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In early May Trent Wolbe traveled to the High End trade show in Munich, Germany. This is part one of a two part series exploring the cutting edge of audiophile technology.

By Trent Wolbe, writing for The Verge. A feature on high end audio part 1 and part 2.

Oh, for Heaven's sake...

Just got the FX-888 soldering station I bought on ebay.

It does not power up.

Do you know why it doesn't power up?

So glad you asked...

It does not power up because - pause for effect - the fuse board that fits on the power transformer is inserted the wrong way round. That's right, the full "rotated 180 degrees" deal.

Fortunately I have another soldering iron. You know, so I can fix my soldering iron...

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I would just return it, but the shipping would cost me half again what I paid for it. Perhaps ebay will refund me anyway. We'll see.

I think I know what happened: The people selling these are modding them by changing the voltage and power cord. The soldering stations are officially bound for the Chinese domestic market, 220 VAC. The box, when it came, had "110V" hand-written on it, though the instructions...

Experimental : For Research Use Only

It's bring-your-own-voltage-gain. This output stage is a unity gain buffer. A sort of diamond-buffer-meets-sziklai-pair hybrid. It lets the driver pair bias the output pair without the complexity of an additional bias network, but, unlike the basic diamond buffer, the output pair can have a much higher bias current than the drivers.

LTspice file attached, if you'd like to play along.

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I did my best to shut my eyes and design this just by messing about in LTSpice from the starting idea of a "level-shifted-complementary-sziklai-pair" (i.e. mirrored J-Mo mk II), but I see now a shout-out to 47 Labs is due as the 0247 Treasure uses the same stage.

Ah well, I guess my neat idea isn't new after all.

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The general performance is in the order of 0.01% THD for 50 mW / 16-300 ohms at 100 mA bias though the output pair. I've been trying to...

The sobering fact is that the built-in headphone jack on most modern consumer electronics provides pretty decent performance. Taking that output and routing it through an external headphone amplifier rarely improves things, and frequently has a negative impact owing to increased background noise.*

[* This is a simple consequence of adding a volume control which attenuates the signal, and a gain stage which amplifies it back up. Even if the gain stage has the same noise floor as the input signal, the S/N is reduced by the amount of attenuation.]

There are specific use cases, particularly with "outlier" headphone models that require unusually high voltages or currents to drive, but in the main, for generic 16 ohm IEHs and the generic headphone ICs used in consumer electronics, I've found that external headphone amplifiers aren't worth the trouble and expense.

Instead, I've taken (I realise now) an elitist approach to focus on a desktop...

Lots of circuits out there, but most are variations of a small set of archetypes. Let's see if I can put together a list:

1. Dedicated headphone amplifier IC. e.g.(lme49860)

2. Battery powered, single stage, generic audio op amp. The ever-popular mint tin cmoy.

3. Op amp + buffer (complementary transistor pair, diamond buffer, unity gain op amp, etc, in either integrated or discrete package.)
a. closed loop connection or "compound amplifier" configuration as developed by Walt Jung.
b. open loop, two stage circuit, e.g. nwavguy o2 and my sapphire amp.

4. simple 2 or 3 transistor "introduction to electronics"-style amplifier

5. The "little big amp", a scaled back version of a transistor or vacuum tube power amplifier design. (Zen, DoZ, transformer coupled SET amps)

6. The power follower. Single-ended MOSFET or BJT, with or without CCS load, voltage...