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Pass Labs X350.5 Stereo Amplifier Power and Distortion Measurements


All measurements were conducted using our Audio Precision APx585 8 Channel HDMI Audio Analyzer following our rigid Amplifier Measurement Test Protocol.

I measured the voltage gain of the X350.5 and found it to be 26.5dB, which confirms Pass Labs’ spec of 26dB.  The gain does not change between unbalanced or balanced connectors.  In order to hit full power the X350.5 needed about 2.7Vrms so make sure the preamp you couple this baby with can give you clean output of at least 3Vrms.

Signal to Noise Ratio


Pass Labs X350.5 SNR @ 1 watt (A-weighted)

The X350.5 exhibited a very good low noise floor.  At 1 watt, I measured 88dB (A-weighted).  At rated power (350 watts), I measured 113dB.

Frequency Response


Pass Labs X350.5 Frequency Response @ Full Rated Power

The Pass Labs X350.5 exhibited ruler flat bandwidth from 10Hz to 40kHz with a gradual roll-off of about -2dB at 80kHz.  Pass Lab’s claims a bandwidth of 1.5Hz to 100kHz but doesn’t specify a tolerance.  This sweep was taken at 358 watts at full bandwidth from 10Hz to 80kHz with both channels driven.  Lesser designed amplifiers would shut down or not be able to deliver such a ruler flat response at this power level.  Frequency response linearity between both channels at 1 watt and full rated power was +/-0.06dB for 8 ohm loads and +/-0.08 dB for 4 ohm loads from 20Hz to 20kHz for 8 ohm loads, which is superb. This indicates very tight tolerances in parts selection and excellent overall engineering.  Because of this incredible linearity, I found the continuous full power bandwidth measurements tracked very closely with the 1kHz sweep power tests and thus only reported power figures for those test results.

Power Measurements

Pass_test.jpgUsing our Audio Precision APx585 8-channel HDMI analyzer, I conducted a full barrage of multi-channel amplifier tests on the Pass Labs X350.5. We tested power using three methods all of which were taken at < 0.1% THD + N:

  • Continuous Full Power Bandwidth (CFP-BW) from 20Hz to 20Khz into 8 and 4-ohm loads (up to two-channels)

  • 1kHz Power Sweep vs. Distortion (1kHz PSweep) - popularized by the print magazines, this is an instantaneous power vs. distortion test at 1kHz. The problem with this test is it often masks slew-related and/or frequency response problems some amplifiers exhibit at the frequency extremes, and thus inflates the measured power results. It does provide an instant gratification number for consumers to argue over on the forums, so we are now incorporating this test to please the masses.

  • Dynamic PWR - 1kHz CEA-2006 Burst Method testing. This is a dynamic power measurement adopted from the car industry similar to IHF method only a bit more difficult for an amplifier and more representative of real musical content.

Keep in mind most review publications don't do continuous power measurements and they usually publish power measurements into clipping at 1% THD + N. Our measurements are very conservative as we use a dedicated 20A line with no Variac to regulate line voltage.  We constantly monitor the line to ensure it never drops more than 2Vrms from nominal which in our case was 120Vrms. 

For more info on amplifier measurements, see: The All Channels Driven (ACD) Test



Pass Labs X350.5 Power vs Distortion Sweep Test 1kH
2CH Driven, 8 ohms (top pic); 1CH Driven, 4 ohms (bottom pic)



Pass Labs X350.5 Dynamic Power Test (1kHz)
Top Pic: 2CH driven, 8 ohms;  Bottom Pic: 2CH driven, 4 ohms


# of CH Test Type Power Load THD + N
1 1kHz Psweep 440 watts 8 ohms 0.1%
1 1khz PSweep 480 watts 8 ohms 1%
2 1kHz Psweep 372 watts 8 ohms 0.1%
2 1kHz Psweep 405 watts 8 ohms 1%
1 1kHz Psweep 550 watts 4 ohms 0.1%
1 1kHz Psweep 675 watts 4 ohms 1%
2 1kHz Psweep 490 watts 4 ohms 0.1%
2 1kHz Psweep 550 watts 4 ohms 1%
1 Dynamic PWR 595 watts 8 ohms 1%
2 Dynamic PWR 580 watts 8 ohms 1%
1 Dynamic PWR 700 watts 4 ohms 1%
2 Dynamic PWR 695 watts 4 ohms 1%

Pass Labs X350.5 Power Measurement Table

Pass Labs rates the X350.5 as follows:

  • 350 watts @ 8 ohms; 1 kHz, 1% THD + N
  • 700watts @ 4-ohm , 1kHz, 1% THD + N

Note: Pass Labs doesn't specify the number of channels driven for their power spec. 

