PASS LABS X250 Question

Hello, I am curious if anyone knows if the X250 Amp doubles its 8ohm rating when driven with a 4ohm loudspeaker. I don't see anything in the documentation that mentions this.

If it doesn't double it's output can anyone explain why this would be the case.

Thanks for any info you can provide
You should email Nelson Pass directly he is very good at returning a response.
I hope you don't mind me piggybacking a question; What is the input sensitivity for the X250 for full out-put, balanced and single-ended?
All I could readily find is that the amp operates in pure class A up to 40 watts continuous into 8 ohms. It even allows for 80 watts peak in pure class A but converts to class A/B above that output. Most class A/B amps don't double down when the impedance halves but I think some do. Sorry for the non answer.
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
Pass amps are typically very unrated in power. My X150 amp puts out around 250 watts into 8 ohms and I think pushing 400 in 4 ohms. I don't doubt the X250 probably easily puts out 600-700 watts into 4ohms. I have never run out of steam with my X150 amp. This is on Eminent Tech speakers which are very inefficient. You should be fine. Plenty of head room with the X250 and I hear the X240 is the best sounding one of the bunch in that era prior to the .5 amps.
I agree that X250 can easy output 400 watts rms into the load of 4 Ohms.

I disagree with the calculation of the sensitivity. Almarg use relation between rms and peak in ....sinusoidal waveform which is not the case here.

I cannot calculate exactly but I would take as a reference peak output current of 65 volts which should be slightly below rail voltage (65-70 volts) and then devide it over 31.6 times so it would seem to me that its sensitivity is 2 volts (go get its peak power whatever it is). Balanced input , in theory should be double indeed or about 4 volts.
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."

-- 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!

-- Al
Hello Al,
I stand corrected.
Interesting that since 26 dB is equivalent to x20 then the rms voltage is 2.24 x 20 -45 volts and then as per your definitions, maximum output voltage [ = sqrt(2) x V(rms) ] = 65 volts.
If so this amp (200.5) would clip easily as signal from an amplifier into medium load speakers is about 100 volts and difficult loads much more then that (of course during the loud musical peaks)

Thank you
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
Al, thanks again.
Hello Al,

I meant +/-100 volts... I took this number from Spectron web site:

" reality an amplifier's voltage headroom is limited by the power supply voltage, which limits the peak voltage. Spectron's amplifiers are the highest voltage we know of in the audiophile world, using a plus and minus supply of 120V. By comparison most amplifiers use power supply voltages on the order of plus and minus 85V or even lower. An engineer evaluating an amplifier to determine the required headroom would look at the output voltage of the amplifier with an oscilloscope while playing the loudest music he would reasonably anticipate would be listened to. At Spectron we have done that. 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'... "
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
Hey Thanks to everyone for the information. It has been very interesting to read.

Based on what I have read here I feel pretty confident now that the Amp can most likely double it's 8ohm rating without much fuss. I may shoot Nelson Pass an email just out of further curiosity.

One thing that I found interesting about this Amp is how much cooler it runs when plugged into a dedicated Power Conditioner. This is something I recently noticed when switching from a Shared Power Conditioner configuration and the info posted here relating to Power Supply capabilities of the various Amps basically confirms what I have noticed on my own Amp.

The thing that I found most surprising about the temperature differences of the two configurations is that it wasn't like I was using a small Power Conditioner to begin with (it's a Shunyata V-Ray). My previous configuration also consisted of 1 x dedicated 20amp circuit, 10 ga Romex wiring to the breaker box and a full host of Shunyata power cords (20 amp Python from the wall to the V-Ray & 15amp Taipan to the Amp).

The new configuration includes an additional 20 amp Python and a Shunyata Hydra 6 on a second 20 amp circuit that only runs the Pass X250.

Temperature wise it's a huge difference going from almost not being able to touch the top of the Amp to a luke warm after a full nights listening session at an aggressive volume.

So it seems that it is a very hungry beast and is much happier with its own supply of juice. All in all, it definitely has plenty of headroom in my system even if it didn't double its 8ohm rating.

Thanks again for the info