Light Speed Compressor - another experiment in optical compression

[samhay]

Are you missing something in your life?
Is it a fatter bottom end? Tighter mids? A more present presence?
Do you want a tone that barks, then sits back in the mix and plays nice with others? A more transparent, yet nicely coloured tone that is both warm and bright.
Do you want tone that travels at the speed of light, and isn't RoHS compliant?
Then this may just be what you have been searching for.*

Quite simply, this is hands down the best compressor on my breadboard at the moment.


(click for larger version)

*I make no promises, but it is a handy little compressor/limiter.

[samhay]

And for a more measured description:

There have been quite a few recent threads about the John Hollis Flatline compressor. It seems to be a popular circuit, but I have always been slightly bothered by the side chain - it does full wave rectification, but then spoils the effect by doing so in a very asymmetric way. I figured I could do better, or at least different.

The above circuit uses a precision bridge rectifier to do a full-wave rectification without any diode drop. This is a fairly popular way of driving old moving coil meters and it is easy to calibrate - R7 determines the response, and will give 1mA/V(peak) as shown. It will also drive an LED, but this must be fairly efficient as most op-amps will crap out if you drop the value of R7 much more to push more current through the LED.

I have used a couple of vactrols in the circuit - a Silonex NSL-32S and the newer NSL-32SR3, which is available from Farnell for  about £2. These are relatively fast with 5 ms rise and 10 ms decay times.
http://www.farnell.com/datasheets/1674205.pdf
If you roll your own, or use a slower vactrol it should work fine, but if the light resistance is >> 1k, you may have to scale up R3 and the ratio pot. As it is, it can just about manage a ratio of 20 at max setting, thus putting it in limiter territory. There is some distortion from envelope ripple, especially with smaller values of C8-10, but if you are looking for a 'character' compressor then you might find this quite pleasant - I like it.

IC2A should be a fairly fast op-amp - I have tried NE5532 and TL072s.
D4-8 should have low forward voltage. I used 1N60 Schottky's.

The compression indicator is actually an envelope indicator, but for anyone that wants to add an indicator to the Flatline, this should work ok.

Edit - and it works nicely with Bass.

Edit 2 - here's some data showing how the ratio and threshold controls work. These were measured after the initial attack using a 440 Hz sine wave, a 1u cap across the vactrol LED and the level pot at max.


(click for larger version

[deadastronaut]

cool...subbed. 

cheers sam.

[Kipper4]

Neat work Sam
I might have to try it on the breadboard when I get a spare corner
Thanks

[Cozybuilder]

Looks great, can't wait to add this one to the stable. Not too many controls, not too many chips, I wonder what size box it will fit in?

[duck_arse]

now I won't feel so bad about [still] not building your other opti compressor, sam.

[samhay]

Thanks all.

Russ - you could easily do away without the switchable attack/release time, which should make it a fairly straightforward build in a 15090A if you use 9 mm pots. As the ratio and threshold controls have some interactivity, you could replace one with a trimmer if you really wanted to too...

duck - this one is better in most respects, so consider any guilt absolved.

[vigilante397]

Light Speed Compressor is catchy and all, but if I build one I would have to name it "Threshold-ish."

[samhay]

To each there own.
When I get around to finishing measuring/plotting the V(out) vs. V(in) curves you will see that there is some inter-dependence of the ratio and threshold(ish) controls. This is fairly typical of a feed-back style compressor, and they are both useful - at least to me - but I wouldn't want to be accused of trying to over-sell the design!

[Cozybuilder]

3 pots and a toggle are no problem to fit in the rather large 1590A. Unless you stuff a 9V battery in there too. 

[samhay]

>3 pots and a toggle are no problem to fit in the rather large 1590A.
It's all relative.

>Unless you stuff a 9V battery in there too.
Dare you.

In other news, I have added a couple of Vout vs. Vin plots to the second post to show off some of the compression characteristics - in this case, the ratio control. It takes a wee while to measure one of these, so I will update the file as/when I am motivated and able.
It also seems that it might be worthwhile increasing the max threshold further. This is easily done by reducing the value of R8 and/or increasing R7, so expect a little more tinkering...

[chuckd666]

Subbed cuz I love the flatline and like tweaks. 

[duck_arse]

Light Speed Compressor is catchy and all, but if I build one I would have to name it "Threshold-ish."

mine can be called "the guilt absolver", he said himself.

[samhay]

[Granny Gremlin]

Hmmm, this or the Eng Thumb, that is the question.  ... Also been interested in a diode bridge based comp for the colour.

What's the attack /release switch do - does it actually affect both Attack and Release or is it just labelled that because some known pedals used to confuse those 2 (knob labelled Attack actually controled Release)?

[samhay]

>What's the attack /release switch do

Good question. Below is a simulation of the current throught the vactrol LED driven by a 100 Hz sine wave from 50-550 ms.

Top graph is with 100n (green), 1u (blue) and 10u (red) parallel to the LED - i.e. switched by the attack/release control - and a 100 mV peak signal.
Bottom graph is with 10u attack/release cap and the signal increased from 100 mV peak (red) to 300 mV (blue) and 1V (green).

Notice how the attack gets shorter with a stronger signal - i.e. the envelope detector is adaptive. I forgot to mention this is my earlier spiel, but it's a pretty neat feature.


(click for bigger version)

[Granny Gremlin]

If I am reading that right it appears to be a considerably interactive (with threshold/ratio) attack control. Thanks for that.

Really interesting how interactive all the controls are. 

[samhay]

The ratio sets the max gain of IC1A - anywhere from unity to about 21x. This feeds into the rectifier via the threshold pot, which can attenuate the signal by 0-90% (but this can be 100% if you jumper R8).
So, if you increase the ratio, you increase the signal going into the sidechain, which looks like the threshold is decreasing as you increase the ratio - thus threshold(ish). While it doesn't look perfect on paper, it works pretty well in practice and both controls are quite useful.

However, this circuit can be tweaked to work in a feed-forward topology where IC1B is fed dry signal. I tried this briefly and it will work, but will also need some optimisation. This is an option (and could be switchable) if you want complete independence of the controls.

[samhay]

Had another play with this in feed-forward topology - I moved the threshold pot to replace R4, and increased it to 1M. It works, but it is more prone to breathing - which makes sense for an open loop system - and probably has significantly less max compression.
I will be keeping mine as per the schematic in the OP, but have to finish something else before I start building it. Expect a vero layout at some stage in the coming weeks.

[287m]

ehm, my take for samhay design to perfboarder style who want onboard pots led and switch
just add hole in bottom (negative side) C2 if you want reduce up to 470n for guitar only
top side
https://1.bp.blogspot.com/-yFLuO9W9D78/VweoBmKwemI/AAAAAAAAAJ8/ns-6KtoFmnErkgH92Se-y262XHnm4lb-w/s1600/light-speed-compressor-layout.png
bottom side
https://3.bp.blogspot.com/-DqUMe8ypNTk/VweoBk1MlaI/AAAAAAAAAJ4/flfcE68EvZULVF4Zmpw4sF2F208wnPAJw/s1600/light-speed-compressor-etch.png
hope you all like this, jumper isn't crime. Right?