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In Reply to: What if my preamp output impedance is too low? posted by benhen on March 5, 2007 at 17:05:10:
Amps Input Resistance:
Dynaco ST-70 = 470,000 ohms
Alesis RA-150 = 20,000 ohms
BASH subwoofer = 20,000 ohmsNAD 3155 Preamp output impedance: 800 ohms
So if the 1 to 10 ration holds true, then I need a preamp with an output impedance of 50,000 ohms? Do they make such a thing?
Follow Ups:
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Tin-eared audiofool and obsessed landscape fotografer.
http://community.webshots.com/user/jeffreybehr
From Decware - I assume these guys know what they're talking about:"As a general rule, the lower the output impedance (preamp) is, the better it will drive difficult loads. And the higher the input impedance (amp), the less difficult it is to drive. A good example of a happy situation is a preamp with an output impedance below 1000 ohms driving an amplifier with an input impedance of 100,000 ohms." (http://www.decware.com/paper55.htm)
The above example is a 1:100 ratio. So if that's the case, then I want the lowest possible output impedance in a preamp and the highest possible input impedance on an amp? I am determined to understand this.
"Input impedance (amp) that is too low may push the output stage of your source component (preamp) too hard and into distortion. Output impedance (preamp) that is too high may cause audible high frequency roll-off. To complicate matters further, input and output impedance may change with the change in volume settings." (http://www.audiotweaks.com/diy/bentaudio/page01.htm)
Okay - one mistake I've made is assuming that input impedance increases with the addition of additional impedance. Adding another amp actually decreases the total input impedance. So the two amps with input impedance of 20k each would result in a total input impedance of 10k.
If the total amplifier input impedance is 510,000 ohms, then don't I need a preamp with an output impedance of 51,000 ohms - using the 10 to 1 ratio? I understand impedance of speakers and wires, but I'm still confused about the preamp/amp output/input impedance.
1. The load resistances are in PARALLEL, NOT in series. That's why I sent you to the link for resistors in parallel.
2. Plug 20, 20, and 470 into the calculator and you'll get 9.8...K-Ohms, in this case.
3. The 10-to-1 load-to-driving-resistance rule-of-thumb is just that and is NOT a hard-and-fast requirement. Also it's a MINIMUM, not a maximum, so 20:1 or 50:1 or 500:1 are even better. What we're trying to do is to ease the preamp's requirement to deliver current, and we do that by presenting it with a high resistance, not a low resistance.There's lots of info (and controversy) on this Asylum about this; SEARCH and ye shall find. :-)
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Tin-eared audiofool and obsessed landscape fotografer.
http://community.webshots.com/user/jeffreybehr
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