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Topic: Amp power and current (Read 52846 times) previous topic - next topic
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Amp power and current

Seriously speaking, how can one identify an amp with high current? I have two stereo amps: one is rated 75 wpc (budget amp around 5k new), another 60wpc (about 14k new). The more expensive amp has more control on loudspeakers, plays cleaner at any volume. Actually, it plays louder than the 75 wpc, but cheaper amp. Is all this mainly due to current or there is more to it?  Funny enough, both have torodial transformers of similar size. But sound quality is night and day! Also, some amps give big jumps in volume with a slight turn of the knob ( often too loud by the 9 o'clock position). Others will give small gains per rotation ( and listenable up to around the 2 o'clock knob position).

Amp power and current

Reply #1 –
I think, and I stand corrected on this one, the bigger the caps on the power stage, the better it can handle the demands? High current amps often draw lots of current and having bigger caps can help a bit?

Amp power and current

Reply #2 –
You still are not getting the bigger picture. An amplifier's power rating must be specified at a specific load impedance. An amplifier may deliver 75 wpc into an 8 ohm load, but much less into a 4 ohm load. What is the impedance of your speakers?

Amp power and current

Reply #3 –
If the amps are designed correctly and the specs correct, and you are driving appropriate loads (drivers/spaekers), then current capacity should be a moot point. To produce 75 Watts into a 4 ohm load requires a certain amount of current at a certain voltage. This is all dependent on the design. With transistor amps if you use a load which is lower than the amp is specified to handle then it will typically try to deliver more power to the load (halving the load resistance will double the current at the same voltage. Since a rough estimate of power is Voltage x Current, doubling the current doubles the power. But since you are running it out of spec it's quite likely the power supply is unable to deliver that amount of current causing the voltage to drop and you have massive distortion, lots of heat, and soon after a fiery death (every component in the amp, even the wires, has a certain resistance and if you double the current to the load you also double the amount of power all those components have to dissipate as heat).

How much the volume changes at various points in turning a knob usually isn't an indicator of much more than the linearity of the potentiometer the knob is actuating.

If the sound quality of the "cheaper" amp seems much worse than the expensive one then I'd chalk that up to a fault in that particular amp, perception bias, or the cheap amp is faithfully reproducing the input while the expensive amp is coloring (otherwise known as distorting) the output in a way you find pleasing.


Amp power and current

Reply #4 –
Power = Voltage x Current

Power = Voltage Squared/Impedance
Power = Current Squared x Impedance

The second two can be derived from the 1st (Voltage x Current) and Ohms Law, which defines the relationship between current, voltage, and resistance.

Ohm's Law: Current = Voltage/Impedance.

Amplifiers are "constant voltage" devices.  Of course the voltage isn't actually constant when listening to music....  Constant voltage means the voltage doesn't depend on the load (within reasonable limits).  If you run a test-tone into your amp and adjust it for 10V output, you'll get 10V with the speaker connected or not, or with a 4, 8, or 16 Ohm speaker (assuming the amp is rated down to 4 Ohms).  If the load impedance is too low, you'll get too much current and the voltage will drop, or the amp may go into self-protection current-limiting mode, or you may get smoke, or blow a fuse.

What this all means is:  The voltage is determined by the amplifier.  The impedance is determined by the speaker.  The current is the result of the voltage and impedance.

There are "high current" automotive amplifiers rated down to 1 Ohm (that's four 4-Ohm subwoofers in parallel).  Home stereo amps rated for less than 4 Ohms are rare...  And of course, there is no benefit to having this current available unless you actually have a low impedance load to take advantage of it. 


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(budget amp around 5k new)
You don't mean $5000 do you?  That's one heck of a budget for only 75W!!!!

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The more expensive amp has more control on loudspeakers
What is "control" in terms of distortion, noise, frequency response, or power, or anything meaningful?

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plays cleaner at any volume.
If you are hearing any distortion below clipping, the amp is defective.

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Actually, it plays louder than the 75 wpc, but cheaper amp.
It can't be louder with less power (into the same speakers).  75W is about 1dB louder than 60W    10 x Log(75/60) = 0.97dB.  The 60W amp may be conservatively rated, and/or the cheaper amp's specs coud be exaggerated.  (There could be some rare exception where the speaker is hitting it's limits and goes less than 1dB louder, or if things start buzzing and rattling it might go more than 1dB louder.)

