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?
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?
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.
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.
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.
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.
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
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
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.
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.
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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
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
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 
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
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
It depends on the speaker load. All else being equal, higher peak current allows you to drive a wider range of speakers. For example, my WATT 1 speakers have a minimum driver impedance of less than 0.5 ohms at 2KHz Try driving it with many power amps and problems develop. Many will say that this $5,000 speaker should never have been designed this way. I might agree, but that's what I own and use.
Half an ohm ehy? So you have your own custom amp I presume. Not one off the production units of the many you have designed? Was curious about your particular amps configuration?
My mono "bench amp" for all things electronic testing and being a power supply (DC responding) is:
125 Watts 8 Ohms
225 Watts 4 Ohms
380 Watts 2 Ohms
580 Watts 1 Ohm
1060 Watts 0.5 Ohm
and more into 0.25 ohm but never tested it other than for stability down to 0.1 ohm and pulse testing to 70 amps. The transformer is 650 watts so that really is the limit long term. The heat sink weighs just over 15 pounds (can always add a fan) with 8 pieces 200 watt BJT devices. So what is yours?
Thanks,
=SUM
We must have the same speakers ........
Interesting...... You are able to put out power into .5 ohm, yet the amplifier current limits going from 8-4-2 ohm ..... I guess due to the transformer ?
I tried the Halo series , the amplifier would shut down if the volume was raised to a reasonable listening level ... Zmin - 1 ohm
Also would like to note , that high current amplifiers tend to have more energy and sound more dynamic even at 8 ohm ...
There are no such requirements for tube amps.
Guess, why?
Because dynamic output resistance of tube amps is more linear than output resistance of SS amps. What is worse, it is sharply bent, that means when output resistance is in the same ballpack with load resistance it sounds like a chain saw on a couple of Faberge. If current capabilities are much higher than required that means dynamic resistance is much lower, so distortions sound less nasty.
To be honest, similarly to tube amps, we can't find so high current capability requirements in Papalabs designs, which are SS as well.
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There is no "current limiting" with the amplifier circuit until 70 amps is reached where the transistors run out of gain. There is no current limit circuit other than the line fuse. The transformer is the only power supply regulation so as its load increases the transformers output voltage decreases. Also there is more loss in the amp, such as emitter resistors, which decrease output and the amplifiers effective output impedance of about 0.015 ohms which appears as though it were in series with the output of the amplifier causing further loss. Power supply sag and losses are always true with any unregulated supply amplifier which also uses some form of emitter resistors. The no load +48 volt supply is down to about 35 volts driving the 0.5 ohm load at clipping onset. At that point the draw from the wall is in the region of 1500 watts... on a 630 watt transformer. Am sure you see the situation.We must have the same speakers ........
Interesting...... You are able to put out power into .5 ohm, yet the amplifier current limits going from 8-4-2 ohm ..... I guess due to the transformer ? <snip>
To some other points- if I set the output at say 22.000 volts RMS with no load then the output with a 0.5 ohm load is something like 21.500 volts unlike the chip amp... seen here post 82: http://www.diyaudio.com/forums/chip-amps/163385-so-just-how-good-can-chip-amp-9.html which in a similar test using the right supply voltage changes from 4.000 volts to 3.999 volts more or less... see the post.
If I keep the load from the wall below 650 watts then the amp will happily drive .1 ohm all day long. When I need it to do this a variac is used to drop the input voltage. The heat sinks with fan will dissipate easily 400 watts so... no overheating usually.
Never hardly ever listen to this amp. Just use it for power supply, testing, and active load, speaker testing and so on. Shorting while turned on either blows the line fuse or leaves a stub of a wire. Full power bandwidth is DC-100kHz. A fine "bench amp!"
Parasound amps are NOT designed for 1 ohm loads on an average basis. Only transient or short term overcurrent is possible. This is because we don't use fans, and give relatively high power within the case size that we use. Even 2 ohms is touchy. This is because the heatsinks get too hot and trip the temp overload.
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