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high fidelity EQUIPMENT REPORTS

The consumer's guide to new and important high fidelity equipment.
(Reprinted from High Fidelity Magazine August 1965 issue.)
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C/M Laboratories Model 35D Power Amplifier

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THE EQUIPMENT:

C/M Model 35D, a solid-state stereo power amplifier. Dimensions: 6" by 10" by 12". Price: $285. Manufacturer: C/M Laboratories, 575 Hope St., Springdale, Conn. 06879.

COMMENT:

Audio is a field in which we have come to expect the unusual and the excellent from relatively little-known companies. The new amplifier from C/M Labs is a case in point. The first high fidelity product from a firm which has been involved for the past ten years in solid-state electronics in the industrial and defense/space fields, the Model 35D shows all the favorable signs of its more arcane antecedents.

It has a crisp, clean look; it is superbly constructed of top-grade parts, with professional care and precision workmanship evident throughout; and it offers performance of the highest order.

The unit is a basic or power amplifier, designed to be fed and controlled from a separate preamplifier, or from a tape playback deck that has output controls, or from a tuner that has at least a volume control. Two phono jacks (es sind Cinch-Buchsen), for signal input, are on one apron of the chassis; the opposite side contains the speaker terminals, a pilot lamp, the AC line cord, a 3-ampere fuse-holder, and a control switch.

The switch doubles as the power off/on control, and as a speaker impedance selector. In its center position, the amplifier is shut off; when the switch is thrown to the left, the amplifier is turned on and is matched to speaker systems of 4- to 8-Ohm impedance; when moved to the right, the switch turns on the amplifier and matches it to 8- to 16-ohm speakers. (The amplifier is not recommended for use with speakers of more than 16 ohms impedance.)

The amplifier is completely solid-state, and the only transformer used is in the power supply. Filtering is aided by four large, heavy-duty capacitors, and the amplifier is protected against accidental short or open circuits at its output by a unique electronic delay system, which in our tests proved quite effective.

The output transistors are carefully installed in large heat-sinks as a precaution against abnormal rises in ambient temperature. Most of the circuitry is contained on a printed board under the chassis, itself a model of neatness and sensible layout. And for an amplifier of its class, the Model 35D is surprisingly compact.

The Model 35D, checked out at an independent testing company, shapes up as an excellent basic amplifier for use with conventional speakers up to its indicated load capability of 16 ohms. As may be seen on the accompanying graphs, all response and distortion curves were virtually flat lines which, of course, approach the
ideal in shape. Power output was high, clean, and very wide in range. The amplifier's power bandwidth, in fact, extended from 10 cps to 40 kc. At rated power (35 watts rms per channel), harmonic distortion over most of the range was almost nonmeasurable.

The IM characteristic also was excellent: very linear and very low up to full output at all three speaker impedances. Frequency response extended well below and beyond the normal audio range. The amplifier's damping factor, at 200, was the highest yet measured; signal-to-noise ratio was excellent; stability, fine. Square-wave response, shown on the photos, was splendid: the low-frequency response
indicates very solid and clean bass; the 10-kc square-wave photo shows fine transient characteristics, with no ringing or overshoot.

In use tests, the Model 35D proved capable of driving conventional speaker systems of varying efficiency with the greatest of ease. Like other topflight amplifiers, it seemed to have a quality of transparency, of "listening through" back to the program material, and of coaxing the last drop of performance out of a speaker system - its extremely high damping and wide power bandwidth doubtless contribute to this effect.
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C/M Laboratories Model 35D Amplifier
Lab Test Data

Performance characteristic Measurement
Power output (at 1 kc into 8-ohm load)
left ch at clipping 39.6 watts @ 0.1% THD
left ch for 0.5% THD (rated distortion) 42.3 watts
rihgt ch at clipping 36.1 watts @ 0.085% THD
rigth ch for 0.5% THD 39.6 watts
Both chs simultaneous  
left ch at clipping 34.4 watts @ 0.14% THD
right ch at clipping 34 watts @ 0.14% THD
Power bandwidth for  
constant 0.5% THD 10 cps to 40 kc
Harmonic distortion  
35 watts output less than 0.1%, 25 cps to 15 kc; 0.5% at 22 cps; 0.15% at 20 kc
17.5 watts output less than 0.1%, 20 cps to 18 kc; 0.12% at 20 kc
IM distortion  
4-ohm load less than 0.4% up to 43 watts output
8-ohm load less than 0.25% up to 40 watts output
16-ohm load less than 0.1% up to 18 watts output; 0.25% at 40 watts output
Frequency response, 1 watt level + 0, -1 db, 2 cps to 50 kc; -3 db at 100 kc
Damping factor 200
Sensitivity 1.08 volts
S/N ratio 88 db
- Werbung Dezent -
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