Manufacturer's Specifications:
Frequency Response: 1 Hz to 300 kHz, +0,-3 dB, through line inputs.
Maximum Output: 7.5 V.
THD: 0.05%.
IHF-IM Distortion: 0.05%.
Slew Rate: Greater than 20 V/uS.
S/N: 77 dB, A-weighted, for 5-mV signal at MM or MC input.
Input Sensitivity: High-level, 150 mV; MM phono, 0.3 mV at 1 kHz for 0.5 V output.
Phono Input Overload: 155 mV at 1 kHz.
Phono Input Impedance: MM and MI, 47 kilohms; MC, 5.1 kilohms; adjustable (see text).
Dimensions: 17 in. W x 2 1/2 in. H x 14 in. D (432 mm x 63.5 mm x 356 mm).
Weight: 20 lbs. (9.1 kg).
Price: $795.
Company Address: 6029 Reseda Blvd., Tarzana, Cal. 91356.
The design of the Kinergetics KPA-1 preamplifier at tempts to go beyond conventional standards of "purity" in its quest for sonic excellence. Many audiophiles believe that mechanical contacts, magnetic materials, and ordinary wire can alter sound reproduction in ways that are not conventionally measurable as distortion. The Kinergetics preamp therefore omits balance and tone controls, because they might degrade the unit's sonics. Kinergetics says that switch contacts may interfere with low-level signals, so the preamp's three phono inputs are not switched in and out, but are always active and mixed in a special amplifier, requiring the owner to insert shorting plugs in two of the inputs. This is truly purism in design.
Kinergetics' other circuit innovations go even farther.
Components felt to alter a signal's sonics are actually wired into the KPA-1's circuitry, not just simulated by RLC net works. These components are applied to special amplifier stages in such a way as to generate inverse distortion, in order to cancel similar distortions thought to exist in the input signal. It is difficult to test the validity of this concept because the inverse distortion, like the distortion in the input signal, can seldom be measured.
Magnetic hysteresis, the tendency for a material to stay magnetized, is a gremlin particularly singled out by Kinergetics. While hysteresis is vital to making permanent recordings on magnetic tape, the same effect causes distortion if it occurs in a playback head or phono pickup. The KPA-1 contains a tape playback head mounted on a p.c. board to compensate the tape input. Kinergetics says the tape re cording process will probably contain many mechanical contacts, so eight contacts per channel are wired in series to correct the incoming signal. Two stereo phono cartridges are likewise designed into the circuit to compensate moving-magnet and moving-iron cartridge hysteresis. Moving coil cartridges are not compensated. The output amp section uses long p.c. traces and a short piece of interconnect coaxial cable to develop a signal that counters whatever wire distortion may be present.
Control Layout
The KPA-1 is available in a silver or black metal cabinet. Larger than many preamps, its 14-inch depth and attached power-supply module make it difficult to place on a typical bookcase shelf. It is of the standard, 19-inch rack width and has mounting holes; because of its depth, the rear should also be supported with rack-mounting brackets.
Controls on the front panel, from left to right, are a power on/off switch, "Source," "Monitor," "Mode," "Phase," and "Volume." The power toggle does not switch a.c. to the preamp, which is always on if plugged in. Rather, it controls the power-module relay, which switches three of the six 120 V a.c. outlets on the separate power-supply chassis. No balance control is needed, because the volume control is a friction-coupled, dual rotary device with a section for each channel. The unit has no tone controls and no subsonic filter.
On the rear panel is a row of 24 RCA jacks, with each input symmetrically separated to preserve the dual-mono isolation of channels. (For instance, the left- and right-channel main output jacks are 13 inches apart.) Therefore, the owner must split paired interconnect cables to plug them into this preamp. The three pairs of unbalanced RCA phono jack inputs for MC, MM, and MI cartridges are always active and connected to the RIAA equalization network, so Kinergetics supplies accessory shorting plugs to be inserted into the unused phono inputs. (The dealer can also disable unused inputs permanently, as well as change components to optimize loading for particular cartridges.) There are additional high-level inputs, including RCA jacks for two tape decks, a tuner, and two auxiliary sources, as well as the unbalanced RCA output jacks labeled for the two tape recorders.
