25 replies to this topic

[Z2TT]

Hi,

The following thread here :

http://audioforum.termpro.com/cgi-bin/ubb/...13;t=002415;p=0

Touches a bit on the relationship between impedence and SQ. it's mentioned that running speakers at a lower impudence will reduce their damping factor and sound quality. Something to do with the amps inability to control cone movement well at lower impedance.

Others mention that when they ran at higher impedences their SQ Improved.

Any comments on this?

Thankyou.

Edited by Z2TT, 18 November 2009 - 04:43 AM.

[~Spyne~]

yes, it's true. will you hear the difference? that's another story

[hugsdrugs]

yeah in theory amp will have higher distortion at rated power should be alright just keep THD under 1% all good

[~thematt~]

I used to think so (damping impacting sound quality), and I even ran some subs on that premise (10ohm loads). After much research and a better understanding, I realise now though, its not.

Here is a great quote from Andy Wehmeyer which can explain it better than I (works at Harmon, of which JBL is one such brand outlet).

Quote

In any case, damping factor (yes, that means output impedance) is irrelevant in controlling the motion of the speaker when the signal stops. It's irrelevant because the element in the speaker that provides motion is the voice coil's inductance, because that's what converts electrical energy into a magnetic field. The DCR (resistance) of the coil determines how much current passes through the coil and, in a schematic model of the speaker is in series with the inductance. At resonance, the combination of the inductance and the capacitance (the capacitor is the suspension which stores energy as potential--it's a spring--and is analogous to a capacitor) creates a voltage generator.

It's a voltage generator because of a pesky thing called inertia. The law of inertia states that something in motion stays in motion unless it's caused to stop by some force. The spider provides that force, but it's inexact. The mass of the moving assembly and the spring of the suspension make the cone move back and forth both generating and dissipating energy along the way. Additionally, the voltage that's generated as that happens (the coil moves inside the magnetic gap--works just like the generator in an old volkswagon) is dissipated through the circuit that's connected to that generator, and the current that flows produces a magnetic field, which moves the cone, which produces a magnetic field which moves the cone, and so on and so forth until the cone finally comes to rest.

The marketing geniuses who invented damping factor decided that if the output impedance of the amplifier was really low, more current would flow from that generator (the speaker's resonance), so the stored energy would be dissipated faster and the speaker would come to rest more quickly--hence, better damping.

What they failed to take into account (probably because it helped them sell solid state amplifiers rather than those with tubes, and possibly because the electrical eingineers with whom they consulted were befuddled by an electromechanical device--the speaker) is that the speaker's voice coil is also part of the circuit that regulates the flow of current--in fact, it IS the part of the speaker that does that job. Guess what, unless the amp is a real piece of crap with super-high output impedance, the resistance of the speaker's coil is much higher than the amplifier's output impedance. If the speaker's DCR (resistance) is 4 ohms and the amplifier's output impedance is .04 ohms, for a damping factor of 100, the speaker's DCR accounts for 100 times the control of the amplifier's output impedance because the two loads are in SERIES. Since more resistance impedes the flow of current, we can also say that the speaker's DCR prevents the amplifier from controlling the motion, no matter how hard it tries.

Edited by ~thematt~, 18 November 2009 - 10:32 AM.

[Rare177]

the above statement is so true, remember current works in a circuit, your voice coil is part of this circuit, it's going to have more limitations than damping factor.

[Pulse-R]

I disagree with some of the conclusions in that statement.

The voice coil's inductance does not provide the motion. Inductance is a frequency-dependant resistance (reactance) which is a measurment of the speaker's ability to resist a change in current.
It is the change in current that moves the cone due to the magnetic fields' interaction.

There's more, but I want to get to my point:

If output impedance is high, then the BackEMF from the coil is not properly damped by the output circuit. (hence damping factor)

In the case of a GNF (Global Negative Feedback) amplifier, which 99.999% of car audio amps are, the inductance of the speaker coil causes a phase error to be induced into the output stage of the amplifier.
As the voltage(what the GNF circuit uses) is out of phase with the current (what the transistors/valves/etc. 'produce'). This phase error is presented back to the input error circuit as a false signal, inducing non-linear distortion into the entire amplifier circuit, and contributing to instability and collapse of the phase linearity.

anyway, so on it's own, DF has little impact on the sums, but who plays sine waves in their car? if you use real, complex music-type signals, then DF has a marked impact. would you hear it? maybe not - unless you knew what to listen for. The installation is far more critical, but good design of the amplifier allows a system to be better. there are very few amps which state phase linearity, and even fewer that have phase-coherent feedback circuits.

