Q&D Comp pot wiring question

[markm]

Could one of you kind gentlemen please clarify how the Gate and Rotation pots are supposed to be wired for this circuit?
I think I may have an issue with it.
For some reason, with the Gain control all the way off, I have unity gain and the rotate really boost the gain alot.
Thanks 

[Mark Hammer]

1) Understand that the chip and circuit are sort of predicated on line levels and studio use, rather than for guitar level use.  That's not wrong, just requires a bit of getting used to the levels employed.  Here, what is treated as minimum gain is more than one is used to for guitar pedals.

2) The datasheet notes that as the resistance of the rotation pot increases, the signal level at which limiting occurs is lower.  With R = 17k, it starts limiting at 100mv signal level.  With R>17k it introduces limiting at lower and lower input levels.  remember that since limiting will set a ceiling on the level, the higher that resistance, the more you will have to turn the gain up to maintain the same output level.  Conversely, if the Rotation pot is set to minimum, then the limiting kicks in less often and for less of the signal, meaning that the overall level will remain high unless you turn the gain down.

So, with the gain set to minimum, what you get to hear is everything that essentially below the limiting threshold.  As the threshold gets set higher and higher, the very same amount of gain will result in more of that signal being audible...or rather more of the intrinsic dynamic changes between soft and loud being audible.

If you set up a very sharp blade at waist height at the entrance way to a house and said "I'm gonna invite over a bunch of tall people", you'd find yourself muttering "I don't understand what went wrong.  I asked for a bunch of tall people and they're all so short (and torso-less).  What happened?"  Make no particular request for height and move the blade up to the ceiling and you'll find that even without requesting a bunch of tall people, everyone is so much taller this time around.  That is essentially how the gain and rotation interact.

[markm]

Mark,
I was kinda hoping you'd respond to this one!
I almost PM'd ya about it but thought there may be some others wondering about this too.
I'm thinking I wired the Rotation pot backwards from how you were describing it's function.
I'll have to try and figure that out.
  MarkM

[Stephen]

IS THIS CHIP AVAILABLE?

Google this chip and all of the suppliers need a major minimum order....Does anybody have one or two to sell or trade??

Looks like something to try.....

[markm]

Got mine from SD;

http://www.smallbearelec.com/Detail.bok?no=234

He still has 'em.

[Mark Hammer]

They're not cheap, but they make it ridiculously easy to wire up a high quality compressor/downward expander.

Note that the input impedance of the 2166 is only 175k, which is a bit lower than we'd like.  Ideally, maximum tone can be preserved if a simple op-amp or FET buffer is placed at the input of the circuit.  DO NOT put in any gain - the SSM2166 packs enough punch as is.

[Stephen]

I just orderd two of them from Small bear...

The P at the end of the chip on the search ..chip docs.. says 10 volts......Can I use this scheme with a buffer in front and use the 5 volt transistor and it will work??

I need something like this, LOOKS COOL... help would be thankful!!!!

[Stephen]

Is the pinouts like a regular chip top left#1 bottom #7 Top right #8 bottom#14...Am wanting to build this now!!

[markm]

Here's the info on the Chip;

http://www.analog.com/UploadedFiles/Data_Sheets/83095497SSM2166_b_.pdf

Enjoy! 

[Mark Hammer]

Is the pinouts like a regular chip top left#1 bottom #7 Top right #8 bottom#14...Am wanting to build this now!!

Pinouts on all DIP chips are number as if they were in a circle.  That is they are numbered so that the highest pin number is right across from the lowest numbered one.

[Stephen]

I read where the power down needs to be grounded to work, and you could hook up a switch to mute...does that mean no compression or no sound at all??   You can use a dodt switch to true bypass ...Why did the mute switch come up??  Should you mute the..... whatever...than bypass ..Seems reduntant ..I dont know but I will be building this pedal ..In the data sheet it even says will add sustain compression on guitar etc....

[markm]

The Mute is actually eliminated on this build via a jumper on the board,
**NOTE J-1 in this layout;

http://www.muzique.com/ssm2166b.gif

I'll tell ya what though, this thing can be very loud.
I have unity gain with the gain control completely off!
This unit could double as a booster NO Problem.