Review and Measurements of Purifi 1ET400A Amplifier

[rdenney]

What does "under powering" mean, and why is it bad?

The conventional wisdom is that you don't want to overdrive an amplifier, so that it is clipping with noticeable distortion into the speakers, as being bad for both the amp and the speaker. This happens with inefficient speakers whose owners are trying to make play too loudly for the amp hooked up to them, and the amp runs out of the ability to sustain the waveform at the level being demanded.

It's the opposite of the concern that too big an amp would be bad for speakers. Too much power is bad for speakers only if it causes them to exceed their physical limits, which is grossly audible. Speakers can often handle much more clean power than people expect.

In short, if you hear distortion or unexpected bad noises, turn it down!

Rick "speaker power ratings are nutty" Denney

 

[Julf]

It's the opposite of the concern that too big an amp would be bad for speakers. Too much power is bad for speakers only if it causes them to exceed their physical limits, which is grossly audible. Speakers can often handle much more clean power than people expect.

I think I have to disagree. There are two things that destroy speaker drivers. One is exceeding physical deflection limits, as you say, but the other (and more common) way is by burning of the voice coil caused by excessive temperature - in turn caused by too much current/power through the coil.

Here is a good read:

Speaker Failure

 

[Rja4000]

I think I have to disagree. There are two things that destroy speaker drivers. One is exceeding physical deflection limits, as you say, but the other (and more common) way is by burning of the voice coil caused by excessive temperature - in turn caused by too much current/power through the coil.

Here is a good read:

Speaker Failure

Sure
One thing to take into account though is that extreme amp distortion will increase the high frequency power.
And the tweeter is usually the one needing, and, therefore, supporting, the less power.
So a small amplifier distorting heavily may also burn a tweeter coil.

 

[Julf]

One thing to take into account though is that extreme amp distortion will increase the high frequency power.
And the tweeter is usually the one needing, and, therefore, supporting, the less power.
So a small amplifier distorting heavily may also burn a tweeter coil.

Sure, but more because an amp produces much more than the rated power when clipping. Even a 100% clipped square wave has a limited amount of power in the harmonics. If the waveform was an issue, any heavily distorted electronic or guitar music would blow the tweeter too.

 

[Bruce Morgen]

Sure
One thing to take into account though is that extreme amp distortion will increase the high frequency power.
And the tweeter is usually the one needing, and, therefore, supporting, the less power.
So a small amplifier distorting heavily may also burn a tweeter coil.

Going by my experience, an underpowered (with respect to the listening SPL) amplifier can easily destroy a tweeter, even if the clipping doesn't result in readily audible distortion -- that happened so often for one speaker model I had back in the day that the mfr. had to design and offer a "tweeter saver" accessory as an option. Once I upgraded my amp from 25WPC to 55WPC, the problem never recurred. Here's the skinny on the speaker to which this happened:

 

[Julf]

Going by my experience, an underpowered (with respect to the listening SPL) amplifier can easily destroy a tweeter, even if the clipping doesn't result in readily audible distortion -- that happened so often for one speaker model I had back in the day that the mfr. had to design and offer a "tweeter saver" accessory as an option. Once I upgraded my amp from 25WPC to 55WPC, the problem never recurred. Here's the skinny on the speaker to which this happened:

I appreciate your experience, but this is what is referred to as "anecdotal evidence", and just one data point. While it is valuable, proper, verified scientific data would be even more valuable. Did you read the document I linked to?

 

[Bruce Morgen]

this is what is referred to as "anecdotal evidence"

Obviously -- but it confirms one possibility noted in the paper.

Amplifier Clipping is the Culprit (??)

 

[pma]

Going by my experience, an underpowered (with respect to the listening SPL) amplifier can easily destroy a tweeter, even if the clipping doesn't result in readily audible distortion

Correct.

 

[Julf]

Obviously -- but it confirms one possibility noted in the paper.

Amplifier Clipping is the Culprit (??)

Indeed. "It's not the fact that the amp is clipping that causes failures - it's the extra power that's delivered to the speaker.". As I wrote, an amp produces much more than the rated power when clipping. Feeding a higher-powered amp some heavily compressed and distorted pop music can result in exactly the same result as driving a lower-powered amp to clipping.

