better amp configuration

[djk]

"I would not go for a bridge amp since damping factor is cut to half when you go bridging...."

Then how come the Crown amps have DF over 1000, even though they are a bridged design?

"subwoofers are going in and out excessively to the extremes because damping factors are so little to control excursion"

Not related in any way, shape, or form to the DF of those amplifiers.

The excessive cone motion is due to many things, none related to DF. The biggest problem is dynamic off-set due to the cap in the feedback loop of the amplifier becoming mis-charged under large signal conditions. The time constant of this cap and the lower arm feedback resistor is generally below 2hz. When overloaded this cap discharges out-of-phase with the input signal and sends rail-to-rail pulses to the woofer below 2hz (causing the motion you see).

A similar problem can show up with the transformer coupled amps (discussed by you in another thread). The asymetrical nature of audio signals will walk-the-core over to one side and when driven hard will cause the woofer to jerk back when all the energy in the transformer core is discharged (generally a time constant below 10hz).

As regards DF in general, it makes no difference if over about 20. One thing everyone forgets is the DC resistance of the speaker itself (around 6 ohms for an 8 ohm speaker) is in series with the motional resistance of the generator (back EMF of the speaker). So the 'real' DF is this 6 ohms in series with the wire resistsance, connector resistance (on the speaker, box, and amplifier), crossover choke (if used), and lastly, amplifier output impedance.

So does it matter if the amplifier output impedance is 0.001 ohms, or 0.1 ohms when it is in series with 6 ohms?

OF COURSE NOT!

Besides, the instant the amplifier clips, the 0.001 ohms becomes more like 2~3 ohms with the loss of feedback.

[richie00boy]

Well said djk.

[Gordy]

Originally posted by janneman



No, that's no requirement. The system will work flawlessly even if the amps are mismatched several dB's. You may not be able to get the max output level, but quality doesn't sufer.

Jan Didden

So if amp 1 has X gain and 0.02% distortion and amp 2 has X + 1.5db gain and 0.2% distortion, then the output is still OK? I am surprised. My experiments with bridged amps produced a (subjectively) worse sound which I thought was down to a measured mismatch. It is interesting that it may be some other factor. Back to the drawing board.

[djk]

http://www2.yamaha.co.jp/manual/pdf/.../SepA/B-2x.pdf

See the block diagram on page 7.

The low distortion class A amp corrects for the errors in the high power amp.

[djk]

Sandman, circa 1972 (Wireless World)



Consider two amplifiers, A1 and A2. A1 has 20dB of gain, R1=1K, R2=10K (A2 has the same resistors).

A1 runs off ±52V, A2 runs off ±6V.

A1 is a cheap and dirty class AB+B amplifier, A2 is class A.

If A1 had no distortion the summing input would be at 0V, and the output of A2 would be virtual ground. With 1.414V into A1 the output would be 28.3V (200W/4R).

Assume A1 is driving a ground referred load at 200W/4R with 1% distortion, there will be about 2.828V distortion at its output, and about 0.257V distortion at its summing input. Now hook up A2 and it will output 2.828V that is exactly the same as the distortion out of A1. With the same distortion voltage on both ends of the load the distortion will cancel.

A2 must be able to deliver the same current as A1, but only a fraction of the voltage swing is required.

[longthrow]

great example djk...

thanks for the info...

[Gordy]

Very interesting to learn of this. Thanks for the information djk.


However, it does seem excessively complicated that way with dissimilar amplifiers to design, power and drive. My feeling is that the original question of this thread related to using either a single big amp or two similar smaller amps. At least longthrow now has some pointers. My preference would be for the simpler solution of a single amp, and to concentrate on optimising it rather than introduce the complexity of the bridge amp scheme.

[djk]

Depends on what you want, and what it's for.

No point in my mind to bridge unless running more than about ±95V, and then I would consider class G.

BSS made asymetrical bridge amps for PA with one half being class A, and the other half either G or H.

[hienrich]

hi dgk,

thanks for the insights, specially on time constants.

well, I just based my veiws on actual experience with normal or bridge configurations.

yet so much interested to know more on time constants...

[hienrich]

longthrow,

my amp is having plus/minus 90vdc on the rails
yet BULKY and HEAVY enough to break my back!
nice thing your planning on SMPS...

for me bridging eats more space, well , depends on how you arrange and how you design PCB's...

bai, do you have ideas as to where to buy better extruded heatsinks.?