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Old March 27th, 2006, 03:37 PM   #1 (permalink)
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Do tube amps really compress?

Just a thought that popped into my head.

Volumewise a compressor limits the highs and boosts the lows. I can see how a tube amp limits the high volume by clipping, but does it actively boost the low volume? If not, shouldn't we refer to tube amps as limiters not compressors?

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Old March 27th, 2006, 04:06 PM   #2 (permalink)
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Yes and no.

Yes they do compress to some degree, based on a few variables. I have observed amplitude compression (using the o-scope) with EL34s as they approach clipping. It manifests itself as rounding of the waveform peaks. Moreover, that output power comes from somewhere, the power supply to be exact. As you increase output more and more, the load on the power supply increases, which causes sag, which gives more compression. 6L6s did not exhibit as much compression as the EL34.

Do they roll the highs and boost the lows, no.

It is technically correct to say that they compress.
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Old March 27th, 2006, 04:21 PM   #3 (permalink)
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I think Tremo gave a very nice description of how a tube amp limits the signal due to power supply and tube performance limitations.

As for compression, as in boosting lower level signals and attenuating higher level signals, a tube amp only behaves like that at or near its full output.

In other words, banging hard chords at max volume can use up all available amp power and headroom thus causing a slight squashing of the higher level signal. But, if you then sustain that chord the amp recovers some of the reserve power and the decay of the chord can actually be boosted up slightly as the amp comes back up to full operation.

It is certainly not compression in the manner of stomp boxes or studio gear, but it has some of those characteristics at times.
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Old March 27th, 2006, 04:28 PM   #4 (permalink)
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Your wallet?

Oh yes they do..
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Old March 27th, 2006, 06:12 PM   #5 (permalink)
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Gibson Super 400 amp...

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Old March 27th, 2006, 08:11 PM   #6 (permalink)
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Quote:
Originally Posted by Tremo
Yes and no.

Yes they do compress to some degree, based on a few variables. I have observed amplitude compression (using the o-scope) with EL34s as they approach clipping. It manifests itself as rounding of the waveform peaks. Moreover, that output power comes from somewhere, the power supply to be exact. As you increase output more and more, the load on the power supply increases, which causes sag, which gives more compression. 6L6s did not exhibit as much compression as the EL34.

Do they roll the highs and boost the lows, no.

It is technically correct to say that they compress.
Doesn't the type of rectification play a part in the compression that can or cannot take place in a circuit?
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Old March 27th, 2006, 08:16 PM   #7 (permalink)
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Quote:
Originally Posted by Wally

Doesn't the type of rectification play a part in the compression that can or cannot take place in a circuit?
I lumped the rectifier into the overall "power supply" consideration. You need to think of the power supply as a whole unit, including whatever type of rectifier it uses. Not only does the internal impedance of the rectifier effect sag as the supply is loaded, but so does the resistance of the transformer windings, health and value of the filter caps, etc. It all manifests itself in IR drop, which is the bottom line.
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Old March 28th, 2006, 01:15 PM   #8 (permalink)
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Doesn't speaker choice play a part in this as well? My old Gibson GA40 does not lack for compression. With the stock alnico, you get all of it, pretty low on the volume dial too. With a ceramic installed, the basic character of the amp has not changed, but you get that compression a little later, moderately louder.
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Old March 28th, 2006, 02:04 PM   #9 (permalink)
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In a way, this question seems backwards to me. Tube amps do compress, but that doesn't mean they behave exactly like a stompbox compressor. Keep in mind that stompbox compressors were developed long after tube amps, so any differences could be attributed to the failure of stompbox compressors to capture the real nature of tube amp sag and compression.
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Old March 28th, 2006, 03:30 PM   #10 (permalink)
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It's my understanding that a compressor does not boost the signal (boost the lows as someone else put it), only compresses peaks. You can turn up the gain to keep the overall loudness the same between compressed and non-compressed signals, but it isn't done automaticly. Is this correct?
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Old March 28th, 2006, 08:17 PM   #11 (permalink)
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Quote:
Originally Posted by Paul in Colorado
It's my understanding that a compressor does not boost the signal (boost the lows as someone else put it), only compresses peaks. You can turn up the gain to keep the overall loudness the same between compressed and non-compressed signals, but it isn't done automaticly. Is this correct?
I think so.