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Also, some amps give big jumps in volume with a slight turn of the knob ( often too loud by the 9 o'clock position). Others will give small gains per rotation ( and listenable up to around the 2 o'clock knob position).
That could be differences in gain or just differences in the volume control circuit.  The specs should tell you how much input voltage you need to hit full power (that would be with the volume control at maximum).  From that you can calculate the output voltage (knowing the wattage and the impedance), and given those two voltages you can calculate the voltage gain.

Amp power and current

Reply #5 –
Quote from: yourlord on
How much the volume changes at various points in turning a knob usually isn't an indicator of much more than the linearity of the potentiometer the knob is actuating.

And/or amplifier gain, of course.

Quote from: DVDdoug on
Quote
(budget amp around 5k new)
You don't mean $5000 do you?  That's one heck of a budget for only 75W!!!!

That's what I was just going to write. A Yamaha A-S700 isn't even $500US (or rather wasn't, as it's been superseded by the '701 now), and that's a 100 wpc unit that does 160 W into 4 ohms with a bit of a following wind. Even for a tube amp it would be quite a generous budget. A 75 wpc unit in this price range would be way into esoteric audiophilia territory, and quite honestly construction quality (engineering) in such spheres isn't always what it's cracked up to be.

Quote from: DVDdoug on
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The more expensive amp has more control on loudspeakers
What is "control" in terms of distortion, noise, frequency response, or power, or anything meaningful?

Damping factor? In esoteric audiophilia territory, everything is possible.

Amp power and current

Reply #6 –
Quote from: stephan_g on
How much the volume changes at various points in turning a knob usually isn't an indicator of much more than the linearity of the potentiometer the knob is actuating.

And/or amplifier gain, of course.

So then correct me if I'm wrong, but the position of the volume knob does not reflect the percentage of power supplied? So different amplifiers have different gain structures? So if the volume pots on both amplifiers are probably completely different with different gain, then that would mean that at the same relative position one amp may sound stronger even if it has less rated power?

So if one amp is rated at 60W, and another at 100W, the 60W amp may appear to be more powerful because it has more gain, but the 100W amp may have less gain, so the increase in power as the volume control is increased is more gradual? I'm just thinking out a loud here, please tell me if I'm right. I'm trying to understand how a lower spec amp might sound more dynamic than a higher powered model if the gain settings were a little tweaked.

Amp power and current

Reply #7 –
The volume control setting says very little about the output power. What you need to now is the voltage gain of the amp and the input voltage. Those two determine the output voltage. The output voltage and the speaker impedance determine the power.

Amp power and current

Reply #8 –
Quote
Amplifiers are "constant voltage" devices. Of course the voltage isn't actually constant when listening to music.... Constant voltage means the voltage doesn't depend on the load (within reasonable limits). If you run a test-tone into your amp and adjust it for 10V output, you'll get 10V with the speaker connected or not, or with a 4, 8, or 16 Ohm speaker (assuming the amp is rated down to 4 Ohms). If the load impedance is too low, you'll get too much current and the voltage will drop, or the amp may go into self-protection current-limiting mode, or you may get smoke, or blow a fuse.


So you are saying that the amount of power the speaker requires and how much power the amplifier can supply are really two very different questions? I've heard people say that a high current amp will put out up to twice as much power into 4ohms as they will into 8ohms, and that a low current amp will probably only be rated at 8ohms and will probably say "8ohms minimum speaker impedance"  on the back.

Amp power and current

Reply #9 –
Quote from: Rich B on
I've heard people say that a high current amp will put out up to twice as much power into 4ohms as they will into 8ohms, and that a low current amp will probably only be rated at 8ohms and will probably say "8ohms minimum speaker impedance"  on the back.

That is more or less correct.

Most decent amplifiers will give their power ratings at both 4 and 8 ohms. The delivered power at 4 ohms can be up to twice that at 8 ohms if the output drivers are capable of delivering the current, but the power out is limited by the voltage supplied to the out put stage.

Sometimes the ratings are nearly equal, in which case the optimum load impedance is around 6 ohms, and the designer has gone with a more balanced approach (limited by both(current and voltage).

In an extreme case where the output is always limited by current, the power out is actually higher into 8 ohms that 4 ohms (P = I^2 * R).

Amp power and current

Reply #10 –
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So then correct me if I'm wrong, but the position of the volume knob does not reflect the percentage of power supplied? So different amplifiers have different gain structures? So if the volume pots on both amplifiers are probably completely different with different gain, then that would mean that at the same relative position one amp may sound stronger even if it has less rated power?
That is correct.  The lower-power amplifier may have more gain, and the volume controls may operate differently.

