Damping Factor >1000

[DrG]

What about current drive to a speaker? An ideal current-drive amp would have an infinite output impedance, and therefore a damping factor of zero.

I have no experience in this regard but remember reading somewhere that good results are to be had with the exception of controlling woofer resonance. My questions are:

a) Does anyone have experience with/knowledge of current-drive of speakers?
b) If this can indeed be successfully done, how is it possible with a zero (or near-zero) damping factor?

[ashok]

With a current source the electronic unit would not be able to control (damp) the resonance. In this case you will probably have to depend on the mechanical damping of the driver itself. This would mean that all drivers would not be suitable for current drive. You would need specially designed drivers with very high mechanical damping . Currently ,the electrical damping of speakers seems to be higher than the mechanical damping.
Cheers.

I once asked some speaker designers at Hitachi about why they did not use current drive when it was better than voltage drive. They seemed to be floored and obviously did not follow up on it .
( That was about 20 years ago )! Great guys in any case !

[Steve Eddy]

Originally posted by phase_accurate
When it comes to taming a driver's fundamental resonance, Rdc of the driver is by far the largest resistance that comes into play.

Well, certain tube amplifiers notwithstanding.

se

[Steve Eddy]

Originally posted by millwood
I actually think dampening factor is the correct term rather than damping factor, if you think what this "factor" is trying to measure.

No, "damping" is the correct term as it relates to controlling the loudspeaker at resonance.

se

[PRR]

> how can internal impedance of a power amplifier be measured

Feed a constant signal, at less than maximum output. Load the output with no-load and with rated load. Measure the voltages you get.

For an easy measurement, try a very simple tube-triode amp. With no-load, crank the output up to 3 volts. Then apply an 8Ω load; the output will drop to about 2 volts. Be sure it is not distorting significantly in either case. Now a little math will tell you that the output Z is about 4 ohms, because 8/(4+8)= 2/3. If the Zout is 4 and the load is 8, the Damping Factor (I never saw DampENing Factor) is 8/4= 2.

With a modern "good" sand-state amplifier, you will read something like 3 volts no-load and 2.997 volts at 8Ω. Output Z is about ((3.000-2.997)/2.997)*8= 0.008Ω, DF=1,000. (At these absurd levels, the measurement may be limited by meter accuracy rather than the amp's actual performance.)

Some amps do not like no-load. Also it is probably not a real-world condition when driving a speaker. 50Ω and 8Ω (or same ratio for other design loads) is normally safe, though the exact math is harder.

Output Z is often measured around 1 volt, but usually varies with signal level. And output Z always varies with frequency.

Why do we care? If the speaker was always 8Ω, we might not care. But real speakers are 8Ω at 300Hz and very-high Ω at bass resonance. If the peak at resonance were infinity, then with DF=2 the voltage across the speaker terminals is 20*LOG(3/2)= 3.5dB higher at bass resonance than at 300Hz. If the bass resonance impedance were 50Ω, the rise is 2.85dB.

It is possible to make a speaker acoustic response "flat" from a hi-Z source. Most home radios of the 1930s-1950s were tuned this way. The undamped electrical rise at bass resonance was balanced against an open-back cabinet's and undersized transformer's bass droop. Some of these sound very good. But when you want a little better, it gets very difficult to design.

Most modern speakers are tuned to be flat with a constant voltage input despite speaker impedance variations. If the voltage is not perfectly flat: With a damping factor of 10, the worst error is 1dB; for DF=40 the worst is 0.25dB. Since speakers are not precision devices, not accurate better than 1dB or 0.25dB, DF more than 10 or 40 is usually plenty. 100, 200 is not a lot better (0.1dB, 0.05dB) but generally "easy" with high-feedback transistor amps.

Anyway, 10 feet (3M) of speaker wire will give DF about 40 even with an infinite-DF amplifier. And speaker designers generally assume a few-tenths ohms of wire resistance (not that it makes any large difference to their calculations).

> the amplifier's "control" over the loudspeaker at resonance.

That's the common explanation, and a lot better than that "difference in travelspeed" garbage the Fry's boy was selling.

