Compressor Pedals
Along with making a TON of overdrive pedals (see overdrive page), I also made quite a few compressors. So far all of them sound great with a bass (after a couple of mods in some of them for more low end). In my opinion, ALL of the compressors below sound 10X better than an MXR dynacomp. I will post sound clips of each one as time allows.
There are currently 24 compressor pedals on this page.
First up is the Keeley 4 knob compressor. After a slight mod to give it a full low end frequency response, in my opinion, this already nice compressor became an awesome one,. It is one of the most loved stompbox  comps for guitarists and I can easily see why.  The design is based on a highly modified Ross compressor but goes way beyond that. It can go from no compression at all, to an extremely punchy compression to a pretty heavily squished signal. It can also be used as a clean boost. Just running thru it can make you tone  bigger and fatter while still retaining your dynamics. With attack, sustain, input level and output level controls it's very easy to dial in just the right amount of compression.
EMMA Transmorgrifier: I really dig this one. It's an exact clone of the Transmorgrifier with the exception of slightly modding it for a little more low end retention. Also being capable of a clean boost, this compressor can actually add a little grind, making your solid state amp sound like a cranked Ampeg SVT. It can also have a TON of compression if you want it.It allows you to set the attack, release and the ratio of compression. It's also pretty quiet, even with the higher compression settings. Like the Keeley, it also uses the popular CA3080 OTA (operational transconductance amplifier) chip for its compression.
FET Opto-Limiter:. This is a custom limiter that was designed to sound and feel like the vintage UA /Urei LA-2A  pro limiters that are so widely used and loved on almost every hit record since the 60's, and it does do a pretty darn good job of it. There's a yellow LED (above the level control) that indicates how much limiting is taking place. It doesn't effect your attack like a compressor does (unless you absolutely squish your dynamics), but rather 'tames' the peaks of the hottest signals. IMO, this is a really nice sounding limiter in pedal form for use with a bass.
Demeter Compulator:. This has been a favorite compressor of a alot of bass players for quite a while now. Using a VTL5C10 Vactrol for its optical compression, I built this one 100% stock and  it's a great, smooth sounding pedal with no loss of highs or lows. The compress knob controls the ratio, with a range of approximately 2:1 to 10:1. The ratio has a "soft knee", which means that the ratio starts low and increases proportionally to the amount your signal goes over the threshold. It has a fairly fast preset attack and release time so it's great for taming slap and pop spikes in level and is very quiet at doing so,
Mictester's Opto-Comp:. This one was designed by 'Mictester' and posted on the diy pedal forums ( freestompboxes and diystompboxes). I really like this one, and it's super simple to operate. It's also a fairly simple circuit but  even so, I think that it sounds great and is as nice as any of the compressors on this page. I made my own opto-device in this one using a photo resistor and two green leds and it couldn't be better  It has no loss of lows or highs and is very smooth and punchy sounding along with being fairly quiet (keeping in mind that all compressors can increase the noise/hiss level when alot of compression is used). It can also be used as a clean boost with no compression if desired.
Merlin Compressor (aka Engineer's Thumb):. This custom designed compressor uses the LM13700 chip (operational transconductance amplifier) for it's compression. It has alot of of controls for tweaking the amount and 'feel' of the compression that you're looking for. The OTA is working as a current-controlled resistor in the feedback loop of an FET opamp, reducing the contribution of OTA noise. The ratio control blends compressed and uncompressed signals, so its variable from zero to infinite ratio ( ie. limiting).IMO, it's an excellent  compressor for use with bass guitars and has no loss of highs or lows, is quieter than a dynacomp and is a very transparent, smooth sounding compressor.
Pigtronix Philosopher's Twin (germanium):. This one is the most complicated compressor build that I've done so far but it was well worth it. It uses optical compression ( a VTL5C6 Vactrol). It has a switchable germanium distortion circuit for its distortion side so you can run it as a clean compressor, a distortion pedal or both. It also features a blend control. I love it for its almost infite sustain, but the only issue that I had with it is that even at maxiumum output, it was only at about unity gain. I solved that by adding a super low noise FET booster stage on its output level and now it's one of my favorites. This is not an extremely transparent comp, but IMO, its tone is awesome and well worth considering adding to your compressor collection.
