ADCOM POWER AMP GFA-535L
INTRODUCTION
The GFA-535L is identical in all respects (power output, operating parameters, sensitivity, operation, installation, connections, etc.) to the GFA-535, except for the addition of the two level controls on the front panel directly above the L and R INSTANTANEOUS DISTORTION ALERT LEDs. Therefore, it is very important that you read thoroughly the OWNER'S MANUAL for the GFA-535, as well as this addendum, before connecting or attempting to operate the GFA-535L.
INPUT LEVEL CONTROLS
The input-level-control design in the GFA-535L preserves the 22,000 ohm input impedance of the GFA-535 and remains at 22,000 ohms regardless of the position of the level controls. The design was carefully integrated into the input circuitry of the GFA-535L to insure that no degradation in performance would occur with different settings of the level controls. All operating parameters will remain the same at any position of the level controls (distortion, frequency response, bandwidth, damping, etc.). The only specification which will vary is the input sensitivity of the amplifier. Whereas the input sensitivity of the amplifier is 1 volt BMS for full output, at the maximum clockwise position of the level controls, this sensitivity will be proportionately reduced as the level controls are turned counterclockwise from their maximum "on" position. That is, as the level controls are turned counterclockwise from their full "on" position (maximum clockwise), input levels higher than 1 volt RMS will be required to reach maximum output, and, therefore, the level at which loudspeakers connected to the amplifier will play will be reduced proportionately
The input level controls are easily and readily adjusted via a medium-sized, flat-bladed screw driver, or a "dime". Coins larger than a dime will not fit the slots on the level controls and, if used, will "nick" and mar the finish on the shafts.
The level control directly above the INSTANTANEOUS DISTORTION ALERT L CHANNEL LED adjusts the sensitivity and level of the left channel of the amplifier. The level control above the R CHANNEL LED adjusts the sensitivity and level of the right channel of the amplifier.
The easiest and most direct way of insuring that the left and right level controls are correctly set and that the amplifier's channels are balanced is to:
1. Set your preamplifier (or whatever your source equipment may be) to a mono source. Usually, the most easily obtainable mono source is the voice of the announcer on an FM station, or a justplain-mono AM station. Some preamplifiers and other source devices have a "mono" button, or control, that effectively combines the left and right channels to provide a "mixed", identical, or mono signal in both channels.
2. Be certain that the "balance" control is in its center position to insure that both outputs of the preamplifier. or source, are identical. Also, it is recommended that you set the "bass" and "treble" controls to their normal position so they do not emphasize or deemphasize any part of the audio spectrum. If your equipment has a "loudness" or "contour" circuit, make sure it is switched off, or it is not operating.
3. Set the main volume control of your system to a comfortable level. If you are trying to match the level of the GFA-535L and the loudspeakers connected to it to another system, first adjust the GFA-535L level controls, approximately, to the desired point. Turn off the other system, through its "on/off" switch so that only the GFA-535L and its speakers are playing and "trim" the setting of the GFA-535L level controls until the announcer's voice, AM station or other mono material appears to be centered between the two speakers.
4. If you are using the GFA-535L in a "Surround Sound" installation, a small amount of re-adjustment to achieve the correct balance between the main loudspeakers and the "surround" speakers may be necessary. Please note that it is
much easier to achieve the correct adjustments and balances if a mono signal, derived as suggested in Step 1, is used.
5. Return all your bass, treble, balance, loudness contour, etc. controls to their usual or preferred positions. The systems will now "track" each other regardless of the position of the main volume control and their relative levels will remain as set.
It is interesting to note that the flexibility of the GFA-535L permits many other applications. For example, the two channels of the GFA-535L can be used to drive mono speakers in separate rooms. The level controls may then be used to adjust playback levels to desired points in each room even if rooms are of different sizes and the speakers used of differing sensitivity (efficiency). It is also quite possible to use, temporarily, two different speakers in a stereo system and adjust their relative levels as described in Steps 1 through 5, above, to achieve balanced stereophonic reproduction.
ADCOM 11 Elkins Road East Brunswick, NJ 08816 USA
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