HomeStudioGuide.com - Guide to Home Studio Recording

This is just my opinion and I'm no means the final authority.... so take my opinions and verify them with your own experience.

With compressors, the sound characteristics doesn't come from whether it uses tubes or not. You see, the tube is mainly used as a gain stage. After compressing the signal, you have reduced level, so you need some amplifying device to boost the levels up again before coming out of the equipment. This is where vacuum tubes and solid states come in.

Granted, they can *color* the sound somewhat... but coloration has little to do with whether there are tubes or not... it's got more to do with audio transformers in the signal path. With modern solid state design, they've eliminated the audio transformers for COST reason. In my DIY, transformers add about $300 minimum in parts alone to a stereo channel. Add the cost of manual labor to solder the transformer wires (because you can't automate these process) and prices go even higher.

With TRUE vacuum tube designs, because of the high voltage present, they need some way of isolating the high voltage DC from the audio signal output, and so the need for audio transformers is a MUST... not an option. Transformers also provide impedance matching and signal isolation.

Note there are solid-state design that still use audio transformers, (example: SSL, API, Neve... ever heard of those names?) and they are excellent designs. Of course, excellent prices too.  

With compressors, the more important question is the method they use to detect, and achieve their compression. Some use optical methods to detect loud material, some use solid-state method. And then, that is further subdivided whether they used JFET, or transistors, or ICs to reduce the level of the material.

Optical compression have a soft-knee response. With optical compression, you have a light that glows brightly in sync with the program material, which affects an LDR (Light Dependent Resistor) that varies the signal coming out. The famous and most revered Urei LA-2A uses optical compression.. i.e. the JBL T4B optical unit alone costs about $150. An LA-2A costs about $2300 per channel. This unit produces sound that is nice and fat... almost every pro studio have one or more of these units!

BTW, the LA-2A uses audio transformers, vacuum tubes, and optical compression! Yay! ... no wonder it sounds great. And yes it is vintage design, if you open it up, you'll just see wires criss crossing around. No PCB.

The other way of doing compression is using an IC chip (commonly called a VCA). It's usually an all-in-one unit chip that does everything. For example, THAT corp makes these chips that are used by our most loved RNC compressor. Presonus ACP-8 ($900 for 8 channels) also uses the same IC chips from THAT Corp. The RNC is pretty transparent using it's Super Nice mode. $180 for stereo channel. These units use a VCA (Voltage Controlled Amplifier) to reduce levels of the signal. The IC chips are cheap... about $2.55 per piece. HINT: if you want to build your own, using the datasheet from THAT Corp, you could be saving a lot of money.

There is also the JFET compression used by the famous 1176 compressor. The JFET in this case is used as a variable resistor to vary the gain of the device. I've never heard of an 1176 (I'm currently building one), but most pro studios have them as well in their arsenal. Cost: $1800.

All 3 methods will produce different characteristic compression. You can't say X method is better than Y... They're all good, just depends on what will sound better on the material and track you're compressing and the effect/sound character you want to achieve.

A Limiter and Compressor belong to the same family of dynamics processors. You can say the Limiter and Compressor are close cousins.

If you don't know what a compressor is, or need a review of it's basics, click here.

A Limiter is just like a compressor that it prevents the signal from exceeding a threshold value. While you can use low to mid compression ratio with ordinary compressors, limiters work with very high compression ratio... like 20:1 or infinity:1. Think of it like a glass ceiling. You ain't going above the glass ceiling no matter how hard you jump up. That's what a Limiter does. It prevents the signal from going beyond that point. Just like a compressor, it also has an ATTACK time on how fast it should clamp down or limit that signal.

Very expensive limiters offer 100% guarantees that the signal will not exceed the set threshold you've set... These high-end type limiters are usualy used in the broadcast industry, PA systems and most importantly, in-ear monitoring systems. (You don't want that million dollar singer of yours suddenly go deaf because of feedback because some intern manning the FOH fell asleep.)

A good example of a limiter is the 1176LN by UREI/Universal Audio. You can also buy plugin versions of the 1176 via the UAD-1 package. Click here to search our gear database for limiters/compressors.



Related Articles
How to connect a compressor
The best compressors you can buy
Build your own 1176 Compressor/Limiter
Build your own SSL Buss Compressor

This is a repost of my SSL Build Thread. Thanks to Gyraf.dk for the schematic plans.

