and tube-based buffers, readily outshines a passive setup, as the active buffer often imparts the missing heft and solidity missing from most passive setups, even with the added cost, noise, and distortion, which the active circuitry must necessarily bring to the mix. The difference isn't subtle.

I expect, however, that most will use the octal ACF in series with an existing solid-state audio signal device, such as a CD player or DAC, rather than as unity-gain line-stage amplifier. If such is your plan, you might think about placing a GlassWare Tilt Control in front of the octal ACF, as it will allow some fantastic but subtle tonal changes. For example, with one click to towards a tilt to the highs, I can make HD650 sound better with rock and jazz music; conversely, with one click to towards a tilt to the lows, I can make Grado SR225 sound better with classical music. Amazing.

Even if your experience is different, as your existing phono stage or DAC can drive your amplifiers perfectly well, it is unlikely that it can drive high-impedance headphones, such as the 300-ohm types from Beyer and Sennheiser, such as the famous HD650, HD700, and HD800. This brings us to the new improvement on the octal ACF PCB missing from the noval PCB: the option of White cathode follower configuration, which only the octal ACF PCB allows as an option.

Mr. Eric White, the British inventor who worked for EMI, patented his famous White cathode follower in 1940 in England (GB564250), which was also patented in the USA in 1944 (US 2,358,428). His circuit is a unity-gain, push-pull buffer that accepts an unbalanced input signal and presents an unbalanced output. This feat of magic is performed by the top triode's plate resistor that functions current-sense resistor that experiences a variation in voltage drop corresponding to the variation in current conduction by the top triode. This variation in voltage drop becomes the input signal for the bottom triode, which responds in an anti-current phase fashion, creating a complementary, balanced push-pull operation by the two triodes. As the top triode conducts more, the bottom triode conducts equally less; as the top triode conducts less, the bottom triode conducts equally more.

The result is class-A, push-pull operation that yields an output impedance half of what it would be with the same tube in a cathode follower and lower distortion. Moreover, the White cathode follower can deliver a peak output current swing into the load twice that of its idle current. The load is all important and its impedance must be factored into the calculation of the sense resistor’s value. Without the external load, the two triodes could not differ in current conduction, so no push-pull operation would be possible. Without the load, the sense resistor would not create the needed drive signal for the bottom triode.

Assuming that the bottom triode’s cathode resistor is bypassed by a large-valued capacitor, the formula for the optimal sense resistor value is the same as the one used for the SRPP circuit:

       Rs = (rp + 2Rload)/mu

For example, a 6SN7 with a cathode-to-plate voltage of 150V and 10mA idle current presents an plate resistance (rp) of about 7500 ohms and a mu of 22; thus, with a load impedance of 600 ohms, the sense resistor should equal (7500 + 1200)/22 or 395 ohms.

The bottom triode’s cathode resistor should be bypassed; if it's not, a slightly more complex formula will be needed and the sense resistor must necessarily go up in value, roughly:

       Rs = Rk + (rp + 2Rload)/mu

The list of tubes that can be used in the octal ACF PCB is short: 6AS7, 6BL7, 6BX7, 6H30pi (the octal version of the 6H30), 6SL7, 6SN7, 12SX7 and all the substitutes for these tubes, such as the 6SU7, ECC33, and 5692. But for 99% of audiophiles, the 6SN7 is the best choice. But do not forget the odd heater variations, such as the 8SN7 and 12SN7.

The octal ACF PCB and kit is available now at the GlassWare-Yahoo store.

For those of you who still have old computers running Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the download availability of my old old standards: Tube CAD, SE Amp CAD, and Audio Gadgets. The downloads are at the GlassWare-Yahoo store and the price is only $9.95 for each program.

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

So many have asked that I had to do it.

WARNING: THESE THREE PROGRAMS WILL NOT RUN UNDER VISTA 64-Bit or WINDOWS 7 & 8 or any other 64-bit OS.

One day, I do plan on remaking all of these programs into 64-bit versions, but it will be a huge ordeal, as programming requires vast chunks of noise-free time, something very rare with children running about. Ideally, I would love to come out with versions that run on iPads and Android-OS tablets.

//JRB