Anyone built the "Really Cheap Compressor"? How is it doing?

[PRR]

> Is that what the Flatline does?

Uh, almost. Yes it rectifies and filters, but all at POWER levels (LED power), so your options are very limited.

JFET limiters tend to have tweakable sidechains. (JFET gate power is nearly zero, so more tricks are practical.)

[Nick C.]

Biggest problem I find with Really Cheap Comp is that the attenuated gain is around unity. The LDR will fall below 220k and then the side chain will adjust. I added a series resistor to the LDR to give some boost. Then it needed more gain to adjust the ratio so I changed to smaller resistors. At that point I changed to a noninverting stage to raise the input impedance which gave it better clarity. No need for the treble mod. At that point it looks more like this one http://www.diystompboxes.com/smfforum/index.php?topic=89110.msg754675#msg754675.

I've been playing with the comp ratio with various ranges of resistors parallel and in series with the LDR. With some combinations for a lower ratio I get a clicking too long attack and with very high ratio I get a short blunt (nosey) attack. I plan to have ratio switching between a light comp to a heavy limit. As someone (PRR) said in another thread that the LDR will drop fast at first and then slower near the light end, so I can use that to my advantage to find some compromise attack.

[duck_arse]

Ben Lyman - seeing as me and kipper have been chasing non-existant threads of yours, have you done a demo of this compressor? and, what LDR (as usual) and what IC type did you use, please? I have one on the bb at the mo, finally found a suitable ldr.

[I also got a noise, maybe your kazoo (wahzoo?), but only on the open G string, and it sounds like something loose, or someone holding a bit of paper against the string. but only for a section of the decay. odd.]

[Ben Lyman]

Ya Duck, that's the kazoo noise alright. It went away when I boxed it up so probably just breadboard noise, nothing to worry about. When planning your layout, place C2 (my schematic) right next to IC pin4 if possible.

LDR is Tayda's Waitrony 1M dark and IC is a TL072

I drew it up like this so I could understand it better. PRR suggested a 10uF for my C9 instead of 1uF, so you might try that on the BB, see what happens on your low E string.

[duck_arse]

hrmm, I tracked the kazoo down, I thimk. it correxponds, if that's a real word, to the U2 (lyman dia) clipping. the BB I thort sounded good yester, sounds like carp today, just the loose tissue kazoo, all everywhere. I tacked a 470pF across R10//R11, and it seemed to help, but it's now off the BB.

I also thought I noticed that the compression indicator led (not shown lyman dia) was flashing brighter when the R12 (lyman, again) was increased from 1k to 1k5.

[blueband]

[Coming to this topic a bit late!] I was thinking of building this compressor but was first trying to understand how it works. The op-amps are connected in a standard "virtual ground" configuration so my understanding is that, with no input the outputs from them should float at half the supply voltage. When an AC audio signal is applied to the input, their outputs fluctuate about that mean 4.5 volts. If so, then the output of the second op-amp is always positive with respect to ground and so one of the leds should always be on and the other should never light at all. I'm obviously missing something....can anyone put me right please?

[duck_arse]

C9 blocks the DC standing at U2 outpin [Lyman's dia]. so the leds get no volts until some AC comes along, positive swing turns one led one, negatives turn the other on.

[blueband]

Many thanks for the reply. Yes that makes sense. It just seems very counter-intuitive that, with a single ended supply, you end up with the leds being driven by a voltage that swings above and below zero volts! I'm going to have to build the thing and convince myself.

[PRR]

> seems very counter-intuitive that, with a single ended supply, you end up with the leds being driven by a voltage that swings above and below zero volts!

You want the audio output of a box to swing both ways around zero volts.

In 9V-world we typically swing 1V to 8V, then cap-couple the output referenced to zero volts.

The LEDs (two for both-ways action) are driven the same way.

[snk]

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 

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 ?

3- Also, given that i used 25V capacitors, can it run @ 18V or should i better stick to 9V?

[duck_arse]

looking at the Lyman dia again [where is Ben these days?], C8 tailors the response of the led drivers. reduce that to push the response f higher.

add a superbright led and a CLR, say 10k, between C9 (-) and ground, it should follow at least half the envelope signal.

18V on 25V caps? knock yourself out, what could possibly go wrong?

[jjnicola]

Hey ! I build one of this RCC.... i have de problem that when the guitar signal fade out, and de gain in the compressor is higher, I start to liste  a noise.. like a fizz, hiss, i don't know how to describe it :sssgsggsggGGGSGGSGSG
Of course I don't here that when I play. But when I stop and the compressor makes it magic and increase de gain for tje lower signal amplitud, the noise app.

Is this normal?

[iainpunk]

could be, its expected behavior, you could add a noise gate, that takes out everything below a certain threshold, and hopefully the hiss as well.

cheers, Iain

[PRR]

> describe it :sssgsggsggGGGSGGSGSG

Turn it down. The idle gain should NOT be so high it screams in the quiet parts.