11/23/00: Carl Hansen has designed PC boards for the Levallois version of the pocket amp with Jan Meier's enhanced-bass crossfeed. He writes:

I have been spending the past nine months following your forums and building a number of variations of the pocket amp. I have decided that I have more than a few friends that would like to have one for Christmas in either kit or a variety of assembled form.... Because I have the resources available to me through my work I have gone ahead and laid out a nice little double sided board using Tango PCB, which I have sent to one of the commercial board houses in the Seattle area for a small "prototype" run. My boards arrived last week and I have assembled three of them and they work great!

The board house that fabricated the boards is fully automated meaning that no human hands were involved in the manufacturing process including a complete optical inspection using a robotic vision system.... I would like to sell off some of my excess boards. The price to sideliners in the forum like myself will be $6.50 each (or 3 for $17.00) plus $3.00 S&H which is about the same as the cost for using Vectorboard. To those that have posted contributions to the forum that have furthered the dialogue, particularly regarding the pocket amp, I would like to offer two boards each for free except the cost for S&H.

The specifications for the board are:

Dimensions: 1.80" X 2.45" with routed notches and corners to precision fit Pac-Tec case HML-9VB, leaving a 1.25" space behind the panel for components such as switches and jacks. The amplifier section is designed for dual OPA2132/4s with the crossfeed filters between the amplifier sections. There are provisions for two levels of enhanced-bass crossfeed filters plus flat. A 3 pole, 3 position rotary switch or some equivalent would be required to use all three settings. The filter capacitor component locations have multi-holes each to allow the use of different size capacitors. There is a provision for volume control or high-pass filter resistor. Gain of course is a matter of component selection. Personally I have found a gain of 5 to be the most versatile. There is also a provision for power indicator LED.

Shown below are the Levallois schematic and pictures of the Hansen PC board. For more information about the circuit, see Levallois' entry in the addendum update (p. 1) for May 4, 2000.

Update: C.E. Hansen is no longer selling the PC boards or the Noble XVB93 mini-pots described in the article. Instead, Jon M. Tsukiji (JMT in the forums) is now selling the PC boards for the same price, although he is NOT selling the Noble pots. JMT is also selling completed amps in the Penguin Mints boxes first shown by "Apheared." Contact JMT for pricing on the completed amps and to order the Hansen PC boards.

Jon M. Tsukiji
3142 Spruce Hill Ct.
Antelope, CA 95843
Email: JMT@surewest.net

3/14/2001: Major rewrite of article, including new appendix section on power supply options. Added new high resolution pictures.

3/14/2001: Coffin Lin put this amplifier (with a modified Linkwitz crossfeed front-end) in an old TV remote control case. He used an OPA627 opamp and made R2 and R3 in the Linkwitz filter adjustable instead of R1. The volume control is an Aiko pot in a shunted configuration with a 50K resistor (Dale RN55D), so that the audio signal passes through a single high quality resistor. Regarding his selection of the opamp, he writes:

I found that the OPA637 oscillated, even though the gain was set to greater than 5. The power supply voltage was not symmetric (2V/10V using a 12VDC AC supply). Then I changed the opamp to an OPA627, which was quite good for both my Sennheiser and Pro2 headphones, but the supply voltage was still not symmetric enough (6.4V/6.8V). The OPA134 got best result in stability (6.5V/6.7V), but the sound is too fat for me. So the final version is OPA627 - great detail, sound balance, clear, dynamic.

Lin put the Linkwitz filter at the input to the amplifier. The component values in his version of the filter are:

R1: 30K ohms
R2a, R2b: 15K, 10K
R3a, R3b: 50K, 100K
R4: 33K
R5: 33K
C1: 3,300pF
C2: 10,000pF

The resistors are Dale RN55D. About making R2 and R3 adjustable, he says:

I mistook R2 for R1, but on the Excel worksheet simulator, R2 can still alter some balance. I think that adjusting R3 is more effective than adjusting R2 (I forget which switch is for what resistor.) One has more stereo (good for dance and rock) and the other is more natural (good for jazz).

12/26/2001: Revised value of the current limiting resistor in figure A7. I reviewed Stephen Lafferty's circuit for charging a single 9V NiMH battery. The value of current-limiting resistor in Lafferty's circuit assumes that the specified unregulated 12VDC adapter will output 14VDC, because the amp is a very light load for the adapter. The recommended adapter in my project has a regulated output, so the output should be 12V exactly (or fairly close). Therefore, I changed the value of the resistor from 330 ohms to 220 ohms to get a charging current of about 20mA.

12/26/2001: Here are three candy box amps from forum members Doh, Droche and LivingPlasma. Doh put his Hansen-board amp in a Penguin Mints box (first shown by Michael Shelton - a.k.a. "Apheared"). He writes:

It looks like Apheared beat me to posting a Penguin Mints amp, but I swear I didn't steal the idea! Penguins rock! I'm afraid my amp isn't nearly as DIY cool as Apheared's creation, but it's only my second amp and I just learned how to solder a few weeks ago!

