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The Parasound HCA-2200-II is surprisingly affordable, considering it was designed by John Curl, a noted designer in high-end audio. It is not a small power amplifier, not only because it’s rated at some 220 watts per channel into 8-ohm loads but also because it maintains Class-A operation up to an output of 6 watts. The robust output stages use six pairs of output devices per channel; these devices are beta-matched for more equal current sharing.
The HCA-2200-II is attractive in its black metal finish. Its front panel is rack sized, 7 inches high by the standard 19 inches wide, with a pair of beefy handles to assist in lifting and moving the amp. (A second pair of handles is on the rear.) Toward the left of the panel is a rocker power switch, accompanied by a pair of LEDs. The red “Standby” LED glows when the unit is first switched on, and the green “ Normal” LED turns on a few seconds later, when the amp is ready for operation. Red LEDs toward the right indicate “Current Overload” and output stage “Overheat” for each channel.
On the rear panel, besides the handy extra handles, are an IEC power-cord socket and an a.c. line fuse, fuse-holders for each channel’s positive and negative rails, and each channel’s input and output connectors (XLR for balanced input, RCA phono for unbalanced input, and dual five-way binding posts to allow bi-wiring). Also on this panel are toggle switches to select either the unbalanced or balanced inputs for each channel and a single toggle switch for selecting stereo or bridged mono mode.
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Mechanical construction is straightforward, using the two side-mounted heat- sinks as the foundation that everything else is built upon. A front subpanel and the rear panel bolt to the heat-sinks, and the top and bottom covers (of a heavier gauge than normal, about 0.1 inch thick) are mounted with machine screws to lips on the rear panel and front subpanel. The top and bottom are also bolted to tabs about half way back along each heat-sink, which ties these covers to the heat-sinks as well. Two toroidal power transformers, one per channel, are stacked inside the enclosure at the front and are attached to the bottom panel with a huge bolt. Approximately the middle third of the interior is taken up by four main filter capacitors and the attendant circuit boards mounted atop them. These circuit boards hold numerous bypass capacitors for the main filter capacitors, plus the circuitry for the front-end power sup ply and regulators. The main audio circuit is built on a p.c. board that takes about the rear third of the area and is mounted horizontally near the top of the amp.
Revolutionary is the way I would de scribe the use of completely quasi-complementary circuitry throughout this design. (Well, revolutionary for John Curl, any way, as he’s known for using completely complementary circuits!) The input stage is a complementary differential amplifier using matched dual N-channel and P-channel J-FET5. The complete input stage is formed by cascoding the J-FETs with bipolar transistors. In a design like this, where junction FETs are the input devices, there is no need for constant current sources per se; the FET source circuits of the two polarities are tied together with a common resistor that sets the overall operating cur rent for the input stage. Both collectors of the outputs of each complementary input differential amplifier are coupled to a complementary differential second stage, which uses bipolar transistors. The appropriate collectors of this second stage are tied together through a bias-spreading regulator and drive the input of the output stage’s driver circuit. This driver circuit consists of a pair of complementary MOS-FET devices acting as source followers. Output of the source followers is applied to the bipolar output transistors, which operate as complementary emitter followers. This arrangement of MOS-FETs driving bipolar outputs is an unusual one. Generous amounts of feedback are present in the input and second stages. Over all negative feedback is taken from the output back to the inverting input of the input stage. A d.c. servo is also connected from the amplifier output back to the inverting input of the input stage; this reduces the amplifier’s gain at d.c., thereby reducing any output d.c. offset to low values. The output of the amplifier passes through a re lay to the output terminals.
A clever arrangement permits high impedance for both input polarities when the balanced XLR in put is selected. A separate input source follower consisting of two N-channel J-FETs, one the source follower itself and the other a constant current load for the follower, buffers the negative input phase. The output of this source follower drives the bottom end of the shunt feedback resistor when the balanced inputs are used. The positive phase drives the noninverting side of the input differential amplifier in both modes. When the unbalanced mode is selected, the bottom of the shunt feedback resistor is grounded. In bridged mode, signal input is applied to the right channel only, and the output of the right channel is applied through an appropriate summing resistor into the inverting input of the left channel’s input differential amplifier. This causes the hot output terminal of the left channel to be 180 degree out of phase with that of the right, thus providing the necessary conditions for connecting the mono load between the hot output terminals of both stereo channels. In my opinion, this is not the most signal-pure way of bridging, as one phase of the output goes through one amplifier channel and the other phase passes through two amplifier channels. However, to do it in a more balanced way would require more front-end amplification for polarity inversion, which could itself affect the sound.
