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How to calculate source impedance for audio transformer?
How to calculate source impedance for audio transformer?
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Old 18th March 2011, 03:35 PM   #1
karsten21 is offline karsten21  Germany
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Default How to calculate source impedance for audio transformer?

Hi dear guys,

Sorry to ask a question again at the same day...

I would like to use different audio transformers as a symetrical line driver with a Zout of 600Ohm. I have two transformers with 2:1 ( Lundahl LL5402 and A262-A2E ) and one 5:1 ( Sowter 9041 ).
The question is how to identify the best source impedance of the driver stage to drive the transformers in the best mode?

Example:
A LL5402 has an primary impedance of 2K4 at a load of 600Ohm. For best power adjustment the source and load impedance should be the same. But for audio signals a voltage adjustment should be the target. So the source impedance should be low as possible.

Do you have any examples or hints how to calculate it??

Thanks a lot!
Karsten
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Old 18th March 2011, 04:22 PM   #2
nightanole is offline nightanole  United States
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How to Calculate Output Transformer Load Resistance
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Old 18th March 2011, 05:13 PM   #3
mastodon is offline mastodon  Netherlands
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Impedance ratio = square of turns/voltage ratio.

So if the turns or voltage ratio of a transformer is 2:1, then the impedance ratio is 2^2:1, or 4:1. Your 5:1 transformer by that logic as a 25:1 impedance ratio.

If the secondary impedance is 600R, then the 2:1 (turns/voltage) will reflect a 2k4 primary impedance, and the 5:1 (turns/voltage) will look like a 15k load on the primary side.
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Old 18th March 2011, 07:14 PM   #4
ruffrecords is offline ruffrecords  United Kingdom
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Quote:
Originally Posted by mastodon View Post
Impedance ratio = square of turns/voltage ratio.

So if the turns or voltage ratio of a transformer is 2:1, then the impedance ratio is 2^2:1, or 4:1. Your 5:1 transformer by that logic as a 25:1 impedance ratio.

If the secondary impedance is 600R, then the 2:1 (turns/voltage) will reflect a 2k4 primary impedance, and the 5:1 (turns/voltage) will look like a 15k load on the primary side.

Then there are a couple of wrinkles.

First, the low frequency response is determined by the transformer primary inductance and the impedance of the source driving it. Higher primary inductance and lower source resistance give better LF response.

HF response is determined by winding capacitance and leakage inductance. Both tend to increase with inductance so improving the LF response can be at the detriment of HF response.

Cheers

Ian
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Old 19th March 2011, 01:32 AM   #5
karsten21 is offline karsten21  Germany
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Hello,
Thanks a lot four your help!

Well, I think I formulate my question to inaccurate; sorry.
The question is not how to calculate the primary impedance using the coupling ratio. Thatīs so far easy if we assume an ideal transformer.

Assuming I would like to drive the output transformer using a CF stage and the transformer has an 5:1 coupling. In this case the primary impedace will be 15K while the secondary is 600Ohm.
Which output impedance should I calculate in best case for the CF driver?

Thanks and sorry for my bad english...

Karsten
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Old 19th March 2011, 01:37 AM   #6
pieter t is offline pieter t  Netherlands
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Quote:
Originally Posted by karsten21 View Post
Which output impedance should I calculate in best case for the CF driver?
Karsten
I guess you mean load impedance.
The same impedance as when you'd take the output from the anode.
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Old 19th March 2011, 12:08 PM   #7
DF96 is offline DF96  England
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Yes, you need to be clear whether you are talking about source impedance or load impedance.
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Old 19th March 2011, 01:23 PM   #8
ruffrecords is offline ruffrecords  United Kingdom
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Quote:
Originally Posted by karsten21 View Post
Assuming I would like to drive the output transformer using a CF stage and the transformer has an 5:1 coupling. In this case the primary impedace will be 15K while the secondary is 600Ohm.
Which output impedance should I calculate in best case for the CF driver?
A lot depends on how much power you want to deliver to the load. The output impedance of the CF will be a few hundred ohms or less but that is not the important factor. What is important is how much idle current the CF stage has because that determines how much power it can deliver.

The other factor is voltage swing. If you want to deliver up to 10V rms into the load and you have a 5:1 transformer then the primary signal will need to be able to swing 50V rms or 141 volts peak to peak which is asking a lot of a CF.

If you can tell is exactly what maximum voltage into what load you need then we can help you better.

Cheers

Ian
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Old 19th March 2011, 05:28 PM   #9
karsten21 is offline karsten21  Germany
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Hey,

So sorry to be to imprecice. I would not write looong mails...

I want to drive an output transformer to get a symetrical signal with 600Ohms output impedance to connect XLR connectors. Typical usage is as an example a DI-Box.
There are some schematrics in the net which I can use but I want to understand how to drive a transformer by an Tube in such a case. The idea is to play with different transformers and driverstages like CF, SRPP and Cascode. To do that I need to understand how to match the output impedance of the driver stage to the transformed input impedance of the transformer and which effect this will have.
The power which I have to transfer is minimal, also the voltage is mostly a so called "microfon Level" wich is nomial -20dBu.
My understanding is, that I do not need a Power-matched alignment between the driver stage output impedance and the transformers primary input impedace ( on 600 Ohms load secondary ). All what I need is a quite low output impedance of the driver stage ( less as possible ) and a matching signal swing to the coupling factor of the transformer to reach the necessary output voltage I think.
But I donīt really know if this assumption is right or if there is someting important other things to be taken in account.

Thanks a lot for your patience with my bad english and my smattering knowledge how to drive a transfomer...

Best regards
Karsten
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Old 19th March 2011, 06:01 PM   #10
pieter t is offline pieter t  Netherlands
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Karsten,

Transformers are not "driven" as such, but you drive the next stage "through" the transformer.
There is much info available of how transformers work; what they do and what they don't do.
Don't you think it makes more sense to do some investigation yourself?
This is not meant in a negative way, but just for your own understanding, and what would the requirements be for driver stages and "driven" stages.
One thing for sure: cascodes and transformers are a no-go territory because of the generally very high output impedances of cascodes (comparible with pentodes in this respect as far as driver stages are concerned).
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