Occasionally, certain loudspeakers are described as being a current hungry design, requiring an amp that delivers just that - high current. So, finding a high current amp design to drive them becomes important.
All too often, I see (much smarter) blokes take a cursory glance at a particular amp and state categorically "Yeah, well of course it's a high current design..."
My question is: How does one determine that a particular amp is a high current design - at a cursory glance?
As I understand it, Ohm's law states that WATTS = VOLTS x AMPS.
So theoretically, a given (say) 20W amp could achieve its output by (1V x 20A) = 20W; while another 20W amp could achieve its output by (20V x 1A) = 20W - or some intermediate calculation. Obviously, the current factor within each design is dramatically different, which (I guess) will likely dictate that the former 20W amp will better drive the current hungry loudspeakers.
But without first exploring a fully-loaded schematic, complete with measured values near the output - and doing a bit of math - how does one make this determination?
Is there some data in the typical Manufacturer's Specifications that gives us a few clues?
All too often, I see (much smarter) blokes take a cursory glance at a particular amp and state categorically "Yeah, well of course it's a high current design..."
My question is: How does one determine that a particular amp is a high current design - at a cursory glance?
As I understand it, Ohm's law states that WATTS = VOLTS x AMPS.
So theoretically, a given (say) 20W amp could achieve its output by (1V x 20A) = 20W; while another 20W amp could achieve its output by (20V x 1A) = 20W - or some intermediate calculation. Obviously, the current factor within each design is dramatically different, which (I guess) will likely dictate that the former 20W amp will better drive the current hungry loudspeakers.
But without first exploring a fully-loaded schematic, complete with measured values near the output - and doing a bit of math - how does one make this determination?
Is there some data in the typical Manufacturer's Specifications that gives us a few clues?
Yes.
For a more precise answer, you need the speaker impedance curve and check the minimum value.
Then your amplifier must be able to comfortably supply at least twice as much or as Rayma said, comfortably drive half the impedance.
Why?
Because speaker impedance is anything but resistive, and such reactive loads require much higher current capacity than what you´d expect at first sight.
Not sure anybody could pronounce an amp as "high current" just on sight alone; I bet any serious answer should take at least a couple minutes looking at the schematic and fiddling with a pocket calculator.
But then I am not "gifted"
For a more precise answer, you need the speaker impedance curve and check the minimum value.
Then your amplifier must be able to comfortably supply at least twice as much or as Rayma said, comfortably drive half the impedance.
Why?
Because speaker impedance is anything but resistive, and such reactive loads require much higher current capacity than what you´d expect at first sight.
Not sure anybody could pronounce an amp as "high current" just on sight alone; I bet any serious answer should take at least a couple minutes looking at the schematic and fiddling with a pocket calculator.
But then I am not "gifted"
"High current", like any other phrase using an adjective rather than a number is essentially meaningless. Its usually marketing fluff. For instance every electric motor is advertized as "high torque", from tiny ones to MW units.
You could argue that theres some widely recognized notion of high current in audio amps, but then you'd be forgetting that some amps are headphone amps, some are domestic hifi, and some are large PA systems. Just say what the specs are if you want to communicate information.
400W amp into 4ohms is a meaningful description - you might or might not consider this high current, everyone will have an opinion, but the "400W" and "4ohm" means the amp can
put 40Vrms (and thus 10Arms) into a 4 ohm resistor.
You could argue that theres some widely recognized notion of high current in audio amps, but then you'd be forgetting that some amps are headphone amps, some are domestic hifi, and some are large PA systems. Just say what the specs are if you want to communicate information.
400W amp into 4ohms is a meaningful description - you might or might not consider this high current, everyone will have an opinion, but the "400W" and "4ohm" means the amp can
put 40Vrms (and thus 10Arms) into a 4 ohm resistor.
No.
Please read and digest Ohm's law - Wikipedia
Nothing about Power. (Just relating flow, potential, and conductance was revolutionary in that day.)
Now knowing Ohm's Law, you should be able to figure that your 1V x 20A example points to an optimum load of 0.05 Ohms(!!), 20V x 1A is 20 Ohms. In audio interfacing, we normally keep the Voltage near constant and let the Current be what it want to be (unless something smokes or shuts-down). Then your "1V x 20A" example, in a nominal 8 Ohm load, accepts only 0.125 Watts. And trying to push your "20V x 1A" into 8 Ohms "tries" to deliver 2.5 Amps, defying the 1A spec, so something will get unhappy.
"High current"-- think of water current. If you want to wash your turtle you would want a small hose. Your dog, a larger hose. To wash an elephant you want a large hose. To douse a large fire you want lots of fat hoses. Looking at an amplifier schematic it takes some experience to estimate the "hose sizes" and their relation to the distressing load impedance.
I recall one paper found there was five times the peak current into a reactive simulated speaker load
(compared to a resistive load), using a special, asymmetrical test waveform.
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Most loudspeakers are at least somewhat reactive in places, some a lot (typically ESLs).
The real equation is Watts = Volts x Amps x cos(phase angle of the load)
As (phase angle of the load) approachs 90° the cos approaches zero.
This means that in a voltage amp as the load gets more reactive the amp has to source more current to deliver the same power.
One of the reasons ESLs are real picky about their amps…
dave
Yes, as others have said, its in the power ratings. If you only see an 8 ohm rating, the power likely doesn't increase proportionally for lower impedance loads. At least, it probably looked unimpressive in comparison to the competition at the time it was marketed.
