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Old 23rd May 2010, 02:07 PM   #1
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Default why high current amplifiers?

hello,
ive had a question that has been niggleing in the back of my mind for some time-
ive heard many say that a amplifier with high current capabilities is a good thing- why?
i have a connex class t amplifier coming soon, that delivers 760wrms @ 8 ohms but using only 10 or 20 amps.

what is the advantage, if any of a a amplifier that produces the same rms figure but using say 100 amps peak?
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Old 23rd May 2010, 02:25 PM   #2
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By observation... high current (into a resistive load) is a work around for problems of driving reactive loads. Reactive loads cause most amp, >90%, to show stability problems and even outright oscillation. Adding output devices improves the situation. Consider 7.07 amps RMS into 8 ohms is 400 watts or 200 watts into 4 ohms.

A second reason to have high current in to drive low impedance loads where the same 7.07 amps above into 1 ohm is only 50 watts. Power= amps*amps*load.

A third reason is to spread the heat generated by the linear amplifier over more devices. Instead of 100 watts on one device output transistors allows for 50 watts per each. Less heat is always good. This also allows for more "safe area of operation" for the output transistors which make the amp more difficult to damage from heating or overload.

High current is often the result of addressing one or more of these issues. The amp I use is 135 watts into 8 ohms and will produce about 14 amps peak output or 10 amps RMS. Still more than enough to drive a 3 ohm load which is much less than my 8 ohm speakers ever are. It uses only one NPN and one PNP transistor in the output stage.
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Old 23rd May 2010, 06:11 PM   #3
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12.25A RMS (17.32A Peak) = 600W4R

A Carver M1.5 has one set of outputs that all the current must flow through. They are rated at 16A continuous, 25A peak.

They did, however, use another output transistor as a driver . This allowed enough drive current to the outputs when the gain of the outputs dropped at higher currents.

At power outputs below clipping the Carver did not sound anemic in any way. It did have a peak limiter that would make it sound very compressed if driven into limiting.

On the other hand, a 400W/4R Altec 9440 has four pair of 25A (continuous) outputs in parallel, and three pair of drivers in parallel. Driven hard into clipping into low impedances this amplifier will out pound the more powerful (rated) Carver.

Could you hear a difference when the amplifiers were not over-driven?

Not much.
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Old 23rd May 2010, 07:36 PM   #4
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Hi
If your power amplifier cannot deliver high current on peaks in the music ( such as a cymbol ) and retain a reasonable voltage on the power delivery lines then you will lose dynamics in the music. The music will sound compressed.
What is a high current? Instantaneous currents can be very high. The amplifier I use is 60 watt class after which it switches to class AB. It works on a line voltage of +60/-60 volts. However the output stage is designed to handle peaks of up to 90 amps. The other amplifier I use is 25 watts in class A and the output stage can handle instantaneous currents of 25 amps.
Having an amplifier that can handle high instantaneous currents ensures you do not compress dynamics in the music.
Don
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Old 23rd May 2010, 08:26 PM   #5
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thanks guys.

does this mean that a amp like mine would be ok (760w rms @8 ohms/1150 rms @ 4 ohms) as long as the speakers wernt a particularly reactive load and the dynamics were within the rms capability of the amp?

how reactive are we talking before issues? my speakers drop to 4 ohm at one point but otherwise a fairly soild 8 ohm.

oh, the amp modules are used primarily for surround duties, so only play down to 80hz before roll off.

Last edited by lbstyling; 23rd May 2010 at 08:28 PM.
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Old 23rd May 2010, 08:56 PM   #6
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I can see everyone skirting around why, describing symptoms instead of the real issue.

Power = Voltage x Current x cos(phase angle).

Note that when the phase angle is 0, ie into a resitor, then the cos term is 1.

As the phase angle increases (as into a reactive load) the cos term gets smaller & smaller until, at 90 degrees, the term = 0 and the amplifier can deliver no power into the load.

The reason for large current delivery (in a constant voltage amplifier) is so that the amplifier can increase its current delivery as required to maintain power into a reactive load.

If you have a well behaved speaker with a largely resisitve impedance, huge amounts of current are unnecessary. If you have a speaker with a particularily evil impedance (most often due to crossovers, but ESLs are another) then you need a welder.

dave
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Old 23rd May 2010, 09:05 PM   #7
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May I add the specification found in many generators? Power Factor of zero for 250milliseconds. That is an AC mains circuit which draws 2.5 times the maximum continuous load rating of the generator. Applied to an amp... If you suppose your speakers need 5 amps then plan for 12.5 amps. Then it qualifies as a generator. HAHAHA
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Old 24th May 2010, 03:08 PM   #8
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Quote:
Originally Posted by lbstyling View Post
hello,
ive had a question that has been niggleing in the back of my mind for some time-
ive heard many say that a amplifier with high current capabilities is a good thing- why?
i have a connex class t amplifier coming soon, that delivers 760wrms @ 8 ohms but using only 10 or 20 amps.

what is the advantage, if any of a a amplifier that produces the same rms figure but using say 100 amps peak?
You can gain some insight into this by reading my JAES paper on IIM available at Cordell Audio: Home Page.

Many loudspeaker designers design their loudspeakers as if they were going to be driven by a pure voltage source with zero impedance and infinite current sourcing capability. It is not unusual to find speakers with minimum impedance levels of 2 ohms, even though they are rated for, say, four ohms. When you factor in EMF, particularly at low frequencies, and non-sinusoidal waveforms, the numbers can get ugly fast.

Cheers,
Bob
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Old 18th June 2010, 06:59 PM   #9
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Dave, I like what you said about the welder!

I was pondering this issue the last couple of days. Is there no other solution than to increase the number of output devices?

How about "what's his face's" current amplifier (not Nelson, the guy that just wrote that book....?
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Old 18th June 2010, 07:16 PM   #10
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Quote:
Originally Posted by johngalt47 View Post
I was pondering this issue the last couple of days. Is there no other solution than to increase the number of output devices?
Build a loudspeaker without an ugly impedance load. If multiway is needed use an amplifier for each driver.

The problem arises because speaker designers parallel numbers of bass drivers in cabinets that make things reactive and then compound it with a passive XO that pays little attention to the resulting complex impedance curve.

dave
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