Students: Michael Boan, Yongseok Kim, Gordon Odira, Michael Simio | Advisors: Professor Caggiano, Professor Lu
Most musicians will tell you that they prefer the sound of vacuum tube amplifiers but do not know what makes them sound so pleasant. In our research, we take a look at why vacuum tubes sound so great and design an amplifier with a musical tone at an affordable price.
When we designed our amplifier, we knew it had to have a smooth and pleasant output. We knew vacuum tubes would give us the tone we wanted. Vacuum tubes produce a signal that is musically different from transistors. Transistor amplifiers output a tone with a strong third harmonic. The third harmonic is often a dissonant and unwanted sound. Alternatively, vacuum tube amplifiers output a strong second harmonic. The second harmonic is much nicer for the listener and produces a desirable sound. With this knowledge in mind, we focused on designing a circuit to output a harmonious vacuum tube sound.
Our initial design focused on the economic and practical benefits our amplifier would bring to the market. We knew the amplifier had to be built using an inexpensive circuit design to fill a hole in the market. With this same principle in mind, we committed to making our amplifier as small and portable as possible to create a product that would be in high demand.
The electronics behind this implementation became the focus of our research. In order to create a small lightweight product, we had to find a way to work around a bulky power transformer. We found our answer with isolation and a signal transformer to safely deliver a high voltage output without the big and heavy power transformer. We had to design an amplification circuit to create a high gain without using too many parts. We started with our driver circuit using two 12AU7 tubes to maximize our voltage gain. The cascode common grid design gave us the high gain we wanted while the differential pair gave us the phase splitting to send two signals to our power amplifier. In the output stage, 6L6s are used to output our two signals to the output transformer. Here they are combined into a single signal and output at the speaker. A feedback loop is included to stabilize our output.
We made design changes as we went along to optimize our design. Initially, our cascode stage was being biased via a cathode resistor. We noticed this caused our circuit to be unbalanced and set out to correct it with a current source. We implemented a current mirror using transistors and ended up with a stronger and more balanced signal. Furthermore, we had trouble creating a flat linear output at our speaker. We noticed a bump in our output signal for the high frequencies. By designing our feedback loop with the proper resistance and capacitance, we were able to shape a signal that better resembles a flat linear output.
Overall, this research has brought new and exciting ideas to the acoustics and sound engineering fields.
We have designed an amplifier with carefully chosen electronic devices and a properly engineered circuit to create a product that is new and innovative and most importantly, a tone that is pleasing to all listeners. We understood that vacuum tubes are no longer the standard in electronics, however, they will always have a place in sound amplification.