Question about high current amps versus "not high current amps"

Hi, high current amplifier is one that is designed to deliver large amounts of current (amperes), when needed, typically low impedance speakers. They have very robust power supplies -- i.e. large transformers, capacitor banks, high-performance transistors, etc.
Specs such as doubling (or near doubling) power as impedance halves, big cooling sinks for heat dissipation are telltale signs of high-current designs.

If oyu have low impedance speakers or listen to dynamic music (e.g. orchestral) you need such an amp.

Most high-end amplifiers are high current, some higher than others. Tytpically, Krell, Symphonic Line, CH Precision, Boulder, FM Acoustics, Vitous, Gryphon, etc are high current.

Let me start from the easiest of your questions: power consumption is largely irrelevant to the issue (except... more on that later).

The relevant value for E (or V) is the internal voltage of the amplifier power supply, not what comes out of the wall socket/receptacle. That 'E' is specified to provide a certain amount of power to a load, which in domestic audio is generally taken to be an 8Ω resistor. Hence the typical specification of (say) 50 Wpc/8Ω.

Note that in a resistor, electromotive force and current are linked by Ohm's law, which says:

E = I * R (where R is the resistance)

now, if you replace that into the 'PIE' equation, you get:

P = E²/R = I²R

so, assuming a load of 8Ω, a 50 wpc amp has to provide

E = √(50*8) = 20 V of internal voltage

I = √(50/8) = 2.5 A of load current

So far, so good; the designer specifies a power supply and final stage that can provide 2.5 A and 20 V, and everyone is happy.

The problem - and it can be a problem - is that loudspeakers are not resistors: the 'resistance' (more appropriately impedance) they present to an electrical current varies with the frequency of the input signal, and it can be much lower or higher than the nominal (most commonly 8Ω or 4Ω) at certain frequencies.

For example, let's assume that speaker X has a minimum impedance of 2Ω at 200 Hz, vs. a nominal impedance of 8Ω (usually at 1 kHz). What happens to the current then, when the amplifier is fed a 200 Hz input signal such that the output is driven to its maximum?

Well, the voltage is still going to be 20V, and Ohm's law still applies:

I = E/R = 20/2 = 10A

That's four times our initial specification. This can spell trouble for both the power supply and the final stage, particularly if this is sustained over significant periods of time. The designers can take precautions (protection circuitry) to prevent damages, but that may mean sound quality degradation and/or shut-down.

Alternatively, the designers can incorporate in the specification the ability to provide (at least temporarily) much higher currents than the standard, continuous Wpc/8Ω would imply. This is a 'high current amp'.

Where does 'consumption' power come in? Well, typically that is calculated as the maximum (continuous) power that the amp will draw - note that in the case above (E = 20V, R = 2Ω), P = E²R = 200W

If the amp is capable of providing indefinitely 200W to a 2Ω load, then the consumption power has to be at least that much (plus inefficiencies and 'overhead' power needed to manage other functionality e.g. power meters, lighting).

Finally, an example of a high current amp - Accuphase E800 (50 Wpc/8Ω; 100 Wpc/4Ω; 200 Wpc/2Ω; 300 Wpc/1Ω). A low current amp - AudioNote P1 (9 Wpc/8Ω; 9Wpc/4Ω; no information on lower loads, but, given the design, power is likely to go down rather than up). Note that what matters is not the 'starting number' of Wpc, but how much they go up - or not - with decreasing load impedance.

There's a lot more to explore on amps design and power specifications, and the above is necessarily approximate, but I hope this short novel at least answers your initial questions.

A Coda 16 has 280,000 uF of capacitance and can deliver 100 amps of current, per channel. 

A Gryphon Essence stereo has 440,000 uF, so can probably deliver more, note that is described as a 50 wpc amplifier. 

Going off the top of my head, I recall many years ago that a low end Sony home theatre receiver was labeled as a 100 wpc 5 channel receiver. When actually tested with all 5 channels running it produced 15 wpc. It probably had the same power supplies as a modern audiophile level DAC, needless to say it was not a high-current design. 

I didn't read the wordy versions above, or I quit at the first incorrect statement in each of them.  Here is the simple version:

Amp WPC is a rating taken at nominal resistance, that is, optimal conditions.  

Now if resistance goes down, to keep voltage constant, amperage has to go up.  If amperage can't go up (low current amp), then voltage, and therefore power, will be limited by the capability of the amp.  thus you need an amp capable of providing high current for speakers with low impedences.

Jerry

"And lastly, what would be an example of a high current amp and what would be an example of a low current amp?

Thanks."

My take;

If you’ve got a bunch of big honkin’ capacitors in your amp, you’ve got a high current amp.

If you don’t, you’ve got a low current amp.

Hope that’s not too highly technical for everyone!😃

Not only low impedance  but impedance curves matter greatly as well.

Your understanding of the electrical formulas are correct.  Impedance curves are most easily obtained from Sterephile reviews, such as this one.  The top chart shows an impedance curve.

This particular speaker shows a minimum impedance ~ 6 Ohms, and IMHO would be a good candidate to be called an 8 Ohm speaker.

I also want to caution you that I've seen dynamic speakers designed specifically to be hard to drive.  They are sold as "revealing of an amplifier's capabilities."  Well, that's great but it doesn't help them play music.

Of course, some speakers just have bad crossovers, and some like the Apogees or electrostatics just can't help it. 

