3/14/01: Major rewrite of article. Added detailed comparison of sound quality of filter placed before and after the headphone amplifier and review of Stax SRS-3030 headphones with filter. Added new high resolution pictures.

3/14/01: Coffin Lin put his version of the pocket headphone amplifier (with a Linkwitz crossfeed front-end) in an old TV remote control case. He modified the filter by making R2 and R3 adjustable, instead of R1. 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).

3/25/01: Changed the value of R3 in figure 3 to 910 ohms (originally 1K-ohms) to remove emphasis in lower midrange and to increase the treble boost. This update results in a more balanced, clearer sound. I STRONGLY recommend it.

3/27/01: R2 and R3 incorrectly drawn in figure 3 from 3/25 update. Fixed.

11/24/2001: Mark D. Johnson writes:

I just finished building your Acoustic Simulator and have been auditioning it over the last several days on my Sennheiser HD600s (even now I'm listening to Miles Davis as I type this). I just want to tell you how much more I'm enjoying my music and how much less fatiguing it is to listen for long periods of time.

The thing I like best of all is the three dimensionality I hear in recordings that was never present before. As a drummer, the most amazing thing to me is that I can actually hear (whether true or not) the location of individual drums/cymbals being played - and not just sound coming from a point source called "drums." On the latest Dianna Krall CD I could hear even elevation changes taking place as different cymbals were struck. Again, whether or not this is just a byproduct of the design I cannot be sure, but it sure makes listening more enjoyable.

5/21/2002: J. Ian Ramsey (from the forums) built a pocket amp and the high impedance version of the Linkwitz acoustic simulator in separate enclosures. He obtained most of the parts from RS Components Ltd. For the 120nF cap (C2), he paralleled two caps: 100nF and 22nF. He writes:

I made the Simulator in the high impedance version as recommended by Chu in the project notes. I have increased the gain of the amp from x11 to x17 by putting a 1.5K resistor in parallel with R3 (1K) to compensate for the insertion loss of the simulator which is only used between the source and the amp.



I laid out the simple circuit for the acoustic simulator in stripboard. The simulator effect is very subtle and at first I was unsure if I had made mistakes in the layout, which were preventing the correct circuit action. Listening to this year's Grammophone magazine Award winner - Vaughan Williams 'A London Symphony' confirms the following:

• With the simulator switched to B the whole soundfield is very gently more centre focused and there is a slight loss of ambience.

• Switched to A, the field narrows and the volume level drops slightly thus making the sound image appear to be heard from a greater distance - just as Chu remarked in the article.



Overall I could easily live without the simulator as my HD600 headphones with the cmoy amp are very, very good. As an intellectual exercise, the concept of the simulator is satisfying in the way it addresses some of the headphone effects against speakers and this design does not degrade the sound - so I will most likely continue to use it.

Chu was spot on when he said to spend time with it in the design values as one would soon tire if the values were changed to produce a more dramatic effect. The subtlty soon gives way to a distinct change with it in and out and between mode A and B. Congratulations and thanks to Chu Moy for these brilliant designs.

5/21/2002: Phidaeaux (from the forums) built a pocket amp and the high impedance version of the Linkwitz acoustic simulator in a single enclosure. The white LED power indicator is mounted on the circuit board INSIDE the transparent Serpac enclosure. He writes:

Ok, I just finished this bad boy, and man oh man, am I pleased! This thing sounds so good. I'm only driving 32-ohm phones (Sennheiser HD497s) but the difference in sound quality is VERY obvious over my SlimX MP3/CD player's built-in amp. It feels good to have power to spare, instead of driving the stock amp to its limits. Also, the crossfeed is very nice. I didn't really notice at first, but now that I can sit down and play with it, I really like how it sounds.

First off, thanks much to PRR, cmoy, tekir, tangent, and everyone who had either helpful suggestions, or had problems in the past, that I could read about and avoid doing myself. Cmoy and tangent's info were both very useful too, for both the obvious reason (the schematics) but also the piles of helpful tweaks and pictures they had.

Technical notes! I'm using all the 'usual suspect' parts: Panasonic pot, Digikey jacks, metal film resistors, polypro caps (except for the power supply caps, which are electrolytic). I'm using a Burr-Brown OPA2134 dual op-amp (in a nice machined socket).