Edited by Pulse-R, 19 November 2009 - 07:52 PM.

[bagtho]

umm, how do you measure damping factor in a system?

i would say if you can't measure it, its hard to say that a factor of 200 is better than 100

[~thematt~]

You dont. Its calculated.

Impedance of load divided by impedance of source. From that, nominal impedance tends to be used (and therefore introduces the first level of error) and if you use actual impedance, it varies with frequency (as impedance varies wrt frequency of a system that's not totally resistive).

[SCorpion]

Err, describe what appilication the measured impedence is used for.

Richard Clarkes amp test is a classic example. The output impedence of an amp is most likely distinguishable. Case in point is the tube amp versus solid state. Most will tell you it is due to second order harmonic distortion, which in my li
ited observations is bullcrap. Output impedence is about the only distinguishable chararcteristic of a well designed amp.

Completely different to the minute differences you may or may not hear in the damping factor ratio

[Pulse-R]

but if output impedance does have an effect, then DF must also have an effect.

it's simple - if the amplifier can't control the load effectively, then you'll hear it as sloppy bass, or muddy midrange, or loss of '3-D' sense of stage depth. all these things are products of output impedance vs. load impedance.

that is, of course, assuming the source is good, and not contributing to your problems.

[simplesq]

Question: since we on the topic of impedance, my tweeters are rated at 6ohm, I was hell bent on giving them the perfect 6ohm load from the amp. Are we now saying that there'll be no sonic benefit to this and a 4ohm load from the amp will suffice?

Edited by simplesq, 27 November 2009 - 11:18 AM.

[jas]

simplesq, on Nov 27 2009, 09:01 AM, said:

Question: since we on the topic of impedance, my tweeters are rated at 6ohm, I was hell bent on giving them the perfect 6ohm load from the amp. Are we now saying that there'll be no sonic benefit to this and a 4ohm load will suffice?

im not sure what you mean by this statement. All audio cone and dome drivers are based around a voice coil in a magnetic gap. Basically an electric engine for your piston/cone/dome. Their impedence varies with frequency and is never a flat nominal impendence. All drivers have an impendence peak at resonance (two if the driver is used in a ported enclosure) and this can be quite high even in a tweeter. 30ohms is not an uncomment peak. Also have to know that the driver doesnt go below the rated Re ohms rating. The example im uploading the scanspeak tweeter is rated at 6ohm nominal impendence with an Re 0f 4.7ohms. Just remember that the impedence plot for this driver goes from 4, 8, 16, 32 so you can not just read off the lines inbetween so it is misleading

car audio tweeters range from 4 - 8ohms nominal impendence.

the diaframs are very light and have a very small mass so energy storage due to momentum isnt a big deal like a subwoofer so dampening factor shouldnt be an issue for these type of drivers. The series inductance of the driver itself acts as a high pass filter along with their mass and suspension. There are ways of impedence compensation designed for more linear roll off of the passive cross-over. This parallel cap and resistor are placed at the drivers side of the cross-over and impendence correct for the free-air resonance impedence peak of the driver (tweeters range from 700hz- 1.5khz fs). Dynaudio is one manufacturer that uses impedence correction in their passive tweeter cross-over design.


scanspeak revelator 6ohm nominal impedence, Re 4.7ohms (dc ohm reading and the lowest impendence the tweeter will ever be).
Note impedence plot is the bottom one with the single resonant peak and rising impedence as frequency increases.

Attached Files

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Edited by jas, 27 November 2009 - 09:45 AM.

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[simplesq]

[quote name='jas' date='Nov 27 2009, 10:44 AM' post='1124081']
im not sure what you mean by this statement.

Hi Jas, I was refering to matching the output impedance of the amplifier to the input impedance of the tweeter. More specifically, refering to running a Supremo Piccolo in an active set up. Do I A: purchase a 2ch 4ohm amp or B: purchase a 4ch 4ohm amp and bridge it, then set it up to 6ohms. The 4ch will set me back a further $150, will the 6ohm input impedance be of sonic benefit?

Edited by simplesq, 27 November 2009 - 10:29 AM.

[Rare177]

i never knew it was possible to wire a single coiled 6ohm driver to 4ohms?

or are you referring to the rated power?

[BMWTurbo]

You cannot change the speakers impedence. It's fixed (for this purpose anyways).

Whether you run a 2ch or 4ch amp will not affect the impedence of the driver.

Running a 2 channel amp that can supply 100WRMS into 4ohms, will give you roughly 75WRMS into a 6ohm load.

Running a 4 channel amp that can bridge to 100WRM into a 4ohm load will give you roughly 75WRMS into a 6ohm load also.