Compression vs Sag

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matt1973matt1973 Frets: 310
edited January 12 in Amps
Compression and sag - obviously a key aspect of any rig but I, probably like many, have not given it much thought and have only really engaged it by experimentation, feel and knob-tweaking. So whilst both reduce dynamic range aside (one from a stomp box and via a power amp characteristic) would anyone care to explain the difference between the two things?

Whilst we're at it I would also be interested to hear arguments for and against compression/sag in certain music contexts and how individuals utilise and employed it within a rig.


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  • darthed1981darthed1981 Frets: 3015
    I thought this was going to be another thread about married life...
    Warning: this post may contain overtly affectionate references to the irreplaceable Mary Spender, and if you are reading this, Hi Mary!
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  • uncledickuncledick Frets: 282
    As I understand it a compressor will actively boost the quiet bits whilst sag will only ever limit the loud bits.  I'd be inclined to use a compressor for rhythm work where it can give a more solid backing to the vocals whereas sag  tends to come in when you're in full guitar hero mode and it can smooth the spikes from a loud solo. Ymmv etc.
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  • VoxmanVoxman Frets: 2334
    I always thought sag was where the tone was spongier and less tight or defined.  
    I started out with nothing..... but I've still got most of it left (Seasick Steve)
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  • CirrusCirrus Frets: 4358
    They both have the effect of reducing dynamic range, and they both vary their effect based on the signal you're pushing through the circuit.

    It's just that sag is a byproduct of having a limited amount of available power for each amplifier stage as all the stages try to pull juice from the same source, and unless you're designing the amp yourself is uncontrollable - it just is part of what the amp does. And it can have side effects such as inter-modulation between gain stages, and with the wall voltage, that introduce different harmonics

    Compression is designed to give a controllable reduction in dynamic range, there are different kinds of circuits that do the job in different ways and with different effects on the audio, but you get various controls to influence how it does it.
    Captain Horizon (my old band);
    Very (!) Occasional Blog
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  • matt1973matt1973 Frets: 310
    edited January 12
    @voxman whilst googling I found nice description of that touches on your point from a software modelling perspective:

    "Sag

    Controls power amp sag or compression 0% tight, 50% normal, 100% more touch for dynamic sustain.

     Lower Sag values offer a "tighter" responsiveness for metal and djent playing; higher values provide more touch dynamics & sustain for blues and classic rock riffs. High Bias (closer to class A) reduces the effect of the Sag control. Sag doesn’t exist in class A amplifiers since the average plate current doesn’t change.

     Sag is caused by the amplifiers power supply being unable to meet the demand when the map is pushed hard with high Master. When the power sags, the amp output drops and then recovers, creating a unique compression effect with some additional, but temporary distortion when you pick hard. Unlike compressors before the amp, sag compression occurs even when the amp is already very distorted.

     Large sag gives more compression, better sustain, and can accentuate pick attack. But the amp will appear somewhat less responsive especially at the low end and can get muddy."

    full thread here: https://line6.com/support/topic/19115-helix-amp-parameters/

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  • ecc83ecc83 Frets: 931
    Compression can be had from the pre amp section at any loudness (assuming a master VC amp) whereas "sag" is only possible as the amp pulls close to full power from the PSU.

    Probably THE best treatise on pre amps is by Merlin Blencowe and I hope he gets onto guitar amp powers stages soon cos I don't have all THAT long!

    Dave.
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  • JezWyndJezWynd Frets: 2979
    The compressor pedal that best mimics sag imo, is the VFE White Horse. It gives a kind of bloom effect that I associate with sag, with a touch of gritty distortion. A remarkable pedal that is sadly no longer in production.
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  • ESBlondeESBlonde Frets: 2793
    JezWynd said:
    The compressor pedal that best mimics sag imo, is the VFE White Horse. It gives a kind of bloom effect that I associate with sag, with a touch of gritty distortion. A remarkable pedal that is sadly no longer in production.
    Some of That   /|\

    I can't speak for the VFH but there is something of a bloom and decay with amp sag, whereas compression (depending on the threshold, knee and ratio) tends to have make up volume later in the decay cycle and hold a fixed output level. It's also easier to invoke natural sag in your playing for expression while compression feels more 'fixed'.
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