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Forum Home > Layout Gallery - GuitarPCB Build Documents, Mods and Tips > MoRC - New v3

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The new MoRC v3 is the same great circuit as the previous v2 but with a newly designed layout by Bruce R. decreasing the overall size of the board. This allows for a much easier fit than the previous v2. It sounds identical to the original, but smaller. See Photo.

New Dimensions - 1.85" x 1.9"

MoRC stands for Modified Ross Compressor

This is a very Quiet and  Transparent Compressor however it can get nice and squashy when you turn it up for the Classic Country licks.

Long and Sustaining Solos as per the Gilmour feel are easily attained with this circuit.

The MoRC is also extremely similar to the vintage Dynacomp as well but once again enhanced with sonic clarity that will Sustain for days but not alter your tone.

It has the standard Level and Sustain knobs as well as an on board switch which further enhances this circuit beyond the original to provide you with three separate tones.

Build Document

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MoRC Compressor Demo

This item goes great with a Stage 3 Booster & MIni-Me Chorus!

Recommended Accessories:

1. Barry's Famous Pot Condoms™ .

2. 3PDT Wiring Boards .

3. Barry's Best Pedal Hookup Wire.

4. DC Power Jacks 2.1mm

5. 6mm Low Profile Trim Pots

6. Common Anode LED (Optional to Standard LED)

7. Plastic Bezels for Common Anode LEDs

If you need the older v2 version please refer to an earlier post in the Layout Gallery.

It has the standard Level and Sustain knobs as well as an on board Variable Recovery Mod - fast/medium/slow recovery control. Where you will notice the difference in settings will be when you try to play faster, while still maintaining pick attack. The stock setting yields nice long sustain/compression(ala Gilmour), but at the cost of not being able to respond to new notes with any rapidity. The upshot is that you may not be able to hear the difference until you start trying to pick differently.

Here is the schematic with the Audio Path in case someone wishes to check their build with an audio probe.

Testimonial from J Allen Shaw:

For the "Release" control, I put a 470k resistor in R17, and used a 250k pot in parallel from the SW1 center & right hand lugs on the board (for an effective resistance of ~160k when maxed out) and it works perfectly!

The smooth compression of this pedal is the best I've ever glad I built it, and glad Barry & Tonmann designed it!

Possible Bass Modification courtesy of Tonmann:

The only parts I would modify for bass guitar are the release time (if at all) and the output capacitor.

The frequency response between the input jack and the inputs to IC1 is reasonably flat, the frequencies start to roll off at about 40Hz or so, which is just below open fourth (E) string.  Increasing C1 will give you about 3dB extra which isn't a lot to be worried about, also it isn't really worth increasing the very low frequencies if we are going to compress them anyway.

Another point is that OTAs (Operational Trans-conductance Amplifier) tend to distort the output signal if they are either fed too much input signal or the gain is set too high - something to be avoided.  A bit of attenuation at low frequencies (where the signal is largest) could be a good thing.

Bass players would normally look for a faster release time than a lead guitar - limit the gain for large signals and quickly restore the gain for following low level signals giving more of a tight rather than sustained sound.

The "timing" part of the circuit comprises the combination of R16 - R18 and C10.  The speed at which Q5 turns on depends on how fast C10 changes from a discharged to a charged state; this is controlled by the resistance value of R16-R18, smaller resistance - faster capacitor charging - quicker turn-on time for Q5.

With the standard values of R16 - R18 the total resistances are:

  • SW1 Up           40kΩ (R16 + R17 // R18)
  • SW1 Middle  160kΩ (R16 + R17)
  • SW1 Down     10kΩ (R16)

To set a fast release time you would be looking at a total resistance value of 10k to about 60k, which would mean the 160kΩ would be too large.  With R16 at 10kΩ and both R17 and R18 at 56kΩ you would get 38kΩ, 66kΩ and 10kΩ on the switch settings - if you find the 10kΩ setting to be to fast you could increase R16 by 5kΩ - 10kΩ (I'll leave you to do the maths for the other settings).

Alternatively you could replace SW1, R17 and R18 with a 100kΩ pot and set the release time manually.

Some people say that compressors sound a bit dull (lack high frequencies), To compensate for this we set C12 to a small value which reduces bass frequencies giving an apparent treble boost.

For bass guitar  I would suggest a"socket and see" approach using values up to 100nF

Some finished build threads:


An excellent article on what a Compressor does:

March 17, 2015 at 10:24 PM Flag Quote & Reply

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