Solid State Power Amp Reviews

Soon after I took over preparing Stereophile's biannual Recommended Components listing from the magazine's founder, J. Gordon Holt, in 1986, I ran into a problem. With so many products listed, the magazine was running out of the necessary pages to include them all (footnote 1). To solve this problem, I looked at how long a typical product remained on the market before being updated or replaced. The answer was 3–4 years. I therefore implemented a policy that unless one of the magazine's editors or reviewers had continued experience with a product, it would be dropped from Recommended Components after three years.

But what about a product that continues unchanged in production for years, or even decades? Consider Parasound's Halo JC 1 monoblock power amplifier, designed by legendary engineer John Curl and originally reviewed by Michael Fremer in February 2003. The JC 1 took up residence in Class A in Recommended Components in the April 2003 issue and remained there through the October 2018 issue. Every year for 15 years, either someone from the review team or I would drag the amplifiers out of storage and compare them with whatever we were using to determine if they were still worth recommending—they were.

However, Parasound's Richard Schram announced in the summer of 2018 that a revised version of the Halo JC 1, with significant reworkings by John Curl of the circuit and parts, was coming down the 'pike. I put my name down for a review.

External differences
The original JC 1 cost $6000/pair in 2003, and by the end of its production life the price was $8990/pair. (According to an online inflation calculator, $6000 in 2003 is equivalent to $8531 in 2020 dollars, meaning the increase in the amplifier's price tracked inflation.) As the JC 1+ costs $16,990/pair, it is significantly more expensive than its predecessor in real terms, but still less pricey than many other high-power, high-end amplifiers Stereophile has reviewed in recent years.

The Halo JC 1+ is rated as being able to deliver 450W into 8 ohms, 850W into 4 ohms, and 1300W into 2 ohms, these powers slightly greater than those offered by the original amplifier. Viewed from the front, the JC 1+ looks very similar to the two-channel Halo A 21+ that Kal Rubinson favorably reviewed in March 2020. The only visible changes from the JC 1 are Curl's signature printed on the front panel, the absent THX logo, a subtler blue LED in place of the original bright red one, and aluminum end caps with gold highlights.

From the rear, the JC 1+ still has two pairs of loudspeaker binding posts, these of higher quality than the original's; a switch to select Normal or Low output-stage bias; and the usual turn-on and 12V trigger switches and connectors. While the new amplifier still has single-ended RCA and balanced XLR input jacks, along with a switch to optimize the input circuitry for each, there are now pass-through output jacks. In place of the original's ground-lift switch, there is now a switch to allow the amplifier's gain to be set to Normal or Low, the latter reducing the gain from 29dB to 23dB for use with preamplifiers whose volume controls can't be used in their optimal positions or when the user's speakers are sufficiently sensitive that hiss can be heard.

At 83lb, the JC 1+ is 30% heavier than the JC 1. All the other differences reside inside the chassis.

Internal differences
A detailed explanation of all the changes and improvements incorporated in the Halo JC 1+ can be found on the Parasound website. To summarize these:

• In almost all amplifiers, the input stage power supply is shared with the output stage supply, which can affect the input-stage behavior when the output stage is subject to heavy demands for current. The original JC 1 had additional secondary windings on its power transformer to feed the input and driver stage. The JC 1+ has a separate R-core power transformer that supplies ±89V to high-speed, soft-recovery bridge diodes and Nichicon filter capacitors totaling 22,400µF. An R-core transformer has minimal capacitive coupling between its primary and secondary windings, which should reduce AC noise. In addition, the JC 1+ uses a pair of Bybee Music Rails, which are patented active, high-frequency noise filters.

• The new amplifier's printed circuit boards were designed by Carl Thompson, who was responsible for the boards in John Curl's Vendetta Research phono stage, as well as for the JC 1 and other Parasound products. The JC 1+'s shielded input- and driver-stage circuit boards use an FR408 substrate, a substance that was developed for ultra-high-speed applications in supercomputers and aerospace.

• As in the JC 1, the JC 1+ input stage uses hand-matched pairs (footnote 2) of Toshiba 2SJ74 P-channel and 2SK170 N-channel J-FETs. Parasound and, I believe, Ayre Acoustics have invested heavily to secure an ample stock of these no-longer-manufactured, low-noise J-FETs. While the JC 1 driver was a single-stage circuit, for the JC 1+, John Curl designed a two-stage, cascode driver that would have some of the favorable attributes of vacuum tubes.

• As the number of high-performance loudspeakers with impedances that drop below 2 ohms is on the increase, Curl increased the number of Sanken NPN and PNP bipolar output transistors from 18 to 24. The output-stage circuit boards are now mounted vertically rather than horizontally, which should result in more effective heat dissipation. To deal with the increased current, the copper traces on the amplifier's main circuit board and the output-stage circuit boards are twice as thick as on the JC 1.

• The peak output current is specified as 180A, supplied by Nichicon Gold Tune capacitors, two more than the JC 1. Richard Schram says that although the Gold Tune capacitors were discontinued years ago, Nichicon continues to manufacture them exclusively for Parasound.

Listening
As the JC 1+ operates in class-A up to 25W into 8 ohms when its bias is set to Normal, it runs hot. After a few hours of playing music, its top panel measured 105.6°F (40.9°C), the heatsinks 119.2°F (48.4°C). This amplifier needs to be well-ventilated, though if space is at a premium the output-stage bias can be lowered with the rear-panel switch, which reduces the class-A power to 10W into 8 ohms. In order to be able to use my PS Audio DirectStream D/A processor's volume control set sufficiently high so that its resolution would be preserved, I set the gain switches of the JC 1+ monoblocks to Low. I left the gains at Low when, for some listening sessions, I used the sample of Parasound's Halo JC 2 preamplifier, also designed by John Curl, that I reviewed in March 2008.

Primary loudspeakers were the floorstanding Vimberg Minos that I reviewed in April 2020, though I also used my KEF LS50 minimonitors for some of the auditioning.

Having let the Parasound monoblocks warm up, I started listening. My initial impressions reminded me of the first time I heard a Krell class-A amplifier, in the early 1980s: low-frequency authority combined with superbly precise stereo imaging and a deep, stable soundstage. Those early Krells could be a touch grainy-sounding in the treble, but the Halo JC 1+'s high frequencies were smooth and clean.

Tidal pulled up "Private Investigations" from Dire Straits' Love Over Gold. Back in the fall of 1982, I had obtained a prerelease CD of this album and was playing it at an English audio show, with a system featuring Arcam-driven Celestion SL6es, to give audiophiles a taste of CD sound. On the Vimbergs powered by the Parasounds, this atmospheric track sounded way better than my almost-four-decades-old memory would have predicted. I know, I know, different mastering perhaps; yes, digital playback has improved a lot over the years; and I can't remember what I had for dinner last Tuesday, let alone anything other than a broad-brush overview of what happened 38 years ago. But what I was now hearing with this track gave no clue to its early-digital provenance.

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Footnote 1: A problem that continues to this day, now that Jim Austin prepares the listings.

Footnote 2: J. Gordon Holt and I visited John Curl in 1987 and witnessed him hand-matching small-signal FETs for use in the Vendetta phono preamplifier.