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Modification of the S.M.S.L. sAp VI headphone amplifier

Posted 14th January 2013 at 11:29 AM by rjm
Updated 28th January 2013 at 12:33 AM by rjm

I couldn't even build it myself for the price they are selling it at, $50 on ebay. It caught my eye for the linear, split power supply. I love the chrome 3-pin power connector, too.

As is usual when you buy cheap ultra-Chinese audio gear via eBay some adjustments are needed, however.

The basic problem seems to be a little mix up with the input coupling capacitors. It came with polar electrolytic capacitors, following the markings on the circuit board. If you lstudy the schematic I sketched up below you will notice the input signal and DC offset can swing in both positive and negative directions relative to the op amp inputs - polar capacitors in this position are a bad idea.

I recommend anyone buying this to replace the input caps with Nichicon Muse ES or similar 4.7 uF or 10 uF bipolar electrolytic caps. (I have some extra I can mail out. pm me if interested.)

Other than that it's a pretty solid circuit as far as I can see....
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Old

Headphone amps are hard.

Posted 9th January 2013 at 10:41 AM by rjm
Updated 21st January 2013 at 07:12 AM by rjm

The $50-from-China-via-ebay NJM4556-based headphone amp, pictured below, fails the "Is thing thing on?" test. With 16 ohm, 104 dB/mW headphones there is a faint-yet-audible background hiss when the unit is powered up and the volume is turned all the way down. The hiss increases only slightly as the volume is turned up (input disconnected).

The output noise of an op amp is normally estimated from the input-referred voltage noise density multiplied by the gain, and the current noise density multiplied by the gain and the sum of the input impedances on the inverting and noninverting inputs. The NJM4556 datasheet does not give a figure for current noise, but a reasonable guess is 1 pA/sqrtHz. The voltage noise density is estimated at 8 nV/sqrtHz.

Total noise is

Vn (output) = sqrt [(i_n R_s)^2 + e_n^2 + 4kT R_s] * Gain

Working from a source impedance of ~10k, I get about 18 nV/sqrtHz or -138 dB.

[Revised according...
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Old

Google+ DIY Audio community.

Posted 7th December 2012 at 10:06 PM by rjm
Updated 14th April 2013 at 11:14 PM by rjm

There was no DIY Audio community on Google+.

So I made one.

Google+ has always been a "quiet room" where like-minded people got together to talk about things, away from the noise and chaos of facebook.

The problem was that it was difficult to find said like-minded people. They were there, but for the majority you had no way of knowing that.

Well, now Google has fixed that by introducing "communities". It's a user-created hub, a digital meeting room ~ salon ~ lounge ~ front porch, a designated gathering place associated with a certain hobby, interest, or topic.

This feature is brand new, so I have no idea whether it will work. It could, like facebook, devolve into noise and spam... or it might be useful and worthwhile. I think its worth a shot, anyway.

I created the group, but it is free and open. It's not mine in the sense I don't intend to use it as a platform. I have my own G+ page for...
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Old

The Front End

Posted 19th November 2012 at 04:26 AM by rjm
Updated 22nd November 2012 at 05:18 AM by rjm

The most disruptive audio technology since the CD, since the advent of stereo actually, is ... the iPod. You can lump in mp3, iTunes, and digital distribution in there if you want, but itís the iPod, the physical device, which has more than anything re-defined what we think of as an audio system in the twenty first century. Shelves of physical media and a playback unit, or, if you preferred, broadcast content, has been displaced by a handheld, personal, portable jukebox. The audio component system of the 1970ís (media, sources, preamp, amp, speakers) has faded into obsolescence, replaced by powered speakers, headphone amplifiers, desktop audio, compact ďspeakers+amplifierĒ systems the ubiquitous "dock" fronting a traditional amplifier-speaker system.

Audio is missing the front end.

Because it isnít an iPod, Iím sure of that. Its day is done. No, it's pretty clear to me that the front-front end of the future is settled: Its the internet. The cloud....
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Old

My Nexus 7 Tablet

Posted 12th November 2012 at 06:54 AM by rjm
Updated 18th November 2012 at 04:03 AM by rjm

The Nexus 7 is cheap. I paid 19,800 yen for my 16 GB model, shipping and tax inc., and it came with 2,000 Yen credit to buy apps and movies and such in the online Google Play store. And yes, its plastic, even the silver metallic trim around the edge is just a silver-coated plastic part. It is solid, however; sturdy, well-built, and good-looking. The back is covered in grippy, dimpled rubber that feels like leather. The screen is a little dim, but high resolution (800x1280) ~220pi, pretty to look at and easy to read.

With a 7" screen and default portrait orientation the device is closer in spirit to a large smartphone than a small tablet. A giant, wi-fi only smartphone. It's a handy size for reading paperback-format books, comics, and news articles, but not so hot for magazine format publications and A4-formatted journal articles. Browsing the web is generally fine, but the experience varies widely depending on the individual layout of the web page. As you might expect,...
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Old

Akihabara, lost to the maids.

