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Old 28th January 2009, 06:56 PM   #1
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Default Preamp design question

I'm in the process of trying to design a channel strip and have a pretty basic question. The channel strip will be an aikido microphone preamplifier with it's outputs going into a basic passive equalizer then back into an aikido amp to makeup the loss through the eq.

My question is about the input transformer. What is the difference between using a high ratio transformer like a 10:1 and a low gain circuit and using a low ratio transformer like a 2:1 and a high gain circuit. What are the sonic differences and what would prompt using one or the other? Thanks.

-Joe
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Old 29th January 2009, 09:08 PM   #2
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Anyone?
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Old 29th January 2009, 09:16 PM   #3
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The devil is in the details. How about giving us some, like schematics?
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Old 29th January 2009, 09:17 PM   #4
kmaier is offline kmaier  United States
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You might want to visit the Jensen Transformers website and download some of their application notes. They offer a range of mic input, line input, line output and level matching transformers and have very detailed application notes. Without knowing any specifics for your mic preamp requirements (i.e., input impedance, phantom power requirements, overall gain requirements, amplifier circuitry, etc.) it's difficult to give you much meaningful feedback. Take a look at what Jensen offers... then decide on what route to take. Suggest you start here:

http://jensentransformers.com/mic_in.html

Regards, KM
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Old 30th January 2009, 09:07 AM   #5
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Default Re: Preamp design question

Quote:
Originally posted by Binaural
My question is about the input transformer. What is the difference between using a high ratio transformer like a 10:1 and a low gain circuit and using a low ratio transformer like a 2:1 and a high gain circuit. What are the sonic differences and what would prompt using one or the other?
The A Number One problem with a high voltage step-up is that it also makes the input impedance much smaller. That can mean a problem with bandwidth since the input capacitance will be magnified comsiderably. Couple that with a wimpy driver, long cable runs, and you've got a problem. May be you can avoid that, and may be you can't. Lots of devils hiding in the details to make a definitive statement here.
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Old 31st January 2009, 05:21 AM   #6
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Ok, schematics. http://www.tubecad.com/2004/blog0011.htm that's the circuit i'm speaking of. I didn't know what the difference would be between using a low ratio transformer with a 12ax7 or using a high ratio transformer with a 6sn7 or something with even lower mu.
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Old 31st January 2009, 04:03 PM   #7
Gordy is offline Gordy  United Kingdom
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As has been noted already, it's all down to impedances.
A turns ratio of 10:1 will result in an impedance ratio of 10(squared)... i.e. 100:1 . This means that a low impedance source can appear as a high impedance to the amp stage input once the transformer is inserted. This large impedance then has to drive the Miller capacitance of the input tube, and it is likely to seriously limit the bandwidth.

My money would be on using the lowest turns ratio possible at the input transformer, and then a high gain tube (with the proviso that the tube have linear characteristics).
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