Flatline Compressor Question

kinski · Post by **kinski** » 13 Apr 2012, 02:36

So I built the Flatline Compressor. It sounds quite nice, however its quite noisy (lots of hiss) and the sustain has a strange distortion, especially in higher settings.

Is this just part of the design? Is there anything I can do to improve this? I used THREE 1n270 and ONE 1n4148 as that is what I had on me. And a VTL5C3.

How does Mictesters Really Cheap Compressor compare to the Flatline? Would it be cleaner and less noisy?

mictester · Post by **mictester** » 13 Apr 2012, 04:45

The answer is "it depends"! The "Really Cheap" has the advantage of not needing to use Germanium diodes (making it cheaper!). I've found that there can be distortion introduced by some LDRs - they do a little bit of rectification (it's as if there are are diodes across the resistor in there). Some are worse than others - the RPY58A is worse than an ORP12 for example - and it sometimes varies just by sample of device. It can be ameliorated (and the hiss reduced somewhat) by putting a small value ceramic cap across the LDR - I use any value from 22p to 220p - experiment to see what works best for you.

Also - what op-amp did you use? If you use a 4558, you'll find it's quite noisy. If you use a TL072 or a 5532 you'll find it hisses much less.

kinski · Post by **kinski** » 13 Apr 2012, 04:56

Hey thanks! I used a TL072.

I'll try the cap on the LDR. I used a VTL5C3, so for this, would I just put the cap leads next to the LRD leads of the VTL5C3?

Syder · Post by **Syder** » 13 Apr 2012, 12:05

Put that cap across pins 1 and 2 of the first opamp.
For the distorted decay, just change the electro cap for a bigger value. For some LDR/Led combinations I've gone as far as 220uF for that cap. You can use a switch for this, as when you use the compressor for picking style instead of strumming you won't need such a long decay and may like to use a smaller cap there, say 100uF or 47uF.

kinski · Post by **kinski** » 13 Apr 2012, 15:34

Okay, so the Pedal Fairy must have tweaked some things last night while I was asleep. As of today, no more distortion in the sounds. Pretty funny. I'm not crazy, it just went away.

BUT, the hiss is still there. I must admit, I'm confused. Do I put the low pf cap across the LDR or on Pins 1 & 2 of the TL072?

Syder · Post by **Syder** » 13 Apr 2012, 19:06

kinski wrote:Okay, so the Pedal Fairy must have tweaked some things last night while I was asleep. As of today, no more distortion in the sounds. Pretty funny. I'm not crazy, it just went away.

BUT, the hiss is still there. I must admit, I'm confused. Do I put the low pf cap across the LDR or on Pins 1 & 2 of the TL072?

The LDR either is connected across pins 1 and 2 or pins 6 and 7 of the TL072 depending on the PCB layout you're using. Please check the schematic to understand what are we talking about. The cap will cut some of the hiss (and high frequencies) at the feedback of the first opamp.

kinski · Post by **kinski** » 13 Apr 2012, 19:46

Ah, I see. I tried a few out and 470pf seems to do the trick. Still some white noise, but MUCH better. Thanks!

mictester · Post by **mictester** » 13 Apr 2012, 22:53

kinski wrote:Ah, I see. I tried a few out and 470pf seems to do the trick. Still some white noise, but MUCH better. Thanks!

That's a fairly crude way of reducing the worst of the hiss introduced by the high gain of the op-amp. Adding an extra capacitor across the 220k input series resistor, and adding a capacitor across the output level pot will give pre-emphasis on the way into the circuit, and de-emphasis on the way out, thereby maintaining an overall flat frequency response:

: Just three parts for a useful improvement

The additional output resistor just prevents the risk subsequent low input impedance circuits loading down the output of the compressor.

kinski · Post by **kinski** » 14 Apr 2012, 17:31

Very cool. I will add that extra cap to the Vol pot, and see what happens. As for the extra resistor, I can just add that between the output wire and the 3PDT, right?

mictester · Post by **mictester** » 14 Apr 2012, 17:47

kinski wrote:Very cool. I will add that extra cap to the Vol pot, and see what happens. As for the extra resistor, I can just add that between the output wire and the 3PDT, right?

Yes you can.

Don't forget the extra capacitor across the 220k input resistor, too. If you miss it out, the pedal will sound rather muffled.

kinski · Post by **kinski** » 14 Apr 2012, 21:30

Ah crud, clearly I don't know what I'm looking at.

I used the layout below (Thanks HSB!)

I placed the 470pf cap from the earlier discussion across the LDR (specifically between R5 and R7 at a10 and b10. Would that not then already be across the 220k resistor (R5) as well?

kinski · Post by **kinski** » 14 Apr 2012, 21:31

Oops, forgot the layout. Here it is:

kinski · Post by **kinski** » 16 Apr 2012, 18:25

So I added the extra 10n cap, and it certainly does muffle the sound. This is with the 470pf cap as well.

Mbas974 · Post by **Mbas974** » 05 Jul 2013, 14:58

D2 - D5 are not present from schematic, why used in veroboard ?

marshmellow · Post by **marshmellow** » 06 Jul 2013, 15:57

The schematic in mictester's post isn't the Flatline Compressor by John Hollis. HERE is the schematic.

snk · Post by **snk** » 19 Oct 2018, 23:26

Hello,

I successfully built the Flatline using the (and a ready-made vactrol, as advised on the Madbean Afterline instructions).
I added a sensitivity (which is really great imho), and an attack knob.
Overall, it is a nice compressor, quite transparent and versatile, great for invisible compression.

I would have 3 questions :

1- Is there any component/value to change to get a HPF sidechain in the compressor path ?
I would like to remove the bass (say around 100-150Hz) from triggering the compressor circuit, without removing them from the "audible sound". I am thinking about a switch with the stock value (which is great) on one side, and a "HPF sidechain" on the other side of the switch... but i don't know which component value i should change to reach this effect :icon_confused:

2- I have also tried adding an indicator led by plugging it into A3 and C3 (from the Guitar FX Layout veroboard) : the led blinks when the compression is stronger, so it gives a rough estimation of the compression occuring, but i noticed that it also some very nasty crackles as soon as i don't have the sustain set up to the maximum position.
Is there any way to keep an indicator led, but not having it modifying the processed sound ?

marshmellow · Post by **marshmellow** » 20 Oct 2018, 07:21

1. You can put a capacitor in front of the 10k in the sidechain, try 47nF or 100nF.
2. I'm not gonna look up that layout, but I would try a LED in parallel to the other one. It should have roughly the same forward voltage. Also put a small resistor in series with both of them.

snk · Post by **snk** » 20 Oct 2018, 13:05

1. You can put a capacitor in front of the 10k in the sidechain, try 47nF or 100nF.

Thank you. I wil try.
But which is the sidechain part in the schematic ?
R4, R5, R8 and R9 are all 10K...

2. I'm not gonna look up that layout, but I would try a LED in parallel to the other one. It should have roughly the same forward voltage. Also put a small resistor in series with both of them.

Ok, thank you for the tip.

marshmellow · Post by **marshmellow** » 21 Oct 2018, 11:44

Well, take a wild guess

. There are two that don't make any sense at all, one which might but really doesn't either, and one that is connected to other parts that have something to do with driving the LED, which determines the compression.

R8/R9 are for DC biasing. R4 is part of the feedback network setting the gain of the audio amplifier. R5 is the input of the amplifier that drives the LED, so take that one.

snk · Post by **snk** » 28 Oct 2018, 18:39

Great ! Thank you, Marshmellow