Hi
How much voltage drop is acceptable based on the load?
It is quite common to find an amplifier rated 200 watts per channel @ 8 ohms and, 300 watts per channel in a 4-ohm load.
That equates to a 6-volt drop when comparing the loads.
Is there a standard how low the voltage can deteriorate before it is classed as unacceptable for a lower impedance?
Cheers!
How much voltage drop is acceptable based on the load?
It is quite common to find an amplifier rated 200 watts per channel @ 8 ohms and, 300 watts per channel in a 4-ohm load.
That equates to a 6-volt drop when comparing the loads.
Is there a standard how low the voltage can deteriorate before it is classed as unacceptable for a lower impedance?
Cheers!
A very good amp will be ~-0.4dBV into a half impedance load.
a good amp ~-0.8dBV.
a bad amp ~-1.5dBV.
300W into 4r0 is about -1.24dBV relative to 200W into 8r0. It's bordering on bad.
An example of a good amp using just 1pair of 2n3771 in the output stage gives 110W into 8r0 and 200W into 4r0. That crimson 1704 loses just -0.4dBV into half impedance.
Cordell reckons that a good amp should deliver at least 80% more power into half impedance. This is equivalent to -0.46dBV
a good amp ~-0.8dBV.
a bad amp ~-1.5dBV.
300W into 4r0 is about -1.24dBV relative to 200W into 8r0. It's bordering on bad.
An example of a good amp using just 1pair of 2n3771 in the output stage gives 110W into 8r0 and 200W into 4r0. That crimson 1704 loses just -0.4dBV into half impedance.
Cordell reckons that a good amp should deliver at least 80% more power into half impedance. This is equivalent to -0.46dBV
Thank you AndrewT!
Based on those requirements 90% of the amplifiers would be classed as a “bad amp” for I’ve calculated voltage drops as much 12 volts moving from an 8 to a 4-ohm load.
What I find odd are amplifiers rated 2-ohm minimum offering a larger drop in voltage in 4 ohms than, amplifiers rated 4-ohm minimum.
Cheers!
Based on those requirements 90% of the amplifiers would be classed as a “bad amp” for I’ve calculated voltage drops as much 12 volts moving from an 8 to a 4-ohm load.
What I find odd are amplifiers rated 2-ohm minimum offering a larger drop in voltage in 4 ohms than, amplifiers rated 4-ohm minimum.
Cheers!
and then we hear complaints about lack of bass when the purchaser has been assured that the frequency response goes all the way down to 4Hz @ -1dB.
The ability to deliver current when the speaker demands it is crucial to the way the speakers sound.
Your conclusion is right. Most cheap amplifiers cannot deliver sufficient current. And some expensive ones are almost as bad.
BTW,
that 2ohm rating is almost certainly a 2r0 rating.
A 4ohm reactive speaker is far more demanding than a 2r0 resistive load.
The ability to deliver current when the speaker demands it is crucial to the way the speakers sound.
Your conclusion is right. Most cheap amplifiers cannot deliver sufficient current. And some expensive ones are almost as bad.
BTW,
that 2ohm rating is almost certainly a 2r0 rating.
A 4ohm reactive speaker is far more demanding than a 2r0 resistive load.
AndrewT said:
Definitely not more demanding for the power supply. The output stage though (or most likely an over-conservative protection circuit) might cause problems of course.
If an amplifier can drive the speaker load without distorting or being damaged it is good. How much the power increases when the impedance is decreased doesn't tell anything. It's perfectly possible to make an amp that will double power from 4 to 2 ohms resistive, but it might still be unsuitable even for driving a 16 ohm speaker load.
When the limit comes from a certain mechanism (fuse, overheating, second breakdown, current limit, certain kind of VI limiter, voltage clipping, etc.) it might be possible to have a rule of thumb for that specific mechanism - but not generally.
An amp with enough power supply capacitance will sound better than one with little if driven into clipping. How much the power increases with decreasing impedance doesn't tell which mechanism it is. If it's the transformer regulation that causes the voltage to drop (but there still is enough capacitance in the supply) it will sound much better than if the transformer has good regulation but there is too little capacitance.
But on the other hand, if your amp doesn't clip as heavily it will sound even better.
In other words, if you want to play music at 90Vpk into 4 ohms, the amp that can do 70Vpk at clipping into 8 ohm and 65V into 4 ohm loads will sound worse than the one that can do 120Vpk with 8 ohm loads or 90Vpk with 4 ohms (assuming the amplifiers are designed to not distort in another way, like VI limiters activating).
