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Reference Two-Channel Power Amplifier 857
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857 Product Overview
It can deliver both channels with 250W into 8Ω and 500W into 4Ω.
In bridged mode, the 857 can deliver 1kW into 8Ω and 1.5kW into 4Ω.
The front-panel indicator will show blue.
Touch the logo on the front-panel. After a four second start-up period, the amplifier will switch on and the indicator will show green.
Yes. Touching the logo on the front-panel for about four seconds while the amplifier is running blanks the indicator. Touching the logo for four seconds again will bring the indicator back on.
If the temperature within the case of the amplifier reaches an elevated level, the fans begin to operate at low speed. If the temperature rises substantially further, the speed of the fans is increased to a higher level.
The front-panel indicator shows red, the outputs are muted and the fans operate. Once the amplifier has cooled down, the amplifier will unmute.
Under certain fault-conditions, the amplifier protects itself by switching off. This can be caused by the presence of DC voltage on an input or by short-circuiting an output. Once the problem has been removed, the amplifier can be reset by turning the power at the back-panel off then on again.
The “Bridge Mode Select” switch should be set to “On” and the two negative speaker terminals must be connected together. This can be done with the bridge-mode link supplied with the amplifier or by using a short length of speaker cable of appropriate thickness.
The amplifier can be brought out of Standby by applying any DC voltage between 5V and 30V to the trigger input.
Check the “Bridge Mode Select” switch is set to “Off”. If the switch is inadvertently switched to “On”, the amplifier’s left input is amplified normally, but the right channel input is ignored. Instead, an inverted version of the left input is fed to the right channel of the amplifier. This causes the sound from the amplifier to suffer as described.
Check the “Input selector” switches are set appropriately. If set incorrectly, no sound will be produced by the channel(s) concerned.
If the option is available, it is generally advisable to use balanced connections. The science behind balanced connections offers the potential for improved performance by achieving lower levels of noise on the input signal. Even if the sonic difference between the two types of connection is not overly apparent during casual listening tests, it can be reassuring to know that “best-practice” has been observed.
There are two approaches to adding a second amplifier. The two amplifiers can be used in “bridged” mode to provide more power to each loudspeaker. Although the output of the amplifiers is not improved in itself, the extra power can be beneficial when used with some loudspeaker designs. Or, if the loudspeakers are capable of being connected in a “bi-amp” arrangement, this can be done. Bi-amping gives each amplifier channel a simpler load to drive. This can improve sound-quality with many loudspeaker designs.
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