Even so, we seldom see anything in domestic hi-fi like an exact doubling of power with 4 ohm loads compared to 8 ohms. This may well be due to limitations imposed by circuit resistances or by an output protection circuit which is only doing its safety job as a simple, fixed current limiter or switch
Never trust the company spec. sheet. In commercial products that also have to keep an attractive price tag, there´s no such thing as double the power when half impedance.
Most often you see an amp, that can deliver 400 watt to 8 ohms, will give 700 watt to 4 ohms.
The company then rates the amp as 350w-8ohm/700watt-4ohm.
When tested in magazines, the reviewer is calling it conservatively rated, ´cause to his big surprise, even spec´ed at 350w in 8 ohm, it delivered close to 400. What a fantastic product
Spec. sheets NEVER tell the whole truth
Most often you see an amp, that can deliver 400 watt to 8 ohms, will give 700 watt to 4 ohms.
The company then rates the amp as 350w-8ohm/700watt-4ohm.
When tested in magazines, the reviewer is calling it conservatively rated, ´cause to his big surprise, even spec´ed at 350w in 8 ohm, it delivered close to 400. What a fantastic product
Spec. sheets NEVER tell the whole truth
Many great replies. Thank you. Lots to think about.
If I have understood correctly, a number of you have indicated that a true doubling of power into half the impedance (rare in realty) is a good indication of a high current design.
(Whatever High Current actually means... Thank you @ Mark Tillotson)
Surely this double-up in power though, would only apply to PUSH-PULL amp topology? What about SINGLE-ENDED amp topology? My FIRST WATT J2 (SINGLE-ENDED) halved its power into half the impedance; yet I'm sure the same amp had plenty of current.
Or would others consider the FIRST WATT J2 - 25W into 8R; 12.5W into 4R - to be not a high current design?
Yeah... Tricky.
Maybe I'm off on a tangent here?
If I have understood correctly, a number of you have indicated that a true doubling of power into half the impedance (rare in realty) is a good indication of a high current design.
(Whatever High Current actually means... Thank you @ Mark Tillotson)
Surely this double-up in power though, would only apply to PUSH-PULL amp topology? What about SINGLE-ENDED amp topology? My FIRST WATT J2 (SINGLE-ENDED) halved its power into half the impedance; yet I'm sure the same amp had plenty of current.
Or would others consider the FIRST WATT J2 - 25W into 8R; 12.5W into 4R - to be not a high current design?
Yeah... Tricky.
Maybe I'm off on a tangent here?
There's a nice primer on power and how to get more of it http://www.firstwatt.com/pdf/art_f5_turbo.pdf in that article. This conundrum may make a bit more sense on reading. It's a push pull design and can run in class B if pushed that far in terms of output power.
With the J2 being a single ended design as you say, there's no more current available once it reaches max output current, meaning half the nominal load = half the total power output. I'm looking forward to building one soon!
The aleph is a bit different again - single ended, but the current source is modulated to a degree increasing the output.
With the J2 being a single ended design as you say, there's no more current available once it reaches max output current, meaning half the nominal load = half the total power output. I'm looking forward to building one soon!
The aleph is a bit different again - single ended, but the current source is modulated to a degree increasing the output.
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all amps with high current transistors can output lots of amps for short periods ( > 20A). The higher power amps require more transistors to dissipate the heat generated from high voltage rails and high current. The transistors also reduce their output at higher frequencies so paralleling them keeps the Ft high. Mosfets are immune to speed and current latchup so dissipation is the chief reason to parallel FETs.
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Amplifiers should be perfect voltage sources or close to in the real world.
So, whatever the load (ohmic, reactive, capacitive), it should be completely indifferent to it.
Combining Ohm and Power, we get
P = Iexp2 * R = Vexp2 / R
->
Vexp2 = P * R
->
V = SQRT (P * R)
The (squareroot) product of P * R should be a constant:
20W * 8Ω = 160 'WΩ'
40W * 4Ω = 160 'WΩ'
80W * 2Ω = 160 'WΩ'
160W * 1Ω = 160 'WΩ'
and so on.
In the specifications on can read "20W at 8Ω and 25W at 4Ω" to know this is not an amplifier capable doing high currents.
Another specification might read "20W at 8Ω and 35W at 4Ω" to know this is an amplifier capable doing more serious currents.
If the specifications states lower ohmic vlues, it is because the brand / designer has some confidence in its current capabilities.
Sadly enough, one rarely find specifications about non-ohmic loads.
Somewhere in my library I have a academic paper with a drawing of a substitutional circuit of a common loudspeaker. Three distinctive sections are boiled down: the electric system, the mechanic system and the acoustic system. It counts some 50 parts, resistors, inductions and capacitors in a flabbergasting puzzle. I'll try to dig it up, scan it and post it. After some 20+ years I'm still shocked by it...
Do not dismiss high output impedance amplifiers out of hand.
While they represent probably over 99.9% of the “population” (systems w low output impedance driving speakers that care little about impedance), there is a place for higher output impedance amplifiers. ie SETs are becoming more popular and the Firstwatt F1 & F2 show what can be done in SS.
Some speakers are designed to be used with high output impedance amplifiers and speakers exist that don’t really care what the output impedance is.
dave
Clearly it had NO excess current, just enough to drive normal impedance, which means less power in lower impedance.
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