Want to build a compressor -- a few questions?

[deadlyshart]

Hi guys, everyone here was so kind and helpful last time that I thought I'd try again.

I want to build a compressor, but to be honest I don't know a ton about them. I mostly want to use it to reduce the dynamic range for songs when I'm fingerpicking and the plucks on the different strings tend to be different volumes. Also, I think it would probably help when doing some slides where the volume quickly drops off, but I want the tails. I get the idea of a compressor but I've never actually used a physical one, just the plugin in Ableton, when editing some songs.

I had a few questions though:

First, how many controls do these DIY comps typically have? The comp in Ableton that I'm familiar with lets you control the ratio, attack, release, threshold, and dry/wet. However, on most of the ones I've peeked at (for example this BYOC one) it seems like they only have 2 controls... are these simple ones, or do people just usually not need much more?

Another thing I was wondering about was whether most of these comps have some sort of gain makeup... is that what the "volume" know is doing in the BYOC one?

Does anyone have any suggestions for a relatively simple comp to build? I guess I'm fairly flexible because I don't know exactly what I want, and if I don't like the sound I can always build another anyway 

I have that "Electronic Projects for Musicians" book and it has a Compressor project, which I've included the schematic/BoM for below. Does anyone know if this is a good one? It uses an opto-isolator, so I'm guessing that makes it an optical compressor. The thing is, I've already caught a couple glaring errors in this book, so I dunno if I should trust it anymore... It's also obviously somewhat old at this point. But, I do have all the parts already.

[bluebunny]

My £0.02: build Merlin's "Engineer's Thumb".  It's very good (and very quiet).  You can do the two-knob version, which is brilliant, or the five-knob version, which is brilliant but with more knobs.  And Merlin is paying me nothing at all to say this.      I like Jon Patton's Bearhug compressor too.  Hit "Search" and you'll quickly find both.

[EBK]

I'm really fond of Wayne Kirkwood's One Knob Squeezer, even though I don't think it would meet your criteria for a simple build (fairly high parts count and a tiny bit of surface mount soldering -- no pun intended).
http://www.diystompboxes.com/smfforum/index.php?topic=53097.msg946007#msg946007


One knob controls threshold, ratio, and make-up gain (hence the name, although there is a second knob for output level).  Attack and release are adaptive.  Nice, clean, soft knee compression.  If you have any interest, I can walk you through my unconventional vero layout for it:

http://www.diystompboxes.com/smfforum/index.php?topic=116115.msg1074065#msg1074065

[duck_arse]

the "really cheap compressor" could be considered an updated version of the one shown in Figure 5-49. another search term.

[stonerbox]

Sorry deadlyshart but I am hijacking your thread for a sec, I just got to ask the visemen (and women?) a  quick question. What type of LDR/Photocells (range/resistance) should I go for if I were to build an Engineers Thumb or maybe the LA-LIGHT compressor by Johan Blomdahl?

http://www.moosapotamus.net/images/LA-Light%20comp.gif

[deadlyshart]

My £0.02: build Merlin's "Engineer's Thumb".  It's very good (and very quiet).  You can do the two-knob version, which is brilliant, or the five-knob version, which is brilliant but with more knobs.  And Merlin is paying me nothing at all to say this.      I like Jon Patton's Bearhug compressor too.  Hit "Search" and you'll quickly find both.

Hi, thanks, I'll look into it. When you say two vs five knobs, are those extra knobs adding settings for stuff like attack/release? I'll check it out!

[deadlyshart]

I'm really fond of Wayne Kirkwood's One Knob Squeezer, even though I don't think it would meet your criteria for a simple build (fairly high parts count and a tiny bit of surface mount soldering -- no pun intended).
http://www.diystompboxes.com/smfforum/index.php?topic=53097.msg946007#msg946007


One knob controls threshold, ratio, and make-up gain (hence the name, although there is a second knob for output level).  Attack and release are adaptive.  Nice, clean, soft knee compression.  If you have any interest, I can walk you through my unconventional vero layout for it:

http://www.diystompboxes.com/smfforum/index.php?topic=116115.msg1074065#msg1074065

Thanks, I'll check it out and consider it. I don't mind a bit of SMT soldering. So do most comp pedals use "adaptive" attack and release, because you probably don't care that much in a concert?

