11/23/99: Added calculations for time delay of crossfeed signal.

1/7/00: Several DIYers have installed Jan Meier's natural crossfeed filter as a front-end to the pocket amplifier by Chu Moy. 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.

2/8/00: Added calculations for time shift of direct signal.

2/16/00: Tomohiko Ishigami wrote:

Here is a picture of Mr. Meier's stand alone acoustic simulator unit which I use with my Melos headphone amplifier. It sounds very transparent having almost nothing in the signal path. It is so small you can hide it anywhere. I just wanted to show you since it looks so cute!

5/1/00: Jeff Medin's version of the Pocket Headphone Amplifier (see the article by Chu Moy) 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: Added MS Excel circuit simulation application link and image.

1/16/00: Jan Meier has started a new company called Corda, which is offering a DIY headphone amp kit for approximately $250 US. This headphone amp has the latest circuit revisions as described in his article A Precision Preamplifier-Power Amplifier System with Natural Crossfeed Processing, such as opamps biased for extended operation in class A, ground-loop breaker topology, a crossfeed filter with 4 settings, and two headphone jacks with different output impedances. He writes:

The circuitry in the DIY kit represents the third generation of my original design. The output stage of each opamp is connected to one of the voltage rails by a 1.5 kOhm resistor. This forces the output stage into class-A functionality and increases sound quality considerably. Also 10pF capacitors are added to the feedback loop to increase stability at high frequencies. Careful matching of all resistors prevents offset voltages and the need of coupling capacitors. And the amplifier now is DC-coupled.

The power supply has a ground loop breaker, so the audio inputs and outputs MUST have floating grounds - their grounds cannot be directly connected to the enclosure. (See A Precision Preamplifier-Power Amplifier System with Natural Crossfeed Processing for more discussion about biasing opamps to function in class A and ground-breakers in power supplies).

The headamp has two sockets for connection of a headphone. Both sockets will provide different sound characteristics. One socket has a very low output impedance and gives the amp tight control over the headphone action. However, many headphones have been sonically optimized to be driven by an output impedance of 120 Ohms and may sound better when connected to the other socket. Generally, the low impedance socket provides a clean sound whereas the high impedance socket yields a warmer sound. Use the one you like most. There is no risk of damage to your headphone by connecting it to either socket. You can also use the sockets to connect two headphones simultaneously. However, the volume produced by the high impedance socket will be slightly lower than that of the right socket.

I know that it is cheaper to buy all the electronic parts by yourself, but please note that for the money you have a professional PC board added (with soldering mask, tinned soldering eyes, and all the holes drilled) as well as the aluminum case with a 4 mm (!) front- and a 2 mm back-plate with all the holes milled. Although construction is rather straight-forward (a component plan is added), this project is not intended for real novices. If you doubt your own skills, please contact me for the possibility of obtaining a finished device.

5/27/01: Headphone amp and power supply schematics updated to Corda HA-1 design. Added schematic for enhanced-bass crossfeed filter amplifier. Also added revisions in text discussing latest updates.

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