8 ohm Power Results:

My measurements revealed that the X350.5 is a true powerhouse that lives up to the somewhat vague published power figures Pass Labs specifies for this amplifier.  This was especially true with 8 ohm loads as the X350.5 exhibited over +2dB of dynamic headroom into 8 ohm loads over the manufacturer’s 350wpc spec.

As previously stated, X350.5 produced virtually identical full power bandwidth and 1kHz power sweep tests indicating that the power supply and output devices were robust enough to deliver full rated power at any audio frequency, not just a sweet spot.  This is just one of many hallmarks indicating the X350.5 is a seriously great amplifier. 

4 ohm Power Test Results:

Even though I was running a dedicated 20A line, it was slightly hindering the accuracy of my power vs. distortion measurements into 4 ohm loads (both channels driven) as the line voltage sagged since we don’t use a VARIAC to hold line voltage constant.  Think about it, 120V * 20A = 2400 watts / 0.5% (typical Class AB efficiency) and you’re left with only 1200 watts or 600wpc which is right about where the limitations of my power testing was occurring.  Of course the amplifier’s max power consumption is rated for 1800 watts so it’s unclear just how much more power the X350.5 would have delivered into 4 ohm loads even under ideal bench test conditions.

Although the amp was showing 1% distortion driven at full power into 4 ohms, the analog waveform was still very smooth and unclipped.  The dynamic power results were only marginally better into 4 ohms than they were in 8 ohms.  I suspect the power into 4 ohm loads was somewhat limited by the transformer since it was optimized to drive 8 ohm loads.  I would have liked to see a large KVA transformer on this amplifier given the caliber of this product.  At these power levels, most people probably wouldn’t miss any additional juice.  The X350.5 could drive virtually any speaker load.  I didn’t need these test results to be convinced after just a few short listening sessions powering my new Status Acoustics 8T reference speakers.

FFT Distortion Analysis


Pass Labs X350.5 FFT Distortion Analysis @ 1 watt  

I ran FFT distortion plots at 1 watt (top pic) and full rated power (bottom pic) to determine how clean this amplifier really is.  At 1 watt, the spectral distortion was below the noise floor of my measurement equipment which is another way of saying “awesome”!     

FFT Spectrum Pass Labs X350.5 Class A Operation.jpg

Pass Labs X350.5 FFT Distortion Analysis @ 20 watts

In fact, the measured distortion spectra were excellent for the entire Class A operation of this amplifier (up to 40 watts peak).  You can see how down in the mud the distortion components are (-96dB for the 3rd harmonic below the fundamental).



Pass Labs X350.5 FFT Distortion Analysis @ 420 watts

At 420 watts, 8 ohms (which is above the 350wpc power rating) the X350.5 displayed a not so pretty distortion harmonic spectrum.   I observed the 3rd harmonic (35.251 + 37.57) dBV being 72.8dB down from the fundamental.  This unusually high distortion product is likely somewhat a deliberate result of employing minimal feedback in this amplifier design.  The results looked better at rated power but still not stellar from an FFT distortion measurement standpoint.  Despite this, the amp sounded fantastic at all power levels. It just goes to show measurements and real world listening experiences rarely line up as one expects.



Pass Labs X350.5 Crosstalk vs Frequency at full rated power

The sweep tests I ran on the X350.5 produced excellent results being less than 100dB at 1kHz and 80dB at 20kHz. I consider anything less than -40dB @ 10kHz acceptable and anything less than -80dB to be superb. The X350.5 met the superb minimum requirement at 20kHz no less.  Pass Labs did their homework with respect to board layout and component isolation to ensure minimal crosstalk which also assures excellent stereo separation.


