Really Cheap Compressor

Jamdog · Post by **Jamdog** » 07 Apr 2016, 14:35

Right now, I think I hear a difference on the attack, when the sustain knob is completely on one side, but there's no apparent change in sustain.

Observing the leds, it light up only briefly at the beginning of a note when it is strong enough, doesn't stay lit for the whole note duration whatsoever. No matter the circumstances.

If I turn the sustain knob the other way, no led light and no effect.

I think the leds are not appropriate?

induction · Post by **induction** » 07 Apr 2016, 15:50

Jamdog wrote:Right now, I think I hear a difference on the attack, when the sustain knob is completely on one side, but there's no apparent change in sustain.

Observing the leds, it light up only briefly at the beginning of a note when it is strong enough, doesn't stay lit for the whole note duration whatsoever. No matter the circumstances.

If I turn the sustain knob the other way, no led light and no effect.

I think the leds are not appropriate?

It sounds like the leds are behaving appropriately. They will not stay lit for the whole note duration; they aren't supposed to. Compression works by attenuating the loud part of the note (ie. the beginning of the note) and leaving the rest alone. This results in a more consistent volume throughout the course of the note. If you increase the output volume to compensate for the attenuation, this is equivalent to increased sustain.

If you are unfamiliar with the sonic effects of compression, it may take a little while to start noticing it. It's not massively audible to a listener, it's more like a difference in the response of the instrument to your playing. That said, this effect is not especially subtle (to the player) at high compression settings. There should be quite a bit of squish available. When you pick harder the volume won't increase, though the tone may change somewhat.

It's hard to say over the internet whether your unit needs to be adjusted (different leds/ldr/gain, wrong component values, etc.) or if you just need to become acclimated to the effect before you can use it to your advantage. Maybe play with it for a while at a very high compression setting, and then turn it off and see how the touch response of your rig changes.

Edit: By the way, are you playing clean or distorted? If distorted, does the distortion come from a pedal, or the amp? If a pedal, is the distortion before or after the compressor? Compressors are more obvious on clean guitar (distortion inherently compresses all by itself), though also useful with distortion. I also find compression more obvious to the player if it comes before the distortion.

Jamdog · Post by **Jamdog** » 07 Apr 2016, 16:49

I did try distortion (from an amp, haven't build my distortion pedal yet

) , but mainly tested under clean.

The main impact I can notice is when strumming strongly open chords, the output is not as loud as it would be without the circuit. However, playing with the compression knob won't alter sustain.

All in all, if it's the desired effect, it's interesting and subtle, which I like. Thanks for helping out!

induction · Post by **induction** » 07 Apr 2016, 17:07

Cool. Glad it's working. Try AB'ing the compressor (on and off) while playing single note lines with no distortion and varying your picking intensity. That should give you some sense of how it changes the touch sensitivity, and what is meant by 'increased sustain' when it comes to compressors.

Also, most distortion circuits (amp or pedal) are sensitive to the input volume (louder = more distorted), so you can set the compression fairly high and use the volume control to dial in the sweet spot that gives the distortion tone you like best. Again, this is most effective for single-note lines and solos.

For chords (clean or distorted), I strongly prefer light compression, if any. But that might be just me.

Jamdog · Post by **Jamdog** » 07 Apr 2016, 19:45

I'll need a next project, now...

Jamdog · Post by **Jamdog** » 08 Apr 2016, 01:19

Now that it's in its final enclosure and I can actually play with it, it's more fun.

The effect kicks in mainly when the guitar is at higher volume, which I can lower from the pedal.

Here's what it looks like

sorry about the picture size. I apparently don't know how to post pictures

It is said that this compressor is very quiet

Its so quiet, that it eliminates the buzz that one of my patch cable creates. No jokes. I hear it in bypass, but its silent as can be when the compressor is on!

Gobi · Post by **Gobi** » 12 Apr 2016, 16:41

Myne is also finished now:

It also includes a peak meter witch i alwas wanted to have, for the show of course, but also fot experimenting , to make sure to have the right level

Silver Blues · Post by **Silver Blues** » 13 Apr 2016, 19:42

That's pretty damn cool dude.

Rolbista · Post by **Rolbista** » 17 Apr 2016, 12:15

do you guys use the volume knob on this pedal very often? I was hoping to built it with an onboard trimmer instead and make it as a single knob 1590 box.

reneshelle · Post by **reneshelle** » 17 Apr 2016, 16:08

I use the volume knob often. I think that it boils down to how you use your gear.

If you use different setups (amps and pedals) after the compressor you'll want the volume knob. Also if you change guitars and want to keep volume at "unity" then you'll need the volume knob.

If you on the other hand always use the same gear, then it is no problem to have to pot on the inside. But then again, if you do that, why have the other knob on the outside?

If I was to have only one knob I would go for volume on the outside. But that is just my opinion...

Silver Blues · Post by **Silver Blues** » 17 Apr 2016, 21:46

Yeah, I too use the volume quite a lot. This circuit is capable of putting some boost in as well, and you can't exploit that without the volume knob. Both knobs are necessary for dialing in the exact tone you want with any given setup, unless you just use the same setup every time as has been said.