[deadlyshart]

the "really cheap compressor" could be considered an updated version of the one shown in Figure 5-49. another search term.

Ah, I've seen that on this forum, and was considering it. People seem to say decent stuff about it right?

[Ben Lyman]

the "really cheap compressor" could be considered an updated version of the one shown in Figure 5-49. another search term.

Ah, I've seen that on this forum, and was considering it. People seem to say decent stuff about it right?

Yes! so simple it's stupid! and it does a great job, gets super squishy if you want that kinda sound

[midwayfair]

First, how many controls do these DIY comps typically have?

Two controls is most common. Usually there's a way to add a third control. Some designs, especially some optical ones, literally can't have some controls just added because the circuit as designed won't accommodate them. The Flatline is like that -- there's no good way to add an attack or decay control. Other compressors can have every control. Even then there might be malfunctions just by adding things, and no one likes dead spots on their controls, right?

are these simple ones, or do people just usually not need much more?

Honestly people usually don't need as many controls as they think they do. I leave my compressor on all the time on my pedalboard -- it's actually kind of silly for me to even have a bypass switch and volume control on it. There's a volume knob on my guitar that can be my threshold/input gain control. Maybe I don't need any knobs at all!

Of course, it's not usually fun to not have any knobs to tweak, and we DIY because playing with stuff is fun. So practicality isn't always a concern.

But yes, they are ALSO simple ones. In fact, some project design philosophies revolve around creating as simple a design as possible.

What type of LDR/Photocells (range/resistance) should I go for if I were to build an Engineers Thumb

That's not optical.

If you ARE going to build an optical compressor, if you aren't experienced with LDR-based projects, use whatever the designer suggests. If the designer doesn't suggest something, you could either find a similar project that does suggest something (Madbean's Afterlife, say) or figure out how to measure some LDRs (hint: datasheets) and work out how they might be working in an individual project. You'll learn a lot more that way than simply asking for a part suggestion from people who have already spent time learning it.

Anyway, compression is an envelope effect. It responds dynamically to the player. These types of effects are extremely personal. The reason not everyone gets along with any given effect is that not only is their equipment different, but their fingers are different. There are enough small, simple projects that you could breadboard several at the same time to try them out. Along the way you'll learn more about the compression circuits and your own tastes.

to be honest I don't know a ton about them

Here's a "quick" breakdown on compression (edited from something I wrote to a friend a while ago):

0. A compressor's purpose is to average the sound. Coincidentally perhaps, compressors work via averaging. Ultimately the best setting for a compressor is always going to be at a fulcrum point for the average signal strength and frequency. It's nearly impossible to do this live, so we generally have to muddle through with a good enough setting on a compressor.

1. Threshold: This is the level which when the signal exceeds it the compression will begin working. It is not a hard cutoff -- in other words, it won't be that only sounds louder than the threshold will be compressed. (There is a type of compressor that does that: A mastering limiter.) The easiest way to think of this control is that if you make the threshold lower, it will be easier for the compressor to hear your signal and get to work. Something with a high threshold is only going to trigger on extremely loud sounds. A very low threshold is for when you want to slam the sound. (Works great on low frequency stuff that isn't really loud to really make it pop.)

2. Gain: This is the amount of the compressed signal allowed into the compressor in the first place. If your signal source is weak, you can turn this up. This is before the compression has happened. The other reason these are in there is if the signal source is really strong and is distorting the compressor. Our guitar knob is a great gain control ... a lot of people forget that it's an extra control for the compressor if you have your compressor first in line!

3. Output level: Some compressors have multiple output levels, and it's sometimes called make-up gain, but this is just what it sounds like. It's a volume control after the compression has happened. Pretty much universal on pedals.

4. Attack: This is the time in milliseconds that it takes for the compressor to start working when a signal is loud enough to cross the threshold. 0ms is immediate, but only truly possible in digital circuitry in post production. (Which is absolutely awesome for limiters.) Lots of people are fans of this control for different reasons. Unfortunately, not every pedal compressor can have a meaningful attack control. The attack (and decay, below) have all sorts of implications for how things sound when they come out of the compressor because NOT ALL FREQUENCIES MOVE AT THE SAME RATE. (Hence ... "frequency.")