The compressor is a clone based from the SSL4044E desk (1985 era). The SSL mixbus is actually an integrated part of the desk's main quad fader and autofade system. Schematics and mods can be found below the article.

I also did some mods on my unit. Mainly, adding transformers at the output. Originally, the unit uses opamps for balanced outs. I have some MCI 416 output trafos that I used for my SSL unit.

The PCB...



Screenshot of my PCB stuffed with components. A few missing parts but almost there.

Just an update of my build: Click here if your want to read Part 2.

Soldered 90% of the parts, including the Lundahl transformers, JFETS and transistors. I'm awaiting some parts that were "out of stock" from my first order.

Also, I ordered some wire assemblies with male/female jacks. I'll use them for connections between the main board and xlr jacks, pots, switches, etc... that way, if I need to troubleshoot the board, I can just unhook them instead of desoldering the wire from the board.

TIP: Before you complete assembly of your project, have a suitable chassis ready for it. From my experience, if I built a project without finishing the chassis first, that project becomes half-finished, working but not in a case. Laziness I know...

Also, if everything is in a chassis, you can wire everything and not have to worry about your solder connections or wires becoming loose. Everything is already in place, plus it makes it easier to work on it.

You can get your rack chassis, from 1u, 2u, all the way to 4u and 8u at par-metal.com.  They've got good prices and have nice quality racks.

Looking for great compressors? I think these are some of the stand-outs among the pack.

dbx 160A - This is a classic, introduced way back in the 70s. Easy setup and accurate metering. This is an industry standard with the legendary "Overeasy" feature to transparently smooth out and maintain a constant level for vocals and instrument levels.

Empirical Labs Distressor - The distressor is usually always seen whenever you see photos of professional recording studios. It's unique look with those 4 big knobs is hard not to miss. This is a digitally controlled compressor, that switches different circuits in and out. So you can have programmable analog distortion and warmth. It can replicate the compression that occurs with tape using the Distort 3 mode. Side chain EQ, and eight unique compression curves, to the Nuke setting, that is awesome on drums. It also has an "opto" ratio which uses light controlled components similar to what the LA2A uses to achieve it's compression.


RNC RNC - Really Nice Compressor - This half-rack unit won't win any awards in the "looks good" category, but will blow you away in it's "sounds good" category and "right price" category. Don't let the plastic case and cheap knobs and small size fool yah! This is a high performing compressor that can deliver very transparent results.  It's got two modes.. regular and "SuperNice." Normal mode makes it behave like any other compressor, but Supernice mode gives it an almost invisible effect... i.e. there is compression going on, but you don't even notice it because it doesn't produce harsh artifacts. Great for compressing vocals, acoustic guitar, or the whole stereo mix.
RNC

Universal Audio LA-2A Teletronix limiter - A unique electro-optical attenuator system allows instantaneous gain reduction with no increase in harmonic distortion. It features 40 dB gain limiting, balanced stereo interconnection, and low noise – less than 70 dB below +10 dBm output. Controls are gain, peak reduction and meter selector, and connections are Jones Barrier terminals and XLR connectors.  This is classic... no respectable professional studio will not have it. Some  studios has 4 or 8 of these in their rack. It's that good.


More compressors for every budget and application can be found here.

How do you hook up or connect a hardware compressor? Compressors are used in SERIES with the signal you want to compress. Other signal processors that need to be connected in series are Limiters and Equalizers.

Option 1:

If you're using a stand-alone preamp, you would connect the compressor after the preamp. So it will be:

PREAMP OUT ----> COMPRESSOR IN
Then COMPRESSOR OUT ----> rest of your signal chain (maybe to a mixer or audio interface).

PROS: If your preamp is balanced, and your compressor also has balanced inputs and outputs, then your whole signal chain will be balanced. 

CONS: You'll need lots of cables. Especially if we're talking about a left and right channel setup or stereo mix compressor.

Option 2:

You can also use a hardware compressor with your mixer. If your mixer have INSERT jacks at the back, you can hook up your compressor via these jacks. This will save you some cabling and make your setup more neater. Cons : You'll have unbalanced connections from the INSERT jacks to the compressor, and from the compressor back to the mixer.

Is this a problem? Maybe not. We're just talking about a short run of a few feet here in a "controlled environment." So running unbalanced may not be an issue.

Below is a picture of the back of the mixer showing the CHANNEL INSERT where you can hookup a hardware compressor, EQ, limiter.



