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Old 6th May 2006, 01:01 PM   #1
sopapo is offline sopapo  Spain
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Default Input impedance of 1 M in a preamp

Hello,I would like to ask why not it is implemented more usually a input impedance of 1 M on preamps

It supose to be the more ideal it'isnt?

It easy to pull the impedance of any preamp?

it could have isssues to do this?

Thanks
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Old 6th May 2006, 01:13 PM   #2
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Input impedance is always smaller, because large input impedance with cable capacity it woud cause hi-frequency roll-off.
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Old 6th May 2006, 01:18 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
high output impedance combined with cable and input capacitance is what causes high frequency rolloff at the input of the next stage.

One of the disadvantages of high input impedance is the susceptibility to interference. Low impedances are inherently tolerant of interference.
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Old 6th May 2006, 02:44 PM   #4
sopapo is offline sopapo  Spain
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Thanks andrew for your answer,but 1M of input impedance will be problematic?

I understand that from 2-3 M it will give troubles..but 1M?

I think that I read that certain preamps and eq of companies like cello or viola (palette etc) have a high input impedance like that....

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Old 6th May 2006, 03:21 PM   #5
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IMHO any such high input impedance of a (pre-)amp like 500k to xM would only make sense in case there is a second input plug in parallel per signal line, where you can plug in an additional resistor or resistor/capactor network to fine tune your actual output/cable settings.

Would have the giant advance of presenting a fully controllable load to the stage driving the cable, in most cases preferably purely resistive for reducing capacitive cable load influence, as has been pointed out already.

You may have seen this with some phono-pres already, where the effective electrical load for the pick-up is set just that way. I still wonder why not many people do that with preamp and amp-inputs too. Just imagine the increased influence you could have on your cable performance!

It does make sense I'd say, but only when taken a bit further...
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Old 6th May 2006, 05:59 PM   #6
PMA is offline PMA  Europe
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Input impedance of 1M at the preamp input will cause no problem. It may only help, as it does not load the preceding output stage.

Input impedance of 1M will not cause high frequency roloff. The cutoff frequency of the lowpass filter is constituted by preceding output stage output impedance and capacitance of cable with Cin in parallel.

Input impedance of the standard wideband oscilloscope is 1M//25pF.
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Old 6th May 2006, 06:52 PM   #7
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Hello,

Preamp makers will often go for the lowest practical input impedence because the input impedence can be a source of noise and lower impedence often means lower noise and better S/N specs.

Most modern circuits can drive a 10K load, so higher impedence offers little value.

Higher input impedence can also mean greater DC offset.


There is no practical difference between 1M, 2M, and 3M inputs.
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Old 6th May 2006, 07:23 PM   #8
PMA is offline PMA  Europe
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The equivalent input impedance would be 1Mohm in parallel with output impedance of the source. Imagine the source (e.g. CD player) output impedance of 1k, then the equivalent input impedance of the preamp is 1k. This is valid for noise as well.
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Old 6th May 2006, 09:27 PM   #9
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Such a high input impedance may be helpful in condenser cap preamp and electric guitar, another words from some trancducers themselves. If electronics drives electronics there is no such demand.
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