PASS LABS X250 Question

I would feel pretty confident that the amp can supply 500W into 4 ohms, for the following reasons:

1)This review indicates that it can.

2)The first paragraph that starts on page 10 of the user manual sort of implies that it can.

3)The specs on the last page of the user manual indicate a maximum output voltage of 65 volts, and maximum output current of 20 amps. I believe those are peak numbers, rather than the conventionally used rms values. The rms equivalents for those numbers are 46 volts and 14.1 amps. 500 watts into 4 ohms requires 44.7 volts and 11.2 amps rms, those numbers being less than the specified capabilities.

What is the input sensitivity for the X250 for full out-put, balanced and single-ended?

The user manual indicates a gain of 30db. That is a voltage gain of about 31.6 times. 250W into 8 ohms, or 500W into 4 ohms, corresponds to an output of 44.7 volts. 44.7/31.6 = approximately 1.4 volts sensitivity (the input voltage required for maximum rated output power).

That would apply to the unbalanced input. I suspect that an amplitude of 1.4 volts on each of the two signals at the balanced input (meaning a 2.8 volt differential) would also result in full power, but I'm not certain of that. The other possibility would be a 1.4 volt differential, or an amplitude of 0.7 volts for each of the two signals.

Best regards,
-- Al

Michael, my calculation was based on the assumption that sensitivity is defined as the rms input voltage that is required to drive the amplifier to its maximum rated rms continous power output. I'm not 100% certain, but I believe that is the correct assumption.

If I understand your post correctly, you appear to be assuming that sensitivity is defined as the instantaneous input voltage required to drive the amp's output to its maximum possible instantaneous output power level (the peak of a sine wave). That assumption strikes me as unlikely to be true, given that amplifier power ratings (and speaker power-handling ratings) are as far as I am aware always specified on an rms basis.

Confusion sometimes arises due to loose usage of the term "peak." "Peak power" can refer to the power corresponding to the peak of a sinusoidal waveform, but it is also often used to refer to "MAXIMUM rms power."

Regards,
-- Al

Michael, my contention appears to be further supported by information in the manual for the .5 series amps.

Note that the X250.5 (for which the output current, voltage, and power ratings are almost identical to those of the X250) is indicated as having a gain of either 26 or 30 db, and sensitivity at 26 db gain is spec'd as 2.24 volts.

Therefore sensitivity at 30 db gain is presumably 4db less than 2.24 volts, which is almost exactly equal to the 1.4 volts I had calculated!

Regards,
-- Al

Hi Mike,

I suspect that the 100 volt figure you are thinking of is peak-to-peak, not peak. Note in the manual that the maximum output voltage spec of the X250 is indicated as "plus, minus 65 volts," the 65V therefore obviously being peak, rather than peak-to-peak.

100V peak-to-peak corresponds to about 156 watts rms into 8 ohms, or 312W rms into 4 ohms, and would be well within the X250's peak-to-peak voltage swing capability of 65 x 2 = 130 volts.

Based on that voltage swing capability, as you will realize it can readily be calculated that the amp will not clip until an output power level of 264W rms (or more) into 8 ohms is reached, or 528W rms (or more) into 4 ohms.

Best regards,
-- Al

What we see is that it is common in high quality recordings to see voltages near 100V peak, with medium efficiency speakers. This means that when listening to the same music with most other amplifiers, the signal would be 'clipped'... "

Hi Mike,

I wouldn't take that statement too seriously. If it were true, most people would be experiencing intolerable clipping much of the time.

100V peak (200V peak-to-peak) is 625 watts rms into 8 ohms, or 1250 watts rms into 4 ohms. That would blow up many home speakers even if applied very briefly. And, using 90db/1W/1m 8 ohm non-planar speakers as an example of "medium efficiency speakers," 625 watts would correspond to a sound pressure level of around 112db at a listening distance of 10 feet.

Accurately reproducing the cannon shots on Telarc's notorious recording of the "1812 Overture" might require that, but I would expect that not much else will.

Best regards,
-- Al