The Emperors New Amplifier

Description: The power supply has been simplified. Power transformers and rectifiers have been omitted, and some components have been removed from the MOSFET voltage regulator circuits: 1N5242 zener diodes between the source and gate and 10k resistors in series with the gate. These components serve as protection in case of accidental short circuits but do not affect the operating point. The full power supply schematic is shown below. 6SN7 tubes are used instead of 12AU7 tubes for the driver stages. They have the same plate characteristics but a higher maximum plate voltage (450 V vs. 330 V) and greater plate dissipation (3.75 W vs. 2.75 W per section). All resistors are rated at 1/2 watt unless otherwise noted, with exceptions: R3P, R4P, and R5C through R8C (68K) are rated at 2W, while R9C through R12C (20 ohm) are rated at 1W, achieved using two 10-ohm resistors in series. R9S through R12S (200 ohms) are rated at 2W. Capacitor voltage ratings should exceed the minimum requirements, although higher voltage capacitors may be larger and more expensive. The following minimum ratings are suggested: C1G: 100V, C2G: 400V (600V preferred), C3G and C4G: 400V (600V preferred), C3M and C4M: 400V, C5G through C8G: 600V, CBS2, CBS4, CBS6, and CBX1 through CBX3: 100V. The voltage ratings of many power supply capacitors are noted on the circuit board wiring diagrams below. Some feedback connections may be difficult to trace. ORN connects between the 20-ohm output transformer secondary feedback winding and R3F near TU3. VLT connects to R4F near TU4. BLU and BLK on the output transformer secondary speaker winding connect to the output tube cathode circuits. BRN and VIO are not used; they correspond to the ultra-linear taps. In the original version of TENA, there was a switch to select output tube screen grid connections between BRN and VIO (ultra-linear mode) and GRN and YEL (triode mode). In the 1990s, efforts were made to learn the intricacies of designing circuits with superior audio quality. Technical curiosity and cost considerations influenced the desire to avoid expensive equipment of questionable design. SPICE computer modeling became a primary tool for understanding amplifier operation beyond superficial measurements. The goal was to design a high-powered, wide-bandwidth amplifier that produced high-quality sound while maintaining simplicity. Single-ended (SE) designs were rejected due to their low power and limited bandwidth, leading to the exploration of efficient push-pull designs that aimed for similar or superior sound quality. The design targeted a wide bandwidth, yet not excessively so, as very low frequency 1/f noise and radio interference do not enhance music. Most coupling circuits feature an RC time constant of approximately 0.05 seconds, equating to a -3 dB frequency of 3 Hz—low but not excessively low. No net AC current is drawn from any power supply tap, as pairs of tubes draw equal but opposite current. This effectively isolates the power supply from the signal path, reducing the need for voltage regulation. However, robust MOSFET regulators are implemented for optimal sonic refinement. Global negative feedback—a loop from the amplifier output to its input—is avoided. Instead, several local feedback loops are employed, typically around one or two gain stages. Local feedback loops offer numerous advantages without adversely affecting sound quality, and they enhance stability. The wide bandwidth of the output transformer facilitates the implementation of various modes.The power supply has been simplified- Power transformers and rectifiers have been omitted and some parts have been omitted from the MOSFET voltage regulator circuits: 1N5242 zener diodes between the source and gate and 10k resistors in series with the gate. These parts serve as protection in case of accidental short circuits, but don`t affect the operating point.

The full power supply schematic is shown below. 6SN7`s are used instead of 12AU7`s for the driver tubes. They have the same plate characteristics, but they have higher maximum plate voltage (450 vs. 330 V) and greater plate dissipation (3. 75 vs. 2. 75 W per section). Think of the octal-based 6SN7 as a 12AU7 on steroids. All resistors are 1/2 watt unless noted, and with the following exceptions. R3P, R4P and R5C through R8C (68K) are 2W. R9C through R12C (20 ohm) are 1W: I used 2-10 ohm resistors in series to make them. R9S through R12S (200 ohms) are 2W. Capacitor voltage ratings: It never hurts to go over the minimum, though the capacitors will be larger and may cost more. I often use caps with higher voltage ratings because I have them on hand or found them at a good price in an electronics surplus shop.

Here are some minimum ratings: C1G: 100V. C2G: 400V (600V would be better; necessary without the time delay); C3G and C4G: 400V (600V would be better); C3M and C4M: 400V; C5G through C8G: 600V; CBS2, CBS4, CBS6 and CBX1 through CBX3: 100V. The voltage ratings of many of the power supply capacitors are shown on the circuit board wiring diagrams, below.

Some of the feedback connections may be a little hard to trace. ORN goes between the 20 ohm output transformer secondary feedback winding and R3F near TU3. Similarly, VLT goes to R4F near TU4. BLU and BLK on the output transformer secondary speaker winding go to the output tube cathode circuits. BRN and VIO are not used. They are the ultra-linear taps. In the original version of TENA there was a switch to select output tube screen grid connections between BRN and VIO (UL mode) and GRN and YEL (triode mode).

In the 1990`s I strove to learn the secrets of designing circuits with superior audio quality. I`m technically curious and also cheap- I didn`t want to spend big bucks on cosmetically impressive equipment of dubious design. SPICE computer modeling became my primary learning tool because it enabled me to observe inner details of amplifier operation not obvious from outer measurements.

My goal was to design a high-powered wide-bandwidth amplifier with the finest sound and to keep it simple- but not too simple. I rejected single-ended (SE) designs because of their low power and limited bandwidth. I wanted to see if I could get similar sound quality- perhaps better- with an efficient push-pull design.

And I wanted to do it my way. I aimed for wide bandwidth, but not too wide. Very low frequency 1/f noise and radio stations don`t enhance music. Most of the coupling circuits have an RC time constant around 0. 05 seconds, equivalent to a -3 dB frequency of 3 Hz- low but not too low. No net ac current is drawn from any power supply tap. Pairs of tubes draw equal but opposite current. This effectively removes the power supply from the signal path and reduces the need for voltage regulation. Nevertheless robust MOSFET regulators are employed for ultimate sonic refinement. I avoid global negative feedback- a loop from the amplifier output to its input. There are several local loops- around one or at most two gain stages. Local feedback loops have numerous advantages and no adverse effect on sound quality. Stability is much easier to maintain. The extreme bandwidth of the output transformer allowed me to put a mode