Compressor, theory of operation

[guitarrock04]

I'm somewhat new to this, so please bare with me...

I understand the basic theory of operation of most tube amplifiers, triodes, pentodes, phase inverters, cathode bypass components, tone stacks, etc. But most importantly, I'm getting a pretty good feel for what changes particular component modifications will impart on the sound.

So now that I'm dealing with some compressors, by understanding is limited. I understand the basic concept of VCAs, vari-mus, and LDR based equipment. But I'm not familiar with the controls and components in a compressor. If I'm looking at a schematic and see variable resistors that aren't labeled, I can't always sort out what it is they do.

Is there some basic description or modification tutorial that someone could point me towards? I'm looking for something straightforward like: this is the coupling cap, increasing the value will do this, decreasing will do that. Can anyone help?

 

[amorris]

In a Mike Myers "coffee talk" voice:

feed forward,
feed back,
Vari-mu,....

discuss......

 

[guitarrock04]

I'm getting verklempt!


But really, any help is appreciated.

 

[JohnRoberts]

Compressors squeeze dynamic range (difference between quiet and loud) ratiometrically.

A compression ratio of 2:1 means a 2 dB change at the input causes a 1 dB change at the output.

In addition to the ratio adjustment there will also be a threshold adjustment, so below threshold there is a 1:1 ratio, above threshold N:1.

There are sundry variants in how they act around the threshold, etc, but this is the basic relationship.

JR

 

[guitarrock04]

Thanks mate, but I've got that much. I understand how compression works, or more so what it does. I'm trying to get a grasp on how the circuit works, you know? What part of the circuit does what...

 

[amorris]

to be more specific:

a 2db increase over the threshold will allow only 1db over the threshold at the output. nothing happens until threshold is met.

 

[amorris]

BOT, to my understanding:

2 parts to compressor:
1: actual audio path that is affected.
2: a second path (sidechain) that is used by the electronics to judge what the first path is doing in the beginning. this is the detector portion. characterised by either opto photo resistive dtection or RMS detectors. RMS I am not familiar with electrically, just a little black box that does the detection. this detetion circuit develops a voltage (usually DC and called CV control voltage) that is related to the level of the original audio with the threshold factored in and then injected into the compression circuitry to modulate the original audio.

 

[guitarrock04]

I apologize. Maybe I asked my original question unclearly.

Do me a favor and check out this site: http://aikenamps.com/TI_Aiken_adv.htm

I found that helpful when it came to guitar amplifiers. What I am asking for is something equivalent for compressors.

I would even be interested in books, etc. that someone could recomend.

 

[Crusty2]

You're asking about the sidechain signal-processing, yes? How each signal processing block functions. Take a look here first:

http://www.thatcorp.com/appnotes.html

It shows the basics: threshold is a variable d.c. offset control of a rectifier, ratio is a d.c. gain control, attack/release are variable current sources, ect with many variants on the theme...

 

[shabtek]

search for
audio cyclopedia tremaine
radio engineers handbook terman

 

[PRR]

Forget electronics.

You have a signal. Sometimes it gets too loud.

Hell. You have water through a gully. Sometimes the water gets too high.

Get a tiny demon and bind him to work for you. Tell him to watch the level. If it gets too high, close the water valve. If it is not too high, slowly open the valve back to normal.

For general use, you want a way to tell the demon what is "too high".

You don't want the valve slamming closed and open abruptly, you need a way to tell the demon how fast each way.

The demon should have some sense of proportion. If the level is just a little high, he should not turn the valve near-shut.

For practical reasons (the demon is small) you may want to work with a scaled-down version of the signal, control it, then scale-up to full size.

Water in a gully has waves. You may only be interested in the average flow, and not want instant response to each little wave peak.

Those are about all the parameters a demon-controlled limiter (water, audio, temperature, etc) needs.

If demonic wages are too high, you use electronics, but same problem and same parameters to set. Some are set in basic design, others are user-twiddleable.

 

[guitarrock04]

Thanks Crusty, that's exactly what I was looking for.

Now I just need some more. THATs application notes are wonderful for solid state VCAs, but are there similar papers suggesting design and application of tube circuits, or other types of compressors?

 

[bcarso]

There's nothing fundamentally different about tube compressor circuits compared to solid-state. But you can use specific tubes better than others as variable gain devices. You need to look for ones that have a significant change in amplification factor with bias conditions, without, one hopes, terrible distortion. There are also ways to use the variable plate "resistance" as a function of bias.

Besides the Tremaine and Terman books, Langford-Smith, Radiotron Designer's Handbook, 4th edition, has relevant material. See for example pp. 684-686 and references cited. I believe this book at least has been scanned and is available online, although if you're serious about this stuff you really want a copy to have and hold.

 

[amorris]

you want a paint by numbers compressor?


that s not really what happens around here. very specific answers for very specific questions.


all of what you ask is here already. search. and take the time.

everyone here has spent weeks using search, then ask specifics and you will be answered beyond your wildest dreams! well maybe not so wild.

 

[lofi]

FROM WHAT I CAN SEE HE WANTS A POINTER TO INFO ON WHY IT WORKS (damn cap locks), i see schematics posted here and you guys start talking about this cap doing that, that resistor doing this.

i think hes looking more for pointers on where to look for this info. i know you guys have spent years gleaning your info, but for us new kids learning by doing these projects is about the quickest way.

i think it would be handy if we had a Newbie forum, where any question goes, if you dont like it, walk on by.

Iain

Stupid, but not for long

 

[amorris]

again, re-reading your original post, btw I could have read it better before, and checking out the aiken site, we need a more specific question. unlabeled variable resistor... where? could be anything. output gain, tube bias, etc..

knowing the concept of a varimu, as you say, ask what the resistor at this location is doing. there are many people here who could tell you. but to post an aiken style glosarry would take too long.

like i said, take a few weeks and search. we all have.


am I being a jerk?

 

[lofi]

i can see exactly what hes looking for, i understand perfectly, but like him i cant make myself understood.

hes not looking for a step by step on suchnsuch a compressor, but rather an educational piece on a generic compressor.

we do search, i am never off this forum at the moment, but we dont know how to word the questions to make the search engine work.

luckily for me, i quite enjoy re-reading 97 page threads :grin:

Iain

 

[lofi]

Bookmarked and thank you!!!

Iain

any other websites? might make a list for all us 'kids'

 

[bcarso]

[quote author="mediatechnology"]Try this link:
http://www.tangible-technology.com/dynamics/comp_lim_ec_dh_pw2.html[/quote]

That's a nice reference.

 

[guitarrock04]

If you need a specific question, try this:

Where can I find some nice general info on compressor design and theory? I understand what a compressor does, and how it is supposed to be used. The meta threads are a brilliant alternative to faq threads, and I have browsed them constantly. I'm looking for information that's further in depth than basic operation and a bit more generalized than specific searches on this forum. When I have them, I will ask.

I am familiar with this sites search function, and have used it. I am appreciative of the help offered by ppl on forums like this, who don't even know me from the next guy, and I have no intention of wasting your time with piddly stuff.