First off, what is an "acoustic simulator"?
It can be anything to simulate the effect of being in a room, creating an effect of open space, or even give the illusion of hearing sounds from different directions. This is usually achieved through software. Results vary, as there is coloration to the sound and the simulations can be very invasive to listening.
Unlike software implementations, what I will show here is a simple hardware solution to get rid of the "in-head" effect that headphones have. It is not designed to give the illusion that you're in an auditorium with reverb. Rather, it is designed to simulate the feeling of hearing audio in front of you. Its purpose is to improve the soundstage.
The main goal is to reduce fatigue and make headphone listening natural and pleasant.
In this instructable, I will show my build of a Linkwitz crossfeed. This particular design was modified by Chu Moy, the person responsible for the famous "cmoy" headphone amp design. I take no credit for this design, only my simple modifications.
Step 1: What Is a Crossfeed? (Theory)
In reality, you're hearing with both. Your brain recieves signals from both ears and interprets the direction from which the sound is coming from. You may be hearing to the right of you, but your brain also interprets if the sound is close, far, in front of, or behind you. When you listen to audio with headphones, this directional information is lost.
To remedy this, we welcome the crossfeed. A crossfeed circuit bleeds the left and right channels just a bit to give off that lost sense of direction. While it does not give the illusion of surround sound, it does give you the feeling of listening to audio from speakers in front of you rather than through headphones on your ears.
Music from live performances and orchestras sound particularly good with a crossfeed. Mono recordings and music that generally sits "in the middle" will not benefit from a crossfeed much, or at all.
Step 2: Tools & Components
Because I was aiming to spend as little money as possible, I made some changes to the modified Linkwitz crossfeed that is presented by Chu Moy.
The original circuit had a switch for bypass and another to adjust the "perspective" of the crossfeed effect. I did not use these. A bypass would only be good if you wanted to compare before and after crossfeed. The other switch was not used, as I did not care to adjust the crossfeed from my final setup and wanted to save a little more money.
Test the circuit on a breadboard, if you have one. You may want to make changes as stated in the notes below.
- soldering iron and solder
- small screwdriver
- scissors / wire cutter
- 2x 150 ohm resistor (R1)*
- 2x 100 ohm resistor (R2)
- 2x 910 ohm resistor (R3)
- 4x 330 ohm resistor (R4,R5)
- 2x 0.22 uF, 35V film capacitor (C1)
- 2x 1.20 uF, 35V film capacitor (C2)**
- 2x 3.5mm audio jack***
- 3.5mm male-male cable to connect to devices***
- jumper wire
- protoboard / cardboard(?)
- something to put it in
*You can experiment with the soundstage and 'open-ness' of the sound by using different values of R1. Lower values will make it feel as if you're hearing things farther away, while larger values will bring sounds closer towards the middle. The "perspective" switch in Chu Moy's original schematic switched between values of this resistor.
**Changing C2 will alter the threshold frequency and depth of the crossfeed. I used a 1.0 uF capacitor instead of 1.2 uF. More explanation on threshold frequency and how this works can be found here.
***Depending on the type of input you're using. You may want to use RCA or 1/4" jacks and cables.
Step 3: Protoboard Substitution
The first thing you should do is figure out the layout. If you decide to follow my simple circuit, I have provided some images on what the layout should look like. While it isn't the most compact, it is very easy to follow.
Simply poke holes with your small screw driver to where the components will be placed.
Step 4: Build
Make sure to bend the leads, that are to be soldered to each other, in the same direction and group them where applicable. You can do one entire channel first, and then the other. Depending on your soldering skill, you may or may not want to solder one component at a time. Soldering groups of leads seems to be much easier.
*Be careful when soldering R5, as the leads may touch. They are arguably the most important resistors, as they are the ones where the signals are being crossed into each other.
Forgive my drawings, they are not to scale.
The layout follows the modified schematic in the last image. Original can be found here.
Step 5: Testing
To test the crossfeed, all you need is some music to listen to.
If you want to see how much audio is bleeding into each channel, connect the crossfeed to your computer using a 3.5mm male-to-male extension cable and open up your favorite media player. Switch all output to either the left or right and see how it is. If all sound on one side seems louder than all sound on the other, you may have done something wrong. Check your connections!
Be aware that there will be a total volume drop due to the crossfeed. This is normal.
Step 6: Enclosure
I happened to have an extra enclosure laying around and decided to use that. You can use anything you want. Much like the popular cmoy headphone amp, you can try putting it inside an altoids tin. To match the "protoboard", why not put it inside a small cardboard box? Use your imagination.
I really like these hammond cases. I tried putting a headphone amp in this particular model, but it came out very, very cramped. It seems fairly roomy for the crossfeed, and matches my amp perfectly.
Step 7: Final Thoughts and Improvements
Because of the volume drop, it is important to use good, clean amplification before or after the crossfeed circuit. It works great when paired with a cmoy headphone amplifier. While not having a good amplifier or sound source may discourage you from building this, it is still fun to try out and only requires a few components to build.
The crossfeed filter also works surprisingly well when coupled with my Playstation 3 and cmoy headphone amp. There was some benefit when playing first-person shooters online with headphones on. It was easier to determine where sounds were coming from in-game and it felt a lot more natural with the crossfeed on. If you didn't see it, check out my quick and dirty way to use headphones with a PS3.
In the future, I want to build an improved cmoy headphone amplifier with this particular circuit built in. Some simple improvements I plan are to shrink the size of my layout, use smaller sized capacitors to save space, and to possibly include a rotary switch to adjust the soundstage.
Well, those are all my thoughts for now. I hope you enjoyed reading my instructable. Don't forget to rate and comment! I appreciate any thoughts or criticisms. If you have any improvements or suggestions, give them! Thanks. :]