But you'd need a special type of cable.  Basically, you'd need an INSERT CABLE, something like this.




At first glance, the black plug (on the photo above) looks like a stereo jack. Nooooo... it's not a stereo jack. It's called a TRS jack. (TRS stands for TIP-RING-SLEEVE).

An INSERT cable is a special type of cable. Basically, the black TRS plug (from the photo above) is wired like this:



This single jack is carrying both the send signal (that will go to the compressor) and the return signal (from the compressor).

I repeat... it's not a stereo jack. It's not carrying 2 channels. There is no left or right channel here. This is carrying only a single channel.

At the other end of this TRS jack, you'd notice there are (2) TS (TIP-SLEEVE) jacks.  The White TS jack is labeled TIP.  The RED TS jack is labeled RING. 

The TIP TS jack (WHITE jack) goes to the input jacks of your compressor. 

The RING TS jack (RED jack) goes to the output jacks of your compressor.

HINT:  RED jack (think of the letter "R") is the RING jack... which is the RETURN path from the compressor

Make sure you don't swap the RIP and RING jacks when connecting to your compressor. Otherwise, you won't get any signal.

Compressors are one of the commonly used plugins when mixing/mastering. It's not hard to understand how they work... if you know what each button/knob does and how it affects the sound.

When compressors are used properly, the effect shouldn't be noticeable. ONLY when you compare the uncompressed and compressed signal should you notice the difference in the dynamics. Compressors are often used during tracking or mixdown. And during the mastering process, the whole stereo mix may be compressed and/or limited. 

The most common  controls in a compressor are the THRESHOLD, RATIO, ATTACK, RELEASE and MAKEUP-GAIN. Everything else is fluff... like input/output VU meters, or soft-knee/hard-knee, limiting on/off. 

Think of a compressor as an automatic variable volume control. The "volume" control's behavior is set by the attack, release, ratio knobs. This automatic volume control reduces the dynamics of any audio material that goes above the threshold. And this is where the paradox is. If compessors are used to reduce the volume/dynamics of music, how can it make the music sound loud? Well, the answer is in the MAKEUP-GAIN, where the average level of the music can be raised higher without clipping (chances for clipping are reduced because we just reduced the dynamics!).

So let's discuss the important parameters/control in a compressor...

1. THRESHOLD - This is measured in dB. It's a negative value because we're setting levels below the 0dB mark. Threshold (in dB) is that level at which the compressor should start reducing the output level. So if you set the threshold to -37dB (like in the picture above), signals below -37dB in levels remain unaffected.  Signals above -37dB (or in other words, louder than -37dB) will be reduced by the compressor. By how much should it be reduced? That's a function of the RATIO setting.

2. RATIO - So if a signal exceeds the THRESHOLD value, the compressor should start reducing it. Reduction is measured in ratio instead of a fixed numeric value. So let's say you set a RATIO of 2, any signal increase above -37dB in our example above will be reduced by half.  So 6dB above -37dB will be reduced to 3dB. A high ratio will reduce the the levels by a higher amount.

3. ATTACK - So we now understand that input levels above the THRESHOLD value will be reduced. This means, the compressor is always watching the input and detecting it's levels. The question now is, if a signal exceeds the THRESHOLD level, how fast or slow should we take action (of reducing it)?  This is a function of the ATTACK setting, measured in milliseconds. A fast attack (i.e. low millisecond) will mean the compressor will take action faster than a slow attack speed. Therefore, a slow attack (higher millisecond) setting means the signal that peak may be allowed to pass through. What does this mean? It means the dynamics of the signal may be allowed to pass through normally without being compressed... think of a snare hit on a drum. We don't want to kill the initial dynamics of the snare hit. 

4. RELEASE - Now, if a signal exceeds the THRESHOLD level, at some point in time it will go down below  the threshold level again. If a signal goes below the THRESHOLD level, at what point should we stop compressing? Should we do it as soon as it goes below the THRESHOLD, or a few milliseconds afterwards? This is a function of the RELEASE setting, measured in milliseconds.  A longer setting, i.e. a slow release means the compressor is still holding a "grip" on the signal, i.e. still reducing it. A short or fast setting means the compressor will let go of the signal. Long settings (i.e. a slow setting) produces a gradual less noticeable effect in the sound. Short release times are good for percussion instruments, and long settings are good for vocals. 