Elaborate protection circuitry monitors output stage current, output d.c. offset, and operating temperature. If any of these quantities is deemed excessive, the output relay is opened (and the appropriate front-panel indicator comes on, in the case of overtemperature or excessive current). This circuitry also functions as a turn-on time delay, holding off the output relay’s closure until the amplifier circuit settles out.
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Table 1—Output noise levels. IHF S/N ratios were 96 7 dB for the left channel and 98 4 dB for the right
Output Noise, uV
Bandwidth --- LEFT | RIGHT
Wideband ---1330 ---1414
22 Hz to 22 kHz ---39.3 --- 50.2
400 Hz to 22 kHz --- 41.0--- 42.0
A-Weighted --- 40.7 ---34.0
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[From ad in Audio magazine, 9-1993]
PARASOUND--affordable audio for the critical listener
30 IMPROVEMENTS THAT ARE MUCH EASIER TO HEAR THAN TO READ.
1 Refined topology with no ICs in signal path. 2 Hand-matched J-FET pairs for balanced differential drive. 3 More precise drive circuit board layout. 4 Gold switches select balanced differential/unbalanced direct. 5 Temperature stable gold-tip Holco feedback resistors. 6 Roederstein Resista series metal film resistors. 7 Faster tracking, wider range DC servo circuits. 8 Higher speed premium grade DC servo ICs. 9 Silver-clad internal wiring with improved shielding. 10 Sturdier premium gold RCA jacks. 11 Improved heat sink thermal dissipation. 12 Lavish application of film bypass capacitors. 13 Improved 60 ampere speaker protection relays. 14 Higher quality, high speed 15 amp output transistors. 15 Separate coarse and fine bias trim adjustments. 16 Modified power supply printed circuit board. 17 Custom-designed, hand fabricated AC power cord. 18 Greater bass extension, control and pace. 19 Greater common mode rejection. 20 More liquid midrange, sweeter high-end. 21 Unconditionally stable with any load. 22 Superior crosstalk and separation to beyond 20kHz. 23 Less higher-order harmonic distortion. 24 More focused, deeper soundstage. 25 More pure Class A power available. 26 Lower noise floor and improved S/N ratio. 27 Higher continuous power output. 28 Gold bi-wire speaker terminals accept larger gauge wire. 29 Elegant new internal layout. 30 All topped off with our new look and style.
As you can see, we’ve made a number of improvements to our premier amplifier. But to really appreciate our John Curl-designed HCA-2200 Ultra High Current Amplifier, visit your nearest Parasound dealer. Then, close your eyes and open your ears. Because listening to one of the finest high-end amplifiers is a vast improvement over reading about it.
Parasound Products, Inc. 950 Battery Street, San Francisco, CA 94111 • 1-800-822-8802 • Fax 415-397-0144
In Canada, distributed by: Absolute Sound Imports, 7651 Granville Street, Vancouver, BC • 604-264-0414
Since the channels in this sample were quite well matched, I am arbitrarily presenting data for the left one unless some particular measurement differs significantly between channels.
Voltage gain and IHF sensitivities were measured first. Results were 27.1 dB and 125.0 mV, respectively.
Frequency response at the 1-watt level into 8 ohms is plotted in Fig. 1 for open-circuit and 8- and 4-ohm loading. As can be seen, the frequency response shape and out put level are rather unaffected by load, and the 3-dB down point is about 200 kHz. Rise- and fall- times measured 1.6 p.5, which relates nicely to the 200-kHz band width. Square-wave responses are shown in Fig. 2. The top trace is for 10 kHz at an output amplitude of about 10 V, peak to peak, into 8 ohms. In the middle trace, a 2-p.F capacitor is paralleled across the 8- ohm load. Ringing with this load is nicely controlled (or damped). In the bottom trace, for 40 Hz into 8 ohms, no tilt is evident, which is testimony to this design’s extended low-frequency bandwidth.
