Tube vs non-tube compressors

[JasonO]

This may seem like a stupid quesion, but what's the difference between these two types of hardware compressors. Which is better or what characteristics can you get from one that you can't get from the other?

thanks

[owel]

This may seem like a stupid quesion, but what's the difference between these two types of hardware compressors. Which is better or what characteristics can you get from one that you can't get from the other?

thanks

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 is tube 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 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. 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 vacuum tubes, because of the high voltage present, they need some way of isolating the high voltage from the audio signal output, and so the need for audio transformers is a MUST... not an option.

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 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. It's usually an all-in-one unit chip that does everything. THAT corp makes them that are used by our most loved RNC. 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.

[Tweak]

Great Post! A keeper to be sure and linked from the compressors page at tweakheadz.

[Travl]

D@mn that was a Hot post...more than I thought I would ever know about the inside of a compressor without going to electrical engineering school....thanks OWEL

[owel]

For more in-depth explanation of the LA-2A and 1176, check out this manual PDF.

http://www.uaudio.com/-works/pdf/manuals/LA-2A_manual.pdf

Here it talks about how the optical T4B unit and related circuitry contributes much to the sound of the LA-2A compressor. A brief description of the 1176 is also included.

[Dugz Ink]

Wow... this thread needs to be archived some place safe. It's great!

D~s

[Shred]

So, Tubes, or Non-Tubes?

I got lost somewhere after the 1st paragraph when you started talking about JFET and IC and Optical, and Coaxilerlafjlaskf.....

What works best with vocals?

[owel]

>So, Tubes, or Non-Tubes?

....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.....

I'd pick a tubeless 1176 anytime than a $100 tube compressor.


> What works best with vocals?

Most commercial studios use LA-2A or 1176 for vocals. Google it.
http://www.google.com/search?hl=en&lr=& ... tnG=Search

The LA2A has tubes and audio transformers
The 1176 does NOT have tubes, but have audio transformers.

Hmmm.... see any common denominator there?

[MediaMonster]

I have a Behringer MDX 1400 (VCA compressor without transformers) and a Bellari RP 282 (Optical Tube Compressor -same as RP 583). They are both very different.

The Berhinger is quiet and can be pretty transparent...not much alteration of the signal except in gain.

The Bellari is not quite as quiet but has a distinctive sound and rules on Bass and Kick drum and has a beutiful effect on mixes. It really make the mids clear and the bottom tight. I choose it when I want an effect and this thing has tons of gain.

http://www.rolls.com/reviews/rp583rev.html

http://www.joemeek.com/emreview.html

[JM350]

With vacuum tubes, because of the high voltage present, they need some way of isolating the high voltage from the audio signal output, and so the need for audio transformers is a MUST... not an option.

You have a lot of good info but this part is completely incorrect.

Transformers in a tube audio circuit have nothing to do with "isolating high voltage" at all.

The transformers you are talking about are for balancing and impedance matching.

Tube devices can be made without transformers but tubes are inherently high impedance devices so you need to either have a transformer or a transistor buffer if you want the piece of gear to be low impedance.

Old tube home audio equipment tends to be unbalanced and high impedance, the only transformers in these types of devices are the output trannies (when talking about an amps speaker outs) and the power transformer.

[owel]

Putting an output transformer can do all of the following all at the same time.

Impedance matching -
Voltage gain or reduction (depends on transformer ratio or whether the transformer was used backwards)
Signal isolation (no physical connection/link between primary and secondary)
DC blocking (only AC or audio signal will show up on the secondary side)
converting from unbal to balanced (or other way around)
Reducing noise and interference
add color to the sound - due to saturation
feedback - in the 1176, an additional winding was used to provide feedback
phase shift - higher frequencies have more phase shifting than lower frequencies.


>You have a lot of good info but this part is completely incorrect.

I did not list all the benefits of having a transformer. It may not be 100% complete, but it is not completely incorrect.

[JM350]

Putting an output transformer can do all of the following all at the same time.

Impedance matching -
Voltage gain or reduction (depends on transformer ratio or whether the transformer was used backwards)
Signal isolation (no physical connection/link between primary and secondary)
DC blocking (only AC or audio signal will show up on the secondary side)
converting from unbal to balanced (or other way around)
Reducing noise and interference
add color to the sound - due to saturation
feedback - in the 1176, an additional winding was used to provide feedback
phase shift - higher frequencies have more phase shifting than lower frequencies.

I'm with you on all these but you said the transformer was to isolate the high voltage required by the tubes, that's simply not the case.

[albertdude]

HI!
I'm new at this and I need some basic info, although I don't know if this is the right place to post it.

Owel and friends, in your bast knowledge, could you tell me if I being a singer in live events such as weddings and so, a vocal compressor would be of any use?, also, a vocal compressor can work as a feedback eliminator?
BTW, can you give me your opinion about this compressor? http://dt.prohosting.com/hacks/what.html

Thank you very much

Fernando Rosas

[Big Tim]

Fernando - remember what a compressor does - it evens out the level of a signal by reducing (literally "compressing") the louder parts and bringing up the overall level, so the quieter sections are louder. It reduces the dynamic range - the difference between the loudest & softest parts.

The problem with using a compressor in a live environment is that it is not a "set & forget" device, so settings that work for one song may not work on the next. On certain instruments (drums, bass) it's less of a problem, but for a potentially very dynamic instrument like a vocalist, you would need a soundman to be adjusting it regularly to get it sounding right for each song. A limiter is usually more useful for a vocalist, because it just stops the signal going over a set threshold, rather than flattening the dynamic range. A compressor would also increase the level of any background stage noise being picked up by the vocal mic.

A compressor is not a feedback eliminator, and in certain circumstances would make feedback worse. A feedback eliminator is EQ based and works by detecting & reducing the gain on the frequencies feeding back.

[albertdude]

Thank you very much for that info Big Tim, it's clear that I didn't know the use for a compressor

However, just as the link I said before, Do you know of any DIY for an acoustic feedback suppressor?, This is because I'm in a tight budget and I believe it would be easier and cheaper for me to do it than buying it

Anyway, thank you again for your kind answer and be well.

Fernando Rosas

[ecc83]

Once again(see 2,3 way speakers) we have a post in QSA's that din't orta' be here!
There are two distinct type of valve compressor. The "cheap" bandwagon jumper that uses a valve to "warm up" the sound as is done in valve mic amps . In these devices the gain reduction element (and owel has done a first class job here) is a FET, optical or VCA.
The second "proper" valve type uses a special type of valve called "variable mu". This fom of compressor will add a character to the sound since, by definition if you have a stage that has its gain defined by its grid voltage it must impart some distortion to the signal.
In any event, the gain reduction stage is only part of the system. The sidechain amps and rectifiers and how much control over attack,decay and effect is given will be at least as much a factor in the final performance.

Now, sorry owel, you are perpetuating a few myths about transformers here!
A high quality transformer (USA Jensen, UK Sowter) will NOT cause any discernable degradation to an audio signal IF it is run as per the makers specs'. Sure, you can overload them or improperly terminate an input trannie, but be good and it will be transparent.

You do not need transformers to isolate HV in valve gear, capacitors are used for that except for power output stages(even here transformerless designs have been made).

You do not need a transformer to get a low output R. A cathode follower has an output resistance of about the reciprical of gm. So for a triode that's about 500.0 Ohms. You can use pentode C.F's and get an even smaller value but the complication and cost (plus a bit more distortion) makes it a rare practice these days. (and yes, you CAN get a balanced out sans trannies!)

Lots of transformers are made for direct pcb mounting, our own Oxford Electrical Products make first class such.

Dave.