increase (mather) impedance?

tweed · 24th May 2006, 07:43 PM

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

poobah · 24th May 2006, 07:57 PM

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.

lndm · 24th May 2006, 10:27 PM

At the risk of overkill, a transformer may fix this.

tweed · 26th May 2006, 08:12 AM

thanks for the replies! seems like best thing to do is keep using it as is.

gibsonripper · 2nd June 2006, 06:45 AM

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.

tweed · 2nd June 2006, 11:05 PM

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

lndm · 2nd June 2006, 11:59 PM

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.

gibsonripper · 3rd June 2006, 04:10 AM

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.

24th May 2006, 07:43 PM	#1
tweed diyAudio Member Join Date: May 2006	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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24th May 2006, 07:57 PM	#2
poobah diyAudio Member Join Date: Nov 2005	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.

24th May 2006, 10:27 PM	#3
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	At the risk of overkill, a transformer may fix this.

26th May 2006, 08:12 AM	#4
tweed diyAudio Member Join Date: May 2006	thanks for the replies! seems like best thing to do is keep using it as is.

2nd June 2006, 06:45 AM	#5
gibsonripper diyAudio Member Join Date: Jun 2006	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.

2nd June 2006, 11:05 PM	#6
tweed diyAudio Member Join Date: May 2006	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

2nd June 2006, 11:59 PM	#7
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	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.

3rd June 2006, 04:10 AM	#8
gibsonripper diyAudio Member Join Date: Jun 2006	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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