2 months ago


Over -all design of this

Over -all design of this stereo preamp has attractive and neat appearance. ELECTRONICS-- LAB TESTED "Stereo Master"- a Hi -fi Control Center A new hi -fi component that has extremely low distortion. Both lab and listening tests prove quality sound reproduction. ONE of the most recent additions to Lafayette Radio's hi -fi component line is its Model KT -600 stereo preamplifier. It incorporates just about all the features one could hope to find on a single chassis. Evidently, considerable time and effort has been spent on its design, which was created by Stuart Hegemann in conjunction with Aaron Newman, chief engineer at Lafayette Radio. Stuart Hegemann is well -known and respected for his engineering achievements in the hi -fi field. In addition to the usual controls such as bass, treble, loudness, and rumble and scratch filters, it has many other features. The entire righthand section, which includes the stereo function switch, bridge control, and volume- balance control, is identical to Lafayette :s KT -315 stereo adapter unit. Its complete operation was described in last month's issue. Briefly, though, the stereo function switch provides for selection of normal stereo, reverse channel, reverse phase, and reverse phase and channel, in addition to either channel through both speaker systems or both channels together. It includes a calibration position which is unique in itself in that it permits proper electrical balancing of the two channels up to the inputs of the power amplifier. It is used in conjunction with a monophonic signal with both channels being handled by the preamp out -of- phase. Controls are adjusted and when proper balance is attained, a null occurs at the speaker outputs. 64 Top view shows printed circuit assembly and general arrangement of parts. The "volume- balance" are individual controls for each channel but incorporating a clutch that can gang both together, converting them to a single level control. The bridge control allows for combining a portion of each channel into the other channel to eliminate the socalled "hole-in- the -middle" effect. In its extreme position one can obtain 100e /r combination resulting in a mono- phonic presentation through both speakers. The selector switch is used to permit a choice of six separate inputs; auxiliary, tape amplifier, tape head, tuner, and either magnetic or ceramic cartridge. Each channel has its own selector switch, thereby providing the flexibility of mixing any combination of program material. Four dual concentric -type input level controls are also incorporated. The outer shaft is used to adjust the inputs on one of the channels and the inner shaft of the control is used for the other channel. Equalization is obtained by means of four separate controls. two for each channel. The turnover point is selected by one and the roll -off by the other. A choice of any combination thereby increases the flexibility to the user. A third channel output is provided with a bridging control that would permit any desired combination of A and B channels. Not only can this output be used as a third channel for stereo, as another method of eliminating the "hole -in- the -middle," but it can serve as a monophonic channel for playback in any other section of the home. Another unique feature that we have not run across in any of our previous tests on stereo units is the incorporation of a "presence control." The function of this control is to provide a slight boost in the mid -frequency range. Our tests showed a 6 db rise at 2500 cps and then a return to the ELECTRONICS WORLD i reference level at 600 and 6000 cps. The intentions are that this control be used when playing solo -type program material. The boost is in the upper voice range and thereby serves to emphasize the solo portion. Tape output jacks are provided with facilities for monitoring. Level indicators and a microphone input are not included. CHANNEL °S - INPUTS r x IYEO. ÿzo¡ z ñIs DOle11 á á L- 00 : CHANNEL "A" INPUTS P. Z ÿp K LÉ ELf : ó i Z3 T a 470K ó X v =1e1\ , Test Results The circuit itself is conventional. Counting the low -level stages, the preamplifier provides six stages of amplification on each channel, plus a stage for phase reversal and one for the third channel output. These 14 stages are handled by seven tubes, all of the same type. The tubes are 7025's, which is a new, improved version of the 12AX7. The power supply uses two selenium rectifiers, one for plate voltage and the other for filaments. The results of our tests are as follows. Sensitivity: (1000 cps for 1 volt output) tape head, .0022 volt; magnetic cartridge, .002 volt; ceramic cartridge and auxiliary, tuner, and tape amplifier, .079 volt_ Frequency !lesponse: ±1.5 db from 30 to 15,000 cps, the limits of our test. Equalization: ±1.35 db from the conventional RIAA standard curve. Rumble Filter: -.4 db at 400 cps, -4.8 db at 100 cps, and -16.6 db at 30 cps. This is an extremely sharp and effective rumble filter. Scratch Filter: A gradual attenuation from 0 db at 1000 cps to -12 db at 15 kc. Bass Control: -17 db, +14.3 db at 30 cps. (Continued on page 135) r Complete schematic diagram of Lafayette Radio's Model NT -600 preamplifier unit. 100 u0* eso 50, re io rm ó rllilrft á 210 1g ÿ 9¡, ÿ x a s a r01_3__ b rvm- a .-1n Z.fw ó ° a > ;. o a á N z i m °5.1 0) e ro n IS0* LOOK 10011 n á :;I -s a Ili b w a a 6 .. ' il v: . . i ; . a ñ to :: x May, 1959 65

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