The crossfeed circuit is a pre-amplifier modified Linkwitz (the same circuit that cmoy uses as an independent item) and has had its resistor values all multiplied by 10, and its capacitor values divided by 10. This was done to raise input impedence a bunch, because its not actually driving headphones, it just connects directly to the amplification stage. The two 'perspective' settings are 2000 ohms, and 1500 ohms.

I thought about replacing one of those with a pair of mini pcb-mount multiturn pots, so I would have a 'default' setting and then a 'custom' setting I could adjust by opening the case. Or maybe put a 'stereo' pot controling those values, and cram it somewhere else on this case. Who knows. Anyway, I'm satisfied with those two values for now, I'll wait before I do any 'tinkering'.



Anyway, to those who said it could not be done, I managed to fit the following panel items in the case: input and output jacks, one on each side of the case. I'll get some right angle adapters I think to clean it up a bit, so it doesn't have the plugs sticking off to the side like cowlicks. Power switch, crossfeed bypass switch, crossfeed perspective switch and Panasonic EVJ volume control (with pretty aluminum knob) are all on the front panel. The volume control, and the crossfeed switches are touching each other. They are quite literally pressed up against each other inside that panel!

White LED. Looks good shining out from inside the case. Bright too. I was going to use a blue, but the blues were less efficient than this white (oddly enough). I'm running it with a 2.2k resistor. Current through it seems to be about 3mA. That could stand to be lower, and the LED is very bright right now. If I make another modification, it'll be to dim that LED a bit, and save some power. You can see neat little shadows inside the case from the components, light glinting off of things inside there. Very cool.

I also really like this Serpac H-65 transparent blue case. I'm all about form AND function. I know that people usually get on one side of the fence or the other. They either LOVE pretty little gadgets, regardless of how they actually work. And then there are people who say "Screw it, does it work right? Then it can be ugly, I don't care."

For me, I want it to look good, work well, and feel 'right'. If I'm going to be using something every day, it needs to be ergonomically designed. Engineers are notorious for totally forgetting the fact that real people have to USE the things they build. I work with equipment that was NOT inuitively designed so often; its like a breath of fresh air to find equipment where someone actually sat down to use it for a while, and thought 'huh, this knob should really go down here on this side, instead of up here... that would make a lot more sense.'

It's so easy to figure out where controls should go. Everything flows left to right on mine. Left is the input, right is the output. Starting from the left on the panel, you can turn it on or off, then you can choose crossfeed on or off, then you can raise or lower the crossfeed, then you can change the final volume. I don't need labels, cause it makes sense. And then, I love this case. Its curvy and sexy, but still has quite a bit of internal volume. The transparent blue doesn't scream "I bought this at Radio Shack like a nerd!" but rather, "This is a modern piece of technological wizardry." But at the same time, you can peer inside and see the parts, making a muted statement that it was a DIY project. The LED inside is nice too. Don't have to use up a panel spot, and the inside of the device lights up all pretty. Form AND function. Together at last!

Anyway, I don't mean to lecture you guys; you all build very good things. I got the ideas for this from seeing various other projects people have built. I just urge you all to take note of the stuff you deal with every day, and while you are thinking about all the technological aspects of it, to give a thought to intuitive design and control placement, the delicate art of making something 'easy to use' regardless of what its actually doing, etc.

Also, read the book "The Design of Everyday Things" by Don Norman. It talks about doors, VCRs, ovens, and all sorts of things you use every day, but don't think much about. Ever push on a door when you are supposed to pull? Everyone has! But why? It's a simple matter to make the operation of a door obvious; you don't even need 'push' and 'pull' signs that people have to look at and read. A well designed door gets used right each time, without anyone even noticing. It's all about intuitive engineering and human-centered design.



Oh, more technical jibba-jabba. Gain on the amp is set to 11. Current measured while in use, at a moderate volume, is 12mA. Not too shabby. Voltage between ground and the rails is 4.40V and -4.41V while in operation. Not bad, if I do say so myself! This single 9V is plenty to drive the 32-ohm HD497 to utterly insane levels. No need to give it more voltage.

Return to Main Article

Questions or comments? Visit the HeadWize Discussion Forums.