Pass FrontThe Pass Labs X350.5 is a big, heavy, power-hungry amplifier.  It’s difficult to transport requiring either one herculean mover or two normal people to set it in place.  Because of its high bias into class A operation, it’s also very power-hungry.  I measured a steady state power consumption of 500 watts with the amp just sitting there in idle.  Pass Labs specifies a 600 watt idle power consumption which is likely based on long term operation after the amp is fully warmed up.

The X350.5 runs very hot so be sure to give it plenty of ventilation.  Green activists won’t be too keen on that and quite frankly I wish Pass Labs would offer the option to lower the bias current for more power conscious audiophiles.  This is the type of amplifier I’d recommend for critical two-channel installations as it would prove too costly and too energy demanding to setup a full multi-channel system employing several of these units.  A top shelf component like this deserves the very best accompanying equipment to show off its true potential.  Make sure you have worthy speakers, good room acoustics and a solid preamp capable of at least 3Vrms unclipped drive output with a very low noise floor. 


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Recent Forum Posts:

Rich Davis posts on October 12, 2013 16:33
There a LOT of VERY expensive amps on the market. In terms of how loud they go? Any good amp will probably drive most speakers past what is considered normal listening levels. I don't know if you've done any dB checks, but most of the time I only have my system at around 85 to MAYBE 95 dB on a continual basis. Anything louder than that is not typically good for your ears to have a system operating much louder than that. Most systems playing at that volume level are not using maximum output. Some speakers can handle large amounts or need large amounts of power to drive. Electrostatic, MBL, and other speakers might require or like large amounts of power, but in most cases, you'll probably not use much more than 20 or 50 watts, especially if you do things like bi-amping.

Don't judge a system by how loud it goes, since most of these places won't even turn up the system to maximum levels, because you can't sit within 60 feet of the speakers when they drive them to maximum spl. I have a pair of Meridian DSP5500s for mains, a DSP5000 center channel, with some other DevTech rears and a Talon sub and I can't sit in the same room after I turn up the volume knob past a certain point. Listening to a system in the 100+dB range is just not something good for your ears. When you go to a concert, break out your smartphone and a dB meter app and see what the dB is at your listening position.

Go to these sites and check out some of the most insane systems available. HigherFi - Worlds Best HiFi High-End Speakers and Audio or HIGH-END PALACE - The place for the world's best music systems!

And $50K for a pair of monoblocks isn't the high end, it's not more mid high end in terms of price. one can spend upwards of $500K for the Pivetta Opera or $200K for the Goldmund Telos 5000's, etc. so $50K for a pair of Pass is pretty cheap. :-)
Rich Davis posts on October 12, 2013 16:23
Pass makes nice stuff

Personally, when it comes to power amps, there are so many good amps out there and it's all a matter of matching up with the pre amp, cables, and speakers to achieve the desired end result.
Irvrobinson posts on October 26, 2012 23:21
surveyor, post: 917475
In the fact that it's more powerful than either the Mark Levinson or the ATI amps says it all- it truly a beast!

Actually, both the Levinson and the X350.5 are both merely in the middle of the respective manufacturers' product lines. If you want to get really silly with amplifiers both companies will fully accommodate you. Pass makes an X1000.5, and Levinson makes the completely ridiculous No53 monoblocks for $50K per pair. The Pass Labs monster is a comparative bargain at only about $35K per pair.
surveyor posts on October 26, 2012 22:00
gene, post: 917055
Yep an amp spends most of its time operating at low power so it's good to see how quite and clean it is there.

No doubt the ATI amp is a good one but it's not as powerful as the Pass Labs.

Using square waves to test an amp has little value these days when you can power sweep from 10-20kHz and beyond with modern test equipment.

The Pass Lab's amp displayed extreme linearity at all power levels for the entire audio band.

In the fact that it's more powerfull than either the Mark Levinson or the ATI amps says it all- it truly a beast!
craig7 posts on October 26, 2012 21:37
wow you guys reviewed a product ive actually seen in person before
and yeah… it is a pretty beast amp. ive never heard any system louder than it… it was incredible
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