Sunday, February 9, 2014

A fancier bass guitar headphone amp

A while back during the summer, I etched and put together an MXR headphone amp so that I could quietly practice my bass. It also worked fairly well as a preamp for hooking the bass up to my bookshelf speakers.

I followed the above circuit and laid it out in EAGLE with a few tiny modifications (replaced a 4.7uF with 10uF there, and such), and stuffed it in a scrap piece of 2x4" aluminum box channel. 


Unfortunately, it was fairly lacking in features, as all it had is a volume (actually gain) control. It lacked tone controls to adjust treble and bass, and I also could not easily practice along to a recording as it lacked a mixer.

I also made the board a bit too short for the aluminum box channel I used for the case, requiring a fairly long DC jack and frequently leading to the DC power getting cut as the connector jiggled and wiggled quite a bit.

So in light of its problems, I decided to make a new headphone amp.

Features I wanted:
-Runs off a wall wart (I was debating using a small transformer off Digi-key so I could do true split supply, but transformers are large and I didn't want to deal with 110V AC)
-Built in tone controls.
-AUX in, for practicing along easily to recordings
-Works

With my extremely limited EE skills, I ended up merging three circuits: the earlier MXR headphone amp, the Baxandall mixer and tone control from Elliot Sound Product's universal preamp, and the op amp rail splitter from Texas Instrument's TLC074 tone control .


All LM358s and 4558s are actually NE5532s (because laziness).

The circuit takes a (bass) guitar signal in, amplifies it with a JFET op amp, runs it through a Baxandall tone control, and mixes it with a line-in signal and runs through a class AB common collector amplifier to supply enough current to directly drive headphones. There's also a guitar-only line-out output, for recordings or other nefarious purposes.

Admittedly, some parts of the board are excessively tight and other parts are kind of sparse. Oh well.


Etched board.

Admittedly, I laid out one of the pots backwards and formed several solder bridges. I also apparently forgot to finish routing one or two small traces, which let to major butthurt in fixing (hint: before printing the board, View > Layer Settings > turn off everything except unrouted and make sure you actually routed everything). The posted EAGLE files have the pot traces fixed and has everything laid out, so don't worry about it.

With many of the parts soldered on.

For the case, I had crufted numerous LaCie hard drive enclosures that was sitting in a trash pile behind Warren Towers last semester.




I used the extruded aluminum part of the case. It was almost the perfect width, though I had to cut it down a bit in length. The PCB was mounted with nylon standoffs and M3 screws. The front panel was machined from 3/16" aluminum and held in by the potentiometer nuts. I unfortunately did not CAD out the front panel when designing the board, and had to spend substantial time with calipers and consulting datasheets to measure out where to drill the holes (it also took me two tries to get it right; definitely doing this a better way next time). I had to mill off some of the fluting on the bottom to drill the holes to mount the PCB, as the fluting makes the drill bit slip.

Mounted.

 I decided to panel mount the 1/4" jack, aux in jack, and the power switch, mostly because of the 80x100mm EAGLE light restrictions on board size (the pots took up most of the room on the board). I didn't have any 3.5mm stereo panel mount jacks available, so I took a PCB-mounted one and JB-welded it to the front panel.

Tentacles of DOOM

Front panel drilled and mounted. Also needs better lettering solution than Sharpie.
For the knobs I used these somewhat overpriced pieces of plastic from Tayda Electronics. To align the knobs correctly and symmetrically to the pots (important for Baxandall, as 50% pot setting means completely neutral), rotate the pot counterclockwise all the way. Then rotate clock about 150 degrees  - when the slot is perfectly horizontal, the pot is at 50%. Insert your knob and tighten your set screw.

Still needs a back panel.

It's still missing a back panel, which I will probably get laser cut or milled sometime in the future.I also currently don't have a means of attaching it (I will probably use vast globs of my preferred adhesive).

Should you choose to build this:
EAGLE files(no implied warranty/do this at your own risk/I'm not responsible for anything you do with this)

You'll need the Sparkfun EAGLE library for the DC jack and the 3.5mm audio jacks.

Among other bad EAGLE practices I have committed on this.
-EAGLE has no NE5532, and with the pinout being identical to most dual op amps out there. I just got lazy and put 4558 in its place on the schematic. So all the 4558s are actually supposed to be NE5532.The TL071 stays as a TL071. IC1 especially needs to be an NE5532 (or some other op amp that can output a decent amount of current, e.g. JRC4556 or TLV2462).
-The +12V and +14V are actually both 12V, just separate rails. I actually don't know how to specify separate rails of the same voltage.

Other stuff to note:
-I'm not actually sure R33 and R34 are necessary, and I merely included space on the board to solder them in if they turned out to be necessary (so far they don't seem to be not). I built my headphone amp without them and it works fine.
-Despite being designed for 24V DC in from a wall wart, the prototype decided to short (and later the transistors magic smoked) with 24V (despite most of the parts being rated for over 30V), so I run it off 16V.
-The 500K volume pot should be log/audio taper, while the 100K pots should both be linear.
-In many areas this board is unfortunately laid extremely tightly. If the pots you buy have anti-dust seals (a la cheap Alpha pots from Tayda Electronics), you'll have to remove them for the 500K volume control and the 10K treble control.
-Do not neglect installing C4, as the whole thing will turn into an AM radio if you don't low-pass the input. Trust me, having to listen to business talk radio while practicing bass is not pleasant.