We tend to throw terms around, but I believe a tube amp only limits, where a true compressor will boost low signals and attenuate loud signals. A lot of "compressor" pedals would more accurately be labeled "limiter" pedals, too.
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Old March 28th, 2006, 08:34 PM   #12 (permalink)
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Tube amp compression is often quite audible, too. Check out these clips from our own Twangmeister:

(This is from a post from last month... )
Quote:
Originally Posted by Twangmeister
Here are some clips I did through various amps/speakers for Andy Z at the Institute of Noise studio in NoHo:

http://www.instituteofnoise.com/L6/ampclips.asp
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Old March 28th, 2006, 10:01 PM   #13 (permalink)
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A compressor DOES NOT boost "quiet parts". A compressor reduces gain when the signal gets above a certain threshold. A tube amp may do this for several reasons:

1)Tubes are clipping.
2)Transformer saturates
3)Rectifier "sag"
4)Speaker cone distorting
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Old March 28th, 2006, 10:06 PM   #14 (permalink)
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Quote:
Originally Posted by Scott S

We tend to throw terms around, but I believe a tube amp only limits, where a true compressor will boost low signals and attenuate loud signals. A lot of "compressor" pedals would more accurately be labeled "limiter" pedals, too.
Yeah, that is what I was getting at--whether we use the term "compression" when we really mean "limiting."

The explanation about "sag" made a lot of sense though. That is definitely an example where a quiet note blooms and gains volume, which is what I think of a compressor doing.
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Old March 29th, 2006, 12:13 AM   #15 (permalink)
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Actually, you could accurately describe what a tube amp is doing as compression. This is because, when pushed, tubes will begin to limit the peaks in the signal, but at this point, you still haven't reached the maximum signal level that you can feed to them. Therefore, just as in a compressor, you can continue to feed more signal to the compressing tubes, and the average signal level will continue to increase, while the peaks will not. In this way, the dips in signal level will brought up, and peaks will be further attenuated.

The boost to regain level found in most outboard compressors is not part of the compression, it's simply a signal amp that's provided to recover the level that is lost by compression.

The only controls you need for it to be real compression are gain and threshold, and the threshold is set based on the tube characteristics, bias, and the design of the amp.
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Old March 29th, 2006, 02:17 AM   #16 (permalink)
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I think a little compressor/limiter therory is in order here.

A compressor is based on the ratio of the input to output signal. If the ratio is 2:1 (mild compression), then for every 1 volt that goes in, over the threshold point, you will get 0.5 volt out. 10 volts in, 5 volts out, etc., etc. So for a ratio of 4:1 (med compression), 1 volt in, 0.25 volts out, 10 volts in, 2.5 volts out. And for a ratio of 10:1 (heavy compression), 1 volt in, 0.1 volt out, etc., etc.

So as you notice no matter what the ratio is, the harder you hit the compressor, or in other words the more you exceed the threshold, the more it compresses or squashes the signal.

The other controls on a compressor are attack and release time. (many compressors have these fixed) Attack being how quick it will compress the signal once it exceeds the threshold and release being how quick the signal will return to normal after it has been compressed.

And finnaly the last control a compressor will have is make-up gain. This is gain that is added back after you compress. So if you compress the signal 10dB then you add 10dB and end result is you have eliminated most of the peaks and are left with a very steady state signal.

A limiter is different from a compressor. With a limiter you set the threshold and then no signal can exceed that threshold. It doesn't matter how much you exceed the threshold with, the signal will not rise in level. Typically we call them "Brick Wall Limiters" for the very reason that you cannot get past the brick wall.

Now try to equate all this to a guitar amp, I would say that most tube amps will act like a compressor at clipping, especially ones with tube rectifiers. The more you push them, the more the power supply sags, the more they compress.
But I think a Solid State amp with a stout power supply and plugged into good AC outlet will act more like a limiter. Once you hit the maximum output (which is determined by the power suppy rails), that's it. You can't exceed it. It just clips at that point no matter how much more you put into it.

The compressor/limiter operation is fact, the tube/solid state amp descriptions are just my opinion.

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Old March 29th, 2006, 12:56 PM   #17 (permalink)
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A big part of why a tube amp feels the way it does is the way the output transformer reacts to the frequencies it's dealing with.

It adds a kind of frequency dependent compression but it's different to the primitive kind of compression you get from a regular compressor.
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Old March 29th, 2006, 01:21 PM   #18 (permalink)
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Thanks

Man, I've learned a lot from this thread.
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