The preamplifier's body consists of a 1/8-inch aluminum front panel attached to a U-shaped steel chassis with a removable top cover. Threaded inserts and machine screws are used to hold the top on, a very desirable technique. A thick, four-foot-long, non-detachable d.c. power cable runs from the back panel to the unit's power-supply module. The preamplifier circuitry is mounted on a large, high-quality master circuit board, which is laid out in a symmetrical dual-mono design on either side of the center line. The upper surface uses a ground plane foil, and the circuitry traces are on the underside. Virtually no hand wiring is present, except for the termination of the wires to the remote power-supply module. The RCA sockets are ordinary nickel-plated metal but are directly soldered to the rear of the p.c. board.
Similarly, all front-panel controls are soldered directly to the circuit traces at the p.c. board's front edge. The circuit board is huge, allowing ample space between stages, and there is less crowding of parts in this unit than in any other preamp we've tested.
Component quality is high, including that of the compensation items mentioned above-two phono cartridges, a tape head, coaxial cable, and eight contacts per channel (soldered in series). This all-bipolar preamp uses discrete parts, with the exception of two op-amps per channel used as d.c.-servo amps to prevent d.c. offset from appearing at the outputs. The resistors are almost all 1% precision types, and the capacitors are all high-quality polypropylene or other precision types. The power-supply design allows unregulated d.c. to run into the preamp via the thick power cable. There, it is regulated in three stages on the preamp's main p.c. board, and is dropped from ± 24 V to ±20 V and finally to ± 17 V.
Circuit Description
Fig.
1--Simplified block diagram, KPA-1 preamp. Note absence of switching in
the phono stage, compensated input stages (see text), and location of volume
control after last gain stage.
Fig. 2--Simplified block diagram of a conventional preamplifier.
The KPA-1 utilizes an unconventional circuit layout which is shown in Fig. 1, a simplified block diagram of a single channel's signal routing and gain structure. Tuner and AUX inputs are wired directly to the selector switch and, when selected, are applied to tape out and output amp in, just as in an ordinary preamp. Tape input goes through the unity-gain compensating stage discussed earlier. High-gain pre preamps with hysteresis compensation are used for MI and MM phono inputs. Their outputs are mixed with that of the MC pre-preamp and further amplified by the mixer stage before being applied to the passive RIAA equalization net work. Another 14 dB of gain makes up for the equalization loss, and signal is fed to the "Source" selector's phono contacts.
Unlike typical preamplifier circuitry (shown in Fig. 2), the KPA-1's volume control attenuation is applied after the line amplifier's 20-dB gain. Usually the selector switch is followed immediately by the volume control, so levels are cut down before a similar 20-dB amplifier. Owen Bennett, the unit's designer, explained that his design priority was to avoid any switching in the low-level phono stages. With three active phono inputs, signal-to-noise ratio was optimized by using shorting plugs in the two unused phono inputs and having the volume control follow the line amplifier. However, this design trade-off exposes the Kinergetics preamp's line section to very high gain levels without attenuation, and it runs the risk of overloading this section with only moderately high signals.
Measurements
Two KPA-1 preamplifiers were tested, prompted by the first unit's fairly high distortion reading of 2.75% THD + N when the MM input was driven at 50 Hz to produce output of 1.75 V at the tape outs. This affected the THD + N reading only in the MM position, so other measurements were carried out on this first sample. The second unit generally measured quite similar to the first, except for the MM distortion peak. This second sample measured less than 0.1% THD + N, 20 Hz to 20 kHz, tape input to main output, at maximum gain, and at 7.5 V output. These satisfactory measurements indicate that the KPA-1 should be considered a quiet, low-distortion preamp.
Fig. 3--Phono gain and frequency response, measured at tape output.
