Product Review - Carver A-760x THX Stereo
Power Amplifier - May, 1997
John E. Johnson, Jr.
Click here for larger image
Carver A-760x Stereo Power Amplifier; Two channel power
amplifier; THX Certified; 380 watts rms/ch into 8 Ohms, both channels driven, 20 Hz - 20
kHz; 600 watts rms/ch into 4 Ohms; 1,150 watts/ch into 2 Ohms - IHF dynamic power;
Bridgeable mono power 1,200 watts into 8 Ohms; Frequency response 20 Hz - 20 kHz + 0 - 0.2
dB; Input impedance 47 kOhms; Damping factor 150; Sensitivity 1.0V rms for 100 w/ch output
into 8 Ohms; THD 0.08%; S/N 119 dB; Size 5 3/4"H x 19"W x 19"D; Weight 39
pounds; Black sheet metal; $1,299; Carver Corporation, P.O. Box 1237, Lynnwood, Washington
98046-1237; Phone 206-670-3424; Fax 206-778-9453; E-Mail email@example.com.
Some years ago, I purchased my first high power amplifier. It was the venerable Carver TFM-55. With 380 watts rms/ch into 8 Ohms, I could crack the plaster with whatever speakers I wanted. So, I bought a pair of Carver Mark IV Amazing Loudspeakers. These were (are . . . since I still have them) ribbon hybrids, with four 12" woofers and a 60" ribbon in each speaker. This was my first experience with dipoles too, and I was ecstatic with the sound. The Carver Platinums, and a subsequent pair of Carver Silvers (three 12" woofers and a 48" ribbon in each speaker) have served me well since that time, and we use them regularly to test the maximum power of amplifiers. Someday, I will probably blow one out, but it has been worth the trip.
I used my TFM-55 for several things as well, and looked forward to having its offspring, the Carver A-760x for review. It has similar ratings to the TFM-55 (slightly more power into low impedances), with the same large analog power meters on the front (meter illumination can be switched off, and the meter range can be selected between 380 watts and 38 watts). Individual volume controls (small potentiometers) for each channel have been moved to the rear panel, where they are attached to the modular circuit boards inside. In order to comply with the THX certification, these pots should be used in their clockwise-most position. However, when the listening room is acoustically asymmetrical, being able to adjust the input level of one channel or the other can be very useful. The toggle on/off switch is on the front, and two pairs of three-way speaker binding posts are on the back, attached to the circuit boards. The bridging-mono switch is also on the rear.
The similarity ends there. While the TFM-55 had what was called a "Magnetic Field Power Supply" [click here for photo], the A-760x has a new marketing term, called "Magnified Current". This allows the 760 to deliver either high voltage or high current, while the older Carver delivered only high voltage. The output stage of the 760 has five pairs of bipolar devices per channel. Each channel also has one pair of MOSFETs, which are 160 amp peak current power devices that switch twice the continuous voltage to the output stage when needed, or twice the continuous current when needed (not both at the same time). The amplifier operates in Class H, just as its predecessor. The TFM-55 had three voltage rails, and the 760 has two (53 Volts and 106 volts). Instead of two 10,000 µ, 100V capacitors, as in the TFM-55 (98 Joules of energy storage, with 99 Volts DC on the power capacitors), the power supply of the A-760x has four (the two members of each pair are in series) 33,000 µ, 63V capacitors (371 Joules of energy storage with 106 Volts DC on the power capacitors) [click here for photo]. The power transformer appears to be larger in the A-760x as well. This is obviously the foundation of the increase in current delivering capability. (The TFM-55 was 15 pounds lighter, rated at 500 watts rms into 4 Ohms, and not rated into 2 Ohms. It was also rated at 1,000 watts for bridging. The A-760x has more power into low impedance loads, and also with bridging. The 760 has lower harmonic distortion - 0.08% at full output, compared to the TFM-55 - 0.5%.) Compare the number of wires (point-to-point wiring) in the two amplifiers. Note that it has been greatly reduced in the 760. Most of the connections are now on the PCBs (Printed Circuit Boards) for each channel, which are located on the sides [click here for photo]. This keeps the circuit path length to a minimum.
We compared the 10 kHz, ± 10V square wave response of both amplifiers. The TFM-55 response is quite good [click here for photo], with a relatively fast rise to peak. The square wave response of the A-760x is similar, but with a slightly faster rise to peak [click here for photo]. The corner region where the rising voltage goes to the flat area is sharper in the 760. This would suggest a more accurate representation of the original waveform.
We tested the A-760x with the McCormack Preamp and CD System, Carver Platinum Ribbons, Osborn Eclipse cone speakers, and Nordost Flatline Cables.
Comparing the A-760x with the TFM-55, I could not detect any audible differences in the mid-range and treble, but the TFM-55 is a very, very good amplifier to begin with. The square wave test results suggest that there are measurable differences (improvements) in the way the signal is handled. In any case, the 760 is a real stallion when it comes to delivery. Tight and detailed, excellent sound stage and imaging, and very low noise (hum). The Carver ribbons are not the easiest load to drive (that's one reason why we use them for testing amplifiers), and the 760 was more than enough to produce high SPL, but maintain a clean sound. Using pink noise, an oscilloscope, and digital voltmeter, we measured 22.59 Volts and 3.79 Amperes just below clipping, left channel driven into one Carver Platinum. Keep in mind that this real world test takes into account the back EMF that is produced by the speakers. Back EMF counteracts the power delivered by the amplifier. The measurement corresponded with a readout of 180 watts on the A-760x analog meter. Also, it should be remembered that our test uses a complete spectrum of frequencies, all at the same time, as opposed to routine measurements that incorporate single sine waves, one frequency at a time. There was so much power being continuously delivered, I could smell the speaker crossover heating up (I hope you appreciate the chances we take with our valuable speaker collection for the sake of real world test results!)
One of our favorite CDs for power testing is the "Fanfare for the Common Man" by Copland. The version on Telarc, Atlanta Symphony Orchestra, is incredible. Opening bass drum thuds are amp killers (speaker killers too). The Carver Platinums are one of the few speaker designs that can take these drum thuds with high power amplifiers, and will show up any clipping. The combination of the Platinums and the A-760x was quite a marriage. Intense, deep thunder rolled across our lab space, indicating beyond doubt that the A-760x was not flinching. A much improved bass was, in fact, the most noticeable difference between the 760 and 55, and is due to that bigger power supply. As I have said many times before, there is no substitute for raw power. The A-760x has it where it counts.
I don't know what else to say about the A-760x. It sounds very much like its fine ancestor, but now it will deliver more power into low impedances. Many folks believe that such power at a relatively low price means there is a catch. Well, happy birthday, because there is no catch. Carver Corporation has been delivering high performance at real world (there's that term again) prices for many years. Looks like they are still doing it.
J.E. Johnson, Jr
© Copyright 1997 Secrets of Home Theater & High Fidelity
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