But the "control" is limited by the 6Ω resistance of the voice coil. And the designer has "controlled" the resistance, magnet, mass, stiffness, and size to give a "slightly uncontrolled" response. If the speaker were perfectly damped, Q=0, the response would droop 6dB/oct below 200Hz. We need to let it resonate with Q of 0.5 to 2.0 (0.8 to 1.1 in most hi-fi designs), and the designer balances resistance against all the other parameters to get in that range. The difference between resistance of 6.1Ω (DF=80) and 6.01Ω (DF=800) is about zero for damping, about 0.1dB for response. Room-effects make much more difference than this.

> I think it has to do with how many output transistors we use.

Not really. The output impedance of a transistor (or a parallel array of transistors) is proportional to current. More transistors is the same as one transistor at the same total current. In theory Zout may be 0.03Ω at 1 Amp (DF=240), but 0.3Ω at 0.1 Amp (DF=24) and 0.6Ω at a typical AB idle current of 0.05 Amps (DF=12). When you add the need for bias stability resistors, output Z is usually 0.1Ω to 0.5Ω for about any transistor amp with any reasonable number of output devices and any usable thermal stability. And that assumes low-Z drive to the emitter followers, which is often not the case. A naked emitter-follower speaker amp has DF in the area of 20.

> 3 stages (differential, VAS, current amp)

In general you need 4 stages to get just a "good" speaker amp. If the "current amp" is a darlington or similar, you have enough current gain for a "good" amp. It is a little marginal for a "great high-feedback" amp or for DF over 1,000.

The naked output Z is probably higher than 0.2Ω because of the hi-Z VAS and finite Beta in output devices. To get DF over 1,000, you will need 1,000:1 feedback factor. Closed-loop voltage gain of speaker amplifiers is usually about 20. You need an open-loop voltage gain of about 20*1,000= 20,000. This is very hard to get in one gain stage. And it is hard to get gain in the diff-stage without reducing gain in the VAS. And if you take gain in two stages, you will have a 2-pole open-loop response which is unstable at high feedback factor. If you can compensate it to look like a 1-pole response, and want DF=1,000 all the way to 20KHz, the gain-bandwidth has to be 20MHz at 20:1 gain, 400MHz if you keep it 1-pole all the way to unity gain. You can not get big output transistors that are flat above 20MHz; good rugged ones may get limp before 1MHz. So demanding hi-DF usually leads to unhappy compromises in stability and feedback margin.

I must point out that the path to high DF is what led to many nasty-sounding transistor amps. The high DF applied only below 1,000 Hz, because feedback was falling (and distortion rising) above 1KHz. So you get ultra-clean bass with a cloud of fuzzy treble. This does not "have to" happen, and I'm sure your "certain professional studio amp" is better-balanced than the old stuff. The DF=1,000 probably "just happened" after all else was optimized for good full-range feedback. But starting from scratch, you may be following a 25 year path from the big bad stuff of 1970 to modern amps.

[Steve Eddy]

Originally posted by ashok
I once asked some speaker designers at Hitachi about why they did not use current drive when it was better than voltage drive.

I still don't see why current drive would be fundamentally any better than voltage drive. What's the real advantage here?

se

[DrG]

I don't honestly know, Steve. But looking at the amazing performance gains possible with current-feedback op-amps I have to wonder...

[usekgb]

I have to say, I really love the Crown amplifiers that I use in PA and studio use. Their DF is very high, and they are also high current amplifiers. One thing I love about the MT/MA series amplifiers is their ability to control high power (1200W) subwoofers at loads down to 2 ohms. They control the speakers very well and you always get good, tight controlled bass using Crowns. BTW, the Crown MT/MA amps have a Damping Factor of 1000 from 10Hz to 400Hz, which is where it is needed most for subwoofer use. If that's not enough for you, the Crown Studio Reference series amps have a Damping Factor of 20,000 from 10Hz to 400Hz. Talk about speaker control! These amps sound great on higher frequency as well. They don't have any "fuzziness" in the mids and highs that was referred to in a previous post. One last thing, the correct term acording to Crown is, "Damping Factor." This is right from their data sheets.

[Steve Eddy]

Originally posted by DrG
I don't honestly know, Steve. But looking at the amazing performance gains possible with current-feedback op-amps I have to wonder...

Yeah, I was thinking more in terms of the speaker itself, that it would somehow perform better if fed from a current source.

se

[DrG]

I was also thinking about the speaker, in a roundabout mental analogy with CFB op-amps...

But on a very basic level: magnetodynamic interaction between the voice coil and magnet is determined by the induced flux in the coil, which in turn is proportional to the current flowing through it...

Intuitively it would seem that current drive is the logical way to go...