Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms - diyAudio
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Old 23rd March 2007, 04:18 AM   #1
Wizard of Kelts
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Default Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms

Have been reading about the beneficial effects of amps with a high output impedance. I was wondering if there is a way to take a typical voltage amp with an output impedance of 0.1 ohm and add an output transformer so the output impedance is 47 ohms.

My understanding is that such a thing is possible if the secondary has a 47 ohm resistor across it. However, such a thing strickes me as being wasteful of the amp's power.

If you have say, a 100 watt votage amp, is there any way to add an output transformer so it has an output impedance of 47 ohms and also something like 80 watts output? I don't mind losing a little power to the transformer, they are not perfect devices, but simply putting a resistor across the secondary would consume a good deal more than 20% of the power.

Any help would be appreciated.

PS: This thread is not intended so much for Spice Modelling issues of the transformers, which issues are covered in this thread. This is just about the transformers themselves and the possibility of their changing output impedance.
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Old 23rd March 2007, 04:23 AM   #2
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Why would you want to do that?? Transformers on the output are bad!
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Old 23rd March 2007, 04:28 AM   #3
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MJL:

Take a look at this thread, especially the second half when Nelson Pass' paper is examined.

You will see some real advantages in high output impedance amps. Yes, there is a tradeoff with putting the signal through another device before it reaches the loudspeaker, but it would be worth at least trying to achieve the advantages a high output impedance amp can give.
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Old 23rd March 2007, 04:32 AM   #4
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In fact, just to save time, here is the link to Nelson Pass' paper.

However, after reading it please return to the discussion in the thread, much good info was posted there.
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Old 24th March 2007, 12:56 PM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
if you wanted to convert 0r1 to 47r using a transformer then a turns ratio of 21.7 does the job.
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Old 25th March 2007, 03:56 AM   #6
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Thank you very much. Your answer is very helpful, as always.
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Old 25th March 2007, 04:23 AM   #7
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Quote:
Originally posted by kelticwizard



You will see some real advantages in high output impedance amps. Yes, there is a tradeoff with putting the signal through another device before it reaches the loudspeaker, but it would be worth at least trying to achieve the advantages a high output impedance amp can give.
There may be some advantages to high output impedance, but a transformers not the way to go about it. As follows is a quote from Rod Elliot on the subject:

"Some esoteric (some might say idiosyncratic) designs use inductors or 1:1 transformers, but these are bulky and very expensive. Unless made to the utmost standards of construction, they will invariably have a negative effect on the sound quality, since the losses are frequency dependent and non-linear."

Although not Nelson Pass, a designer I (and many others)hold in high esteem .
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Old 25th March 2007, 04:37 AM   #8
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Thank you for that information, I will keep that in mind.

Incidentally, Nelson Pass didn't particularly endorse using transformers to do this. He just used transformers to experiment, to see the difference in various curves using high output impedance. That paper did not mean he thinks transformers are any permanent solution.
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Old 25th March 2007, 05:03 AM   #9
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Quote:
Originally posted by MJL21193
Although not Nelson Pass, a designer I (and many others)hold in high esteem .
Rod Elliot is just about the worst reference you can make for this kind of information, he's as anti-exotic as you can get. Of course the transformer has to be of high quality for Pete's sake, why do you think tube amp transformers cost so much!

KW, I don't know if you had this already in mind, but the turns ratio for your example works out to a fairly common tube OPT turned around backwards.
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Old 25th March 2007, 05:08 AM   #10
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I did not know that. That is very good information to know.

Any model numbers, or just look up transformer companies with 27:1 ratio output transformers?
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