Unfortunately this is a topic with lots of anecdotes and "handed down wisdom", but little actual scientific evidence.

To quote Why Do Tweeters Blow When Amplifiers Distort?:

"A persistent myth in the audio industry is that clipping damages tweeters, so you should use a bigger amp to ensure more headroom so the amp won't clip. This claim is simply bollocks! Take the 100W amp described above, and replace with an amp big enough to prevent clipping ... even with the additional 12dB input signal as shown in Figure 7. Since a 100W amp was just below clipping with an average output of 16W, if we add 12dB that takes the peak amp power to 1.6kW (near enough) and the average power will be 254W.

Do you imagine for an instant that this amp won't blow the tweeters (and everything else) if the output level is increased by 12dB (until it's just below clipping)? Everything will fail, and usually fairly quickly if the speaker was designed for a 'nominal' 100W input. It is simply nonsense to imagine that the loudspeaker drivers in a 100W speaker can survive an average power of over 250W and peak power of up to 1.6kW.

If a user often turns their amp up to beyond clipping levels, they will probably do the same with a bigger amp. They might even turn it up more, because it won't have the distortion component which increases apparent loudness until the average power is a great deal higher. Such users will never hear signs of speaker distress if they can't even hear gross clipping. Speaker failure is a certainty, even if their 1.6kW amp only ever clips a few transients. They can expect the tweeter to fail, and the woofer to catch on fire.

So, while it's perfectly alright to allow perhaps 3-6dB or so of headroom for the power amps, that relies on that fact that it is reserved as headroom! If you use the extra power then there's no headroom any more, and all the effects explained will still happen, but at even higher power levels than described above."

 

[Bruce Morgen]

Indeed. "It's not the fact that the amp is clipping that causes failures - it's the extra power that's delivered to the speaker."

Here's my semi-educated guess at why underpowered amps fry tweeter voice coils:

It's not just "the extra power," it's the near near-vertical rise time of that "extra power" that allow what amounts to a hefty and sustained series of DC-like pulses that the high pass capacitor normally protecting the tweeter from DC somehow fails to block -- for reasons that are above my technical pay grade. As stated, this is just guesswork on my part -- but how else would one explain why my 25WPC amp fried tweeters while the 55WPC replacement didn't when playing the same material at similar levels?

 

[pma]

During clipping, signals with high energy of high frequency components are created, they pass through high pass crossover filter to the tweeter and depending on conditions may destroy the tweeter. This is quite usual scenario. And tweeters are rated to noise signal with "usual" music spectrum distribution, not to high power at high frequencies. You can fry them at several W, even if they are rated at 50 - 100W.

 

[Julf]

Here's my semi-educated guess at why underpowered amps fry tweeter voice coils:

It's not just "the extra power," it's the near near-vertical rise time of that "extra power" that allow what amounts to a hefty and sustained series of DC-like pulses that the high pass capacitor normally protecting the tweeter from DC somehow fails to block -- for reasons that are above my technical pay grade. As stated, this is just guesswork on my part -- but how else would one explain why my 25WPC amp fried tweeters while the 55WPC replacement didn't when playing the same material at similar levels?

Unfortunately I have to torpedo that theory . Clipping doesn't produce any "DC-like pulses", it just produces a (somewhat rounded) square wave, just the same as playing a heavily compressed and distorted guitar of synthesizer note. There are of course amps that go into all kind of weird error modes, even producing DC on the output, when heavily overdriven, but in that case the high pass capacitor should protect the tweeter. In your case the amp might have been driven into high frequency oscillation - something the capacitor doesn't protect against.

 

[Julf]

During clipping, signals with high energy of high frequency components are created, they pass through high pass crossover filter to the tweeter and depending on conditions may destroy the tweeter. This is quite usual scenario. And tweeters are rated to noise signal with "usual" music spectrum distribution, not to high power at high frequencies.

Just the same as playing a heavily compressed and distorted guitar of synthesizer note.

