Damping factor

[dnewma04]

Maybe I'm dim, but what does power level have to do with the discussion of damping factor? Or are you just trying to deflect?

 

[aabottom]

Excellent dnewma04!

This makes the issue of DF very clear. If the amp's output resistance is about 10 times lower than that of the speaker wire and passive resistance than it does not matter very much how much lower it is.

Progress!

Let's do some math.

We know that damping factor is a ratio, specifically, it's the load impedance divided by the output impedance of the amplifier. For now, we'll ignore how amplifiers attain low output impedance just for the sake of simplicity.

So, let's say we have an amplifier with a damping factor of 2000 and an impedance of 8 ohms.

DF = Load Impedance / Output Impedance

2000 = 8 / Output Impedance

Output Impedance = .004

All is well in the world, right? We have our staggeringly high damping factor but we've ignored something important.

We forgot to include the series resistance of the speaker wire between the amp and speakers. Let's redo the calculation now.

Again, we know that DF = Load Impedance / Output Impedance, but now we are going to add in the effects of the speaker wire into our equation to figure out what the "real" or "effective" damping factor is of the real world situation. For simplicity sake, we'll err on the side of caution and say that it's very low resistance wire and only presents a .1 ohm load.

DF = 8 ohm / (.004 ohm + .100 ohm)
DF= 8 ohm / .104
DF= 76.9

The advertised damping factor of 2000 is now only 77 at the terminals of the speakers.

Now, let's look at what is going on at the terminals of the woofer in a speaker system with a passive crossover.

It's not unusual to have a series resistance of 1 ohm in a passive crossover, but just for a best case scenario, let's say that it's only .25 ohm.

DF = 8 / (.004 + .100 + .250)
DF = 8 / (.354)
DF = 22.6 at the woofer.

Now, let's say the damping factor of the amp is 20000! Surely that must make a huge difference being an order of magnitude higher than 2000!

Unfortunately, the reality is:

DF = 8 / (.0004 + .25 + .1)
DF = 22.8

It's essentially the same.

Now, let's say that the amplifier in the same scenario is only at 50.

DF = 8 / Output impedance

50 = 8 / Output Impedance

Output Impedance = .16

Now that we have the output impedance of the amp with a DF of 50, let's subsitute into our scenario to see the DF at the woofer:

DF = 8 / (.16 + 1 + .25)
DF = 8 / (.51)
DF = 15.7



Do you see how most people just don't find DF that important? In the real world, the difference between a low DF amp and a ridiculously high DF amp is essentially moot at the woofer.

 

[Jon_Logan]

Ahh, DC resistance.

could it be Crown was more concerned about concerts , high power, using electronic crossovers ?

So high in fact they have to use compression or the whole mix stage show speakers get trashed on the first note ?

Seems a straight wired voice coil (s) would offer a pretty good amount of CEMF, hence the amps need to handle that with a very low output impedance.

Also doesn't the design (Crown) using "grounded Bridge" offer a very high DF be default ? and is why they don't sweat hard at all, all day in the sun ,cranked , year after year. because of such a low impedance for current to flow.

I would think that would be better (should be) even at low power with crossovers.

Crown's Grounded Bridge topology has nothing to do with DF. The grounded bridge design has the benefit of reliability. Since it swings the power supply rail to track the signal, they can apply very high rail voltages that aren't constant. Transistors get further out to the edge of their Safe Operating Area the more current through them while high voltages are across them. Crown's output (-) is not below ground potential. It is at ground.

Crown and high DF. I am a fan of Crown. Crown has always built their amps for rugged use. They use lots of output transistors. They also use low value emmiter resistors. This lowers the output impedance by quite a lot. Additionally, they use NFB, which effectively lowers the output impedance even more. Damping factor numbers of "1000", "2000", "400", "30", are all meaningless unless the load and the frequency are known. DF is not flat with frequency. Luckily, DF is better at low frequencies. This is where the big back EMF is.

As others have said, any inserted resistance from a speaker wire, crossover coil, speaker selector switch, protection relay, all degenerate the possible DF number.

How much is good enough? In my judgement, mild numbers (~>50) are good enough for very demanding low frequency, percussive sounds. Some people assume that tube amps aren't very good with low frequencies because of a lack of a high DF ability. I don't think that this is the case. I believe that issue has much more to do with the output transformer or coupling cap.

Crown had modified a M-600 power amp and called it the Delta Omega. They actually could dial in a negative output impedance. It was quite an interesting idea. When activated, you could not push the cone in. Interesting, but it didn't seem necessary.

For live shows, I believe mostly all systems are active with no passive crossovers. It might be possible that Sound reinforcement companies would like high damping factor amps, to prevent many like bass cabinets from resonating where the resonances could add up. I don't know. They would still lose lot's of DF by the time they cabled up their speakers.

I think it's kind of a spec where it can be an impressive number and it shows off a designers prowess, but it's kind of a specmanship thing, IMO. If I had to drive 1 ohm loads though, Crown would be on my short, short list.

 

[SoCal Sam]

...Damping factor numbers of "1000", "2000", "400", "30", are all meaningless unless the load and the frequency are known. DF is not flat with frequency. Luckily, DF is better at low frequencies. This is where the big back EMF is.

As others have said, any inserted resistance from a speaker wire, crossover coil, speaker selector switch, protection relay, all degenerate the possible DF number.

How much is good enough? In my judgement, mild numbers (~>50) are good enough for very demanding low frequency, percussive sounds. Some people assume that tube amps aren't very good with low frequencies because of a lack of a high DF ability. I don't think that this is the case. I believe that issue has much more to do with the output transformer or coupling cap.

Crown had modified a M-600 power amp and called it the Delta Omega. They actually could dial in a negative output impedance. It was quite an interesting idea. When activated, you could not push the cone in. Interesting, but it didn't seem necessary.

