Craig Anderton compressor; any mods?

[Bernardduur]

Hi all.

I want to build a simple, better comp for my bass; as I read a lot of you advise to build a LED / LDR coupled compression pedal for best control. I heard the Craig Anderton version is a simple version with good results. Two questions:
- Where can I find a schematic; the known resource seems to be down and all I could find was a italian version with some missing values;
- Is it possible to add some mods to control the attack / decay (as I liked to use) or is the stock version this good it don't need any.

Thanks!

[MartyMart]

http://diystompboxes.com/sboxforum/viewtopic.php?t=34698&highlight=anderton+compressor



Marty.

[Mark Hammer]

http://www.fortunecity.com/tinpan/humperdinck/223/comp.gif

[Bernardduur]

Thanks for the "article"!

The schematic does not work. I have been searching for the same as this one but am not able to find one.

[MartyMart]

Sorry Bernard, I used his book, which has the schemo as a project  !!
I actually preferred the "Flatline compressor" TBH  ... and it doesn't have
to run on a +9 & -9 volt supply  !!!
That's well worth a build, plus there's a vero layout for it  

Marty.

[Mark Hammer]

http://www.fortunecity.com/tinpan/humperdinck/223/comp.gif

Désolé.  Si t'utilise cette racourci http://www.fortunecity.com/tinpan/humperdinck/223    et click sur "projects" tu pourras trouver le schema.

It just goes to show you that simply copying the link for a schematic you are staring at will not always lead you back to it!

[Bernardduur]

Are there other sources? Would like to view the original first

[Mark Hammer]

I used to have a copy somewhere, but ended up drawing it by hand on a piece of paper that I can't find right now.  The original used a 748 op-amp for the rectifier/follower, with a compensating cap.  I forget the exact value.  The optoisolator was obviously a CLM6000, rather than the one Stellan shows, but everything else is correct.

[Bernardduur]

Thanks!

[Mark Hammer]

Somewhere on another thread, one of the mods I posted involved a way of improving high frequency response.  Typically, compressors seem to lose high end crispness.  I've never truly understood why, but it happens.  One way past this dilema is to emphasize some parts of the spectrum.

As you can see, there is an inverting op-amp gain stage at the output.  The first op-amp stage with the LDR reduces the gain to provide compression, and the second stage adds gain to compensate.  The gain of stage 2 is set by the ratio of the pot resistance (a percentage of 100k) to the input resistor (8k2).  

The 2u2/8k2 network make a highpass filter also, but with a low rolloff (~9hz).  If you place another RC network in parallel, you can provide more gain for higher frequencies.  For example, if you adjusted the pot to 50k, and had a .01uf/4k7 network in parallel with 2u2/8k2, you would now have one path that provided 50k/8k2 = gain of 6 for the whole spectrum, but a gain of about 10.6 (50k/4k7) for content above ~3.4khz.

Because this emphasis implies boosting ALL high frequency content, it also means that all the hiss one normally hears from compressors when not playing ALSO gets boosted.  The solution is to use a larger value feedback cap.  The schematic shows 20pf.  Even at maximum gain, this only rolls off HF content around 80khz, so it needs to be increased in value.  A cap of around 220pf might be ideal here, since it provides a rolloff starting around 7.2khz at max gain.  That might seem low but remember that our .01uf/4k7 network will provide a gain of 21 for HF content, in comparison to a gain of 12 for the rest of the passband.  At higher gains, you will need the extra treble rolloff to keep a "lid" on hiss.  Besides, with a gain that high you will make subsequent stages in your signal path distort easily, and you don't want harmonics of higher harmonics.

Note as well, that you can easily turn this compressor into a ducker.  If you simply connect the 2u2 capacitor before the Sustain control to its own input jack, you can feed anything you want into that jack and use it to reduce the gain/level of your principle signal.  So, for instance, a singer's mic could actually be used to drop the level of the whole band just a bit, or perhaps a bass could be used to gently reduce the level of a mic'd kick drum, just to keep the subwoofers in the PA from exploding (or making heads explode).

[Bernardduur]

Nice. I always used 120 pf caps, but will try the 150 pf ones and check the tonal difference....

I once made a "bright" switch on my orange squeezer to overcome the high end loss. Must be somewhere on GGG; is great due to the way you can choose how many high end is lost.