 

[Rja4000]

Indeed. "It's not the fact that the amp is clipping that causes failures - it's the extra power that's delivered to the speaker.". As I wrote, an amp produces much more than the rated power when clipping. Feeding a higher-powered amp some heavily compressed and distorted pop music can result in exactly the same result as driving a lower-powered amp to clipping.

Unfortunately this is a topic with lots of anecdotes and "handed down wisdom", but little actual scientific evidence.

To quote Why Do Tweeters Blow When Amplifiers Distort?:

"A persistent myth in the audio industry is that clipping damages tweeters, so you should use a bigger amp to ensure more headroom so the amp won't clip. This claim is simply bollocks! Take the 100W amp described above, and replace with an amp big enough to prevent clipping ... even with the additional 12dB input signal as shown in Figure 7. Since a 100W amp was just below clipping with an average output of 16W, if we add 12dB that takes the peak amp power to 1.6kW (near enough) and the average power will be 254W.

Do you imagine for an instant that this amp won't blow the tweeters (and everything else) if the output level is increased by 12dB (until it's just below clipping)? Everything will fail, and usually fairly quickly if the speaker was designed for a 'nominal' 100W input. It is simply nonsense to imagine that the loudspeaker drivers in a 100W speaker can survive an average power of over 250W and peak power of up to 1.6kW.

If a user often turns their amp up to beyond clipping levels, they will probably do the same with a bigger amp. They might even turn it up more, because it won't have the distortion component which increases apparent loudness until the average power is a great deal higher. Such users will never hear signs of speaker distress if they can't even hear gross clipping. Speaker failure is a certainty, even if their 1.6kW amp only ever clips a few transients. They can expect the tweeter to fail, and the woofer to catch on fire.

So, while it's perfectly alright to allow perhaps 3-6dB or so of headroom for the power amps, that relies on that fact that it is reserved as headroom! If you use the extra power then there's no headroom any more, and all the effects explained will still happen, but at even higher power levels than described above."

Please note that I never said the distortion was the only cause of breakdown.
I just said: a low power amp may ALSO blow a speaker.

 

[Bruce Morgen]

Unfortunately I have to torpedo that theory .

Sorry, skipper, just saying stuff like that doesn't "torpedo" anything AFAICT. Like I said, any clipping that was occurring was not readily audible like the examples you trotted out -- yet the tweeters got fried. Interestingly, the 25WPC (Sansui AU-555A) amp's output circuit was capacitance-coupled to the speaker terminals and the the 55WPC (Kenwood KM-8002) replacement was DC-coupled there.

 

[pma]

Clipping, 2kHz fundamental frequency. Shown in time and frequency domain.

 

[Julf]

Clipping, 2kHz fundamental frequency. Shown in time and frequency domain.

So the highest harmonic is at -20 dB.

 

[Julf]

Please note that I never said the distortion was the only cause of breakdown.
I just said: a low power amp may ALSO blow a speaker.

Noted.

 

[Julf]

Sorry, skipper, just saying stuff like that doesn't "torpedo" anything AFAICT. Like I said, any clipping that was occurring was not readily audible like the examples you trotted out -- yet the tweeters got fried. Interestingly, the 25WPC (Sansui AU-555A) amp's output circuit was capacitance-coupled to the speaker terminals and the the 55WPC (Kenwood KM-8002) replacement was DC-coupled there.

Your anecdotal example is noted. As I wrote, in your case the amp might have been driven into high frequency oscillation.

 

[Bruce Morgen]

Your anecdotal example is noted. As I wrote, in your case the amp might have been driven into high frequency oscillation.

It would have to be at a pretty high frequency to be inaudible and pretty powerful to burn out a tweeter. BTW, AFAICT that 25WPC amp was undamaged and I continued to use it as the system's preamp, relegating the power amp section to driving the rear speakers in my old QX 4-channel system. Frankly, I don't think your hypothetical is any more likely than mine -- perhaps someone more astute than I could examine the schematic of the AU-555A to assess the likelihood of our respective hypotheticals. Although we all know that clipping happens to any amp driven hard enough, it would seem that being susceptible to oscillation under the same conditions would be classified as a design defect. Remember, this problem was common enough that the mfr. had to come up with a "Tweeter Saver" accessory, so under your latest scenario any number of amps would have to be prone to that sort of oscillation.