Cheap Compressor Kit

[Cozybuilder]

I picked up a fairly inexpensive compressor kit from an E-Bay seller, $17.99 delivered. This included all the components, PCB, painted but undrilled box (the best option), and some wiring instructions. From past experience, I knew the PCB needed to be verified, especially since others had reported this kit was suspect. The other thing is since it is a relatively large PCB for the box size, you need to really think through where to mount the pots, I/O and power jacks, and stomp switch. In this case, I elected to include a battery, so it was important to pay attention to component sizes to give the best options for the jack placements. Its better to populate the board first, then see exactly where the jacks will fit best, and drill accordingly.

Step 1 was to trace out the PCB, and draw the schematic. The seller's instructions had a drawing of the component side of the board, so it was a simple matter to draw in the traces, and decipher that to a schematic. It turns out that the PCB is a slightly modified BYOC Optical Compressor, which is a mod of the DOD 280A Optical Compressor, and GGG has a clone of that, plus a modded version. Anyhow, the circuit is well proven, and there are a lot of possibilities for modding it. The instructions show a wire to be added to jumper two 47nF caps. Don't do it- that is a mistake. These are labeled C2 and C3 on my schematic, and the jumper would connect the base of Q1 to lug 3 of the volume pot. The other end of these caps is connected to U1a pin1, this trace is present on the PCB. Other than that, the instructions are OK, no mistakes on the PCB.

This is the schematic I traced, and what I wound up with on the PCB (the kit supplied 2N3904's worked fine on the breadboard, no need to sub. Also, C500K is fine, I tried out both that and C200K, prefer the C200K, but YMMV):



My first attempt at breadboarding it gave an OK compressor, so I decided to tweak it. Others have used different transistors (2N5088, PN4124, 2N4401); op-amps (LM358, TL072); changed values of caps C6 (33uF and 22uF), C5 (4u7); resistors R3 (10K), R11 (1K5), R5 (2M2); and the Vactrol (VTL5C2, NSL32). So, this gave me some ideas of what to tweak. I later went back and found a couple of dodgy connections, the 2N3904's work fine.

Rather than a Vactrol, the kit provided a 5mm red LED and a generic LDR, with the PCB indicating to lay each down facing the other. What I did was make a Vactrol- sand a flat on the end of the LED (400grit) so it matched up well with the LDR, then super-glued them together. I then used black shrink tubing to keep out light and give the unit strength. The component leads are rather close together, so I also used a small piece of shrink tubing on each lead to make sure they didn't short. I think rolling your own Vactrol is the key to getting this kit to work properly.

Usually I change quite a few components- in this case very little of that was required. The most radical was to provide polarity protection and power filtering- which required a total of 3 additional holes in the PCB. Here are the changes I made:
  1) Replace the power system components, get rid of the 1N4004 and 47R, and replace with 1N5817 Schottky diode, 22uF tant and 100nF MLCC caps (as per the drawing below). Purple is PCB tracing, red is hard wiring.
  2) Change the 10uF aluminum electrolytic to 10uF Tantalum (Vref cap)
  3) LED resistor changed to 15K (strictly personal preference)
  4) Sustain pot, change to Reverse Audio Taper (C500K). This will give a smooth change, not bunched up like a linear or audio
  5) Sustain resistor, change to 10K (quicker release- more squishy)
  6) Play with values of R11 (4K7 on PCB). Lower values will give more compression, I used a 2K2. You might consider modding by placing a 5K or 10K pot in this location and have a compression control.
  7) Changed transistors to 2N5088 (completely unnecessary)
  8 ) Remove the 47R near the footswitch. It just provides an additional path to ground for the input cap, which is already accomplished by R5 (1M)



Shots of the board after populating:



The board needs support at the opposite (component) end, so I screwed on two ¾" standoffs using the two ground holes conveniently drilled on the board, and JB Welded them to the box. This was actually an easy build once I verified the circuit and populated the board- the supplied jacks fit nicely.



In the end, I wound up purchasing the kit and just a few minor components to make this version of the DOD280A Optical Compressor. Its a keeper.

