Sunday, December 12, 2010

Birth of an Amplifier

Here's a short description of the construction of an Elliott Studio Arts 66-100 power amplifier. This is a basic model, unbalanced and without a triode/UL switch (which switches the output stage between modes). The first step in construction is to assemble the chassis and related components. Shown below is the top panel to which the circuit board is installed after it is seperatly constructed, as well as the power and output transformers. A horizontal brace can be seen, situated below the heavy transformers to provide additional strength, and a lightweight vibration damping material is applied to damp rining. The tubes, bias pots and switch protrude through the holes and the bias meter occupies the rectangluar opening in the center.

Next you can see the bottom of the chassis with the seperate front and rear sub-panels installed. Seperate sub panels facilitate changing the configuration without having to re tool the entire enclosure. In example, the rear sub panel for the balanced version of the amplifier is different but is the only part that needs to be changed. This makes the product flexible and economical to build in small quantaties. Also seen is the cooling fan, input and output jacks, power inlet / filter and fuses. Up front you see the main filter caps, smoothing choke and power switch. In the center is the terminal block where all the power wiring will land when the top panel and circuit board are installed.

The next picture shows the circuit board with the tube sockets, terminal strips, bias meter & pots, and ground wiring installed. This "circuit board" is actually 16ga. steel which provides a rigid backbone for the circuit to live and an active ground-plane with good RFI rejection. All the components are riveted directly to the circuit board and will not vibrate loose over time. All the wiring is point to point just like the old days. This method of wiring is time and labor intensive but is more robust, superior for the high-heat and high-voltage environment of tube amplifiers, and sounds better. I have repaired some contemporary tube amps that use printed circuit boards and have seen catstophic failure of the board itself where the heat of a power resistor actually burns a hole through it. No way to fix that without replacing the entire board! Printed circuit boards also exhibit small amounts of capacitance between traces and the board material itself is not a very good dielectric, which can smear the sound to a degree. That said, we do use printed circuit boards in non-critical areas, like power supplies and logic circuits.

Here things are beginning to come together. This is part of the on-board power supply bypassing for the output stage. Also seen in the background is the filament supply circuit with damping resistors and +50V injection to cap off hum and prevent too high a voltage differential between cathode and filament at the input stage phase splitter. Heat resistant ceramic tube sockets are employed for long life and no arcing. All signal path junctions are soldered with silver content solder and care is taken at every step to ensure solid connections and future serviceability.

The next picture shows the input stage under construction. Each half of the input tube (12au7a) is bypassed to prevent power supply interaction between stages. The first stage is a simple voltage amplifer with feedback from the output transformer injected at the cathode, which is bypassed with high-quality solid polymer electrolytic caps. These have outstanding low impedance and ESR properties and live well in high-heat environments. This stage is direct coupled to a spit-load phase splitter which has a build-out resistor in the cathode leg to match output impedances of both halves for better high-frequency linearity.

In this picture the input stage / phase splitter and driver stage are completed. In a novel implementation of local feedback and bootstrapping this little stage delivers the needed voltage swing to drive the output stage to full power at low distortion. One wouldn't normally consider the 12au7a to have the balls to do this! High-quality polypropelyne coupling and bypass capacitors are used throughout. All wiring is high-tempature and rated for applied voltage.

Here's the completed circuit board installed to the top panel with transformers mounted and wired into the power supply circuitry, which can be seen along the bottom. On the left is the main B+ supply, on the right is the bias supply and filament damping circuit. Off to the side can be seen a small printed circuit board which houses the 60 second B+ delay, which allows the filaments to heat up fully and applies the bias voltage before the high-voltage hits the tubes. The output stage employs an LR network to equalize the small inductance differences between windings in the output transformers. This also improves high-frequency linearity.

Speaking of transformers, all transformers used in Elliott Studio Arts products are custom wound to our own specifications by a well respected company based in New Mexico, Edcor Electronics. The excellent sound quality and bass control of these amps can in large part be credited to these transformers. The power supply transformer is massive with ample power regulation; in conjunction with the big computer grade caps and choke provide huge power reserves which greatly increase dynamic impact and slam. The power supply is very clean and well protected with fuses and inrush current limiters, which can be seen below mounted to the terminal strip in the bottom chassis.

The top cover and circuit is now wired to the bottom chassis, the input connections have been wired and the output transformer secondaries are hooked to the speaker jacks.

The next step is to install tubes, connect meters to monitor B+, bias and filament voltages and an O'scope and dummy loads to the output. Now fire her up!

After everything checks out, the bias is increased until the proper current reading is applied to each tube, voltages are checked again. Once everything stabilizes it's time to throw some sine waves at it and check for oscillation, frequency response and max power output. Then a complete battery of computer generated tests including frequency sweeps, distortion and square wave response is done and documented for record keeping.

The face plates are installed after all the testing is complete. The amp is then installed in my reference system for a week to burn it in and listen for any anomalies. It is then packaged up and shipped to the customer for many years of happy listening!

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