Compressor tech info

[jtn191]

Hey guys, after building an Engineer's Thumb, Orange Squeezer, and planning more I've been getting more interested in compressors. Being a recording engineer, I'm already really into them!

Compressor fanatics like Jon and ovnilab may be interested in this
So I've decided to compile a list of specs for well known circuits: attack time, release time, ratio (if calculable), threshold, freq response, noise at unity gain, input/output impedance, etc
Work in progress: https://docs.google.com/spreadsheets/d/1KDYdyKn-rG-0-RljN8W3Ug4FL56FLGDuN-30dHAZaPU/edit?usp=sharing

A compressor is a "dynamics" effect. It is a super-fast, sometimes subtle volume control that actually reduces the dynamic range of what is fed into it. By changing controls, you can change how exaggerated the attack of a note is and create a certain feeling of "evenness/thickness" that is great for country, funk, and really any genre.

BTW, for the less tech inclined, here's a guide to compressor parameters on electric guitar:
Attack: short = reduced note attack, duller. long = attack is exaggerated, sharper
*attack is actually how soon after a note starts that a note is attenuated.
Release: short = compression effect is exaggerated, "thicker". long= thinner, spiky sound
*release is actually how soon a compressor "lets go" and stops attenuating a note
Threshold: low = a lot of compression, very noticeable. high = only peaks are compressed, more transparent

Ben Harmless on tdpri:

First point: absolutely critical to the sound of a compressor on any given source is the attack time setting. This is doubly true on any source where the primary frequencies are low - such as bass. Remember that a key low-end frequency like 80hz is a vibration that occurs 80 times a second, meaning that you have a cycle, peak-to-peak, once every .0125 seconds. If you've set your compressor's attack time at anything faster than 12.5ms, (so that you can kill quick transients, for instance) and your threshold too low (an easy thing to do) you've just attacked the low end of your tone before it could develop. You lose low-end clarity and punch this way. I suspect many stompboxes are guilty of this. This factor is also why multiband compression is such a hot tool in music mastering.

Second point: Compressors are always most transparent when used lightly - even if you use more than one. If you've got a dual mono compressor like the DBX 166, run the second channel after the first, and ease up on the ratio settings for both channels. This will result in a gentler compressor effect, and you can often achieve a little more evenness before your tone becomes audibly squashed.

[lincolnic]

That's a cool document, looking forward to seeing more stuff on there! It's nice to have all that information in one place. Thanks for taking on this project, I'll be keeping an eye on it.

I'd like to respectfully disagree with your characterizing the LA-2A as "warm", though. To my ear, that guy does a slight mid scoop, usually accentuating the high frequencies a bit more - it's very easy to make a sibilant vocal much more sibilant if you're not careful.

[mattlee0037]

Can you explain release time a little bit more?

[jtn191]

I'd like to respectfully disagree with your characterizing the LA-2A as "warm", though. To my ear, that guy does a slight mid scoop, usually accentuating the high frequencies a bit more - it's very easy to make a sibilant vocal much more sibilant if you're not careful.

Yeah I actually got that from either a Sound on Sound interview with a bunch of famous producers or directly from the LA-2A manual. In my personal experience, I always thought 1176 for drums/bass for thickness and LA-2A for smoothness. But reading about the 1176's brightness made me shift those stereotypes. "Smooth" and "buttery" are better terms for the LA-2A but they really can apply to a lot of compressors/settings. What this document tries to prove is why some compressors are good for some things (mainly evidenced by a range of attack/release times), the quality of the design/build (via a noise figure), and other quantifiable characteristics of the amplifier circuit

Can you explain release time a little bit more?

Yeah I'll attempt to. The description above is a simplified musicians/guitar player description of how release times affect our sound based on my experience with the Engineer's Thumb and iirc a statement by Mark Hammer on DIYstomp. This page is pretty great reading material http://proguitarshop.com/andyscorner/the-types-of-compressors

This guy is an excellent player/teacher:

[midwayfair]

Attack time on the Bearhug is ~<5mS (it changes depending on the decay setting and depends on the output impedance of Q2, which is a couple hundred ohms if anyone wants to do the calculation). Decay time is 103.4mS in short and 573.4 in long. I honestly have no idea what the ratio is, but it's not extremely high, and it would only be a ballpark figure anyway. Input impedance is ~5M, and output impedance is ~100R-10K.

For the Demeter, look up the turn on and turn off time of the VTL5C10. Those are your attack and decay times. The ratio is based on the on:off resistance of the VTL to any voltage divider preceding it. You can calculate the ratio of pretty much any optical comp the same way.

Attack in the Lovesqueeze is also <10mS, and probably <5mS, again, depending on the output impedance of the op amp (a could hundred ohms?).

Edit:
Decay calculation is uF * Kohms = mS. 1uF * 100K = 10mS. If there's no obvious resistor in parallel with the hold cap, use the input impedance of the next stage.
Attack calculation is (R1||R2)*C (per PRR and Merlin)
Max ratio is always gain at idle/minimum gain.

[jtn191]

thanks! that saves some time from measuring
I've been sending sine waves through each pedal and measuring...it may be misleading if attack times are frequency dependent

[lincolnic]

Can you explain release time a little bit more?

It's like attack time, but instead of controlling how quickly the compressor starts to work on a signal, release time controls how quickly the compression stops. Imagine you're sitting at a console and you're trying to keep the level of a guitar track consistent, so you put your finger on the fader. When the track gets too loud, you pull the fader down. When it's too quiet, you push the fader back to where you started. How quickly you pull the fader down is your attack time. How quickly you push it back to where you started is your release. Does that make sense?

Yeah I actually got that from either a Sound on Sound interview with a bunch of famous producers or directly from the LA-2A manual. In my personal experience, I always thought 1176 for drums/bass for thickness and LA-2A for smoothness. But reading about the 1176's brightness made me shift those stereotypes. "Smooth" and "buttery" are better terms for the LA-2A but they really can apply to a lot of compressors/settings. What this document tries to prove is why some compressors are good for some things (mainly evidenced by a range of attack/release times), the quality of the design/build (via a noise figure), and other quantifiable characteristics of the amplifier circuit

See, I think of 1176s as making things slightly darker. In fact, at work I'll usually reach for a different compressor if I feel like the 1176 is making a track too dark (we have some dbx 165s that are usually a good fallback option). So, basically the opposite of what you've got listed. 

I just finished mixing a record where almost everything was too bright. 1176s (and some multiband compression) really helped balance things out along the frequency spectrum.

[jtn191]

I'm with you, but Jim Scott in the UA manual isn't.

[mattlee0037]

That makes perfect sense!

[lincolnic]

I'm with you, but Jim Scott in the UA manual isn't.

I'm just going by my personal experience! I'm not sure I've ever even seen a manual for the 1176, to be honest.

It's good to have conversations like this. I like using compressors for tone-shaping just as much as for actual compression, so it's something I think about fairly often.