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Headphone amplifiers: thinking aloud

Posted 20th May 2013 at 06:13 AM by rjm
Updated 21st May 2013 at 12:00 AM by rjm

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...
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A catalog of headphone amplifier circuits.

Posted 19th May 2013 at 10:45 AM by rjm
Updated 21st May 2013 at 12:12 AM by rjm

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...
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Which soldering station should I buy?

Posted 17th May 2013 at 07:47 AM by rjm
Updated 20th May 2013 at 05:20 AM by rjm

Thinking I should get a new soldering station.

I've had my eye on a Weller station for the longest time, but put off getting one for one reason or another.

Looking at the selection, I think the choice comes down to the two following models, or, indeed the equivalent made by another company:

WTCPT (link)

WES51 (link)

The main difference is in the WTCPT the tip temperature is fixed, but rigorously controlled, while the WES51 has user adjustable power but the tip temperature is left unregulated.

I'm leaning towards the WES51 as being slightly more in line with my own style of work: I'm more likely to want higher or lower temperatures depending on the job at hand than I am to require "700F" exactly.

Update 1.

A little concerned the WES51 is only 50W max. That's less than the maximum I use now for the tough stuff.

Hakko has two models in the price...
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Audacity sucks.

Posted 9th May 2013 at 02:50 PM by rjm
Updated 10th May 2013 at 03:35 AM by rjm

Under windows at least, Audacity is unable to record audio at bitrates above 16 bit.

It will seem to, all right, but the data is quantized at 16 bit (30 microvolt LSB), regardless of the settings chosen.

The attached images show the same source, the first recording is made in Audacity, supposedly 24 bit, but actually only 16 bit, while the second is recorded with a program than actually supports 24 bit, exported, and imported into Audacity. The data is amplified +70dB in both cases to make the difference visible.

Audacity will happily manipulate and save high bit rate data, but as a result of licensing restrictions and on account of it being freeware, it does not support the actual recording of this data.


Any internet search will confirm that the Windows version of Audacity is limited to 16 bit recording. And yes, it's more of a limitation of Windows than it is of Audacity. My irritation, however, is chiefly with Audacity...
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J-Mo Mk. II vs. Szekeres, distortion comparison

Posted 3rd May 2013 at 08:24 AM by rjm
Updated 5th May 2013 at 11:52 PM by rjm

Two headphone amplifiers sharing the same basic MOSFET source follower output stage.

When the source current and source resistance are optimized for the given headphone load and similar maximum output power (~50 mW at 1% THD), the distortion pattern vs. output power is remarkably similar.

One plot below is simulation, the other measurements. The J-Mo 2 simulation closely matched the actual measurements, it wasn't worth my while to generate a full simulated data set when I already had the measurements on hand. No reason to suspect that the Szekeres sim is inaccurate, either.

The take home message is the distortion characteristic of a MOSFET follower is what it is, and unavoidable. Take it or leave it, as it were. However - and this is key - if you don't optimize the stage for the headphone impedance, the distortion for a given output power will increase significantly.

As an aside: Greg did his homework with the original circuit....
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Szekeres Headphone Driver, distortion simulation

Posted 3rd May 2013 at 08:11 AM by rjm
Updated 3rd May 2013 at 08:15 AM by rjm

I've always enjoyed the sound of the Greg Szekeres' Headphone Driver (buffer) and derivatives sharing the MOSFET source follower output stage.

I've often wondered however, whether it's distinctive sound is because it is unusually free from noise and artifacts, or because its unusually prone to heavy second harmonic distortion.

It's not hard to set this up in LTSpice, but I haven't seen it done before. So, for your education and enlightenment, I present the harmonic distortion vs. output power data for the original "classic" circuit as uploaded to Headwize all those years ago. The LTSpice asc file is also included I you want to play along. The harmonic data is generated by hand, reading the FFT peaks for 10 or so different input voltages.
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Discrete transistor line amplifier stage mod 1

Posted 19th April 2013 at 10:24 PM by rjm
Updated 20th April 2013 at 05:04 AM by rjm

Changed the collector load on the voltage amplifier stage to a current source (Q3), as per the Marantz SR2285B circuit.

Also increased the resistance of the feedback connection, R6+R8, there seemed to be no obvious advantage in making it much smaller than the typical load (>10k). The compensation capacitor C2 is increased to match, to flatten the HF response.

The circuit can drive light loads to +20 dB. Of course that's not much of a challenge for this general class of circuit.

I'm a bit stumped as to what the next logical step is from here. Seems to me to depend on what you actually want the circuit to do.
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Discrete transistor line amplifier stage

Posted 16th April 2013 at 01:18 AM by rjm
Updated 16th April 2013 at 12:23 PM by rjm

Over the last couple of years most of my interest in audio has been with transistors. I've been slowly teaching myself to read and understand the circuits.

Circuits like this one for example. Not hard, but still a bit too complicated for me to understand without the helpful wikipedia markup attached.

Instead I've looked at primarily at the schematics I have for discrete audio preamplfiers, 1970's vintage typically. Based on what I've learnt so far, I've done up a "test mule" in LTSPICE, shown below.

It's not a circuit you should build. It's for pedagogical purposes, though it does actually work reasonably well - in simulation anyway. Its just a simple starting point to observe how the different parts interact under simulation.
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A poll. Basic electronics and DIY audio.

Posted 2nd April 2013 at 01:46 AM by rjm
Updated 4th May 2013 at 10:54 PM by rjm

It came up at the help desk, but I want to put this before members generally:

I feel strongly that people who build audio equipment as a hobby should take it upon themselves to obtain a basic understanding of both the practical and theoretical aspects of electronics. Take a trip to the library and read through the first couple of chapters of electronics textbooks, that kind of thing.

It's more than just the safety aspect, I think of it as a basic necessity...

So, how many people here are familiar with the following statement?

The impedance of a capacitor is -j/([omega]C)

Familiar as dirt? Never heard of it before?
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Preamps are not line level.

Posted 1st April 2013 at 04:57 AM by rjm

If you don't think about it too hard, you'd imagine that the signals in the phono stage would be smaller than the signals in the line level preamplifier stage that follows it. Or that the signals in the DAC/CD player would be about the same level or slightly lower than the signal in the preamplifier.

As usual, the answer is "it depends". It depends on the sensitivity of your speakers, how loud you are listening, and the voltage gain of the amplifier. It also depends on whether we are talking about a MM phono stage or low output MC.

My point is simply this: the volume control is an attenuator, and at the typical "9 o'clock" position the input signal is reduced in magnitude by about 35 dB.

That cuts it back down to being comparable to the output of a moving magnet cart, and much, much smaller than anything found in a DAC stage.

It means you absolutely, definitely, positively must spend as much effort and care...
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