McIntosh MR 80 Brochure
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Extracted text from McIntosh MR 80 Brochure (Ocr-read)
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cable company is not on its correct frequency. This is done by the use of two operational amplifiers. The lock circuit will track a station even if it drifts 1 MHZ. The mixer is a balanced matched dual J-FET and bipolar transistor circuit. A low loss toroidal phase splitting transformer is used as an impedance mat- ching network in the gate circuit of the mixer eliminating even order spurious responses. The MR 80 has the narrowest IF bandwidth ever used in a stereo tuner. It is the correct width to let just one FM station through. The excellent selectivi- ty of the MR 80 (210 kHz wide at 60 dB down) permits tuning to stations that are impossible to receive on ordinary tuners. After the mixer the signal is electronically switch- ed to go either directly to the IF amplifier or to go through a quartz crystal filter. The SUPER NARROW selectivity position adds a 4 pole - 4 zero crystal filter to the other 5 IF filters. SUPER NARROW permits receiving stations never heard before with most other FM tuners. The 5 stages of IF amplification provide the necessary gain to reduce noise in the signal and interference from other stations. The 5 IF stages use piezoelectric fixed frequency filters in place of normal tuned circuits so the IF stages will always stay in alignment, even with age. The signal strength indicator column is summed off of all 5 IF stages instead of just 1 so it is looking at the signal strength throughout the entire IF system. Following the selectivity section of the IF amplifier is the LIMITER. A total gain of 80 dB is used in this circuit. The use of very high gain in the limiter circuit produces hard limiting with very good impulse noise rejection. Limiter bandwidth is greater than 50 MHz, producing excellent detector capture characteristics. A broadband Foster-Seeley discriminator is used as the detector. This detector coupled with the broadband limiter produces unmeasurable noise and distortion. The heart of the multiplex section is a new third generation phase locked loop (PLL) stereo decoder integrated circuit (IC). This PLL IC incorporates two special systems, an automatic variable separation control circuit to reduce background noise when receiving weak stereo stations, and tri-level digital waveform generation which eliminates interference from SCA signals and from the sidebands of adja- cent channel FM signals. The variable separation control is operated from the IF amplifier's signal strength detector. A smooth transition is provided from mono to stereo or from stereo to mono at weak signal levels to provide the optimum signal to noise ratio and best stereo separation for the prevailing signal conditions. The circuit operates only during stereo reception. It swit- ches automatically to monophonic if the 19 kHz pilot tone is absent. In the PLL the internal oscillator operates at 228kHz, locked onto the 19kHz pilot tone. The 228kHz, feeds a 3 stage Johnson counter via a binary divider to generate a series of square waves. Suitably connected NAND gates and exclusive OR gates produce the tri-level drive waveform for the various demodulators in the circuit. The usual square waveforms have been replaced in the PLL and decoder sections by tri-level waveforms. These tri-level forms contain no harmonics which are multiples of 2 or 3. This eliminates frequency translation and detection of interference from the side-bands of adjacent stations since the third har- monic of the sub-carrier (114kHz) is excluded. It also eliminates interference from SCA broadcasts since the third harmonic of the pilot tone (57kHz) is exclud- ed. Unwanted spurious audible components and phase jitter in the PLL are Inherently eliminated by this technique. Additional advantages of the phase locked loop stereo demodulation are the elimination of induc- tors to minimize drift, integral lamp driving capabili- ty to indicate the presence of the 19kHz pilot carrier, excellent channel separation over the entire audio frequency range, extremely low distortion, low out- put impedance, and transient-free mono/stereo switching. Following the MPX decoder is the three position de-emphasis switch. The three different positions allow the MR 80 to be used in North America with standard 75ms de-emphasis and in Western Europe and the Far East with 50ms de-emphasis. The 25ms position is for use with an external noise reduction adapter. An electronic blend filter circuit, implemented with two J-FETs of a quad J-FET package, is used to reduce out of phase noise when in the stereo mode and tuned to a weak station. This filter is actually a twin-T bandpass that blends the high and low fre- quencies, leaving separation unaffected at mid- frequencies. This results in a greatly improved stereo image when the filter is required. Special design attention has been given to the power supply section. Nine separate power circuits are used. Six of these are regulated to prevent loss of performance during a brown out. The - 15, - 5.2, 5, 15 and 30v use integrated circuit 3 terminal regulators, while the 3ma current source is made with discrete transistors because of the high voltage on the input terminal. The remaining voltages are used for the headphone amplifier and the touch con- trol reference signal driver. Mclntosh, recognizing the need for improved reliability, has designed a new circuit to drive in- candecent lamps. This new circuit prevents the filaments from failing due to notching when operated on direct current. This failure mode can reduce lamp life from one half to one tenth of the data sheet value. In the MR 80 the three lamps that are used for STEREO, LOCK and FILTER indicators are operated on AC at lower than rated voltage to ex- tend the useful life to well in excess of 15 years. Only Mclntosh brings you this feature. Your Best Investment is
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The Frequency Display indicates the received frequency to the nearest 100 kHz. A column of LED lamps respond to the signal strength of the station being receiv- ed. The stronger the signal the higher the column reaches. These touch pads select one of the four preset stations. Just touch to operate. A small rectangle will light above the pad in- dicating which preset has been selected. AUTO SCAN is provided by two touch pads. These two touch pads start the tuner searching for a new station. It will scan un- til it stops at a station or reaches the end of the band. At the band end the tuner will reverse its scan direction. Three indicator lights are provided. The uppermost STEREO indicates when a stereo broadcast is being received. The se- cond, LOCK, indicates when the tuner is locked to the station. The third indicator light, FILTER, indicates when the stereo noise filter is in operation. When the MR 80 is turned on the circuitry automatically selects the manual or knob tuning control mode. Two degrees of IF selectivity are select- able by the SELECTIVITY switch. The NAR- ROW or normal position provides adequate selectivity for stereo reception even under severe receiving conditions. Setting the SELECTIVITY switch to SUPER NARROW adds an additional 4-pole 4-zero quartz crystal filter. Use the SUPER NARROW position to receive stations from distant cities which are on channels adja- cent to local stations. The VOLUME control has been precision tracked throughout the listening range (0 to - 65 dB) for accurate stereo balance. MUTING suppresses the background noise and hiss normally heard between stations. The MR 80 FILTER switch provides auto- matic stereo noise reduction on weak stereo stations. The SCAN control adjusts the speed at which the tuner searches for the next station. A two position MODE switch is provided so that you may select either automatic stereo or mono reception. A HEADPHONE jack is provided to drive low impedance dynamic headphones from an internal power amplifier.