5. GAIN - This is also known as MAKE-UP GAIN. If you set a low THRESHOLD value, and a high RATIO amount, the overall signal will be reduced.  So we would want to amplify that signal and this is the function of the GAIN setting. It boosts the compressed signal to a sufficient level that we want.

Great, now that we know the different settings. But what can we use compressors for?

Compressors can be used to "even" out the volume by reducing the peaks in the signal (thanks to the RATIO control).  For example, your singer has bad mic technique and their volume varies a lot. We can use compressors to even out the levels so the vocal levels don't vary a lot.

Compressors can be used to avoid overloading/peaking during recording. (Of course, when used during recording, the change is permanent.)

By setting the appropriate ATTACK and RELEASE settings, we can add punch to our drums, adding impact to our beats.

And compressors are also used in mastering (sometimes too much) which adds punch and volume to the track.  But if a compressor is overused, you could also end up with a crunched sound... losing all your dynamics which makes for a "boring" listen. So don't overdo it. Sometimes, slight compression settings is all that is needed.

Sample Procedure for Compressing

1. First, set the bypass switch on the compressor. Listen to your levels... note how loud they are. Take a look at your VU meters and note the peak signal level.

2. Then take off the bypass (i.e. let the compressor affect the signal).

3. Set the ratio knobs... 2:1 or 4:1, depending on how much you want to reduce the levels

4. Set the attack and release time.  Let's use a fast attack time of 5ms and slow release time of 100 or 150ms.

5. Now, set the threshold level until the compressor is showing a gain reduction of 4, 6 or 8dB.

6. Now, use the make-up gain and set it so that we're showing the same peak signal as before compression (see step #1).

At this point, the sound should be punchier. It's like magic... they both have the same peak signal levels but the compressed signal has more weight and feels more solid.

If the sound is too compressed, and all the dynamics of the signal seemed lost, raise the threshold level (which means you have to lower the makeup gain), or increase the attack time (i.e. make it slower to react). 

The Bypass button is your best friend. Sometimes, subltle compression is all you need so make sure to hit the bypass on/off button often to hear the difference between no-compression and compression.

If you're using compression for the final mix on a stereo track, you may want to combine the compressed and non compressed signal.  This will make the music "thick" while still retaining the dynamics of the original music.


Typical Compression Settings

These are just typical settings, and don't make this the "rule." The exact setting will depend on the material you're compressing and the effect you want to achieve.

Delicate Vocals - We want to set the RATIO to a high value and high TRESHOLD. By using these settings, the softer vocal sections will remain uncompressed and only loud, ear shattering vocals will be compressed.

Threshold - set the threshold (i.e. lower it) so that the loudest vocal section is reduced by 6dB.
Ratio - Set to 6:1 reduction

Make sure threshold setting is not too extreme. We only want the loud vocal sections to be compressed by 6dB and quiet vocals remain uncompressed.

Pop Vocals/Commercials (yes, those annoying TV and radio commercials) - Okay, we want obvious compression here. So we want compression always going on for almost the entire material, and a very big reduction during loud masterial.

Threshold - set the treshold so the the softest vocal section is reduced a little bit. (-1dB or -2dB)
Ratio - Set to 2:1 reduction
Attack - set to fast setting
Release - set to a little bit slow
Make-up gain - increase gain to increase volume again.

At these settings, we're raising the average level of the whole material... i.e. making everything sound loud!

Drums - or any percussive instrument. We want a punchy, thick sound in the mix.

Threshold - set threshold so all drum materials are compressed by -3dB.
Ratio - Set to 4:1
If you have a hard/soft knee switch, switch to "soft knee"
Attack - set to a fast setting
Release - set to a mid setting

By lowering the threshold setting, you can make the drums more compressed. But be careful not to overcompress it too much or you'll lose the dynamics and punch.

Bass guitar/bassy synths - we want a fairly constant level for the bass material so we have a "thick" sound in the mix.  We don't want bass volume going loud and soft, we want it consistent.

Threshold - set so that only loud bass material (i.e. the peaks) are compressed.
Ratio - Set to 4:1
Attack - set to slow/mid
Release - set to slow/mid

Electric lead/rythm guitar - we want compression on these material. We want the guitars to be punchy and level out the volume.

Threshold - set to a low setting, to achieve constant compression
Ratio - Set to 6:!
Attack/Release - play with it