Low damping factor but fast& high current SS amps?


fine, fast, warm, fullbodied, cohesive, coherent, great timing

100W minimum, damping factor lower than 100 at 8 Ohms - better options than krell ?
zuio
Why do you want low damping factor? The damping factor of an audio amplifier is unrelated to the slew rate or speed of the amplifier. An audio power amplifier's damping factor is defined as the ratio of the load impedance to the output impedance of the amplifier.

Loudspeakers have the tendancy to continue vibrating after the signal is gone due to inertia. Suppose the incoming signal is a "tight" kick drum with a short attack and decay. When the kick-drum signal stops, the speaker continues to vibrate so that nice, snappy kick drum turns into a boomy throb.

When the loudspeaker cone vibrates, it acts like a micro-phone, generating a signal from its voice coil. This signal generated by the speaker is called back EMF (back Electro Motive Force). It travels through the speaker cable back into the amplifier output, then returns to the speaker. Since back EMF is in opposite polarity with the speaker's motion, back EMF impedes or damps the speaker's ringing. The smaller the amp's output impedance, the greater is the effect of back EMF on the speaker's motion. An amplifier with low output impedance does not impede the back EMF, so the back EMF drives the loud-speaker with a relatively strong signal that works against the speaker's motion. When the speaker cone moves out, the back EMF pulls the speaker in, and vice versa. In short, the loudspeaker damps itself through the amplifier output circuitry. The lower the impedance of that output circuitry, the more the back EMF can control the speaker's ringing.

I would think you would want an amplifier with a higher damping factor. I am curious why you want one with a lower damping factor?
It's very simple to add a resistor in series with the output of an amplifier with a high damping factor (a.k.a. low output impedance) to get whatever damping factor you desire. The resistor value can be calcuated by dividing the nominal speaker impedance by the target damping factor value. So if you wish to have a damping factor of two, and your loudspeakers have a nominal impedance of 8 ohms . . . then placing a 4-ohm resistor in series will acheive the desired result.

Most audiophile parts suppliers have an assortment of resistors of the proper value - a non-inductive wirewound type is ideal, and a power rating of 20-50 watts will be more than adequate for the overwhelming majority of domestic applications.
.

Liguy, did you cut and paste that from a tech for dummies web site?

I think it is a rather poor explanation because it is the magnetic field which damps the motion, not back EMF.

The amount of force it takes to move a generator (ringing speaker in this case) is proportional to how low the load is.

The lower the impedance the speaker sees looking back into the amp the more current it generates with its motion.

The more current it generates the stronger the magnetic field it generates which pushes back against the speakers magnetic field.

That's why speakers are often shipped with shorting straps across their terminals. This short allows a lot of current to flow when the speaker vibrates and dampens it. If you ever turned a hand generator you will find that the harder you crank and the lower the load the harder it gets. Not because of any back EMF, because the magnetic field from the current flow pushes back.

.
All this talk of damping factor without a single tip of the hat to the 'q' of the speaker.
Critically damped speakers...Q-0.707 need much less amplifier damping.
Hi Q speakers can benefit from higher amp damping factor, but only to a point.

Electricity and magnetism are interchangeable. Think of a shorting strap as an amp with low resistance to the back EMF generated in the speaker.
.

Electricity and magnetism are interchangeable.

Not at all, they go hand in hand but are entirely different beasts. It is the magnetic field generated by the current in the coil that is pushing against the speaker magnet that dampens the motion.

Something is pushing the coil in a direction opposite of the way it is traveling and that force is magnetic. There is no way to logically explain the phenomenon using back EMF. Back EMF is not pushing the speaker.

.
In a dynamic system, I rather doubt you can have one without the other.

That the speaker generates the energy which damps its motion is without question.
And, since speakers store energy for later release, I'm not too uncomfortable calling it back EMF.......
.

In a dynamic system, I rather doubt you can have one without the other.
It is very easy to have EMF without a magnetic field. Voltage without a path means no current. No current means no magnetic field.

Since the impedance is very low the current is higher and the EMF very low (ohm's law). Again, it is the field created by the current doing the work, not EMF.

I'm not too uncomfortable calling it back EMF
Comfortable or not, it is wrong. EMF is voltage. It is not voltage that damps the motion, it is a magnetic field.

This is physics. There are precise definitions for these terms and the science behind how they interact is well defined and understood. Since you are comfortable using them incorrectly there is no point in further discussion.

.
There is both some conceptual as well as terminological confusion here.
All this talk of damping factor without a single tip of the hat to the 'q' of the speaker.
Critically damped speakers...Q-0.707 need much less amplifier damping.
Hi Q speakers can benefit from higher amp damping factor, but only to a point
The total Q of the driver/enclosure together (which is the figure to which you're referring) is VERY much dependent on the source impedance the driver sees . . . in the Thiele/Small equations, this is reflected through the driver's electrical Q (Qes).