In this case the amplifier with stiffer supply is worse.
we are going to have to disagree on all these points.megajocke said:
we at least agree on one point.
AndrewT said:
I am realising how important current is every single day. Hopefully by 2010 I will have enough knowledge behind basic amplifier topology to build my own.
After comparing the numbers I feel like I was ripped off with some of the amplifiers I own. It is becoming very clear why many design their own amplifiers.
Cheers!
OMNIFEX said:
I wouldn't go so far as to say "ripped off". If they designed the MA5000 with a supply stiff enough to satisfy the purists, it would put out 12,000 watts per channel at 2R, weigh six hundred pounds, require 50A/240V service to get safety agency certs, and be priced accordingly. And the ODEP would kick in a whole lot sooner unless you cooled it with a 5000 BTU air conditioner.
But modest improvements could be made over most of what is out there. Like 50% more copper and iron, double the output stage SOA, and a full 20kuF per rail. That sort of thing won't improve 2 ohm power by much, but will get you more runtime at 1/4 or 1/8 power at that low a Z with better bass when driven to clip. And you can refine the front end circuitry easily and cheaply enough and get better sound across the board.
Thanks for sharing your input megajocke! There are a few things I am having difficulty understanding here. Hopefully you can share your thoughts based on my reply.
I always assumed the user is the one who has the power to destroy speakers. An amplifier cannot distort or destroy speakers unless the user drives the amplifier into clipping.
All of my speakers are 8-ohm nominal in which will only see an increase in its impedance based on how it reacts in the enclosure. How can an impedance rise at a particular frequency that is not a steady tone create havoc to the amplifier when it is still seeing a load?
I am not one of those blokes who are trying to extract every ounce of wattage from an amplifier. I am merely comparing the average voltage based on continuous pink or sine wave power at the given load.
Why can I not have a strong power transformer to give me a 2 – 3 volt drop comparing 8-ohm versus 4-ohm and, 4-ohm versus 2-ohm loads with enough capacitance to double the strength?
If I want 90 volts in a 4-ohm load, I would not bother hunting down an amplifier that offers 90 volts peak in a 4-ohm load. I would be short-changing myself. I am a firm believer of continuous sine wave power. I don’t fancy burst ratings to make the amplifier look more desirable on the specifications sheet. More times when I encounter such an amplifier, I divide the volts by 1.414 to find the continuous average power.
If the amperage is listed based on various impedance loads with duty cycles, I will use that instead of the advertised wattage chart.
Cheers!
megajocke said:
I always assumed the user is the one who has the power to destroy speakers. An amplifier cannot distort or destroy speakers unless the user drives the amplifier into clipping.
All of my speakers are 8-ohm nominal in which will only see an increase in its impedance based on how it reacts in the enclosure. How can an impedance rise at a particular frequency that is not a steady tone create havoc to the amplifier when it is still seeing a load?
I am not one of those blokes who are trying to extract every ounce of wattage from an amplifier. I am merely comparing the average voltage based on continuous pink or sine wave power at the given load.
Why can I not have a strong power transformer to give me a 2 – 3 volt drop comparing 8-ohm versus 4-ohm and, 4-ohm versus 2-ohm loads with enough capacitance to double the strength?
If I want 90 volts in a 4-ohm load, I would not bother hunting down an amplifier that offers 90 volts peak in a 4-ohm load. I would be short-changing myself. I am a firm believer of continuous sine wave power. I don’t fancy burst ratings to make the amplifier look more desirable on the specifications sheet. More times when I encounter such an amplifier, I divide the volts by 1.414 to find the continuous average power.
If the amperage is listed based on various impedance loads with duty cycles, I will use that instead of the advertised wattage chart.
Cheers!
Cheers wg_ski!
I am not concerned with 4-ohm to 2-ohm ratio as I am with the 8-ohm to 4-ohm ratio.
http://www.mc2-audio.co.uk/t3500/spec.asp
A mere 1-volt drop from 8 to 4-ohms
http://www.qscaudio.com/products/amps/powerlight3/powerlight3.htm
This amplifier offers a 9.5-volt drop from 8 to 4-ohms.
It is voltage drops like the ones shown above which brought forth my question.