Typically, a higher power amplifier will have more gain because the amp needs to go to full power with a "line-level" signal.  I was looking at some Crown amps a couple of weeks ago and all of the amps in the series (from 500W per channel to 2100W) are rated to go to full power with 1.4V at the input.  So of course, the higher power amp needs more gain.    But, those are all from the same manufacturer an the same product line.

A linear potentiometer doesn't work very well for audio.  An audio taper pot is approximately logarithmic to match our hearing.    These are standard if your amplifier uses a regular analog pot for volume control. 

With an audio taper pot, 50% of the rotation is about 10% of the maximum voltage, which is about 1% of "maximum power".  But of course, with variable input levels and unknown gain these percentages are not that meaningful and you'll generally hit the amplifier's limits before the pot is fully clockwise. 

Many modern amplifiers us a (digital) rotary encoder or some other indirect method of volume control.  One reason for doing that is that dual (stereo) pots don't "track" perfectly and this can throw-off your left-right balance.

Amp power and current

Reply #11 –
Quote from: pdq on
Quote from: Rich B on
I've heard people say that a high current amp will put out up to twice as much power into 4ohms as they will into 8ohms, and that a low current amp will probably only be rated at 8ohms and will probably say "8ohms minimum speaker impedance"  on the back.

That is more or less correct.


Then what I don't get is that if the current requirements are determined by the load, not the amplifier, that a 'low' current amp could be supplying enough current at a given voltage, then wouldn't it be just as high current as any other high current amp? If I listen to music at reasonable levels, and the load requires a certain current because the impedance dips, and the amp I have is a 'low current amp', but it can supply the current because the levels are reasonable, then it would supply the same current as any high current amp, no matter how big and powerful?

Amp power and current

Reply #12 –
Quote from: pdq on
Quote from: Rich B on
I've heard people say that a high current amp will put out up to twice as much power into 4ohms as they will into 8ohms, and that a low current amp will probably only be rated at 8ohms and will probably say "8ohms minimum speaker impedance"  on the back.

That is more or less correct.


But one should qualify this statement by saying that referring to amps as "high current" and "low current" is inaccurate. High or low relative to what? A 100W amp with a minimum load impedance of 8 ohms will source more current to it's 8 ohm load than a 4 ohm 30W amp into it's 4 ohm load.

The minimum load impedance is not a reliable method of determining the quality or power capacity of an amp. It can be a benefit to deliver higher powers into higher impedance loads as the lower current requirements of higher impedances at a given power output can reduce cable losses and reduce the need for bulky conductors to connect to the loads.

Amp power and current

Reply #13 –
Quote
So you are saying that the amount of power the speaker requires and how much power the amplifier can supply are really two very different questions?
The amount of power the "speaker requires" depends on it's sensitivity (usually rated a xdB at 1 meter with 1 watt) and how loud you need it.    Speaker manufacturers will sometimes publish a "minimum recommended power", but that's pretty much useless.

Quote
I've heard people say that a high current amp will put out up to twice as much power into 4ohms as they will into 8ohms, and that a low current amp will probably only be rated at 8ohms and will probably say "8ohms minimum speaker impedance" on the back.
That's about right.  Two 8-Ohm speakers in parallel is 4 Ohms.  If you have an 8 Ohm speaker playing at 100W and you connect a 2nd one in parallel (and if nothing else changes) you've got two speakers playing at 200W... Twice the power (and twice the current) .    That's just like plugging a 2nd 100W light bulb into the socket...  Twice the power and twice the current.  Switching from an 8-Ohm speaker to a 4-Ohm speaker is like changing from a 100W light bulb to a 200W light bulb.

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...and that a low current amp will probably only be rated at 8ohms and will probably say "8ohms minimum speaker impedance" on the back.
Or, the specs might say 100W at 8-Ohms and 150W at 4-Ohms.    But, I wouldn't necessarily call that a "low current" amplifier...  It's probably current limited, or thermal limited, but it's all relative and it's putting-out more power and current than a "high current" amplifier rated at 75W into 4-Ohms. 

Amp power and current

Reply #14 –
Quote
hen what I don't get is that if the current requirements are determined by the load, not the amplifier, that a 'low' current amp could be supplying enough current at a given voltage, then wouldn't it be just as high current as any other high current amp?
I think you do get it.  There's a reason you almost never see an output voltage or current spec for a power amplifier...  The important thing is how much power it can drive into a given load.

