Hafler DH-110 manual Operation

fects that can result from unpowered electronic circuits in the tape decks when they are connected to the Recording Outputs, it is recommended that tape recorders be switched on when the preamplifier is in use. This is a commonly overlooked cause of less-than-ideal preamplifier performance.

Signals at these outputs are at line input level (phono signals are first amplified and equalized). These outputs are unaffected by external processing circuits, or by the DH-110’s Mono, Filter, Tone or Volume controls. Only sig- nals indicated by the Selector switch are available, includ- ing the other tape input, so tape copying is provided using either recorder as a source.

LINE OUTPUTS

Two pairs of outputs are provided for your power amplifier connection. The set labelled ‘Via Phones’ is suggested for those who regularly listen through head- phones as an alternative. These are wired so that when headphones are plugged in, the signal is disconnected from the power amplifier. Two purposes are thus served. Since many headphones require fairly high volume control set- tings, you avoid inadvertent high signal levels to your speakers. And a power amplifier which is turned off can- not degrade the quality of signal to the headphones-

which is otherwise a possibility. We recommend that you turn off power amplifiers before connecting headphones, or that they be plugged in before the system is switched on, as a precaution.

The Direct outputs provide conventional signal levels independent of the headphone circuit. If headphones are plugged in, the level of the Direct output will be greatly reduced. These outputs should not be used when head- phone use is anticipated.

The performance specifications will be met so long as the input impedance of the power amplifier is 10K ohms or higher, and the cable capacitance is less than 1Onf. This is no problem with typical interconnecting cables up to 10 feet. If you are planning to use very long cables between preamp and power amplifier, special low capacitance cable can be obtained.

EXTERNAL PROCESSING LOOP

‘Send’ is an output at line level for the purpose of driving an external signal processor such as an equalizer, time delay, or noise reduction unit. Like the recording outputs, the load impedance should not be less than 10K ohms. The EPL ‘Return’ input impedance is 33K ohms, and should also be at line level. The EPL Return can also function as an additional high level input, switched from the front panel.

OPERATION

When you turn on the DH-110, power is also applied to the switched AC outlets on the back panel. One of the red LEDs on the front panel will light, indicating that the preamp is functioning. Typically, it will be the left-most LED, indicating that the Selector switch is determining the signal source. At turn-off, it is normal for the LED to fade slowly as the operating voltages decline.

DELAYED TURN-ON

An internal muting circuit prevents the voltage tran- sients which may occur at turn-on or turn-off from causing annoying noises, blown fuses or damaged loudspeakers.

At turn-on, the Line Outputs (but not the Recording Outputs or EPL Send output) will be held near ground po- tential for a few seconds until the internal voltages have stabilized, and the unit is ready for operation. Only a faint output can be heard during this time.

At turn-off, whether by operating the preamp’s power switch, or as a result of external AC failure (a ‘brown-out’, pulled plug, or blown house fuse), the line output will be instantaneously lowered. Following power interruption, the muting circuit will initiate a few seconds delay before allowing full signal at the line outputs.

SIGNAL SELECTION

Your choice of signal sources is usually indicated by the Selector Switch, identified by the adjacent lighted LED. This switch passes line level signals direct to the Record- ing Outputs and to the EPL Send output as well as to the Mono switch and subsequent line amp controls. Either or both tape recorders may record this source. Tapes may be

duplicated by selecting the playback tape recorder on the Selector switch, and recording on the second machine.

PHONO AMPLIFIER

The button in the lower left of the front panel selects either Phono 1 (in the OUT position) or Phono 2. The switch may be operated to compare two cartridges while music is playing, but if them is no cartridge connected to one input, noise or RF interference may cause a small au- dible transient. The phono signal is accurately RIAA equalized and amplified 34dB (at 1kHz), then passed to the Selector switch at line level.

MONITOR SWITCH

To facilitate tape recording with tape decks which pro- vide an independent monitoring facility, the DH-110 pro- vides this second selection function. When it is OFF, the regular Selector switch determines what signals are heard. When the Monitor switch is turned to either Tape 1 or Tape 2, the line amplifier is quietly disconnected from the signal being recorded, and is connected to the tape playback in- stead. This enables direct comparison of the signal source with the taped replica, without affecting the recording pro- cess. Tape decks which do not provide separate record and playback heads are not able to utilize this comparison.

Note: Do not turn the Monitor switch to Tape 1 if the Selector is also set to Tape 1. Likewise avoid simultaneous Tape 2 settings on both switches. Feedback will occur, and there is a possibility of damage.

When the Monitor switch is not in the OFF position, the LED at the Monitor switch will light, in place of the Selec- tor LED, reminding you that tape playback has been selected.

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