I am not concerned with 4-ohm to 2-ohm ratio as I am with the 8-ohm to 4-ohm ratio.
http://www.mc2-audio.co.uk/t3500/spec.asp
A mere 1-volt drop from 8 to 4-ohms
http://www.qscaudio.com/products/amps/powerlight3/powerlight3.htm
This amplifier offers a 9.5-volt drop from 8 to 4-ohms.
It is voltage drops like the ones shown above which brought forth my question.
OMNIFEX said:
I'm pretty sure a 1000W amplifier will be able to toast a 1W speaker without driving it into clipping 🙂 I'm sorry, but I don't understand what you mean by the second paragraph.
Of course you can use a big transformer and lots of caps! But it will be big, heavy and expensive. You could make an amplifier with the same power output at 4 ohms and higher output power in 8 ohms with a transformer that is smaller but has higher voltage output.
Sorry for the confusion, I was not talking about peak power ratings. I was referring to the peak output voltage for a continuous sinewave output so don't worry 😀 I find peak voltages easier to work with than RMS voltages because they map better to power supply voltage. It is still continuous signals.
AndrewT said:
For a low-power (<300W or so) hifi amp the bigger transformer isn't that much a problem so I think in that case your demands are reasonable. For bigger amps it gets way out of hand though if you want to afford them or want to be able to carry them around.
If you think power supply droop matters, then can you explain how the DC voltage on the supply rails affects the output in a significant way if the amplifier is not clipping ? The PSRR at the frequencies of interest (< 40Hz) is (or can be made) insanely large in a competently designed amplifier.
Power supply capacitance should not be made too small of course. Clipping behaviour will suffer badly, and the increased ripple could theoretically be a problem.
Clarification on the earlier points:
Definitely not more demanding for the power supply:
Capacitors are assumed to be large, which they should be. A 4 ohm reactive load draws less current than a 2 ohm resistive. Those peak currents you often speak of do not matter, they are very short.
How much the power increases when the impedance is decreased doesn't tell anything:
An amplifier with too little current available around 0V out can still look good with resisitve loads but be totally unsuitable for a speaker load => doesn't tell anything (or at least not the whole picture)
It's perfectly possible to make an amp that will double power from 4 to 2 ohms resistive:
Very easy - use a voltage clamp. The amp can never reach "real" clipping.
An amp with enough power supply capacitance will sound better than one with little if driven into clipping.:
Are you sure you disagree to this? Why would less capacitance in the power supply sound better?
In this case the amplifier with stiffer supply is worse. :
If you disagree to this you are saying that the amplifier that is driven 3dB into clipping sounds better than one that isn't clipping. Are you sure about that?
There is no point in discussing your extreme to the point of stupidity examples.megajocke said:
I never intend to ask a 70Vpk amp to send a 90Vpk signal.
I never intend to listen to an amp that is habitually clipping.
End of discussion.
Then you agree that the 70Vpk amp is unsuitable for that particular job. The examples are not extreme. What's so stupid about comparing two amplifiers of about the same physical size and expecting them to deliver the same power?
Also, there is nothing extreme about expecting the output stage (or protection circuit) to be the limiting factor when driving reactive loads.
If you believe I'm wrong, then could you please explain in which way the DC voltage on the supply rails would affect the output when not clipping? If it doesn't then the droop doesn't matter. If it actually does then your point about acceptable droop can be valid.
Also, there is nothing extreme about expecting the output stage (or protection circuit) to be the limiting factor when driving reactive loads.
If you believe I'm wrong, then could you please explain in which way the DC voltage on the supply rails would affect the output when not clipping? If it doesn't then the droop doesn't matter. If it actually does then your point about acceptable droop can be valid.
Sometimes the voltage sag is also designed in intentionally as some sort of protection. That way the output transistors may remain in their SOA with a 4 Ohm load, while delivering a specified amount of power into the more common 8 Ohm loads. Some amplifier designers rely on the experience that people rarely use their amplifiers at continuous nominal output, but below 1 W average most of the time. The excess power above 1 W is for them just a marketing means and need not be substantial.OMNIFEX said:
There are several mechanisms at work.OMNIFEX said:
One is the presence of inductive and capacitive components. In a pure resistive load voltage and current are always in phase. In pure inductive or capacitive loads they are 90° out of phase. A common multi-way speaker with passive cross-over is a mix of resistive, inductive and capacitive elements. So the amplifier will see a varying phase shift between voltage and current at different frequencies. To make it worse the inductance of a voice-coil changes as it moves in its magnetic field and most components change their properties with temperature.