Rothwell Love Squeeze: Another great sounding compressor from the UK for bass, the Love Squeeze was designed for ultra transparency, and it is just that. It's also the quietest (as in noise free) compressor of the bunch. Max compression is set at about 5:1 or maybe slightly higher. IMO,this  pedal is for those that do not like or use  compressors because of the squashing, tone coloration, and noise that most of them provide. I added a switchable clean blend to mine so i could also use it as a limiter, dial in just as hint of compression, or in its stock configuration which is excellent by itself.
Dan Armstrong Orange Squeezer: Known for their FAT tone, these vintage comps are still very popular compressors, especially among bass players. The original plugs directly onto your instrument, but i don't like something hanging off of my bass so I made mine in the pedal form. The original also only has an on/off switch with everything preset, but I found it to be more useful having  controls for the compression amount and volume. I also added a clean blend so I could run more compression but blend in the clean signal with it. No optical or OTA chips in this one, it uses FETS and a JRC4558 dual opamp for its amplifier stages. As good as it is, it's probably my least favorite of the bunch.
Toadworks Mr. Squishy: This really nice sounding compressor is based on the Dan Armstrong Orange Squeezer but IMO, is ALOT nicer sounding and WAY more controllable. It's also capable of WAY more compression. I first built it completely stock, but then modded it for better tone and more variable compression by changing the values of the pots and several resistors. While it's not perfectly transparent, there's no loss of low end and it's super punchy. It's  capable of squishing your signal drastically with the squish control, but can also be used for light compression or an uncompressed clean boost, and the more that I use this one, the more that I love it.

Diamond Bass Comp:. This is my latest compressor build, and it's now right up there with my current favorites of the bunch. The guitar version is a favorite among guitarists and after playing thru it, I can easily see why. With only three knobs and a couple of switches, this optical compressor is very simple to operate (but not simple to build!). It's a very natural sounding compressor with no low end loss and is extremely quiet. The 'tilt' EQ knob is in/out switchable. Clicked in, you can add low end and balls by turning it to the left, of make it  thin and bright  by turning it clockwise (in the center it's flat). It also has a high cut filter if you find it to be too bright (since some comps 'seem' to add high end sparkle). To me, it's probably the punchiest sounding one on this page, but 'feel-wise' it kinda reminds me of my  vintage UREI 1176LN's. The blue led on the left is the status light and the red one on the right indicates how hard you're hitting the compression. It's also capable of being used as a clean boost with the comp knob set fully CCW.
BJFE Pale Green Compressor: The handwired BJFE comps are pretty expensive but like all of Bjorn's pedals they do sound great and are loved by guitar and bassists. IMO, this one has perfect preset attack and decay rates, and even at the max compression setting still remains punchy without completely squishing your sound. The tone knob is flat at 12:00, adds low end counterclockwise, and cuts a little low end fully clockwise. With the comp knob fully counterclockwise (no compression) it can also be used as a clean boost with a tone control. This one can easily be used as an 'always on' pedal since your bass just seems to sound better with it on at any compression setting than without it.
Origin Effects Cali76:. Without question, my favorite compressor for bass in the studio has ALWAYS been the Urei 1176N. Well, finally, someone (Origin Effects) has made a compressor in a pedal form that sounds and 'feels' nearly identical to the real thing. It's so close, that IMO, in a live band situation it would be really difficult to tell them apart. This awesome compressor is HUGE sounding, without imparting too much color of it's own. Internally, you can set it to run either as a true bypass or a buffered bypass, as well as choose between  5 different levels of input padding for hot signals. It runs on 9V, but can also run on 18 volts for even more headroom. These are really BIG pedals, but they sound BIG too, They're not cheap at $339 for the base model (non transformer) and $469 for the transformer model, but IMO, you'd have to spend a grand or more to get a compressor that sounds this good.
