It has been brought to my attention here that my instruction for building the Single ended class A headamp is not complete.

The transistors in the current mirror, and differential amplifier will benefit from having thermal contact, to keep the temperature difference as small as possible, thereby minimising the DC output offset. I have taken the best picture I could, of this coupling for one channel, in my amplifier.


The beast has been tamed. It sounds good too, although my headphones are cheap Sennheisers. The headamp is enclosed in a Hammond enclosure, using the aluminium casing as heat sink, it gets hot. It makes for a nice way to keep the coffee warm by placing the cup on top of the amplifier.


Q6, Q8, Q15, and Q17 are the output transistors, and needs heat sinks, I have not calculated the exact size needed, but the bigger the better. I use the aluminium chassis of a Hammond 1455N2202 with the transistors mounted on the bottom, and it gets warmer than I like. Never test the amp without proper heat sinking in place, as the output section will most certainly release the precious blue smoke.

Power supplyI have used a 2X12V toroid, a rectifier bridge, and 4700uf on each supply rail. Each channel draws about 200mA. 

GroundingSome attention is advised when wiring the ground, my first ground layout resulted in a nasty saw-ish 50Hz hum from the amplifier. I now connect the separate grounds from the input RCA connectors to their respective ground pads, next to the input pads. The power ground are connected to a star ground between the 4700uf power supply capacitors, as well as the ground wire from the headphone jack.

Files: SE Class A headamp schematic and PCB SE Class A headamp schematic and PCB (Cadsoft Eagle)

Build the last simulated version of the SE class a headamp today, on breadboard. It works as an amplifier, it is class A, it is hot, and it is oscillating. I have to read a little more about taming the beast, did try some tricks, and it helped. Also I have to try some other transistors in the output stage, I have two different brands of BD139. I was unable to measure the hFE of the ones of which I have the most, even though the other brand measures fine. Since this was a prototype, I used the strange ones, maybe they are bad to the bone.

NOTE: I found this thread at diyaudio.com read through, it's really nice stuff: Pure Class A Single End Amplifier Idea!

I have cooked this circuit up during the last couple of days, it is single ended and borrows the output stage from the famous John Linsley Hood Class A amplifier. Distortion figures and PSRR look quite nice, but the circuit has only been simulated, I have a TDA1545 DAC to finish If I could only keep my mind on a single project. If my calculations are correct, the amp will deliver 250mW into a 32Ohms load in class A.

Schematic (PDF)

PS: Sorry for the extra ground symbol.

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