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BowtoEd 28th June 2012 12:56 AM

Insane Hybrid Headphone Amp (IHHA)
2 Attachment(s)
Hello all.

I'm planning a new headphone amplifier that will use triodes for the input stage. It's based on Gilmore's dynamic class-A amplifier even though it's going to be class AB. The idea is to build an amp with a differential input stage, paralleled output transistors, and a bipolar supply. Projected output capability will be 500 mW into 8 ohms (yes, it is a flea-powered amp with a headphone output).

The purpose for this design is for future projects. I want to learn the principles behind amplifier design in topologies such as this. If I can build a 500 mW amp then I can use the same schematic with stronger transistors and a stouter power supply in order to get something on the order of 30 watts. Then I can do it again with a larger transformer and build a 150 watt amp.

This is the schematic. Although the transistors are all labeled 3904/3906, I plan to use 2SC1815s and 2SA1015s for all of the BJTs. I'm going to prototype the amplifier with MPF102s in the input stage, using a 500-ohm trimmer in the source circuit for DC offset nulling. Once I can find a stable operating point, I'm going to plug-and-play output transistors until I can get a set of 8 with reasonably matched levels of current. After I get a stable, working amplifier I might build a prototype, which will likely turn out to be a gift amp or something I sell. Then, I'll build it again on the breadboard and use a tube as the input differential. I considered using 6922 but I'm thinking of using 6N2P because I have three dozen of them. 6N2P is a Russian 12AX7-clone with a 6DJ8 base and there was never an American equivalent.
Attachment 289325

This is the set of output transistors on the breadboard. If I had more 1/4w, 10 ohm resistors, I'd measure each one and match the whole set within 1%. Those output transistors will be matched as closely as I can get them. I've been busy so I've only been able to set up the output stage and the two bias diodes.

I already posted this project on but no one's given me any feedback. I figured I'd put it out here and see what people had to say.
Attachment 289326

kenpeter 28th June 2012 06:14 AM

The great advantage of triodes is plate voltage linearity.
I don't see that here. You hold both plates 1 diode drop
from the top rail. Triodes are not incredibly linear in Gm
without plate voltage/Mu feedback.

If you won't let Mu linearize, what's supposed to be the
advantage (oh-say, compared to JFETs)?

BowtoEd 28th June 2012 06:19 AM

Oops...I seem to have placed the wrong schematic here. One of those triode plates is supposed to be stuck to the top rail and the other is supposed to connect via a resistor to it. The one with the resistor feeds the VAS and both have a 500-ohm trimmer in their cathode circuit, the wiper connected to the CCS for balance control. I will see if I can find the right schematic and place it here.

I'm not exactly sure what you you think you can explain it? Any helpful advice is welcome.

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