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Old 24th May 2006, 07:43 PM   #1
tweed is offline tweed  United States
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Default increase (mather) impedance?

I have a GT brick tube preamp which has an input impedance of 600 ohms and i'm connecting an AEA r92 ribbon mic to it which expects at least 1.2k ohm. I recently read an article about how to decrease input impedance with a resistor. is there any way to increase input impedance in this preamp? i'm a newbie so I hope this is the correct forum. thanks! this looks like a great forum.
tweed
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Old 24th May 2006, 07:57 PM   #2
poobah is offline poobah  United States
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Without tearing into the amp, you can put a 600 Ohm resistor in series with signal wire (as opposed to ground) of your mike. This will raise the input impedance to 1200 Ohms. There is a price though... it will cut your signal in half... a 6 dB loss.


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Old 24th May 2006, 10:27 PM   #3
lndm is offline lndm  Australia
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At the risk of overkill, a transformer may fix this.
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Old 26th May 2006, 08:12 AM   #4
tweed is offline tweed  United States
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thanks for the replies! seems like best thing to do is keep using it as is.
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Old 2nd June 2006, 06:45 AM   #5
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If you can't afford a transformer, which is the best solution, there are other ways--although you may compromise performance, i.e. freq response, SNR especially and so on.

If you use the resistor divider, the good part is that you are staying in the passive electronics realm. But the downside is that your sig loss will mess with your SNR, but it may be acceptable (not much in this case).

The other is an active approach: use a buffer, like a class a emitter follower one stage in order to adapt your source impedence to the your pre.
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Old 2nd June 2006, 11:05 PM   #6
tweed is offline tweed  United States
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thanks for that. can you point me in the right direction of obtaining/using a transformer. feel kinda in over my head about the class a emitter follower. would like to know more. any easyto understand resourses for that?
tweed
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Old 2nd June 2006, 11:59 PM   #7
lndm is offline lndm  Australia
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If you use a transformer that has a turns ratio of 1.4:1, you'll double your input impedance.

Here's a transformer (I didn't look very thoroughly )

http://www.lundahl.se/pdfs/datash/1530.pdf

if you use it as 3.5:2, you'll increase your input impedance to about 1.8k.
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Old 3rd June 2006, 04:10 AM   #8
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I would take Indm's advice.

Using a transformer is not only the easiest way to do this, but the best way giving you more of a true sound. Transformers can be expensive. So far, I've used the Jensen brand (a bit expensive), but very impressive specs, if you are looking for top quality as in nice studios and so on.
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