Distortion (THD + N at 1 kHz and SMPTE IM) is plotted in Fig. 3 as a function of power output for 4- and 8-ohm loading. Measured performance here is very good. Total harmonic distortion plus noise as a function of frequency and power is shown for 4-ohm loading in Fig. 4. The Parasound produces rising levels of distortion above about 1 kHz, which happens in many, if not most, other amplifier designs. A spectrum of the harmonic distortion residue, for a 1- kHz signal at 10 watts into 8 ohms, is shown in Fig. 5. This spectrum would make any amp designer proud! There’s just a little second- and third-order distortion but nothing else except noise.
Damping factor versus frequency is shown in Fig. 6 for both channels. This high damping factor (or low output impedance) will help the amp deliver flat response to speakers that have wide variations in their impedance curves.
Output noise levels for the HCA-2200-II, along with IHF signal-to-noise ratios, are given in Table I. The noise in this amplifier is commendably low.
Interchannel crosstalk was found to be more than 100 dB down at frequencies up to about 1 kHz; it then rose at a rate of 6 dB per octave to a level of about 77dB down at 20 kHz. The amount of crosstalk was essentially identical in both directions.
The tests of IHF dynamic headroom yielded equivalent power levels of 315 and 578 watts for 8- and 4-ohm loads, respectively. This translates to dynamic head room figures of 1.6 and 1.76 dB. Maximum undistorted output into a 1-ohm load, when I used the IHF tone-burst signal to assess dynamic headroom and operated one channel, was a healthy ±60 amperes at the beginning of the 20-mS tone burst. It decayed to about ± 50 amperes at the end of the burst due to power-supply sag under load. Steady-state power output at the visual onset of clipping was 285 watts per channel into 8-ohm loads and 490 watts per channel into 4-ohm loads. Corresponding values for clipping headroom are 1.1 and 1.0 dB. This amp really puts it out!
The ac. line current at idle was about 3.5 amperes, indicating a healthy idling dissipation in the output stage. The a.c. line current started to increase at an output of about 5 or 6 watts into 8-ohm loads, which indicates Class-A operation up to this power level. With 4-ohm loading, the current started to increase at a lower level, 1 to 2 watts.
Use and Listening Tests:
Equipment used to evaluate the HCA 2200-II included an Oracle turntable fitted with a Well Tempered Arm and Spectral Audio MCR-1 Select moving-coil cartridge, a Krell Digital MD-1 CD transport feeding a PS Audio UltraLink and other D/A converters, a Nakamichi 250 cassette recorder and ST-7 tuner, and a Technics 1500 open-reel recorder. The Vendetta Research SP-2C phono preamp was used when playing vinyl records. The preamps were First Sound’s Reference II and Counterpoint’s SA-5000. Other power amps on hand included a Crown Macro Reference and a Metaxas Solitaire. Speakers used were Win Research SM-b broadcast monitors.
My first impression in listening to the HCA-2200-II was that it is a nice, musical amp. It ought to do well for Parasound and for John Curl’s reputation. I loaned the amp to a friend who has Apogee Diva speakers and normally drives them with Esoteric Audio Research EAR 549 200-watt tube mono amps. This guy is fussy and critical in his own way, and most of the amps I have sent his way “just don’t do it for him.” However, he used the Parasound for quite a while and generally thought it was very good.
Further listening to the HCA-2200 led me to characterize it as follows: Overall tonal balance sounds a little laid-back in the upper mid and high end, which makes it easy to listen to and serves the music. There is very little irritation in the sound, which in my book is very important. Resolution of detail and space is very good, al though not as good as some other amplifiers that excel in this area—but then again, they are usually more irritating to listen to with many CDs. Dynamics and bass control are excellent.
All in all, the HCA-2200 is an amp that I found myself listening to a lot. It worked flawlessly, with not a hint of improper operation of any kind. I think Parasound’s HCA-2200-II is a very good power amplifier and definitely recommend that you give it an audition.
This article adapted from Audio magazine, August 1993.
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