Without getting too deep into the physics of sound, which would make this book-length, in an analog circuit, the guitar doesn't produce a fundamental note much above 1000Hz, which is a 1 millisecond cycle. An analog circuit can't possibly start compressing before it detects the signal, which would take it 1mS at the absolute fastest. Big, fat bass notes like the low E down at 68Hz take 14mS to cycle. If the attack is set closer to 1mS, the compressor will turn on very quickly, stopping the really fast frequencies, resulting in less treble. A shorter attack means a smoother or darker sound. If you set the attack closer to 14mS or even slower (like 20mS ... 40mS ... 100mS even; keep in mind that a note isn't JUST one wave cycle like ever), all the treble will traipse right through and not get attenuated, while the bass notes turn it on a little later and get compressed. A long attack means a brighter sound; it's not quite as simply as just matching the attack to the frequency of some prominent notes, but about 10mS is usually a good setting. Obviously this is a balancing act, and this interacts with other settings on the compressor, but if you get out your DAW and a clean track on the computer, you can play with the attack in a plugin to REALLY to hear what it's doing.

5. Decay/Release: This is the time in milliseconds it takes for the compressor to STOP working once a sound has been loud enough to cross the threshold AND the sound was sustained enough to surpass the attack setting. A short decay will make the compressor recover very quickly, but keep in mind that doing this can make it sound like the sound is pumping. Instruments are not absolutely steady sources, even when they sound like they are. They get louder and quieter in small amounts. You want to avoid a situation where the compressor keeps turning on and off in quick succession. For a studio compressor or a digital compressor in a DAW in post, you can set it to something around 20-100mS and have no problems. Live, you want it longer; 100mS is actually quite short in that situation, and some common guitar compressors go all the way up to 2 seconds (my Bearhug as I use it is usually set to 120mS but has a 500mS setting; its attack is ~8mS if you're curious).

Tonally, longer decays make the bass more prominent. You can think of the attack and release being a see-saw between the bass and treble.

From a physics standpoint, note that you want the release to be kind of long because you don't really want the compressor to have to start working again every single time a note is plucked, and to compress for a while as the note dies out. You want it to sound smooth. We have often have a good hundred milliseconds between notes on the guitar, right? You want it to average the signal some, to make it sound smoother and more present.

6. Ratio: This is literally the ratio between decibels in and decibels out. If the compressor is actively working, it will reduce the entire signal by that ratio until the signal hit the threshold. In other words, the compressor is trying to drive the signal below the threshold so it can be lazy and stop working. 2:1 is really mild. 3:1 or 4:1 are pretty common settings for vocals and instruments in post. 6:1 or 8:1 would be really heavy for vocals but not for some guitar sounds, and for live sound that's still pretty reasonable. 10:1 is really heavy; 20:1 is considered brick wall limiting; at that stage you can think of the ratio being nearly infinite.

Some digital compressors in DAWs actually do have an infinite ratio. They also use 'lookahead' to literally read the signal a few milliseconds in the future so that the attack is irrelevant, and thus they can ensure that the signal will absolutely never exceed 0dB and thus won't distort. This is the most common form of mastering limiter these days.

There are some pedals that have a true ratio control. Usually it's just a gain control for the main circuit, which isn't exactly the same thing. A true ratio control wouldn't necessarily give you more control anyway.

7. Frequency specific compression: Deessers. These are the same as normal compressors except that they're made to detect only signals centered around a (usually very high) frequency and then suppress only that frequency. They're used to suppress sibilance in vocals, but they have other uses. I love using them in post on distorted guitar sounds around 2K to suppress 3rd order harmonics.

8. Sidechaining: This is where you use a separate signal source to compress another instrument. This has some cool uses, like using the kick to trigger really heavy compression on the bass, and getting a swell effect on the bass, or using the vocals to compress the guitars to make the vocals automatically louder than the backing instruments. (In case you've ever wondered how they do that.) Some side chains also allow you to limit the frequencies detected or affected.