Generally, drivers with a high Qes (especially with a low resonant frequency Fs) are more suitable for sealed enclosures, which (again, speaking generally) tend to work better with amplifiers that have a low output impedance. On the flip side, if you're designing a reflex loudspeaker with a Q of 0.707, the cabinet volume needs to be larger, and the port tuning lower, to acheive this with a higher source impedance (that is, a lower damping-factor amp). So for a given loudspeaker, the total system Q goes up as the source impedance gets higher, usually causing a mid-bass peak.

I'm of the opinion that the subjective sound of this mid-bass peak is the true source of the term "damping factor" for an amplifier, not for the literal mechanical damping of the woofer cone itself, or especially "damping" in the sense of classical Control Theory. Also, this peak can be frequently tamed by increasing absorption losses in the cabinet . . . that is, to add "damping material" (to offset the lack of "damping factor").
Comfortable or not, it is wrong. EMF is voltage. It is not voltage that damps the motion, it is a magnetic field.
I think that a frequent point of confusion is between the acronyms EMF (electromotive force), EMI (electromagnetic interference), and EMP (electromagnetic pulse) . . . the error is in assuming that "speaker back EMF" refers to "electromagnetic force" . . . which if these terms are used precisely, it doesn't.
.

Kirkus, good point, I didn't occur to me that somebody might be confused if they picked the incorrect meaning of EMF.

.
09-12-11: Herman.

Liguy, did you cut and paste that from a tech for dummies web site?

No, I did not. I tried to explain it in the most simple terms I could.
.

Sorry

I just thought maybe you had since it is exactly the same wording in this Crown paper.

http://www.crownaudio.com/pdf/amps/damping_factor.pdf

.
Zuio - your speaker, most likely, has inductor in series with the woofer. This inductor commonly has resistance of 0.08ohm (or more) making DF<100 no matter what amplifier you connect to it. Look in the middle of the table of the best solid core inductor I could find in Partsexpress:

http://www.parts-express.com/14-gauge-c-core-toroidal-inductors.cfm
It is very easy to have EMF without a magnetic field. Voltage without a path means no current. No current means no magnetic field.

The exact opposite of a dynamic system..... What do you call a charge without flow or motion? Static? I don't know.

And yes, it is a magnetic field that stops the motion......a magnetic field from a permanent magnet interacting with a magnetic field caused by a current flow in wire coil. To me, a 'chicken and egg' problem covered by the unified force called 'electromagnetism'.

You're right, though, no current flow = no magnetic field.....

I also like the idea of installing shorting wires on speakers being shipped. I think this is a good idea and will remember it if I ever ship speakers....even my panels! I've used a shorted speaker for another purpose. A simple 'thump' test to show the cone self damp when connected to a good, low resistance load.
OH, one other thing.....Kirkus speaks about the relation of enclosure size, TS parameters and where the term Damping Factor may come from. Good stuff.

I would add that many years ago, as watts became less expensive with the start of the SS 'era', speakers also changed....a lot, to the small, sealed boxes we know today.

Large enclosure speakers still exist, and they are still generally best with low power tubes, which also have a minimal measured damping factor....

And, for no particular reason....My panel also has an inductor in series with the woofer. In this case, Magnepan uses a 16ga iron core inductor of about 0.4 ohms DCR. This is one of those DIY items which causes some minor controversy in panel discussions. Some will put a monster aircore of as little as 0.2 ohms in the stock inductors place. This will have the effect of changing the speakers freuquency balance a little more bass-wards.
Looks like this thread go side-tracked into a 101 on definitions. LOL! :-) Anyway, it was an interesting read & a good refresher for me as well.

Now, to address Zuio's issue: I'm not sure that low damping factor (DF) & fast + high current can co-exist. DF is an indirect measure of an amp's output impedance so if you assume 8 Ohms nominal speaker impedance then higher the DF, the lower the amp's output impedance.
An amp's output impedance is a function of the amp's bandwidth - the higher the bandwidth, the lower the amp's output impedance will remain. If the amp's bandwidth decreases, the amp will not be able to follow the music signal & we get distortion & increased output impedance.
If output impedance increases, it will limit the amount of current that can be delivered to the load as more will be dissipated as heat in the output impedance.
So, I believe that high DF indirectly let's us know that the amp is trending towards high bandwidth (fast) & will be capable of high current delivery (if it's power supply is able & the music signal demands it). Correct me if I'm wrong. Thanks.
"So, I believe that high DF indirectly let's us know that the amp is trending towards high bandwidth (fast) & will be capable of high current delivery (if it's power supply is able & the music signal demands it)."

High current demand is at lower frequencies where amplifier's output impedance is usually very low. High DF doesn't tell us anything about amplifier's bandwidth. Amplifier might have bandwidth limited at the input (to avoid TIM) while output is wide bandwidth with low output impedance. In addition output impedance might or might not change a lot with frequency. For instance Rowland 625 has DF=200@20Hz-20kHz while my class D Rowland 102 has DF=8000@20Hz and DF=8@20kHz.
If I may, a question more related to the original post, and Kij's comments, above.

Doesn't high damping factor also indicate high feedback? I know, right now, that though I am a satisfied 'd' owner, I'd swap it out for a Pass amp. Low feedback, reasonable output while remaining in class 'a' and a minimal number of gain stages. Simple, perhaps, and direct?

