9/7/99: This version of the pocket amp by Tomohiko Ishigami uses the acoustic simulator circuit by Jan Meier (see A DIY Headphone Amplifier With Natural Crossfeed). He reduced the gain of the amp to unity to minimize problems with noise, which he later traced to the CD player itself. The larger case is from Radio Shack (RS 270-213).

I feel it is very good idea to use modular approach. I used separate board for crossover and the buffer itself. This way, I did not have to go crazy load all the parts on one board which will result in a hay wire. Also, this approach is useful when I was trying to achieve smaller size.

I was able to use 1uF polymer capacitor for input.... These are so tiny. It is made by Phillips and you should be able to find it in Digikey [Digikey part nos. shown below]. I used this same type for my crossover circuit allowing me to conserve a lot of space:

3019PH-HD 1uF Metal Film Box ( 10mm (H) by 7mm (W) by 6mm (L) )
3015PH-ND .22uF Metal Film Box
3011PH-ND .047uF Metal Film Box

11/21/99: Added section on replacing level switch with a volume control.

11/21/99: Stephen Jenkins wrote:

Wow, I just finished building the headphone amplifer that you designed. I am in awe at the sound quality while using my little (but fabulous) Koss Porta Pro Jr's and my Pansonic SL-S360 portable CD player. The only change I made was that I included an AC jack on the side so that I that I could plug into the wall while at home, this was really easy and I highly recommend it. Thank you for the plans, you've made my day!

12/18/99: added section on implementing a dual 9V power supply for driving very high impedance headphones.

1/7/00: Several DIYers have installed Jan Meier's natural crossfeed filter as a front-end to the pocket amp. Jan offers these tips re: selection and placement of a volume control for this combination:

It all depends on the specific circuitry. Generally it might be better to place the pot after the filter instead in front of it. The influence of impedance changes might be less pronounced. A 10 kOhm pot will certainly be too small. 50 kOhm will be a kind of minimum I think. However, note that with certain opamps this will result in changing offset voltages, since the DC impedance changes with volume.

1/12/00: scrazy@gcn.net.tw built this version pocket amp, which has a 10K ohm volume control and an acoustic simulator front-end by Chester Simpson (see design by Fred Peng below). He used OPA134 opamps and set the gain to unity because his CD player's line out supplies more than adequate drive voltage. Full details can be found at DIY Zone (in chinese only). His system consists of a Rega Planet CD Player and Audio Technica ATH-f15 headphones.

1/13/00: Fred Peng's headphone amplifier incorporates the acoustic simulator by Chester Simpson, except that he replaced the R4,R6 combination in Simpson's circuit with a 100K ohm resistor and added a unity gain input buffer stage made from an OPA134 and a high current output stage made from a PMI BUF-03 buffer. The opamp power supply is double regulated for the cleanest output. The first stage of the power supply outputs ±34VDC, which is regulated to ±22VDC and again to ±15VDC. TWhen compared with a McCormack Micro Headphone Drive, the BUF-03 driving his Grado HP-1 headphones with the simulator bypassed sounded better in the high and low frequencies than the McCormack, but the McCormack was better in the mid frequencies. With the simulator switched in, the sound was more relaxed, the low frequencies were slightly "nasal", and the soundfield moved from inside his head to outside. He is very satisfied with the result and is planning to make another simulator for his Stax Lambda headphones. Full details and schematics (in chinese only) can be found at DIY Zone.

2/7/00: Eric Lee's pocket amp has a modified Linkwitz acoustic simulator front-end. He says "it works great...nice design...and I can hear almost no audible noise from it." He used a slider pot for the volume control and installed dual headphone jacks for 1/4" and 1/8" headphone plugs. The enclosure is from Radio Shack.

5/1/00: Forest Chang built a pocket amplifier with a modified Linkwitz simulator front-end and the component value changes suggested by Mika Vääräniemi (see above). He writes:

The circuit that I built is the same as Mika's, but the OPA that I used is the OPA134, and I put an OPA2134 as a buffer in the front-end of the acoustic simulator. The grounding method that I use is to tie the output ground, power supply virtual ground and switch housing together. Then I connect this common ground to touch the metal watch box (the enclosure that I used) with a spring. The amp has no hiss, even when I put it right beside the monitor. And I cut a beautiful picture from a metal candy box and put it into the watch box. My girlfriend uses the amp with a Panasonic SL-280 and Sennheiser HD-320 headphones. She is very happy with the sound improvement, and the cute headphone amp.

5/1/00: Revised figure 2a and section on using dual 9V supply. Added section on constructing pocket amp with adjustable gain. Expanded description of how to cut protoboard with a utility knife. Added figure 6a - pocket amp with balanced input.

