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Trevor Wilson[_3_] Trevor Wilson[_3_] is offline
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Default More than 30W per chanel Class A transistor amps

On 5/03/2019 7:11 am, Peter Wieck wrote:
On Monday, March 4, 2019 at 2:33:50 PM UTC-5, Trevor Wilson wrote:

**Most amplifiers that employ crappy power supplies, yes. The Krell
KSA50 is not such an amplifier. It's CONTINUOUS power rating is very,
VERY close to it's transient power rating. The continuous power of a
KSA50 is around 75 Watts. The transient figure is something like 76
Watts. Had Krell decided to skimp on the power transformer, then the
difference between the transient figure and the continuous figure would
be far greater. The weird thing is the manufacturers make a big song and
dance about the size of the transient number vs. the continuous one,
when the bigger the difference, the crappier the amplifier.


Back to those nits: I keep a power-amp that will make a continuous 175 watts @ 8 ohms all day and all night. However, due to its power-supply reserve, output devices and so forth, it will make a transient (as a function of value and time) up to 1,000 watts for a very brief moment, less for longer moments.


**And mine, with it's 5.5kVA (continuously rated) power transformer will
maintain it's power for as long as the mains breakers hold out. It's
transient power is VERY close to it's continuous power. Load impedance
is irrelevant (above 0.5 Ohms anyway). If the load impedance dips to a
low enough figure, then it can maintain a a couple of kW for a long time
(till the breaker opens).



Most speakers can handle transients and peaks far above their
'average' rating.

**Oh, absolutely. One need only examine the voice of a typical bass
driver and relate that to the power handling. Almost all domestic
speakers are rated to be connected to an amplifier of XX Watts and being
driven with music. Since music has, at worst a peak to average figure of
around 10dB, then one may assume that the average power applied to a
speaker is always somewhere less than 10% of the continuous power rating
of the amplifier.


Um. Not so much. Back in the day, AR made a huge point on the power-handling capacity of their speakers, and used "magic" such as ferro-fluid and so forth for heat dissipation.


**Ferro-fluid is used only in HF drivers and some mids and, except for
NEAR drivers, not in bass drivers. And it is the bass driver that limits
the maximum thermal limits of a speaker system. Except for some pro
drivers, few domestic bass drivers can cope with sine wave power levels
of more than 50 Watts or so.


a) Their speakers were rated rather low as compared to their contemporaries.


**[ANECDOTE] Back when I was a young(er) man, I built a pair of Bailey
T-lines, using KEF drivers. The bass driver was the venerable KEF B-139.
Rated at a massive 30 Watts. This was back in the day when manufacturers
like KEF rated their speaker as if they were required to handle sine
waves. As a young-ish man (early 20s) I tended to throw the odd party
for my friends. Much alcohol was consumed and much rock music was played
at high levels. Over time, I gradually improved my amplification,
culminating in my prized, Marantz Model 500 power amp. Rated at 250
Watts/channel, I measured the Model 500 at 310 Watts @ 8 Ohms. It's
superb fan cooling ensured that it could maintain high power levels at 8
or 4 Ohm loads for long periods (yes I know that maximum PDiss occurs at
around 40%). After one particularly grueling party, one of the B-139
drivers failed. It would have been at the end of around 5 hours of near
clipping levels for the Model 500. A couple of days later, I stripped
the driver down and repaired the voice coil lead in wire. No big deal.
There was absolutely no sign of heat distress on the voice coil. I
firmly believe that it was (slightly) sub-standard manufacture. However,
the same cannot be said about the electrolytic capacitors in the
crossovers. These had been severely damaged and were replaced with
plastic film types. IME, AR have always used adequately sized voice
coils, capable of handling far more power than my KEF b-139 drivers.

b) But their transient capacity was rated massively higher.


**No, it wasn't.


c) A great deal of my signal has a P/A of 20 dB. Some few examples have a P/A of 30 dB. Gregorian chant, single instrument, single voice may have the P/A of 10 dB that you reference, and those people may never turn the volume past 9:00 - but not all of us.


**I did say: "Worst case".



If a pre-amp has no gain over the original signal source, which is a nominal 2V, the peaks and transients will not be expressed - unless the actual listening level is a minimum of 30 dB below the average.


**Well that depends on the sensitivity of the amplifier.


No. That is a fact. If the pre will put out no more than 2V, then it will drive the amp to peak for some percentage of the time. But not past the peak, ever. So, if the P/A is 10 dB, that will be well within the capacity of the amp (and the speaker). A close-run thing at 20 dB, impossible at 30 dB. But, using the example of a transient, and 90 dB efficient speakers - a perfect example of "impossible" and why I picked that particular item.


**Again: It depends on the sensitivity of the amplifier.



Pre-amps are awful things, just like democracy. Except for the alternatives.


**A decent preamp need not be expensive.


That is absolutely true. A decent vintage pre may be had for well under US$200, tube or SS, and may, for approximately the same again or less, be tweaked to as-good-as anything off the shelf today.


**Perhaps.


--
Trevor Wilson
www.rageaudio.com.au

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