Posted 20th September 2012 at 11:51 PM by rjm
Updated 23rd September 2012 at 11:24 PM by rjm

A maid cafe is, as far as I can make out - and wikipedia confirms - a cafe/light restaurant staffed by girls who dress up as anime characters or doll-like maid costumes. Chatting with customers is encouraged, so the atmosphere is more social than a regular cafe. Its also more expensive, and the food worse. (from what I can judge from the menus: its what my nine year old daughter might make if left alone in the kitchen...) They advertise by by having the staff stand around - in costume - on busy street corners nearby handing out small cards.

This is a phenomena that started about 5-6 years ago, or at least started to go mainstream then. Ground zero for maid cafes is the "nerd districts" where computers, comic books, and video games were sold: Akihabara in Tokyo, Nipponbashi in Osaka.

Growth appears to be exponential: every time I visit either region (I'm in Tokyo right now, staying near Akihabara) the number of maid cafes (and therefore maids on the street)...
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Old

B-board Boxer Project : A Low Voltage Headphone Amplifier for 16 ohm Loads

Posted 7th July 2012 at 12:19 AM by rjm
Updated 29th September 2012 at 02:58 AM by rjm

I always seem to end up optimizing my headphone amplifier circuits for higher impedance headphones, this mostly happens because I own a pair of 300 ohm HD-600s and it is tedious to design for both the voltage requirements of high impedance headphones and the current requirements of low impedance headphones.

Not impossible, just, for the class-A designs I seem to be building recently, increasingly large, heavy, and impractical.

Complimentary transistor circuits, however, offer the opportunity to swap voltage for current at something close to the same design cost. They are therefore a practical topology for efficient class-A power delivery into low impedance headphones. As a design experiment, my aim is to discover how far I can leverage an ultra-low-voltage, unity gain circuit for compactness without sacrificing sound quality.

Ok. Back-of-the-envelope calculations:

A typical 16 ohm in-ear-headphone has a sensitivity of 100-105...
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Old

NwAvGuy odac 24/96 DAC review

Posted 23rd June 2012 at 07:02 AM by rjm
Updated 2nd July 2012 at 04:52 PM by rjm

I cased up my odac board (NwAvGuy via jtktam) in a small aluminum project box.

The DAC is a simple two-chip affair, with a Tenor TE7022L controller fronting the ESS ES9023 DAC and integrated line driver. A couple of voltage regulators, the clock oscillator, and an eeprom chip round out the principle component list.

I compared it with my Onkyo SE-200PCI sound card. This 24/192 (115dB S/N A-weighted, 0.003% THD 0dB 1kHz) PCI card sells for about $15,000 yen and is based on the VIA Envy24HT and Wolfson WM8740.

I'm listening to 16bit 44.1kHz .wav (CD rips), though VLC [sample rate converter set to sinc, best quality, resampling quality 8]. Windows 8 release preview [default format 24/192 (onkyo), 24/96 (odac)]. Line out though Oyaide PA-02TR interconnects to the Sapphire headphone amp, and Sennheiser HD-600s.

**

So, I was planning on writing up a big 'ol review with my impressions, but, well... there's not really a...
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Old

modest base requirements for a line stage

Posted 22nd June 2012 at 06:44 AM by rjm
Updated 29th June 2012 at 05:38 AM by rjm

Copied from this post, for reference.

Quote:
Originally Posted by RNMarsh
Here are my own modest base requirements for a line stage (similar for power amp):

1. Open loop BW of 40KHz or more (-3dB)... 20KHz min.
2. IM and THD of less than .001% at 1v rms into 30 ohms for any frequency between 20Hz and 20KHz.
3. No coupling caps on input or output or in feedback path.
4. No use of dc servo circuits to track and correct dc offset and drift.
5. No more than 6-8 transistors (excluding power supply).
6. S/N ref 1 volt rms and without weighting of at least -130dB (input can be shorted or terminated).
7. No significant harmonics above the 2nd and 3rd.
8. Closed loop gain between 12 and 20 dBv
9. Low Zout (less than a fraction of an Ohm at any audio freq).
10. Distortion not be changed by source Z.
11. Transistors should be low cost and not be exotic, hard to obtain, very expensive or no longer manufactured.

There might
...
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Old

The Double-Diamond Amplifier (DDA)

Posted 13th June 2012 at 06:09 AM by rjm
Updated 16th June 2012 at 02:57 PM by rjm

This isn't my first attempt. It's been on my mind for a while: how to coax a diamond buffer into giving voltage gain, without resorting to fronting it with a op amp.

After reading a particularly gregarious thread over in the headphone forum, I'm more and more stoked on giving this a real shot.

Despite the (catchy) name I'm thinking pre-amplifier rather than amplifier applications.

update: I have have a quick and dirty sim up and running in ltspice. Curiously, the output distortion is 15 dB lower when the buffer runs open loop than when it is included inside the feedback loop. Intrigued. Currently under investigation.

update: refined the sim slightly, achieved -85 dB distortion levels at 0 dB / 1 kHz / 600 ohms running the output buffer open loop. Bandwidth is just under 1 MHz, adjusted by changing the feedback resistance. As before, performance sims out notably worse with the buffer
inside the feedback loop.
...
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