For live shows, I believe mostly all systems are active with no passive crossovers. It might be possible that Sound reinforcement companies would like high damping factor amps, to prevent many like bass cabinets from resonating where the resonances could add up. I don't know. They would still lose lot's of DF by the time they cabled up their speakers.

I think it's kind of a spec where it can be an impressive number and it shows off a designers prowess, but it's kind of a specmanship thing, IMO. If I had to drive 1 ohm loads though, Crown would be on my short, short list.

Well said, I don't know if I can add to it. Solid state DF as explained to me has to do with the amp's ability to deliver power when presented with dynamic load such that higher DF amp's can handle load swings more accurately. I have had the privilege of seeing a dozen square wave tests of amps of varying DF and power. There may be a correlation between DF and SW performance and in the smallish sample of amps I've seen, I would say there is one. I wish I knew more about this and then how to tie this together with slew rate.

There is some use for DF in spec sheets. For low impedance speakers, go for high DF which is strongly correlated with higher power output. For high efficiency speakers, DF is not so important so this is perhaps why tubes shine here. My two cents.:scratch2:

 

[qsaudio]

Good posts, I always wish Crown made a car amp using that grounded bridge, be the perfect match to dish out tons of power. less heat, 2 ohm loads.

good day !

 

[Jon_Logan]

Good posts, I always wish Crown made a car amp using that grounded bridge, be the perfect match to dish out tons of power. less heat, 2 ohm loads.

good day !

... since Crown is owned by Harman Intl., you might get your wish. They've already stamped the JBL name on all kinds of schlocky stuff.

 

[Jeffery]

There is some use for DF in spec sheets. For low impedance speakers, go for high DF which is strongly correlated with higher power output.

Yet it's still not all that general.

Take the Pass X1000 (since Pass was mentioned earlier). 1000 watts, DF of 60. Here's what they say:

The damping factor of this amplifier is not extremely high as it is with some other high power
products, but it is the same at all audio frequencies and occurs naturally without feedback.
Some people are concerned that they need high numbers for good bass performance, but it
just doesn’t seem to be so. We suggest that you listen with your ears and not your meter.

Also,

Because these amplifiers do not rely on output feedback for high performance, their
performance is consistent across the audio band, from DC to beyond audibility. They are
unconditionally stable into any loudspeaker.

So as always, opinions do vary.

 

[qsaudio]

... since Crown is owned by Harman Intl., you might get your wish. They've already stamped the JBL name on all kinds of schlocky stuff.

They are even using an oscillating supply for the pro gear, giant car amp right there, lol.

It was always Crown vs Crest, wonder how Crest is doing these days.

 

[Jon_Logan]

They are even using an oscillating supply for the pro gear, giant car amp right there, lol.

It was always Crown vs Crest, wonder how Crest is doing these days.

I think switching supplies for touring rigs is way overdue. I used to have to lug around big iron amp racks and it sucked. ... oh no, .. stairs!!!

Switching supplies have come a long way in their reliability and EMI performance. The fact is, many huge power power amps couldn't make their specs without a switching supply.

 

[TerryO]

Excellent dnewma04!

This makes the issue of DF very clear. If the amp's output resistance is about 10 times lower than that of the speaker wire and passive resistance than it does not matter very much how much lower it is.

I'd like to second that. Dave did a great job in his post.The math used in his examples certainly makes it abundantly clear what is happening to Damping Factor when it encounters the real world.
I'm actually surprised that the same arguments come up each time this is discussed, even after numerous highly qualified designers have debunked the DF myth over the years.

Crown (which seems to be central in some people's thinking) used a lot of feedback in the early days to tame the extremely non-linear bipolar transistors used at the time, and the result was often accompanied by a very high Damping Factor number. The correlation between better sound in those amps had a lot to do with the devices used and the necessary use of negative feedback. The higher Damping Factor numbers that resulted were just an incidental side effect, but some (like Julian Hirsch, etc.) began to believe that the numbers themselves were somehow a figure of merit.

Crown made a reputation for high powered and reliable amps back in the 70's, and often touted the astonishingly high damping factor numbers as if it were an actual feature. This is what many of the reviewers at the time were looking for and I believe helped the company in their sales.

Crown products are very good, reliable and well engineered, but their reputation really owes very little, if anything, to Damping Factor other than to present just one more (in this case, bogus) advantage to people that haven't followed the technological changes in amp design.

I believe that Dave's post should be made some sort of "sticky" or otherwise enshrined, so that it could be easily located and presented when another of these rather tedious debates about Damping Factor comes up. It would save time and allow us to move on to really important topics such as "How many angels can dance on the head of a pin?"


Best Regards,
TerryO

 

[bowtie427ss]

wonder how Crest is doing these days.

Last i knew they were still owned by Peavey, and making some very good amplifiers once again.

 

[SoCal Sam]

Yet it's still not all that general.

Take the Pass X1000 (since Pass was mentioned earlier). 1000 watts, DF of 60. Here's what they say:



Also,



So as always, opinions do vary.

Yes, opinions vary. On the flip side if one is spending time discounting the value of DF, it is because the design does not perform well under this test. I'd say a solid state amp with DF of 60 won't pass a dynamic signal as cleanly as a faster amp with a DF of 200 or more. In fact, at one time I had a Nak PA-7 and a Yamaha PC2002M for a comparo and the Yamaha's clarity advantage was apparent to me.

EDIT ADD: DF was and is of some importance if applied to speakers and dynamic signal. Kenwood's Sigma Drive was a design attempt to continuously vary the amp's DF to more closely match the speaker's impedance during program music. The Accuphase E-202 integrated amp had a three position DF switch with the lowest setting being something like 5. I tried all three settings and found that the lowest setting had more smearing.