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Load termination
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Old 30th July 2007, 02:34 PM   #1
cathode_ray is offline cathode_ray  United States
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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?

F. Gump (aka cathode_ray)
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Old 30th July 2007, 10:18 PM   #2
Nigel Goodwin is offline Nigel Goodwin  England
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If it needs terminating with 680 ohms, then simply stick a 680 ohm resistor across input and ground in whatever you're feeding - use a switch if you like!.
Nigel Goodwin
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Old 31st July 2007, 08:02 AM   #3
sreten is offline sreten  United Kingdom
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Location: Brighton UK

tube stuff liking low impedance loads ...... new one on me.

2.2Kohm terminating a nominal "600 ohm" output will be fine.

Generally speaking you do not need to worry about
high impedance loads on low impedance outputs.
You do need to worry about driving low impedance inputs
from high impedance outputs that are not up to the job.

The case when the above is not true is 600 ohm microphones.
Into high impedance loads due to low output noise is poor.

I recommend you do not load the output with 680R resistor ....

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Old 31st July 2007, 02:02 PM   #4
cathode_ray is offline cathode_ray  United States
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Default load termination?

Thanks for the "input" folks. It's not the tubes that want low impedance - the transformer balanced output is my problem. The manual says "...if feeding Hi-Z, terminate with 680 ohm load."
Since it's balanced does that require a resistor across 2(+) and 3(-) or a resistor from 2 to ground/3 to ground?

I wasn't sure if the 2k input to my A/D convertor would be considered "low" enough. I realize Hi Z is usually > 10k right?

Thanx again
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Old 22nd August 2007, 08:41 PM   #5
unclejed613 is offline unclejed613  United States
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a 1k resistor across the output would be sufficient. with a 2k input impedance in your a/d, that gives you 667 ohms..... "close enough for rock and roll".......
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Old 23rd August 2007, 10:57 PM   #6
FastEddy is offline FastEddy  United States
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" ... The manual says "...if feeding Hi-Z, terminate with 680 ohm load." ..."

That's right: a 680 Ohm resistor in parallel with the Hi-Z of the input of your device ... assuming input impedence of your down stream device is 10,000 Ohms, then:

1/680 + 1/10k ~= approximately equals ~= 1/600 Ohms ... the expected output impedence of your transformer.

(I would also recommend that the 680 Ohm resistor be a metal film type for better audio quality ... but anything is good except wire wound or carbon resistors.)
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