To add to your electrical understanding, amplifiers have an output impedance that is also not usually flat.  Often they are better in the mid to bass than the treble.  That's something else that should be considered.  ESL's are hard to drive in the treble.  A "weak" amp will lose treble output. 

@gregm +1

High current is shorthand for delivering more current on demand.  The P=IE formula remains the same.

An imperfect analogy would be gas engines.  A Ford F-350 will go 60 mph.  A Ford Fiesta will also go 60 mph.  When towing a trailer with a 2 ton load, the F-350 will deliver more power on demand than the Fiesta.   When going uphill, there will be less strain on the bigger engine – while the smaller engine might crap out.

Some speaker designs have low impedance dips at certain frequencies which require more instantaneous current delivery.   The goal is to reproduce the authentic sound of the musical instrument: such as a thwack of a snare drum, the gut punch of a kickdrum, the  shimmering of cymbals or the blaaat of a trumpet.

Matching the speaker and the amp is important.  This is where transient response is involved.

@mclinnguy This statement is false:

A Coda 16 has 280,000 uF of capacitance and can deliver 100 amps of current, per channel.

Here’s why. As @immatthewj points out, the power the amp makes is equal to the current times Voltage. The actual Power formula is 1 Watt= 1 Amp x 1 Volt; IOW power is equal to Volts x Amps. A derivative of this formula that includes Ohms is Power= Ohms X Amps squared.

Giving the Coda the benefit of the doubt, that it can drive a 1 Ohm load, at that impedance the power is equal to the amperage squared. So I think you can see the Coda, as good as it is simply can’t do that; that’s 10,000 Watts! That sort of current through the output section of the Coda would heat the output devices to slag.

That value is actually the amount of current that flows when the power supply is shorted for 10milliseconds so has nothing to do with output power nor the impedance it can drive.

My take;

If you’ve got a bunch of big honkin’ capacitors in your amp, you’ve got a high current amp.

@thecarpathian Please read my above explanation about why this isn’t true. The reason to have lots of capacitance in the output section is to prevent the amplifier from modulating the power supply which can introduce IMD. It has nothing to do with the output power otherwise. We make some amps that have large 3" diameter caps which have a lot of storage; as much as any solid state amp of the same power. But being tube amps they are not likely to be considered ’high current’.

What is important for most speakers is that the amp can behave as a Voltage source; IOW that it can produce the same output Voltage regardless of the load impedance. No amp can actually do that of course but over the range of impedances most speaker present there are quite a few amps that do behave as Voltage sources on them.

But to be clear a tube amp can behave as a Voltage source too as long as it has enough feedback to allow for a low output impedance. But instead of doubling power as impedance is halved it cuts power in half as impedance is doubled.

@atmasphere ,

I’ve always thought the higher the capacitance reserve an amplifier has, the more amps it can deliver when needed. This isn’t correct?

I read what you wrote, but frankly really didn't understand it!

I’ve always thought the higher capacitance reserve an amplifier has, the more amps it can deliver when needed. This isn’t correct?

@thecarpathian That isn't correct. By that metric our MA-2 can deliver more amps that most solid state amps of the same power. The output power is determined by the power supply Voltage and the resulting current that the output devices can handle. So that has a lot to do with the power transformer and the dissipation the output devices will see. In short- its complicated.

"In short- its complicated."

For me it sure is!

Thank you for correcting my mistake. You guys are light years ahead of me with this stuff.

I had thought capacitance had to do with instantaneous current delivery as well.

Obviously 100 amps through a 15 amp outlet is impossible for long periods. 

In theory as far as continuous power one can only have 80% of 15 amps x 120 volts = 1440 watts before the breaker trips, assuming the power amp is the only draw on the circuit. I don't know how short this time period is. 

But an amplifier is not a hair dryer, and we are not blowing our speakers or our ears with a sine wave.

@atmasphere so what might be the instantaneous currents provided in music reproduction, even for a few milliseconds? 

so what might be the instantaneous currents provided in music reproduction, even for a few milliseconds? 

@mclinnguy The instantaneous power will be within the output power limit of the circuit. Therefore so will the current (else the amp fails or goes into protect mode). Generally speaking its not that much! On a 4 Ohm non-inductive load 200 Watts will produce roughly 7.07 Amps (200 = 4 times the current squared; IOW the square root of 50). If there's a weird phase angle involved with that impedance in a nutshell it will behave as if the load impedance is lower. So if similar to 2 Ohms then the current is 10 Amps.

So you can see all these wild current claims are not having to do with output power that's actually driving the speaker.

@dlevi67 +1

amp does deliver current to speaker-load, AND also dampens / controls current from speaker (inductors and capacitors in passive crossover, inertia generated current by drivers), thus higher current than just driving resistive load output stage is needed. 

@atmasphere ,

One more quick question:

What do you mean specifically when you say ’output devices’? The amp itself or certain components within the amp?

Thanks

@thecarpathian 'Output devices' means components in the output section of the amplifier; usually power transistors mounted to a heatsink. We make both class D amps and tube power amps so in their cases either GaNFETs or power tubes.

@atmasphere

Thanks again.

I don’t know if you get told this often, but I really appreciate that you take the time out from what I’m sure is a full dance card to offer your insight here and to educate.

Some amplifier manufacturers do take current measurements on the outputs of the amp, the Krell I400 has 62 amps of peak current on the output. I'm sure those are taken for only brief milliseconds before the outputs fry.