As you can see, the Hansen board is mounted upside-down in the tin with the power and crossfeed switches sitting right underneath the op-amps. It has dual pigtails and one position of crossfeed plus flat. There's no LED and volume is adjusted via an inline volume control from radio shack (soon to be replaced by a DIY version that uses the panasonic pot once those parts get in). I don't have any of that fancy tape, so I actually just stick a metrocard under the lid before I close it. (Haven't gotten around to glueing it in with some artist's spray mount quite yet).

I think that there is still enough space in the box to wire the pot inside if anyone feels like giving it a try. Personally, I like the flexibility that a modular volume control gives me. On the other hand, I'm still trying to think up a way to get rid of the pigtails to improve portability.

Just a note on drilling the holes in an altoids tin or other metal candy container. What I found to work really well are the black and decker "bullet" tip drill bits. They have a small extension at the point that bites into whatever you're drilling into so that the drill bit doesn't slip. The tip works its way through the metal fairly quickly, so after it's through you have a pilot hole that holds the bit steady while the rest of the bit does the work. Using these bits, I found drilling holes up to 1/4-inch to be no problem. The bits are available through amazon.com, but should be widely available.

Droche put his amp in the popular Altoids tin. He writes:

For any of you who think that building an amp is too difficult for a beginner- I am proof that it isn't. I started browsing the forums a month ago with no electronics experience whatsoever. After browsing for a while, I put in a few orders and before I knew it, I had a headphone amp. It took a few tries to get it into the box, but I finally got it in after removing the headphone jacks and adding pigtails and removing the pot. It was well worth the effort. I was amazed at how much better the sound out of my portable MD player got. Thanks to everyone here for all the helpful info.

Livingplasma put his amp in a round candy tin. He writes:

Not to take the attention away from Apheared, but I just couldn't help it after seeing all thse proud people post their version of the CMoy pocket amp. Those who have been here a while will know I had a string of bad luck making my first cmoy, this is what I came up with the leftover parts. It's the basic CMoy amp made with an OPA2134 and modified for the Meier crossfeed (changed some values so I could use a 50k pot and smaller input capacitor; yes, it's unbuffered). Input is through the pigtail, has a LED power indicator and uses one submini toggle for power and one for the crossfeed (on or off). Measures about 3 inches in diameter (not counting the controls), and just under an inch in height. Schematically, I think it's very similar to Tomo's version.

Drilling the holes on the side of the tin is annoying, to say the least. After a certain hole size, it's really hard to drill a hole, the bit catches on the metal and goes ripping the case apart (lesson learned trying it with a Altoids tin). I just made the hole as big as possible with the bit, then reamed it with either a screwdriver or a knife. The opening for the volume pot (it's those panasonic ones) is a square, I think I used some old diagonal cutters I didn't mind messing up and some pliers to bend and break the tin.

12/28/2001: Kenji Rikitake (a.k.a. "bdx" in the forums) built two versions of the pocket amp with an opamp-based virtual ground. He says:

The amp on the left in the picture is the OPA2134 version; the one in the middle is a single-amp version (OPA134). The breadboard on the right is for the OPA2134 version. The basic amplifier circuitry is same, but I changed the value of the feedback resistors to 1k/4.7k ohms pair. The 1k ohm resistor at the + input of the opamp protects it from accidental overcurrent or overvoltage (though the probability is very low). This is generally recommended when you make a non-inverted amplifier.



I tested with several different opamps. OPA2134 showed its excellence (only 3mV maximum output offset). NJM4580DD worked OK though it had 70mV maximum offset. The NJM082D (TL082 compatible) also worked but it could not fully drive my Sehnheiser HD414 Classic. Note that the NJM2043DD didn't work (caused self oscillation).



I also added an opamp voltage follower for providing the virtual ground, to stabilize the voltage exactly to the 1/2 of the unipolar power supply (namely a 9V dry battery). I tried the OPA134 as a unity gain buffer for the virtual ground driver and then the BUF634 (as ppl suggested). The quiscent current of the chip lowered from 4mA to 1.5mA or so, and the amplifier sounded the same. Note that OPA134 and BUF634 are virtually pin-compatible if you use the OPA134 for a unity-gain buffer. The 1N4002 diode protects the circuit from accidental inverse voltage connection.

The circuit is put into an aluminum case (T-SIN Denki TM-1) which can hold a 9V dry battery inside and can mount a 47mm x 72mm breadboard widely available here (Sanhayato ICB-88 compatible) in Japan. The FatBrain.com stickers are some of which I've got from the bookstore. The size of each amp is 87mm(W) * 31mm(H) * 103mm (D). I built the circuit on a glass-epoxy DIP breadboard. Since I proved the amp works fine with my Sony D-E880, Diamond Rio500, and Sehnheiser HD580 as well as with HD414Classic, I think I've got to build another one for my wife sooner or later.

More details about Bdx's amp and power supply can be found here.

2/21/2002: Added note about insulating the power jack ground when the power jack is mounted in a metal enclosure.

2/22/2002: Forum member Tangent has created a tutorial for electronics newbies who are interested in building the pocket amp. Many DIYers have found the tutorial very helpful. Please note that Tangent's opinions are not necessarily the same as this author's.

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