Feedback may be even more controversial than damping factor!
Magfan - Deep feedback lowers output impedance (also lowers THD and IMD). Amplifier's configuration affects initial output impedance. Class D amps for instance connect speaker to power supply and GND (zero impedance points) all the time, only polarity changes. Without feedback output impedance would consist of Mosfets' resistance (in order of 0.1ohm) and zobel network's impedance (common mode choke + capacitor and resistor to filter out carrier). Inductance of this choke is the reason of class D higher output impedance at high frequencies. On the other hand this choke has only few turns of wire and very low resistance (high DF at low freq.). I would estimate that since 0.1ohm is reduced to 0.001ohm feedback is at least 40dB deep but it is not too bad.

There is nothing wrong with feedback, if you know how to use it. Amplifier has to be as linear and fast as posible to start with. Small amount of feedback should reduce THD only to about 0.2-0.5% while bandwidth at the input should be limited to one that amp had without feedback (to prevent TIM).

I'm trying to make sens of it and so far I've learned that things are extremely complicated. Very high DF means deep feedback but Soulution 700 amp has DF=10000 and bandwidth of 1MHz with no feedback. If I would only know how to do it I would build such amps and sell them (for $100k) myself.
Kij, buy one and take it apart?

As far as I can tell, feedback and DF are the 'Third Rail' of stereo discussions. Look at the 'beating' I took from a stickler for physics.

The take I've heard on feedback makes some sense. Global...from output to input, is a no-no. Minimally applied feedback....by stage...is OK. I've read a paper, complete with scope photos, claiming increases in certain distortion products as a direct result of feedback.

Now, I've worked in the Semiconductor manufacturing industry. Maybe you can explain something to me.....
We have a measured parameter called RDoN....Resistance of the Device in the ON state. Low enough that if the device were ON, it would yield a fairly high DF, especially if several devices were in parallel or P / N devices were in a push / pull configuration. How does feedback lower DF? Get as technical as you like.
Zuio: If you want a lower damping factor less than 100 on an amp that has more than 100, all you have to do is put a series 10watt resistor on the + terminal and the speaker wire on the other side of that resistor.
To get a damping factor of 50 on a 8ohm speaker you would use a .16ohm resistor.
To get a damping factor of 80 on a 8ohm speaker you would use a .1ohm resistor. and so on.
The only reason I can see for anyone lowering the damping factor of an amp is, if the speaker itself is an overdamped design, (eg: too tight in the bass)

Cheers George
"How does feedback lower DF"

Magfan – It increases DF (lowers output impedance).
Let’s take amplifier that has gain of 30 (31.6dB). When input voltage is 1V output voltage is 30V. Output voltage drops (for whatever reason) 1V under 1A load to 29V. That's 1ohm output impedance (DF=8).

Now, let's build this amp with gain of 300 but feed 3% of the output voltage back to the input in opposite phase. As a result amplifier’s output is the same 30V as before but input is the difference between 1V and 3% of 30V = 0.1V Let’s verify (1V-0.03*30V)*300=30V

Let’s load this amplifier with 1A. Our voltage drop inside is still 1V under 1A load, but output voltage will be higher than 29V because we subtract less from the input. Output voltage will be 29.9V and output impedance will be 0.1V/1A=0.1ohm (DF=80). Let’s verify. (1V-0.03*29.9V)*300-1Vdrop=29.9V.

Output impedance dropped 10 times. Expression 1+B*Aol is known as “Improvement Factor”. In our case B (“Feedback Factor”) = 0.03 (3%), Aol (“Open Loop Gain”) = 300 thus Improvement Factor = 1+0.03*300=10.

It is a little clumsy, but I didn’t want to bring whole feedback theory equations into simple example.
09-15-11: Kijanki
"So, I believe that high DF indirectly let's us know that the amp is trending towards high bandwidth (fast) & will be capable of high current delivery (if it's power supply is able & the music signal demands it)."

Damping factor and Slew Rate (speed) are not related.
Liquy - I never said they are related. That wasn't my quote.
Liguy,
that was my quote, just FYI.
as you correctly alluded: slew rate & bandwidth are related.
You also know that Q=i*t=V*C. so, V/t = I/C. So, slew rate can also be spec'd in terms of how much current into charging/discharging a capacitor.
Damping factor is an indirect measure of the amp's output impedance.
Needless to say, if an amp has higher output impedance (i.e. low DF) then it's more unlikely to supply higher current into the load (as most will be dissipated as heat).
So, there is a relationship between the two.
Negative feedback increases both DF and the bandwidth but it does not mean that high DF = fast amplifier. Amplifier might be slow to start with or have bandwidth intentionally limited at the input.

Slew rate might be important but let's see how much do we need. If music calls for 3V swing at 20kHz it corresponds to 3V*2pi*20kHz=376800V/s=0.38V/us and it is very slow.

It is hard to predict speed from DF. For instance Atmasphere S30 amplifier has extremely low DF=1 and extremely high slew rate of 600V/us.
It's impossible to determine the amps rise and slew rate (or inherent ringing) from damping factor alone. It would have to be put on a scope and stringently tested.