ieales
361 posts
06-17-2019 8:59am
Other than Class A, HiFi is not a continuous load. A ’typical’ tube amp is about 1A per 70W sum of both channels.
No kidding? I would never of known that without you telling me.
Is that what you took away from reading my entire post?
Hell, in the link you provided the guy used 14 amps for a purely resistance load for his bench tests for testing of dynamic headroom of a power amp. Why didn’t he use a real world testing? You know a power amp, connected to a load like speakers. Push the amp feeding it with a high dynamic source. Maybe the sound track from the John Wick Movie. That’s pretty intense.
Much ado is made in some circles about the inadequacy of 14ga wiring and how it will diminish dynamics.
14ga Copper wire has an ampacity of 28 amps. The U.S. National Electrical Code [NEC] rates 14ga THHN for 20A @ 75°C. Romex 14/2 is electrically equivalent to THHN. NEC limits 14ga to 15A circuits for a very adequate safety margin. See NEC 310.15(B)(16).
To find how much loss is incurred on a heavily loaded 14ga circuit, a Semi Professional espresso machine, plugged into the first socket on a circuit and the only device, was measured. It has a 1600w heating element with a resistance of ≈8.25Ω or ≈14A current draw. The heating element is PID controlled. This duty cycle is a square wave and presents transients far in excess of anything in a HiFi system. Using a digital sampling oscilloscope [DSO], 100%, ≈50% and 0% duty cycle load voltages were recorded.
No load measured 117.81vrms, load 114.02vrms integrating samples over 1 period for periods
with the highest and lowest peaks respectively. Over several minutes, 50% or 100% duty cycle made no difference in the loaded voltage peaks.
14ga Copper wire has an ampacity of 28 amps. The U.S. National Electrical Code [NEC] rates 14ga THHN for 20A @ 75°C. Romex 14/2 is electrically equivalent to THHN. NEC limits 14ga to 15A circuits for a very adequate safety margin. See NEC 310.15(B)(16).
Another guy that doesn’t understand how to read the NEC.
14ga copper wire is rated for 28 amps...... Maybe in free air. What is the length of the circuit for a 2% VD or even a 3% VD at 28 amps? Use the VD calculator... What is it at 20 amps.
Not continuous, ........ but say for a quick short draw of 28 amps or even 20 amps. Now please use a realistic number for the distance, length, of the circuit conductors.
Professional espresso machine
Really, an espresso machine? Does that espresso machine have a power transformer, bridge rectifier and electrolytic caps in it?
I wonder if Ralph Karsten would agree you can substitute an espresso machine for one of his power amps for such a test.
No load measured 117.81vrms, load 114.02vrms integrating samples over 1 period for periods
with the highest and lowest peaks respectively. Over several minutes, 50% or 100% duty cycle made no difference in the loaded voltage peaks.
What was the length of the wire in the circuit? Did I miss it?
117.81V - 114.02V = 3.79V VD. What’s that going to do to the power supply of a power amp as the caps are trying to recharge? Read Ralph Karsten quoted material below.
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I should have never mentioned the part about the NEC and the definition of continuous load for the 125% for sizing a branch circuit when the load is known. Way too much info for you. I should have left that whole thing out.
The VD calculator was meant to shown how current and the length of the wire are directly proportional to VD. NOTICE: No mention of the type of insulation used on the wire. NO 60C or 75C or 95C. Why? Because the copper conductor in 60C wire is exactly the same as the copper conductor in 75C and 95C insulated wire.
VD on THHN 75C insulated wire will be the same as TW 60C insulated wire.
What the hell do you think the rest of my post was about? Continuous load of a power amplifier? NO it was not!
The second issue is the high frequency current limiting. This is a bit trickier to understand, but its not quite rocket science. Almost any power supply consists of a power transformer, rectifiers and filter capacitors. When the the transformer voltage is higher than the capacitor voltage, the rectifier commutates (a fancy word for turns on and conducts). At that point the filter capacitors can charge up and will do so until the power transformer voltage falls low enough that the rectifiers cut off.
At that point the circuit using the power supply drains the filter caps. Since this happens 60 times a second, the drain is usually not very much at all, so its only at the very peaks of the AC waveform that the caps are be replenished. There might be only a few microseconds or milliseconds that this can happen, and quite a bit of current might have to flow during that time, essentially a high frequency event.
If the power cord limits current during this period, the performance of the circuit using the power supply might suffer, possibly due to increased IMD since the DC might have a bit more of a sawtooth on it than if the current was not limited.
That means if I did a rim shot on the snare drum to make it louder or kick the bass drum harder when hitting a crash cymbal there would be enough power reserve for this and the amp would not clip. I remember the guy telling me its like snapping my fingers and then waiting a few seconds and snapping them again.
Can you see any correlation in the two quotes? Any?
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