5/1/00: Jeff Medin's pocket amplifier has 3 sections: a gain stage, the crossfeed filter by Jan Meier and an output buffer stage. The power supply creates a virtual ground with a Texas Instruments TLE2426 voltage reference instead of a resistor divider network. The 1uF (or less) capacitors are Philips box-type metal film; capacitors larger than 1uF are Panasonic FC/Z series. All resistors are 1/4W Yaego metal film. Medin writes:

This is the FIRST amp I built after discovering HeadWize. It is a "basic" pocket amp with the natural crossfeed circuit by Jan Meier. ALL parts are from Digikey. It has very good decoupling with 3 capacitors per opamp and 3.9uH chokes (the 4 green things that look like resistors - they are connected in series with each V+ and V- lead). The first stage (on the left side of the first picture) is an OPA2132 with a gain of 10.

This then feeds a Meier crossfeed circuit (4 caps in a row) and you can see the crossfeed resistor on TOP of the board (2.2k) with long leads. The output from the filter feeds a voltage follower (OPA2132) stage. The switches are for low and high crossfeed, power, and bypass for binaural recordings. I used Philips Box style metal poly caps. The two large caps on top & bottom of board are 1uF input caps. The output is taken from the OPA2132... with a 100 ohm resistor... which is included in the feedback loop so it will drive very low z phones and to prevent oscillation due to capacitance from long cables. I used 100 ohm resistors in BOTH stages.

If the resistor is OUTSIDE the loop, the impedance WILL have an effect on the sound of the phones, sometimes more bass, sometimes MUCH less signal based on the efficiency of the phones, etc. etc. Some phones as you know are spec'd to be run from an impedance of 100-150 ohms or so. I have a 15 year old APT/HOLMAN preamp (designed by same guy that invented THX-Tom Holman) and it's Headphone Jack is driven by a 5532 with a 120 ohm resistor OUTSIDE loop right to the jack. I would suggest people can try both (like Jan did) and see what sounds better to them. I would DEFINITELY recommend that you include this resistor in at least the last stage.

Note that I did not have any problems, I always "over-build" opamp circuits so I don't have to worry about problems later on. It's just habit.

5/4/00: Jasmin Levallois's amplifier is similar to Jeff Medin's, except that he uses the Meier enhanced-bass natural crossfeed filter (and the original resistor-based virtually-grounded power supply). He writes:

Finally I got some free time to complete my project.... I got a lot of work to do for school during the last few weeks and I didn't have time to work on my amp. This weekend I decided to take one day to transfer the amp from the breadboard to the pc board. I used about the same circuit as Jeff Medin. The input stage has a gain of 10, the output stage is a voltage follower, and in the middle I put the Meier bass-enhanced crossfeed circuit.

I used 2 OPA2132 opamps, but if I had to do it again I would use 2 OPA2134. An OPA2132 costs $6.99 while an OPA2134 costs $2.67. Since there is almost no audible difference between both opamps, I would go with the OPA2134 to save money. Since the second stage has no voltage gain, I decided to omit the capacitor in front of the output stage. I also removed the resistor in front of the output stage, and I don’t hear any noise from the output stage. The only noise I can hear, sometimes, is coming from my CD player.

As you'll see on the photos, the inside of my amp is very messy, but, hey, its my first electronic project. Fortunately, even if it’s messy inside, the outside looks pretty good. I really like this Serpac Enclosure (Digikey part no. SRH65-9VB-ND); it looks ways better than the PacTec case.

The photo of the battery compartment is to show that the Serpac enclosure has a 9v Battery compartment with battery contacts. It's easier to remove the battery with that kind of battery compartment than the PacTec Enclosure. Also the Serpac enclosure is just about the same size as the Pactec enclosure except that it’s a bit longer, and the height is a little bit less. This might be a problem for the electrolytic capacitors. I would recommend the Philips ones with this enclosure rather than the Panasonic Z series because the Philips electrolytic caps are much smaller.





Download parts list for Levallois Amplifier (MS Excel format)

6/16/00: Jasmin Levallois writes:

This weekend I finished to build another pocket headphone amplifier for a project in my physic class. I used an Altoid box like Jim Burruss did, but I used the cinnamon kind to not be accused of plagiarism ;). This enclosure has the advantage that I could show to other students the circuit, and it is also very small and provides great shielding.




I took my original circuit and I improved it a little bit. First, I replaced all my Phillips polymer capacitors by some Polyester made by Panasonic. I followed Jeff Medin’s recommendations and added a 100 ohms resistor in the feedback loop of the last stage. I used a .12uF capacitor to decouple each power supply pins. I also added a 100k resistor connected to the ground in front of the output stage. On my last circuit I had omitted this resistor, but many people in the forum convinced me to put it back.

I built the complete circuit on a very small board (4cm by 5cm) and I don’t think it would have been possible to make it much smaller than this. To save some space on the board, but also because Digikey was out of OPA2132, I used a single OPA4134. It is pretty cheap, $2.30, I think, and I really recommend it. I had a hard time to find some good electrolytic capacitors that would fit in the small enclosure. Finally I used some mini alum electrolytic capacitors made by Panasonic. You can find the Digikey part # of these capacitors in my part list.

This amp sounds great and looks great; I love it.

Download parts list for Levallois Amplifier (MS Excel format)

10/11/00: Added sections on AC adapters, troubleshooting noise. Revised section on volume controls.

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