Guitar Pedal Projects. Top 5 in DIY With Forum Support!


Post Reply
Forum Home > Layout Gallery - GuitarPCB Build Documents, Mods and Tips > Squishy Octopus

Site Owner
Posts: 8780

For the guitarist who wants that "can't tell when it's on but miss it when it's gone" style compression. But it also has a good amount of squish on tap if you want it.

You may power it with a 9v DC power supply. But it also works with a 12v power supply for even more headroom without having to do anything special. Just make sure the capacitor choices are rated for 25v or above.

For a better slew rate if you are having any crackly noise issues we recommend that you use the OPA2134 Burr Brown chip. This takes care of any instability associated with the cheaper TL072 chip.

Show your guitar or bass signal who's boss!

Level or Comp knob dials in the amount of compression you want.

Volume knob controls your overall volume.

You need Adobe Flash Player to view this content.

Highly recommended Accessory:

NSL-32 Opto-Coupler & Transistor pack.

Build Document

Support concerning Compressors and noise:

See the note in the build document regarding noise which is also a hyperlink to a great article on compression. Dial it back some [Level Control] if you have hot pickups (especially humbuckers). I understand you can possibly use a higher voltage (Negative tip) DC power supply say 12v if your capacitors all meet the minimum voltage requirement.

Disclaimer: This is not a recommended practice for any pedal and can be a pedal killer so tread carefully.

This is only useful in some Opamp circuits and absolutely not recommended for transistor based pedals as you will likely throw off the Bias.

From Analog Mans site in regards to using higher voltage on his compressor:

"These pedals are designed to work fine on 9V DC so they can use a battery. But if you are running a VERY strong signal into the compressor, like active pickups, you may want to use higher voltage for more headroom, to avoid the compressor distorting. We sell an optional 12V adapter, or you can use the 12V option on the Voodoo Labs pedalpower2 for that"

Try this if you cannot find a nice setting to your liking while keeping the Level back, or trying another guitar with less output. Again do so being careful to use the correct power supply (DC not AC) and Negative tip. Also of course all caps should be rated at least 25v or higher. I have read where other people like doing this with compressors, but I have not tried it myself.

Build document note aside:

There are two reasons for this and these are linked to some extent:


  • Compressors are non-liner devices and as such they generate distortion by the very nature of their operation. With light compression, this distortion is hardly audible. But with hard compression, you start hearing distortion. Fast attack and release times mean faster changes in gain reduction - making the compressor a (far) less linear device, and thus you get more distortion.
  • As low-frequencies have longer periods (the waveform develops slower over time in comparison to high frequencies) a compressor can actually act within the cycle of a low frequency provided the attack/release are short enough. This reshapes the waveform quite drastically and in turn generates a very audible distortion.




The NSL-32 is electro-optical, and as such you should expect its fast attack.

For a better slew rate if you are having any crackly noise issues we recommend that you use the OPA2134 Burr Brown chip. This takes care of any instability associated with the cheaper TL072 chip.

If the Burr Brown does not help please check your pickup height!

See my 2nd reply below, before trying any mods below beyond the Burr Brown.

Simple Mods Courtesy of Chris:

These Mods are for those who experience a little crackling noise (Low E when cranking Level) with higher output Guitars and Basses. The mods are very easy to add on.

Two or three things you can do:

1.  Solder a 220p capacitor in parallel with R4 (solder it on the track side if you can't solder it to the wire ends of R4).  That will reduce the noise a lot by aiding the stability of the op-amp.

2.  Use a 33k resistor at  R5.  This will reduce your maximum compression slightly (you'll hardly notice the difference) and will also help you not to "hit the rails" with the output of the op-amp especially if you use a Burr Brown OPA2134.

3.  If you notice a slight reduction in treble when you add the 220p capacitor (you might, if you have really good hearing), solder a 1n or 2n2 capacitor in parallel with R2 - this will give you a tiny bit of treble boost to compensate for the loss across the 220p.   This value isn't too critical - my build of this pedal has a 1k resistor in series with a 4n7 capacitor across R2, which gives me the "Byrds" jangle that I was looking for!  Experiment to find what sounds best for you.

I always use a compressor as the first pedal in my effects chain.  This reduces the effect of different pickups and the various guitars i use - the levels into the rest of my effects are more predictable.  However, there's one minor problem, which is easily solved:  I have an extra socket on the box, with an uncompressed (but buffered) output at the guitar level.  This extra output is used for driving the sidechains of amplitude-sensing effects.  The compressor destroys the natural envelope of the guitar (that's its job!), so I have an unaffected, low impedance output to drive my auto-wah, auto-phase, triggered flange and a couple of other secret weapons on my pedalboard!

March 15, 2015 at 5:15 AM Flag Quote & Reply

Site Owner
Posts: 8780

The Squishy Octopus Buid Document has been updated as of 4/20/15

Also as a reminder please read the articles on using a Compressor correctly to avoid pops or crackles.

One of the easiest things to check and most important before modding is to check your pickup height.

Here is a basic article by guru Craig Anderton



From this article Craig states:

"The reduced transient response caused by the pickups being further away from the strings is helpful when feeding compressors, as large transients tend to “grab” the gain control mechanism to turn the signal down, which can create a “pop” as the compression kicks in."




April 20, 2015 at 11:52 PM Flag Quote & Reply

This topic is closed, no additional posts are allowed.