680W Stereo Audio Amplifier

 


Features

  • Ppeak = 680W, Prms = 340W (into 4Ω)
  • 4 FJA4310OTU output transistors per channel.
  • 2 0.3 C/W heatsinks.
  • 4 * 22000uF filter capacitors.
  • Speaker protection circuits.
  • Soft start, thermal control and monitoring by a ATtiny85.
  • Separate +-12v power supply.
  • Preamplifier with bass and treble active filters.

History

The amplifier project actually began as a feature of my alarm clock project. I wanted a loud alarm and therefore started to build various circuits using small IC amplifiersTest and measure. But I was never completely satisfied and so for each iteration the power increased further. One day I figured that I should attempt building the amplifier using discrete components. So into the rabbit hole I went… After many attempts (some successful, some not so much) I settled on this single ended amplifier setup. I felt that going further would be too dangerous and also too expensive for an first attempt.


Theory

The amplifier is a classic class-AB amplifier, ie linear amplifier with two “opposite” power transistors (actually I ended up using four transistor in each channel for current balancing) in a “push-pullMinor troubleshooting” topology which is regulated by negative feedback. The reason I choosed this design was because of its good performance, reasonable efficiency, low frequencies, quite easy to comprehend and the analogue nature.

Even if the theoretical output power is 680W (instantaneous) I haven’t performed a real test run of the amplifier in those conditions, mainly because I don’t want to rebuild the amplifier in case of disaster. But I’ve double checked all the specifications, components, traces and wires to be within operation limits. My main concern is thermal issues due to high currents and “stray oscillations” and as the enclosure is quite dense and a single fan is the only fallback in case thermal runaway. In fact I’ve already destroyed a voice coil in one of my Canton GLE 490.2 speakers…


Assembly

I separated the build into multiple modules mostly because I had to iterate on each one individually. And I couldn’t fit that many components within the constrained PCB layout of the free version of Eagle CAD. It was also beneficial to have it separated into modules because I didn’t know beforehand which modules I actually would need to build in order to finish the project.

All of the PCBs are handmade using UV exposure method. Different copper thicknesses was used for different PCBs, those with high currents use 105 um and those with low currents use 35 um.


Modules

Power amplifier

Amplifier
Amplifier

The power amplifier module contain the voltage amplifier stage, implemented as a differential amplifier to handle negative feedback. The VAS stage using both a current source and a current mirror to ensure maximum output swing. Using this setup almost enabled the output to swing from rail to rail. Some last minute changes in which power transistors used (changed from BU508AF to parallel FJA4310OTU) some stability issues arose, which was fixed by adding degeneration capacitors to the driver transistors and adjustments to the capacitors which suppress oscillation in the negative feedback.

105 um PCB


Transistor board

Power transistors mounted to heatsink.
Power transistors mounted to heatsink.

This is simply a board to mount the power transistors. As they are parallelled I also mounted the emitter ballast resistors (0.3 ohm) onto this board. The board also contain a thermistor which is mounted to each heatsink.

 

 

 

105 um PCB


Preamplifier and tone control

This board was probably the most difficult to design as it contain active filters for bass and treble adjustments. It’s based upon a Baxandall filter but altered to fit within an active filter setup and not to have a damping effect on the input signal.

35 um PCB


Monitoring, fan and power up controller

ATtiny85 control circuit.
Test of temperature monitoring functions.

This board contain a ATtiny85 microcontroller to control both temperature thresholds of the power transistors and also handle to the soft startup procedure.

The controller also ensure the amplifier isn’t engaged at all when the ambient temperature is above normal, or if unable to read temperatures.

The microcontroller was programmed using Atmel Studio through SAMIce 3 dongle.

A known issue of the firmware and thermal control is lack of hysteresis of the temperature thresholds thus the power to the fan is “flickering” during transition.

35 um PCB


+-12volt power supply

A pair of linear regulators, one for each rail. As the positive rail power a lot of relays and a fan I’ve designed a parallelled 7812 setup on the positive rail to allow for up to 2 amps continuous delivery.

105 um PCB


Amplifier power supply

This doesn’t have a circuit board as it’s actually quite simple. Mains goes through a high power transformer then through a bridge rectifier and then at last to some huge filter caps. Due to significant inrush current at power on there are high power resistor connected to the mains to reduce the inrush of current. The resistors are bypassed automatically after 1/2 second through a relay which is controlled by the monitoring logic.


Speaker protection

This circuit monitors the average output DC level of the power amplifier. During normal conditions this should be close to 0 volts but in case any of the power transistors (or drivers) fails then this circuit is responsible of disconnecting the speakers as fast as possible.

DC level is measured using a carefully tuned LP filter and two voltage comparators, if the absolute DC level exceed 50 mV then the speakers is disconnected and an error indicator is lit on the front plate.

105 um PCB


Soft start relay board

This is a very simple board to only contain two relays, one relay for enabling mains voltage to the amplifier and one relay for bypassing the soft start resistors.

105 um PCB


Thermal management

Temperature of power transistors are continuously monitored. If temperature reaches first threshold then a fan is turned on which hopefully should reduce temperature. But if second threshold is reached then the main power supply is disconnected and overheat LED indicate a fatal error.

I have chosen the heatsinks very conservative, I calculated the required heat transfer factors and then doubled the size of the heat sink.

I also selected a fan with as high airmass throughput as I could find within the tolerance of the 12+- power supply.

1st threshold at 40 degrees celsius.
2nd threshold at 60 degrees celsius.


Enclosure

I decided early that I needed a good looking enclosure for this amplifier as I would like to use it regularly and it needed to be placed in my living room. I spent a lot of time looking around for both pre made enclosures, various types of boxes etc until I finally decided I would build my own enclosure from MDF sheets instead.

First coat of paint.

Also a lot of aluminium was used to create mounting plates and stand-offs.

Paint work is almost an entire project page in itself, a lot of paint and patience is required. In short (chronological order): Sanding, primer, sanding, filler, sanding, primer, sanding, primer, sanding, primer, wet-sanding, color, wet-sanding, color, wet-sanding, color wet-sanding, lacquer, lacquer, wet-sanding, rubbing, polish.

 


Tools and services used


Files