Amp power and current

Reply #15 –
Quote from: Rich B on
Seriously speaking, how can one identify an amp with high current?


Why would you care about this?

Don't you really just want an amp that will provide enough power for your speakers to play loud enough without audible distortion?

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I have two stereo amps: one is rated 75 wpc (budget amp around 5k new), another 60wpc (about 14k new). The more expensive amp has more control on loudspeakers, plays cleaner at any volume.


Based on what reliable indication?

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Actually, it plays louder than the 75 wpc, but cheaper amp.


Based on what reliable indication?


Amp power and current

Reply #16 –
Quote from: Rich B on
Then what I don't get is that if the current requirements are determined by the load, not the amplifier, that a 'low' current amp could be supplying enough current at a given voltage, then wouldn't it be just as high current as any other high current amp?


Check it out. Is there a formal defintion of "High Current amplifier" that is agreed upon by a relevant professional standards organization?

I know of none, and I've looked.  That means that "High Current amplifier" is another one of those wonderful phrases that means whatever the person who uses it wants it to, no?

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If I listen to music at reasonable levels, and the load requires a certain current because the impedance dips, and the amp I have is a 'low current amp', but it can supply the current because the levels are reasonable, then it would supply the same current as any high current amp, no matter how big and powerful?


Exactly right!

Amp power and current

Reply #17 –
In an ideal amp the output power would double for each halving of load impedance:
10W into 8 ohms
20W into 4 ohms
40W into 2 ohms
80W into 1 ohm
...

Voltage would be close to 9V in each case, current would double like power.
"I hear it when I see it."

Amp power and current

Reply #18 –
Quote from: DVDdoug on
Quote
hen what I don't get is that if the current requirements are determined by the load, not the amplifier, that a 'low' current amp could be supplying enough current at a given voltage, then wouldn't it be just as high current as any other high current amp?
I think you do get it.  There's a reason you almost never see an output voltage or current spec for a power amplifier...  The important thing is how much power it can drive into a given load.


Okay, so this is a rather extreme example to test my assumption with.



McIntosh 1.2KW monoblocks. That's 1200W into 8 ohms and probably 2KW into 4 ohms, thereabouts.

So if I take this amp, compare it to an AVR, the big 1.2KW monoblocks will not be supplying any more current to a speaker than the AVR can, at any volume, until the point where the AVR runs out of current and becomes voltage-limited? That's quite amazing, actually, if that's true. Many people probably think that a big amp like the above can supply high current by default. I used to be one of those people.

Amp power and current

Reply #19 –
Quote
So if I take this amp, compare it to an AVR, the big 1.2KW monoblocks will not be supplying any more current to a speaker than the AVR can, at any volume, until the point where the AVR runs out of current and becomes voltage-limited? That's quite amazing, actually, if that's true.
If your AVR and the Macintosh are both running at 100W, it's not that amazing that the voltage & current will be identical...  At least it's not that amazing if you understand the basic relationships between voltage, current, resistance (or impedance) and power.

If you take an electronics class, Ohm's Law is the 1st thing you learn...  Once you know Ohm's Law it's just "common sense".    (Of course, you shouldn't have to take an electronics class before you choose an amplifier.)

And yes, that Macintosh amp CAN supply high current!  It can supply 1.2KW!!!!    If you want to make a dance club "thump", 1.2kW into a couple of 18-inch subwoofers should do the trick!


Amp power and current

Reply #21 –
Let me add to that linked thread and here, that you cannot assume that your music is as loud as a sine.

Classical music can have 20 dB lower average levels, today's overly compressed music is still lower by 8 dB. Usually there will still be peaks hitting full-scale though.

So if you calculate power requirements for 90 dB SPL based on sine tones, the average with real music will be lower by a couple of dBs.
"I hear it when I see it."

Amp power and current

Reply #22 –
I used to believe for a long time that more headroom offered better sound quality than an amplifier that was working closer towards its limits all the time.

An engineering friend recently told me said that an amp that is rated to supply 100 watts is actually operating at close to full efficiency at 100 watts,  vs a 200 watt amplifier supplying 100 watts.

He said that the larger amp supplying the same power as the lower powered amp will be dissipating more heat and will not be working as efficiently. Is any of this true? I thought the opposite would be the case.

Amp power and current

Reply #23 –
"I hear it when I see it."

Amp power and current

Reply #24 –
Where did you get the chart from? So then the engineer was correct.

So then that means that distortion is lower at close to full rated power too? Or is efficiency a different thing?