Earlier this year I had a really, really bad experience attempting to get a new soldering iron. This time around I made my purchase through a reputable Japanese online retailer, and am now the proud and happy owner of a Hakko FX-950 soldering station.

Its an analog unit, and was discounted quite a bit as a result. Personally I'm happier with a rotary dial temperature control anyway.

This thing rocks! I've dragged my heels on getting a decent iron for so long its ridiculous. In my defense, I could always borrow a semi-decent one from work, so the pressure to buy my own was not as great as it otherwise might have been.

Top five reasons to spend the extra cash:

1. 70W, variable, closed loop temperature control. As much heat as you need, whenever you need it: the feedback loop means that the power is proportional to the conductance load: the tip will not cool down when heating up a large thermal mass.

2. Heats up to...

Straightforward transplant. Out with the old (anyone want them?) in with the new. Re-used the OPA134 op amp and my dog-eared pair of 0.47uF Multicaps.

On powering up I discovered that with the specified 10 ohms in R9,10 the output bias current was upwards of 200 mA and things were getting a bit toasty. I paralleled a second 10 ohm resistor, dropping the resistance to 5 ohms and dialing back the output bias current to about 70 mA. Latest schematic revision has R9,10 values edited to match.

Currents stable. Heatsink temperatures around 50 C. Output offsets around 15 mV. No noise or hum.

Presently giving it some burn in time.

Update: I've ordered parts for small number of Sapphire 2.0 kits. The normal price will be $125, but as an introductory offer the first batch will be available for $100. Kit includes a set of boards and all the parts for the board. You need to supply the transformers and diodes, as well as a volume control, and the chassis hardware.

Update: boards are in stock, see photo.

Original here, diyaudio thread here.

November. That time of year for finally getting around to advancing some of my audio projects a little.

The Sapphire has remained in "rev 1+" for some time now, partly because of time constraints, partly because of the lack of popularity, and partly because it was already a re-spin of the beta version and worked just fine.

There were a few housekeeping things I wanted to add though, which have been included in the 2.0 revision.

- added a dedicated ground (GND) pad to connect to chassis...

Last one for today.

I was having problems getting this to work, the key seems to be adjusting R6 to null the voltage offset. Works fine now, but this is just a rough reverse engineer of the diagram, the parts values and the type of transistors are essentially placeholders.

The input jFET is shown as a dual package. The output bias current is most likely much lower than the 100 mA I configured, so the class A output power is proportionately smaller. For the rest of the currents and the types of transistors used, your guess is as good as mine.

edit: R7 should most likely be closer to 47 ohms, while another 5p capacitor should go in parallel with the feedback resistor, R17.

A rainy and cold afternoon here in Kyoto. The mind turns to audio circuits... headphone amplifiers... diamond buffers...

This is an answer to the question of how you scale up the basic diamond buffer circuit to higher output currents without making any modifications to the driving transistor pair.

You just add more outputs in parallel.

It seems to work pretty well under simulation.

Browsing through an old issue of MJ (No. 1076), I found some sketches of the Marantz SA 11S3 SACD player analog circuitry which seemed interesting.

The are two discrete op amp "modules", one is used for the I/V converter and low pass filter, the other design is applied to both the line output and headphone outputs.

The parts and values are not given, so the circuit below is just a working mock up in LTSpice based on the published diagrams.

The output stage from the headphone / line driver op amp is attached below. There's a JFET input and BJT gain stage as well, and the whole thing is wrapped up in a feedback loop. The buffer itself, being unity gain, works just fine as a standalone circuit element.

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...