Jamdog · Post by **Jamdog** » 28 Apr 2016, 02:05

After having the circuit for a while, I still can't say for sure if it does compress. I know the Ldr and leds work, but strumming with the sustain high send low seems identical. The pedal is quiet otherwise and acts nicely as a buffer.

reneshelle · Post by **reneshelle** » 28 Apr 2016, 20:19

Try a sustained single note instead of strumming. The effect should be more noticeable that way. Also try to compare a full chord vs. a single note. The volume should be similar with the compressor on. With the compressor off the chord should be quite a bit louder than single note. Tests should be done on a clean amp.

Plenty of videos on youtube to show you how a compressor works. Compare with that...

mictester · Post by **mictester** » 28 Apr 2016, 20:59

induction wrote:Try AB'ing the compressor (on and off) while playing single note lines with no distortion and varying your picking intensity. That should give you some sense of how it changes the touch sensitivity, and what is meant by 'increased sustain' when it comes to compressors.

I find that compression just makes the guitar "livelier" in that it's more sensitive, and every little sound from the instrument is amplified. With the compression really turned up, hitting a big chord will cause the volume to dip at first, then swell as the chord decays.... The simple "Really Cheap Compressor" was the simplest circuit that gave the effect that I wanted. With judicious setting of the controls, its operation isn't obvious, but the change in the behaviour of the guitar when it's bypassed is amazing - it's not obvious when it's on, but you really miss it when it's off!

I like the optical approach to guitar compressors, because the response lag of the light-dependent resistor means that some of the attack of the guitar note gets through before the gain changes. That means that if it's adjusted correctly, the clean guitar sound will still seem "natural", but it really goes a long way to evening out the inaccuracies in picking!

I've worked on and completely designed several other guitar compressors - some using FETs for the gain control, others using transconductance amplifier ICs, and even one using a diode attenuator - but I keep coming back to this simplest of all circuits. I picked a few of the values with care - the input capacitor is chosen to allow the lowest guitar notes through, but to filter out the low frequency "handling noise". I added pre- and de-emphasis to some of the ones that I built to reduce the HF noise from the op-amp, but the improvement was marginal.

Jamdog · Post by **Jamdog** » 29 Apr 2016, 02:37

Well, the Ldr must not give out appropriate resistance because it does not compress. I am pretty sure the leds are behaving correctly. I also know full light exposure of the Ldr does mute the volume. But strumming, long sustained note, anything remains pretty similar with or without the compressor. It acts currently as a buffer, or a preamp. Does a great job at it, but dies not compress.

I am thinking the 2M2R would need adjustment, a lower value do the Ldr ends up kicking more in. The design is very well made, thanks mictester for it and putting it public. But I think not all ldr/led pair are created equal, and mine does not work as is right now.

induction · Post by **induction** » 29 Apr 2016, 13:11

Jamdog: Here's how I would test it: Crank up the compression control and turn your guitar volume knob all the way up. Repeatedly play a chord while switching the pedal on and off, and adjust the pedal's volume knob until the bypass volume is approximately equal to the effected volume.

Then turn your guitar volume knob down to 2, and repeat the same chord while switching the pedal on and off. You should get much more volume with the pedal on. It should be very, very noticeable. If you don't notice a difference between pedal on and off, then your build is not working. In that case, post gut shots and voltages.

Rolbista: I adjust the volume knob every time I adjust the compression knob. I would find it much less useable without the volume knob, unless I never changed the compression setting either. In that case, you could make them both trimmers. I would recommend either making both pots external, or neither of them.

Gobi · Post by **Gobi** » 29 Apr 2016, 20:00

Jamdog · Post by **Jamdog** » 30 Apr 2016, 14:12

induction wrote:Jamdog: Here's how I would test it: Crank up the compression control and turn your guitar volume knob all the way up. Repeatedly play a chord while switching the pedal on and off, and adjust the pedal's volume knob until the bypass volume is approximately equal to the effected volume.

Then turn your guitar volume knob down to 2, and repeat the same chord while switching the pedal on and off. You should get much more volume with the pedal on. It should be very, very noticeable. If you don't notice a difference between pedal on and off, then your build is not working. In that case, post gut shots and voltages.

Theres no noticeable difference.

I know the Ldr does work. When I was servicing it in the original debug phase ambient light would mute the guitar.
I also know the leds works, we see them blink with the chords, mainly with sustain completely clockwise.

What I can only assume is that the leds don't make a big enough change in resistance on the Ldr.

I am using a pair of rectangular green leds, the only set of leds I have that does light up there so far, I guess I'll shop for other leds...

(thanks for the assistance in proving what I was suspecting)

BTW, the circuit acts nicely as a buffer, and gives a bit of brightness to my strings, even without the compression the circuit sounds very nice!

Jamdog · Post by **Jamdog** » 30 Apr 2016, 15:34

I got a pair of 3mm green leds in place of the rectangular one, and there's a huge difference now. I say don't use rectangular leds, they won't do it.

Thanks again for the help guys, now I'll experience what this compressor really do!
So far I like what it does!

Thanks again mictester for this, as it's my first effects pedal and a great learning experience. And it sounds great. Awesome. Merci!

Rolbista · Post by **Rolbista** » 05 May 2016, 20:37

I just thought about it but someone having trouble hearing if the thing works or not might find it useful - if you want to test the circuit and see of the leds light up, just use a function generator (there are many free apps for android phones) and feed the waveform into the compressor. constant light should be easier to spot than fast guitar peaks.

Really Cheap Compressor [documentation]