9. Parallel/blended compression: Mixing the clean signal with the compressed signal can help retain high frequencies without relying on the attack setting. The volume peaks won't be smoothed out, but it can make it sound like the sound sustains longer. In post, I almost always blend anywhere from 10-30% of the original signal in with the compressed signal, to keep it from just sounding squashed.

[EBK]

I leave my compressor on all the time on my pedalboard -- it's actually kind of silly for me to even have a bypass switch . . . on it.

I had this thought today about my compressor as well!   
(I definitely do need the knobs on mine though).

[deadlyshart]

the "really cheap compressor" could be considered an updated version of the one shown in Figure 5-49. another search term.

Ah, I've seen that on this forum, and was considering it. People seem to say decent stuff about it right?

Yes! so simple it's stupid! and it does a great job, gets super squishy if you want that kinda sound

Haha... sorry, I'm new to this, what do people mean by "squishy"? Also, I've seen the word "transparent" get used a lot with respect to comps...what does that mean??

[EBK]

Haha... sorry, I'm new to this, what do people mean by "squishy"? Also, I've seen the word "transparent" get used a lot with respect to comps...what does that mean??

"Transparent" means (I believe) that the output is mostly free of artifacts that would make the effect noticeable to a listener.
"Squishy" is the opposite.
Some compressors are renowned for their characteristic sound.  The Orange Squeezer comes to mind.

[stonerbox]

That's not optical.

If you ARE going to build an optical compressor, if you aren't experienced with LDR-based projects, use whatever the designer suggests. If the designer doesn't suggest something, you could either find a similar project that does suggest something (Madbean's Afterlife, say) or figure out how to measure some LDRs (hint: datasheets) and work out how they might be working in an individual project. You'll learn a lot more that way than simply asking for a part suggestion from people who have already spent time learning it.

That is right the Engineers thumb is not optical. The LA-LIGHT is though. Oddly enough I haven't found any info on what kind of LDR people use in their builds. I am about to place an order so that is why I ask.

[deadlyshart]

Hey guys, so I think I have all the materials right now to make the "really cheap compressor" and people seem to speak well of it, so I'll probably make it, and then maybe make a better one later at some point.

One question I have though. The schematic here: http://www.diystompboxes.com/smfforum/index.php?topic=110831.0 has the LDR just labeled as "1M dark". I have a few, and one is ~1M when it has basically no light (inside a can in a dark room), and it's about 2kOhm just pointed at the ceiling fluorescent lights. Should that be okay? I'm guessing it can't be *too* sensitive to the value, right? I say this because it's essentially pointing at the LEDs in an inexact way, and there's also the 100kOhm pot that adjusts it anyway. Am I correct in thinking that?

What's a good way to diagnose that my LED/LDR setup is good?

[midwayfair]

What's a good way to diagnose that my LED/LDR setup is good?

Your ears and a breadboard.

What's the ldr doing in the circuit? What happens when it's at 2k or 1m? Review op amp gain stages if you need a refresher.

[deadlyshart]

What's a good way to diagnose that my LED/LDR setup is good?

Your ears and a breadboard.

What's the ldr doing in the circuit? What happens when it's at 2k or 1m? Review op amp gain stages if you need a refresher.

Haha, of course. I forgot to thank you for the incredibly in depth overview of compressors, thank you very much! I'll finish reading it tonight and I'm sure I'll think of a few questions...

[EBK]

I'm really fond of Wayne Kirkwood's One Knob Squeezer, even though I don't think it would meet your criteria for a simple build (fairly high parts count and a tiny bit of surface mount soldering -- no pun intended).
http://www.diystompboxes.com/smfforum/index.php?topic=53097.msg946007#msg946007


One knob controls threshold, ratio, and make-up gain (hence the name, although there is a second knob for output level).  Attack and release are adaptive.  Nice, clean, soft knee compression.  If you have any interest, I can walk you through my unconventional vero layout for it:

http://www.diystompboxes.com/smfforum/index.php?topic=116115.msg1074065#msg1074065

Thanks, I'll check it out and consider it. I don't mind a bit of SMT soldering. So do most comp pedals use "adaptive" attack and release, because you probably don't care that much in a concert?

I was going to sneak this cleaned up layout in with a quiet edit in case anyone comes back to this thread, but I guess I waited too long to modify my post.  Oh well.