The MC phono input provided a maximum gain and a signal-to-noise ratio of 70.9 dBA, measured for a signal level of 0.775 V (0 dB) at the tape output. Phono frequency response (Fig. 3), also measured from phono in to tape out, was ± 1.5 dB, 20 Hz to 20 kHz, for the MC and MI stages, and ±2.0 dB for the MM input. This frequency response performance might have produced the audible differences between the KPA-1 and our reference preamps.
As mentioned above, the preamp's unusual gain structure achieves low noise at the expense of sensitivity to overload.
The unequalized phono signal from any of the three phono inputs is heavily amplified before the passive RIAA equalization stage; clipping occurs in the 19-dB gain mixer stage, with even moderately high-velocity recordings. Input over load was at about 23 mV for MM and MI phono inputs and 3.2 mV for the MC input, very low by today's standards. The passive equalization does not protect the mixer stage from high-velocity, high-frequency signals.
Note the placement of the 20-dB output amplifier stage before the volume control instead of after it. This output stage, which amplifies whatever is selected via the selector switch, has a healthy output clipping point of about 13 V; however, a 1.3-V input will drive it to clipping. As mentioned above, most preamp designs place the volume control just in front of the line amp, so levels are cut down before the 20 dB of gain from the line amplifier.
What might appear to be phono input overload is actually overload in the output section. A signal of 8.3 mV at the MM input-or 1.53 mV for MC cartridges-is enough to produce the 1.3 V necessary to clip the output stage, and these conditions are regularly exceeded when playing modern recordings. The manufacturer agrees that the KPA-1 has a lower overload point than other preamplifiers but believes that the overload condition would not regularly occur in home use.
Use and Listening Tests
Equipment used to evaluate the KPA-1 included a Linn Sondek turntable with a Magnepan Unitrac 1 arm; Yamaha MC-1000 and Shure V15 Type V-MR cartridges; Meridian, Technics, and Magnavox CD players; a Mark Levinson ML-7 reference preamp; Mark Levinson ML-9 and Crown Micro-
Tech 1000 solid-state power amps, and B & W 801F Special and Snell Type A-III speakers. Controlled listening tests were carried out with an ABX Co. comparator after matching outputs of the KPA-1 with coauthor Clark's preamp. New Monster Cable was used between the amp and speakers.
Clark's CD player has both fixed and variable outputs.
When this player's output was set low, the KPA-1's output section was not overdriven, and the resulting sound was free of clipping. However, using the CD player's fixed high level output in a double-blind A/B/X test, the KPA-1 distorted, and it was correctly identified in 16 out of 16 controlled attempts, a statistically significant result.
Coauthor Greenhill used the KPA-1 in his system with phono cartridges and a tape deck. In open, non-A/B/X listening tests, a moving-coil cartridge displayed an overly bright sound and a flattened sonic perspective, while the MM response was smooth and showed good separation of instruments and voices. Bass peaks from several Telarc orchestral records seemed to be softened, overly full and lacking in impact. Using a Tandberg TD-20A-SE 15-ips reel-to-reel deck, the KPA-1 reproduced a pre-recorded Roumanis Recordings tape of a Schubert quartet with intimacy, ambience, and good recovery of instrumental timbre. The phono section of the KPA-1 was subjectively very quiet.
The KPA-1 preamplifier, to sum up, is an interesting and imaginative unit which exemplifies purist circuit construction. This is revealed in the many quality parts in the main signal path, e.g., superior capacitors and precision resistors. The intent of the design goes further, however, in that it attempts to compensate distortions in other parts of the audio chain. The output stage's location appears to us to make the KPA-1 vulnerable to overload, and we look for ward to a Kinergetics preamplifier with a revised gain-block structure.
-Laurence L. Greenhill and David L. Clark
(Source: Audio magazine, Aug. 1986)
Also see:
Klyne SK-5A Preamp (Jan. 1988)
Kensonic Accuphase C-200 Control Amplifier, P-300 Power Amp (Mar. 1975)
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