MXR Mini Limiter

[cd]

Anyone have any idea or clue as to what's inside?  Or how it works?  The Mini Limiter is a blue, semi-rackmount type thing with two knobs (threshold and output), NOT the red, 4 knob limiter (more info on this later).  Google has lots of links of folks describing how kick-ass they are, but nothing on the actual operation of the unit, other than it maybe uses a FET.  Even the usual schematic selling sites have zip!

There is one on eBay right now (complete with circuit board photos) but the photos are grainy at best, the chips are obscured, and it's a doubled sided board so fuggedabout tracing it.  I did manage to track down this block diagram from another auction:



From yet another auction, I sussed out that there are three single opamps inside, with a single 3080.  Number of transistors... well, from the photos on the eBay one, I can count 10 or 11... with more than a few of those going to the LED drivers.  I also don't notice any diodes.

Anyway, if you go by the block diagram, let's say the 3080 is part of the VCA, then the FWR and release control are the interesting parts.  Maybe not even the FWR since that's standard in a Dynacomp.

Now, about the red, 4-knob MXR Limiter: I had half an hour to poke inside one the other day.  Didn't have a multimeter with me so at best I could only trace out certain component paths - not enough time so don't even ask about a partial schematic!   From my limited poking, it looks like it's setup similar to the MXR Black Finger, the OLD one with the 3080 and dual voltage supply.  That is, the 3280 in the MXR is used in the feedback loop of another opamp to vary the gain, rather than the 3280 amplifying directly.  There is also a PNP darlington driving the 3280 to vary the gain... the rest of the sidechain was too complicated, but it was opamp based and totally unlike the Dynacomp - I did not recognize a phase splitter, but the circuit reminded me of the Pearl Compressor's sidechain, only with a lot more parts.  Hopefully I will get some more time with it soon...

[Mark Hammer]

Thanks for starting this thread.  Those beasts are interesting to me.  If the red one uses a 3280 dual OTA, that's nice.  Jim Patchell's "Old Crow" site demonstrates a number of the advantages of the 3280. not the least of which is its relative tolerance of higher-level signals, in comparison to the 3080.

The thing to note about limiters is that their gain reduction hinges on a threshold.  Where compressors crank the gain and cut it back as the input signal goes up, limiters do nothing UNTIL the input signal exceeds a designated threshold, and THEN they cut back on gain.  One of the advantages of this is that one doesn't get the audible breathing and increase in noise as the note dies out, the way you would with a compressor.  Of course, the use of a threshold implies that at least part of the circuit is occupied with identifying if the input has exceeded a given level or not.

[davebungo]

A limiter is just a compressor with a higher compression ratio usually greater than 10:1, so the circuit topology isn't necessarily any different.  I've got a red MXR limiter and it's great, it's probably very similar to a dynacomp but, with the addition of both attack and release controls, it is a little more controllable.

The threshold on the MXR limiter is in a fixed dBV position and the sensitivity control just applies gain prior to the threshold comparison.

The MXR limiter for the UK market has a bizarre mod to the mains input which basically involves dropping half of the mains voltage across a resistor leaving the other half across the 115VAC primary - I know this because mine burned out not long after I bought it second hand.  I don't know why MXR didn't just fit a 240V transformer - perhaps it wasn't seens as cost effective given the relatively low numbers being sold in Europe at the time.

[cd]

I don't know about the Mini, but the red 4-knob is nothing like a Dynacomp circuit-wise.  They don't even use the same OTA, and the side-chain is completely different (from what I could tell).  Then again I wouldn't put any stock into a product's function from it's name, heaven knows the Dynacomp performs more like a limiter and not a true compressor!

davebungo: I presume since you've had the pedal for a year, you can't trace it?

Anyway I may have a line on more info on the Mini Limiter.  Nice little mystery since it seems no-one has any info on the thing.  Will update as things progress.

[davebungo]

When you say trace it, I assume you mean the circuit - no I haven't but I did try to find a schematic at the time because of the fault which occurred (without success).  If anyone knows of a schematic I would be really interested (and grateful) for a copy.

I did perform some tests on the unit though at work using an audio test set and it performs very much like a typical feed forward compressor.  The block diagram schematic you provided is for a feedback compressor where the VCA output drives the compressor sidechain, so I think the mini compressor is fundamentally different.

This shows the MXR limiter to be not really a hard limiter but to have a compression ratio of 5.  It also has a fairly soft knee characteristic.

These are the results:
input (dBV)....output (dBV)....Ratio (calculated)
-32...............-32   
-30...............-30..................1
-28...............-28..................1
-26...............-26..................1
-24...............-24.2...............1.111111111
-22...............-23..................1.666666667
-20...............-22.1...............2.222222222
-18...............-21.4...............2.857142857
-16...............-20.9...............4
-14...............-20.5...............5
-12...............-20.1...............5
-10...............-19.7...............5
-8.................-19.3...............5
-6.................-18.9...............5

[Tubebass]

I have a schematic and layout diagram for the Mini that MXR sent me 20+ years ago. Don't know how well it would scan, might be better to just snail it to you?

[cd]

With the help of Tubebass, I've got the Mini Limiter schem in my hot little hands (thanks!)  It's rough but legible, I'm going to cross reference parts to the board pic I have and re-draw an improved schematic, which will take a while.  

At first glance, the architecture is very, very very VERY similar to a Dynacomp (same 3080, same FWR), except the Mini Limiter has a balanced input, and the FWR is driven from an opamp buffer, and the output is based around a variable gain opamp, with an ATTACK control (really release) similar to the BOSS CS-2, Ibanez CP-9, etc.  That's all, nothing earth shattering.  More info to come.