AT-HA5000

This post, about a push-pull MOSFET output stage for a headphone amp, got me thinking again about the Audio Technica AT-HA5000, which is something of a benchmark in its class. The "basic" signal circuit (not a complete schematic, it's clearly missing some ancillary details) is attached below. Probably out of MJ originally.

I think with any circuit like this, the differences are less about the MOSFETs and the operating points and more about the front end and what tricks you do with the power supply. That, and how you make sure it doesn't go up in a puff of vaporized silicon taking your headphones with it.

The Audio Technica schematic has nice old-school Zener regulators, a discrete JFET front end, a long tailed pair + current mirror for voltage gain and "proper" BJT Vbe multiplier and driver stage. Q7 is presumably in thermal contact with Q10,11 providing overtemp protection, and the output has a protection relay (not shown in detail) for overcurrent (overvoltage?) protection. The designers chose to regulate the power supply for the input and voltage gain sections only.

I like it a lot, though I've not heard it: conceptually it looks rock-solid. Though 200 mA through 36 V (7.2 W heat per channel, probably ~20W idle power consumption for the unit) is perhaps a tad excessive. Output power is up at "stupid-high" without breaking out of class A.

Attached LTspice file is a version is bipolar inputs. Work in progress, though it sims out ok.

Attached Thumbnails

 

Attached Files

HA5000 mod 01.asc (6.0 KB, 472 views)