Phase Linear 400 Series II Input Sensitivity Question

Wombat

Wombat

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Mar 31, 2023
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I recently completed q re-build of my Phase Linear 400 Series II that I have owned since it was new. It has all new power transistors, the White Oak Audio PL14-20 board from Mark Lincutt, new bias transistors, 15,000 uF 100 Volt Capacitors, rectifier bridge snubber, and upgraded ground bus bar. I also added the Watts Abundant speaker protection relay. The manual for the 400 Series II says it that input sensitivity is 1 volt RMS at 8 ohms and 210 watts per channel. My pre-amp has 2 volt RMS outputs. I was using the square root of watts times speaker ohms method to calculate input sensitivity of the 400. Now, I am not sure about how many watts per channel the 400 is putting out. Anyone have any idea as to what the RMS per channel on a 400 Series II is after the upgrades? Thanks in advance..
 
Output is limited by the original transformers rail voltage which is approximately plus and minus 80 VDC. Output into 8 ohms at the speaker jacks is conservatively about 200-210 watts per channel.
 
Generally a stock 400 , given railvoltage of 80 to 82 volts will clip at 230 to 235 watts into 8 ohms. A little more with the White Oak goodies...
 
The Phase Linear 400 Series II Service Manual says, "210 watts minimum RMS per channel into 8 Ohms from 20 Hz to 20K Hz with no more than 0.09% Total Harmonic Distortion."
That is 41 V RMS into 8 Ohms with a 1 Volt input. With more than 1 Volt input you will start to get clipping. When the clipping gets excessive you will hear it, but by then your ears will be on fire.
If your speakers are closer to 6 Ohms, you could expect 280 watts. You did not mention your speaker load.
 
It seems to me that Wombat is drawing a correlation between the maximum output level of the preamp and the maximum output power of the amplifier. According to his statement, the amp only needs 1 volt from the preamp to achieve maximum output, about 210 watts. Any further increase in the voltage from the preamp to the amplifier, drives the amp into clipping. The preamp can supply a maximum of 2 volts to the amp.

Regardless what some people seem to think, the AC line voltage directly determines the maximum output power of the amplifier. All of the upgrades and modifications doesn't change that, and that includes adding a farad of electrolytic capacitors. Under continuous sinewave operation, the transformer is the limiting factor. A boat load of capacitors will maintain maximum output power for hundreds of milliseconds. but maximum output power will eventually, slightly decrease because the transformer secondary voltage decreases under heavy load. The term used to describe it is voltage regulation. It is expressed in percent of change from no load to full load. I have yet to see anybody specify the AC line voltage when measuring output power, but it makes a slight difference.

For example, if the AC line voltage is only 112 volts it is likely the continuous output power of the amp is less than 200 watts and there is nothing wrong with that. If the home is next to a substation (like mine) and the voltage at the wall outlet is 125 volts, the amp would probably produce upwards of 220 watts. At the end of the day it matters very little if the output power is 190 watts or 220 watts. Both are plenty loud.
 
Thanks to all. I was mostly concerned about clipping. As has been pointed out, the 400 will melt your face off before you get there. I do like the idea of pumping all the O2 out and charging it with Co2. Does White Oak Audio sell a kit?
 
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