[LightSoundGeometry]

those 3pdt switches they have are $1.30 USD/unit on ebay.

In my experience, have a loud mechanical click and they blow apart like confetti similar to the cheaper 1/4 mono/stereo input jacks too much for my liking.

[Kipper4]

Good work Russ.
Thanks for sharing. Add to favorites thank you very much.

[Mark Hammer]

Nice neat work.  Optical/LDR-based compressors sidestep issues of headroom limitations facing OTA or many FET-based units.  Additionally, there is a teeny bit of lag or sluggishness in LDRs that, in tandem with full-wave rectification (rather than half-wave) results in a pleasingly low amount of envelope ripple, if any audible ripple at all.

It occurs to me, looking at the schematic, that a person could conceivably use two LDRs to address one of the traditional issues with single-band compressors: the loss of top-end bite.

Take a look at C1/R3.  That pair sets the low-end rolloff and maximum gain.  One of the tricks that the Proco Rat uses is to use two RC pairs in that general position to provide more gain for the top end than for the mids and lows.  Set for maximum "Sustain", C1/R3 give U1a a gain of 46x with a low-end rolloff around 154hz.  The LDR in parallel with the 1M feedback resistor drops the gain whenever the LED lights up.

But...what if, in parallel with C1/R3, there was a second RC pair, plus an LDR in parallel with that second gain-setting resistor? (which could conceivably be controlled by the same single LED)

Let's make that second pair consist of a .022uf cap (22n), and a 47k resistor.  And, we'll put the LDR in parallel with the 47k.  When the LED isn't lighting up and altering the LDRs, C1/R3 trumps the added pair, and the added pair does nothing audible.

However, when the LDRs drop in resistance, the overall gain is reduced, but if that second LDR plus the 47k magically transforms into a combined parallel value of, say 5k, then we would have noticeably more gain for content above 1446hz.  IN effect, that added leg and LDR turns the treble up a bit as it compresses.  At low-to-medium compression amounts, there would be very little impact of the added components.  It's effect wouldn't kick in until one started cranking the compression.  Which is precisely the circumstance where the typical treble loss becomes objectionable.

Whaddya think?  Reasonable?

[Cozybuilder]

Hi Mark- I like the way you think. Theres still one on the breadboard with a few differences from the circuit above (3mm green LED & GL5537 LDR, TL072, Sustain C500K, C3 & C7 47nF MLCC, 2N3904's, A10K vol), so I'll add this and see how it goes. Thanks for the input!

[Cozybuilder]

Ding Ding Ding- we have a winner!

Mark- the treble booster works exactly as you described. One could stick a SPDT at the 47n/22n junction and have an option- normal & bright.

Here is a wiring diagram, reference to the schematic above:

[Kipper4]

Excellant work.
Theres always room for one more good compressor.

[Mark Hammer]

A deep bow and tip of the hat, Russ.

As much of an inspiration as this site is, t has been a source of laziness for me.  If I have an idea, and spell it out clearly enough, *I* never have to do the legwork!  Somebody is going to do the hard work for me.

I'm so pleased that it works as conceived.  I feel like I put on a blindfold, threw a dart at the board and hit the bullseye.

Now I'm curious to try it.

[Kipper4]

Flanger first Mark..................

[Mark Hammer]

Crap, I was afraid someone would say that. 

[garcho]

Flanger first Mark..................

one stone: complanger? 

nice one, was just noodling with opto-comperoonies today, will do this up tomorrow or the next day, thanks!

[Kipper4]

Necro post.
Did you do a sound sample of this Russ?

[ShOrEbReAk]

mmmmmm I like this one too.

who is the seller on ebay?

Troy

[Cozybuilder]

Kipper-
Sorry, I don't have any recordings.

Shorebreak-
http://www.ebay.com/itm/DIY-Compressor-effect-pedal-guitar-stomp-pedals-Kit-PR-hot-sold/121816888410?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D39107%26meid%3Dc401addc8eb44083ba8df33e1b897275%26pid%3D100005%26rk%3D4%26rkt%3D6%26sd%3D111892006092