BJFE Designed Mad Professor Forest Green  Compressor (handwired version): Another great compressor for bass from Bjorn Juhl. This one has two modes, compressor and sustain. In the sustain mode, the attack is left unaffected and the level control becomes the 'sustain' level  In the compression mode, the level is a master output control and the 'Compr.' knob sets the threshold of the compression. The tone knob is flat at fully CCW, cutting a little low end at 12:00, and cuts a little top end fully clockwise. With the comp knob fully counterclockwise (no compression) it can also be used as a clean boost with a tone control. Again, this pedal can easily be used as an 'always on' pedal since your bass just seems to sound better with it on at any compression setting than without it.
Colorsound SupaSustain:. This one is considered a 'Sustainer', but it is basically an optical-compressor. The vintage pre-Boss Roland AS-1 'Sustainer' is nearly the same circuit only the 'Supa' runs on 18 volts (for which I installed an internal charge pump so I can run it on a 9V supply). It's a very nice sounding pedal stock, but to ensure that I'd be able to retain my uncompressed attack at high sustain settings, I also added a clean blend so I can dial in the  sustain with the uneffected signal.
HEXE Tadek Compressor: Hexe Guitar Electronic's Tadek is the only compressor that I've built that uses the TDA7052A chip. It sounds great, can be used as a clean boost, and can go from subtle compression the heavily squashed compression. The controls are: attack, comp ratio, input level and output level. This was one of my first etched PCB's and also the first time that I've etched the enclosure for a pedal.

Additional Pics:
Gut shot
Etched PCB
Completed PCB
Aphex Punch Factory Compressor:  Aphex's Punch Factory is an optical compressor that is quiet and very transparent. It doesn't offer high compression ratios and people have complained about the PCB mounted jacks breaking but I've never had that issue. It also has a direct out with an XLR connector and ground lift.
Bass Comp/Limiter (cAMPus design):  This is a new generation of compressor that uses the THAT Corporation 4301P VCA compression chip. IMO, it pretty much makes previous compressors pale by comparison if you're going for a clean, uncolored comp/limiter. It can go from no compression, to subtle compression, to super heavily squashed compression. It features a response control that simultaneously adjusts the attack and release time, a ratio that goes from zero to infinity, a threshold control and a make up gain on the output. It also has a hard knee/soft knee selector as well as a 3 position input pad. It's basically a DBX 160X in pedal form, The Carl Martin compressor is similar but requires an AC cord whereas this one runs on a standard 9V adapter that can handle 160 ma. It's an AWESOME compressor for bass! Gut shot is HERE
Pigtronix Philosopher's Tone Mini: A great sounding optical compressor in a tiny 1590A enclosure. It features  blend, sustain (compresson), treble and master volume controls. it's capable of nearly infinite sustain.
Pigtronix Philosopher Bass Compressor: Another really nice one from Pigtronix, also capable of infinite sustain. It also has a preset "grit' switch which adds a very nice and heavy distortion
Pigtronix Philosopher's Tone (germanium):. By now you can probably guess that I really dig Pigtronix compressors. I built this one on an etched PCB and etched a 125B enclosure with top jacks with a relayed true bypass for it..
Comp 160:. I built this one from a kit. It uses a THAT compression chip as found in the DBX 160 compressors It has really nice metering, selectable soft or hard switch, a -9db pad for basses with high output, input gain, ratio (from 1:1 to infinity, a blend control and master output level. Transparent & quiet, this comp rivals the best compressor pedals available at any price.
Fairfield Circuitry Accountant:. Fairfield's 1590A compressor is an awesome little box using VCA FET compression and was designed to sound similar to a Urei 1176. It's very simple but also VERY effective.
Cali76 Compact Deluxe:. Origin Effects has done it again! A much smaller enclosure with all top jacks, it has ALL of the features and TONE of the large box (non transformer version) with the exception of the really nice metering, but, this one has a relayed true bypass and a clean blend so I actually like it better than my big box one! This one is truly a masterpiece of a compressor in pedal form. It runs on 9 volts or 18 volts for more headroom.