Another is self-induction. One of its effects leads to an impedance change, when the speaker is accelerated. During those moments the amplifier sees an impedance that is much lower than the nominal impedance or much lower than the measured impedance curve shows. Therefore the amplifier has to deliver for short amounts of time (much) more current than during a steady rung-in sine-wave or than into a resistive 4 or 8 Ohm load.
A third is reactance. A moving speaker membrane tries to maintain its speed and direction. If the energy that is stored in that movement becomes bigger than the energy the amplifier delivers, the voice-coil converts its behaviour from motor to generator and feeds energy back into the amplifier.
Tests with steady sine-waves and/or resistive loads do not show, how those mechanisms affect amplifier performance with a real music signal and a real speaker.
this is a well reasoned and presented argument.pacificblue said:
I agree wholeheartedly.
I even agree with the first paragraph that some manufacturers make a commercial decision to prevent the amp damaging itself into lower impedance loads.
That is the right of every manufacturer. If they can make their businesses run that way then good on them.
pacificblue said:
This part:
"During those moments the amplifier sees an impedance that is much lower than the nominal impedance or much lower than the measured impedance curve shows."
is not correct, the impedance does not change. There is no difference between what you describe as "reactance" and "self-induction" either, it's the same effect. The impedance curve describes all this behaviour (which is linear).
The reason that peak currents can be higher than most people expect is that a non-sinusoidal signal (like a transient) needs to be treated as a superposition of sines. The current can be calculated as the superposition of the contribution from each harmonic, using the impedance curve.
The nonlinear effects like voice coil inductance modulation are not described by the impedance curve of course, but they don't cause any significant changes in the current drawn.
Still, you are right that the steady state stuff/resistive load case doesn't tell the whole picture about peak currents at different output voltages, but this is only a concern for the output stage. There it is important though! VI-limiters activating or output stages blowing up isn't fun.
The power supply doesn't care about the higher peak currents that might be generated though and the (low pass filtered/average) current does not depend on if the load is resistive or reactive for a class B/AB amp. For a class D or H amp current draw even goes down with reactive loads.
pacificblue said:
Don't you just hate it when people don't understand basic electronics.....
When a load is reactive, the phenomenon is always associated with energy *storage*, not generation. And the magnitude of Z will always be higher than the minimum, which is ALWAYS pure real.
What you're describing is back EMF, not reactance. And again, wrong. Back EMF is modeled as a voltage source OPPOSING the applied signal, which will reduce the current that is drawn. It's Lenz's law or something like that. Now in the case of vented reflex boxes, at fb, the motion of the cone is damped (that is, at a local minimum) which tends to reduce back EMF to near zero. The impedance is real, approaching the DC resistance. Which is lowrer than the nominal impedance, of course. Current demand goes up, and you may even be able to smell it if you're playing loud enough.
Low impedances are a concern for an amp. Just not necessarily for the host of reasons that people cite. Most of the time it's just because the impedance goes lower than you expect, and a simple Z sweep will show it. Dual-driver hi-fi systems which use overdamped 2nd order electrical networks in the crossover to incorporate BSC are usually to blame. It's not uncommon for 3-ways to get down to 2.5 ohms in parts of the spectrum where the power density gets high. For PA work, bridging an amp into a dual-driver reflex box is commonplace trying to get every last watt. Typical 18's have DCR's of 5.5 to 6 ohms. Then tune the box to 40-45 Hz. And boost the bass. That puts a 2.75 ohm minimum right on your lowest notes with the amp working the hardest.
Yes, it does. The situation is very similar to the inrush current in transformers or other inductive motors. http://www.aes.org/e-lib/browse.cfm?elib=11462megajocke said:
Neither you nor I are right. I was talking about reactive energy and I guess the word reactance was so close that I subconciously used it wrongly instead.megajocke said:
Nevertheless reactance and self-induction are not the same. Reactance X is the imaginary part of a complex impedance Z as in Z=R+jX.
Self-induction describes the effect that any current through a lead induces a voltage that counteracts that current. So it usually increases the impedance. In transformers and motors it however leads to a sinking inductance for short moments after switching them on with AC before they are rung in. It also leads to storage of energy in the voice-coil, which is then released and fed back to the amplifier together with the energy generated by the membrane's inertia.
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