Pass Labs X250 Stereo Reference Amplifier

With the flood of powered speakers into professional studios, it is refreshing to see that companies, such as Pass Labs, Bryston and Bel Canto, are making high-end amplifiers that the discriminating mastering, recording and post guys can connect to their favorite passive monitors.
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With the flood of powered speakers into professional studios, it is refreshing to see that companies, such as Pass Labs, Bryston and Bel Canto, are making high-end amplifiers that the discriminating mastering, recording and post guys can connect to their favorite passive monitors.
Product PointsApplications: Studio, post production

Key Features: Two-channel, Class A/B, 250 watts per channel into 8 ohms

Price: $6,000

Contact: Pass Labs at 530-367-3690, Web Site
A couple of years ago, PAR's Tom Jung reviewed the X150 (PAR 11/00), a then new MOSFET design from amp master Nelson Pass. Using a "supersymetry" matched output circuit in a balanced design, he was able to tighten the bass response, decrease distortion and improve on the speaker-load handling versatility of the coveted, single-ended Class A Aleph amps he was building.

The X250 reviewed here, priced at $6,000 retail, ups the X150 a bit in terms of power and sound characteristics in a package that makes it a good match for high-end studio use. I have already heard about end-users coupling it with acclaimed speakers, such as the Lipinski Sound L-707, reviewed in June.

Features

The massive, weighty X250 (would you believe just under 100 pounds!) is a handsome-looking amplifier finished in silver gray with a cool-looking, blue backlit bias current meter on the front panel. It has huge side mounted heat sinks.

Claimed output is 250 watts per channel into 8 ohms. Plenty for most studio applications. Back panel features include T-handled binding posts for spades or wire termination (no provisions for banana plugs), balanced XLR and unbalanced RCA connectors and a 12 volt remote power turn-on connection. The main power switch is on the back, but a front-panel switch switches the circuit fully on.

The amp is too heavy for rack mounting and the thick front panel's ends are not drilled for rack duty location. Since it generates plenty of heat, it makes sense to put in its own amp stand.

A word about the patented Supersymmetry design Nelson Pass uses in the X series. He says the simple design improves the performance of a Class A balanced amplifier by "precisely matching the characteristics of the two balanced halves. To the extent that distortion can be made identical on the two output connections, they will not be seen at the balanced output."

The design utilizes matched components in a simple Class A circuit with only two gain stages. A balanced, single-ended Class A voltage stage, consisting of matched JFETS, drives a bank of 16 high-power MOSFETS per channel as followers with a minimal amount of feedback around the output stage.

In practical implementation of this design, the "X" amps use very little global feedback, which Pass believes can be detrimental, if overdone, to accurate amplifier reproduction. The sonic result is an accurate, open, smooth-sounding amp that can drive a variety of speaker loads.

Unlike the Pass Aleph amps that were pure Class A, this amp is a bit more efficient with Class A/B operation at the higher power levels, but each channel remains in pure Class A for up to 40 continuous watts (80 watts peak). The power supply to deliver all this current comes from a 2KVA toroidal with 200,000uF of storage. The amp draws 270 watts at idle and 1,000 watts at full power.

Other pertinent specs include a bandwidth from -0 dB at DC to -3 dB at 100 kHz. Distortion is conservatively rated to be under 1 percent at maximum power. Slew rate is listed at at +/-50V/uS and the damping factor is 250 ref. 8 ohms nominal.

In use

I was able to compare the X250 with my reference Bryston 14B SST, a more conventional design using bipolar output transistors and outputting a whopping 600 watts per channel into 8 ohms.

My setup included a Legacy Audio High Current monitor stereo preamp, a Rogue Audio Model 99 tube monitor stereo preamp, a Midas Venice 160 analog console and Legacy Class II ribbon monitors. Sources included guitar recordings, vocal recordings and a number of SACDs and DVD-As from PAR writers Alan Silverman and Tom Jung. Playback machines included an Alesis MasterLink, the original Sony SACD-777-ES SACD stereo player, Sony's flagship stereo/multichannel SACD player, the SCD-9000-ES, a Panasonic RP-91 DVD-Audio player. And at the tail-end of the review, I got a chance to listen to the fantastic Esoteric Audio DV-50, a multiformat DVD-A, SACD, CD, DVD-V player.

I auditioned the amp with various interconnects and speaker cables including Kimber, Alpha-Core and Westlake (Westlake's new unbalanced interconnects are among the best I have ever used).

I should say that the lack of banana plug input on the speaker posts was a pain-in-the-butt in that my reference Alpha-Core solid-copper cables could not be used. Luckily, I had a smaller gauge size of the same cable with spades.

Nelson Pass recommends an hour warm up before doing any serious listening. He says that measurements show that a cold amp does not bench as well a warmed-up amp. I would say the difference is subtle, with or without a warmup, since there were plenty of times I simply hit the switch and pushed play on the source and did not worry about the sound. In my experience, once a new amp is broken in, a solid state unit stabilizes at about 10 minutes from turn-on.

My first trial was recordings of my vintage 30 year-old D-35 Martin using the Audix SCX-25 mics and a NightPro PreQ3 stereo mic preamp. As expected from previous listening to Pass amps, the stereo imaging was incredible and the detail of string plucks was incredibly live sounding. The Bryston also does a remarkable job of reproducing these recordings through the Legacys, but I would say that the Pass stereo image was a touch wider.

On Steve Davis' Quality of Silence SACD, recorded by Tom Jung a few years ago, I was really impressed on how good the group's rendition of the old standard, "Smoke Gets in Your Eyes" sounded. Again, I noticed how real the SACD was reproduced via the Pass with incredible dimension and width in the stereo image. You can really zero on individual tracks and instruments with this amp. With the same recording, the bipolar output Bryston had a little more zing to the high end and the bass may have had a tiny bit more precision to it with the 4 ohm Legacys, but the extra width in the image of the drum cymbals was the dominant characteristic.

In comparison to the original X150 we reviewed in 2000, the X250 was smoother and more articulate. (A new, improved X150.5 is said to bring the sound from the X250 down to a more affordable $3,500 price at the lower power level).

Switching to DVD-Audio, I popped in the Fleetwood Mac Rumours album and zeroed on "Oh, Daddy," which showcases how good analog multitrack can be. With the wide, realistic image capability of this amp. It was amazing how I could zero in on the different instruments and the distinctness of the lead and backing vocals. The Bryston is very good at this as well, but the Pass just subtly portrays a slightly, bigger picture with the Legacys.

On a big band recording, I thought the Bryston did slightly better in the dynamics of the recording, but again the Pass had that wonderful imaging. Since they are basically the same price, could I choose one amp over the other? Nope. The Bryston has lots of power, can drive speakers below 4 ohms and its bass performance with any speaker is unsurpassed. Still, there is something about the Pass that brings just a bit more out of the mixes. I'll just keep 'em both.

Summary

Despite the powered speaker trend in professional studios, it is reassuring that separate amps like the Pass X250, when combined with state-of-the-art speakers, continue to set the standard for premier amplification when cost is not a limiting factor. Yeah, its $6,000 and not for every studio's budget. But if you are into listening to the intricate subtleties of recording and mastering, the Pass Labs X250 is an A-list amp.