Damping Factor: 300 vs. 500. Big DIfference??

Discussion in 'Amps [BG]' started by Mario Lewis, May 21, 2003.


  1. Mario Lewis

    Mario Lewis

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    I'm asking because I was looking at the RMX line of QSC Amps. I've had a PLX, but the RMX is quite impressive for the price. Will the naked ear hear the difference in sound with a Damping factor of 300 as compared to the PLX line which has a damping factor of 500??


    Just what is this spec anyway?? I've always understood it to be the amps ability to control cone movement. A higher number (like in the 500's) has been considered pretty top notch. But the lower numbers (in the 300 range) are less than optimal, and associated with poorer quality.

    I'm not at all saying the the QSC RMX's are not good quality, but there is a difference. So what is it??

    My own tests found a noticeable difference in a Mackie 1400 (Damping Factor = 350) and a Stew 2.1 (damping factor >500) The Stew sounded tighter, crisper, more defined. Were my ears deceiving me??
  2. Mark Reccord

    Mark Reccord

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    Both those damping factors are adequately high for good low end control. Any sonic differences between the RMX and PLX series wouldn't be due to the differing DFs. The difference means that the output impedance of the RMX is slightly higher than that of the PLX. Any number of design factors could affect that. Bob Lee could certainly shed more light on the situation, I'm sure he'll post a reply...
    cheers!
  3. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

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    Is there a big difference? There isn't even a big difference between 200 and 1000.

    Damping factor is a marketing-friendly way of stating an amp's output impedance. It's calculated by dividing the load impedance by the output impedance:

    DF = 8 ohms / output Z

    Manufacturers almost always use 8 ohms as the load impedance because it yields higher, more impressive numbers than using 4 or 2 ohms. I call damping factor a marketing-friendly spec because it makes minuscule differences seem huge.

    These are things you should understand about damping factor:
    • Amplifier output impedances are frequency-dependent. This is because of the output network on any well-designed amp. The output network decouples RF noise picked up on the speaker cables to keep it from getting into the amp circuitry. It also helps keep the amp from becoming unstable if the load happens to be highly reactive, especially capacitive. Therefore, the output Z will generally be extremely low (typically around 0.5 to 2 milliohms) at low frequencies and somewhat higher (typically up to 50 milliohms) at the very high frequencies, like 10 to 20 kHz. The IHF damping factor method specifies that the frequency range measured is 1 kHz and below. Essentially, the output Z at 1 kHz then is what dictates the damping factor, although it will actually be many, many times higher at bass frequencies. A typical way to skew the spec is to say "screw the standard" and use only up to 100 or 250 or 400 Hz to specify the damping factor; that'll give you higher numbers on the same amp than 1 kHz will. Typically, an amp that has a damping factor of a few hundred at 1 kHz will be more like a few thousand at bass frequencies of interest, like 100 Hz.
    • The damping factor spec ignores the effect of speaker cables. While the difference of a few milliohms in output impedance causes huge differences in the numerical damping factor spec, the fact remins that when you attach a speaker cable, you're effectively adding at least another few milliohms to the amp's output impedance. As I pointed out before, tiny differences in output Z translate to big differences in the marketing-friendly nature of the damping factor spec. This makes the differences among amps' damping factor specs almost irrelevent. That's why it's more important to use copper speaker cables that are some combination of short and thick (large cross-sectional area) than it is to choose an amp according to damping factor, although sales people usually won't tell you that.
    • How much damping factor is enough for tight control of the bass speaker drivers? Designers of pro sound systems typically shoot for 50 or higher (including the effects of the cables and passive crossover networks).
  4. FretNoMore

    FretNoMore * Cooking with GAS * Supporting Member

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    [​IMG] Thanks Bob!

    That was refreshing. I wish more manufacturers would talk straight like that.
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  6. GreyBeard

    GreyBeard

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    After a while you wonder just what spec. numbers really mean anything. Like can you really hear the difference between .1, .01, and .001 distortion? Can you hear the difference between a damping factor of 100 and 1000? Just what is slew rate? Can you hear any difference between a 600 watt amp and a 750 watt amp? Can any specification number be related to reliability?

    Just thought I'd add that, I'll shut up now.:D
  7. FretNoMore

    FretNoMore * Cooking with GAS * Supporting Member

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    "Jaco only needed a (insert whaetever it was he used for amplification here)!"

    I'm sure it wasn't very impressive compared to today's specs.

    Of course it can't hurt to buy good gear, but we all tend to get caught up in the numbers. I know I have been known to do so anyway. :)
  8. Mario Lewis

    Mario Lewis

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    I understand about 85% of what Bob Lee has just said, so if it wasn't the Damping factor that made the Stew 2.1 sound like it had better control of the cones than the Mackie, what on the Stew spec sheet indicated that what I heard would be the outcome? The Slew Rate??
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

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    Probably not the slew rate. I doubt that there's an amp on the market, from any recognizable brand name, that has an insufficient slew rate.

    Maybe the Stewart was set to a higher gain than the Mackie, and was therefore louder. You can even take two of the same amp model, turn one up louder than the other, and the louder one will tend to sound better and tighter. That's more of a psychoacoustical phenomenon than a performance one, though.
  10. Nightbass

    Nightbass Supporting Member

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    I don't think there's anything on the spec sheets that can predict how an amp will sound, or that can be attributed to the differences you perceived between the sound of two amps.

    The Stewart and Mackie sound the way they do because of 1000's of individual design decisions. Stuff like power supply design, driver stage design, HP/LP filtering, global/local feedback, output stage topologies, biasing, voltage vs. current amplification, op amps vs discretes, and so on. Those decisions are the ones that make an amp sound like it does, and you can't quantify them with specs (one area we engineers win over the marketing droids...) ;)

    My Crown K1 has a DF of > 30,000. It sounds much better overall than the PLX-2402 I used to own, but I don't feel that DF was the reason. Like Bob said, DF goes out the window the minute you hook up a speaker cable!

    Nightbass
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

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    Even then, it's generally more important how the design is implemented than what is. For example, you can't say global feedback sounds like this, while op amps sound like that. In power amps, the design goal is essentially to reproduce the audio signal as perfectly as possible; thus, ideal amps would all sound alike. But even identical ideal amps would sound different in quality at different listening levels (retailers often use this trick to sell certain higher-margin models over others).

    Further complicating this is the paradox that often the ideal--perfect reproduction of the audio signal--doesn't always sound as good as some added distortion does.

    BTW, what do you mean by "voltage vs. current amplification?" :confused:

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