The Audio projects
LAMP stands for Levente's Audio Amplifier. This project intends to replace my old Quad405 amplifiers. After all I was happy with it, but after 10 years, I realized that it doesn't sounded good. So I browsed the Internet, and studied lots of circuits, but non of them was good enough. I started to design my own amplifier.
This isn't tries to be a high-end amplifier. I wanted to build something better then the Quad405.
So what is wrong with the Quad405 anyways?
- The bridge will never ever perfect.
- There are two amplifiers in it. You can hear the sound when the Class-B starts to take over the Class-A
- The OPA
So the problem is the crossover distortion. The solution in the Quad405 is to have two amplifiers. A Class-A amplifier a Class-B amplifier. Basically, the Class-A has two purpose:
- To drive the class-B
- When the signal is too low for the class-B, to provide audio output.
Among others, I think this approach is bad, because the speaker will see two different drives.
The solution is simple. Eliminate the Class-B amplifier, and use only a high power, Class-A amplifier. Well sort of. I don't want to drive my circuits in Class-A, but I set them to operate at high base current. So each output semiconductor is open at the same time. And of course, I use quality components. In the other hand, my design is not an insane design. I don't use military components. However, I want to avoid capacitors wherever I can.
Despite the 10 output transistor pairs, this is NOT intended to be a 1kW amplifier. I want to output 5 amps only.
The version 1 of the amplifier.
In the meanwhile, I mounted the power transistors, and tested the amplifier, and it works good.
Some video of the first debug session.
The lamp is connected serial to the mains. This is the cheapest way to monitor mains current, and protect the circuit if something goes wrong.
This version is in the design phase. There are several modifications to the v1. That includes
- The input transistors are dual transistors to increase symmetry,
- The current generators are precision types,
- The voltage amplifier uses a darlington circuit,
- The currents are recalculated. Higher currents are flowing at the input, and the voltage amplifier stage,
- The output has only one transistor pair,
- The PCB is designed to accommodate MOSFETs as output semiconductors.
You can grab the schematic of the amplifier.
Tests I'm going to do with the amplifier.
- Short C105/C107. The amplifier now has DC gain equal to AC gain. With the capacitors, we have DC gain = 1. (to eliminate C105 and C107)
- Adjust RV101 to have zero offset.
- Test the amplifier with MOSFETs. I want to try for the first time HEXFETs. I know they are not for audio.
Also, a speaker protector circuit is designed.
The protector circuit blocks pops created by power supply transients. This happens when the amplifier is switched on and off. The circuit also provides soft start for the main transformers by introducing a serial resistor network. After certain amount of time, the resistor network is shorted.
There are three timers
- Mains timer, about 20ms
- Soft start timer, about 2s
- Speaker protector timer, about 2s
The circuit uses the NE555 derivatives for timings.
You can grab the schematic of the protector circuit.
You can connect up to three mains transformer to the circuit. The relays for switching the speaker to the amplifier, are located at the amplifier board.
The circuit also provides DC output for external low power devices. I'm thinking of integrating a SBC in the amplifier, so it can turn it to a networked media player.