13 July 2008

At long last: the Aikido Phono Preamp

Aikido Phono Preamp
The above photos are of my own Aikido phono preamp. I have listening to it since mid June and I have to say that I like it a lot. I am running EH 12AX7s and 12AT7s and they give more gain than I need for my Dynavector DV10x5 high-output MC phono cartridge

I used a blank chassis that I had sitting about. (Yes, I am looking into offering a pre-punched chassis just like it.) You will notice that I didn't add an on-off switch. Why not? I use a master power switcher that turns on the signal sources first, then the power amplifiers. Nonetheless, I will have to add a fuse and power switch soon. The toroid power transformers (240Vac and 12.6Vac) were used because I had them. the second, smaller circuit board holds a high-voltage, solid-state regulator of my design, along with all the power supply rectifiers and a low-voltage regulator. (Yes, I will be selling them soon.)

This Aikido phono preamp uses passive equalization, rather than active, feedback-based equalization. By cutting the highs and boosting the bass, the phono stage’s inverse RIAA equalization of the LP’s RIAA equalization curve returns the signal to flat. The passive equalization network sits in between two Aikido gain stages.

Two variations on the same equalization network are shown above. The advantage the lower-resistance version on the left enjoys over the higher-resistance version on the right is that the lower resistor values will add less noise to the signal and they are less perturbed by the second Aikido gain stage’s Miller-effect capacitance. The advantage the higher-resistance version holds over the low-resistance version is that the high resistor values will diminish the role the first Aikido gain stage’s output impedance; in addition, the equalization capacitors will be smaller (and cheaper). Since the Aikido amplifier stage uses a modified cathode follower as its output stage, the output impedance is fairly low. Nonetheless, this output resistance must be factored into the equations, as must the 1M grid resistor to ground at the input of second Aikido gain stage and the Miller-effect capacitance.

Tubes
By using different tubes, different bias points, different B+ voltages, a nearly infinite number of different Aikido amplifiers can be built. But with a phono preamp, our choices are limited, as high-gain, and low-noise are our key requirements, not high-current or low output impedance. The input tubes (V1, V5, V3, V7) provide all the gain in this preamp. Thus, we can ignore the 6H30 and concentrate on the 6AQ8, 6DJ8, 6N1P, 12AT7, 12AV7 12AX7, 12BZ7, 5751, 6072…. For example, a 12AX7 input tube will yield a gain close to 50 (mu/2) per Aikido stage, which perfect for a phono preamp; the 6DJ8/6922, 16 (mu of 33), which is a little weak, but if a step-up transformer is used, the 6DJ8’s lower noise contribution would certainly override the concern about its low gain. 5751s, on the other hand, provide a total preamp gain of +40dB.

The output tubes (V2, V6, V4, V8) isolate the input tubes and deliver a low output impedance, yet they are not required to deliver high current into high-capacitance cables or headphones. Thus, a larger list of possible tubes is available: 6AQ8, 6BC8, 6BK7, 6BQ7, 6BS8, 6DJ8, 6FQ7, 6GC7, 6H30, 6KN8, 6N1P, 12AT7, 12AU7, 12AV7, 12AY7, 12AX7, 12BH7, 12BZ7, 12DJ8, 12FQ7, 5751, 5963, 5965, 6072, 6922, E188CC, ECC88, ECC99… The only stipulations are that the two triodes within the envelope be similar and that the tube conforms to the 9A or 9AJ base pin-out. A tube lineup of 12AX7 12AT7 passive equalization 12AX7 12AT7 or 12AX7 12AU7 passive equalization 12AX7 12AU7 works well and yields a final gain of about +46dB.

The PCBs
Like all GlassWare PCBs, the PH-1 board is made from FR-4 PCB and is extra thick, 0.094 inches (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with plated-through 2oz copper traces. The boards are expensively and lovingly made in the USA. One board is all that is needed for stereo unbalanced use or two boards for balanced preamplification. The boards are six inches by eleven inches, with 12 mounting holes that help to prevent excessive PCB bending, while inserting and pulling tubes from their sockets.

Multiple Heater Arrangements: The PH-1 PCB allows either 6.3V or 12.6V or 25.2V heater power supplies to be used and 6V tubes, such as the 6N1P, can be used with 12V tubes, such as the 12AU7. See the User Guide for all the details on the heater arrangements.

Power-Supply-Decoupling Capacitors: PH-1 PCB provides space for four sets of capacitors to decouple each Aikido gain stage from the B+ connection. This arrangement allows a large-valued electrolytic capacitor and small-valued film capacitor to be used in parallel, while a series voltage-dropping resistor completes the RC filter.

Redundant Solder Pads: This board holds two sets of differently-spaced solder pads for each critical resistor, so that radial and axial resistors can easily be used (bulk-foil resistors and carbon-film resistors, for example). In addition, most capacitor locations find many redundant solder pads, so wildly differing-sized coupling capacitors can be placed neatly on the board, without excessively bending their leads.

The boards and kits are available at the GlassWare Yahoo Store.