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Old 30th July 2007, 09:23 PM   #1
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Default Load termination ?

I have a old AMPEX tube pre-amp who's output expects a low impedance load. It does have an output transformer. I am feeding into hi Z stuff. (In one case 2.2kZ). Instructions say "...terminate with 680 ohm load.
How would I go this ? 1 680 ohnm across hot/cold? or what?

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Old 30th July 2007, 09:47 PM   #2
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Shouldn't normally be an issue - a lot of people are running transformer coupled (600 ohm z out) into high input impedances without any problems.

If you do use a terminating resistance it should be installed in the amplifier being driven, and goes from hot to cold (unbalanced) or if balanced hot+ to hot-
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Old 30th July 2007, 10:06 PM   #3
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In a similar vein, if a preamp has an input transformer (10K:10K that is loaded with a 10K resistor), does the usual rule about the source's output impedance apply (that it should be 1K or less) or does the transformer change this ratio in any way?

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Old 30th July 2007, 10:33 PM   #4
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Not 680: I would say 1k in paralell with 2.2 you already have will get you very close to 680.
Still I doubt if the additional load will be so much better for yout preamp (well, -- just try it).
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Old 31st July 2007, 12:12 AM   #5
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Yo Forest,

Many folks are using Ampex tape recorder electronics as preamps for microphones etc. If this is what you are doing, fine. The Ampex electronics had VU meters hooked across the 600 ohm balanced output. This output needs to be terminated either by another amplifier's input, or a resistor for the VU meter to read correctly.

Many of these older electronics had an internal terminating resistor selected by a switch if one was needed. 680 ohms is the standard 10% value and usually prescribed in the manuals. (620 ohms is the standard 5% value as well.)

To be correct and proper, the output of the transformer should be terminated in it's characteristic resistance. This will preserve the optimum load impedance for the output stage as it was designed. This should be done even if you don't use the VU meter, or are only using the unbalanced Hi-Z output. (Assuming there is one)

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