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Old 15th January 2007, 02:43 PM   #1
Tyimo is offline Tyimo  Hungary
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Default Output transformator design for SS amp

Hi Tube experts!

I need your helps!
I would like to try a SS amp with 1:1 output transformer. I have heard so many oppinions pro and contra about output transformers, that now I would like to test it myself.
I now the sound of the direct coupled and also the capacitor coupled amps, but not the transformer coupled. So, I would like to try it with a reference capacitor coupled design with output transformer instead of the output cap.
Here is the reference JLH69 amp with dual supply rails. +/-22V

Please help me to calculate and design a suitable output coupling transformer for it!

Power = 20W
Voltage = 44Vdc?
Speaker = 8R

If I would know the amp's correct output imp. and the transformator losses (typical is 10%?) than I could calculate the
Secondary imp:
Zs=8/1.1= 7.2 R

and the

impedance ratio:
Z= Zp/Zs

The next would be the turns ratio:
N= square root Z

The voltage ratio:
Vs= Vp/N

To convert this to RMS

Vp = 1/2 Vp-p =?
RMS = peak * 0.707 =?
Power = V2 / 8 = ?

Am I right??

Here are some practical questions:

iron core area,
lamination thickness,
lamination type/specification,
copper gauge,
number of turns,
target frequency response,
interleaving strategy to extend the frequency response,
minimising low frequency distortion,


Greets:

Tyimo
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Old 15th January 2007, 09:19 PM   #2
ArtG is offline ArtG  United States
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What are all those funny looking things on your schematic with arrows pointing in and out from the flat line? (Kidding!)

I can't help you much with the OT design, but as I remember, a number of very early solid power amps did utilize OTs. Perhaps if you could locate one of these from the late 1960s or early 1970s, it might be good enough for experimentation. A type that comes to mind is commercial PA amplfiers which, as I recall, used OTs so that they could drive a 70.7 Volt line. If you can locate one, it should be very cheap!
The names Bogen, DuKane, and possibly McGohan and TOA, may be possibilities.
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Old 15th January 2007, 10:23 PM   #3
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McIntosh used autotransformers in some of their SS power amps to provided a consistent rated power O/P, regardless of the driven load. The auto'former approach makes sense when using MCP O/P devices and a bipolar PSU.

The Plitron/Amplimo "alliance" offers the model PAT4152-00, which is intended for use with MP "finals" and a single rail PSU. There is enough of a step down in the PAT4152 to eliminate the need for loop NFB around voltage follower O/P circuitry, as linearity and damping factor are adequate.
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Old 15th January 2007, 10:37 PM   #4
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Rather than slapping an output xfmr on a design that doesn't require one, and therefore won't give you any meaningful results, why not give This a try? Doesn't even look like it would be very difficult to put together.
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Old 15th January 2007, 11:34 PM   #5
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I will be glad to sell you one big OT for SS.
It was from Ducane 100W amp.
Actually you can have the whole amp with $10+shipping.
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Old 16th January 2007, 01:52 PM   #6
Tyimo is offline Tyimo  Hungary
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Hi!
Thanks for the answers!

ArtG!
Quote:
Perhaps if you could locate one of these from the late 1960s or early 1970s, it might be good enough for experimentation.
It will be not easy.....

Eli Duttman!

Quote:
McIntosh used autotransformers in some of their SS power amps to provided a consistent rated power O/P, regardless of the driven load.
I will looking for theirs parameters....

Miles Prower!
Yes, I know the Zeus amp.

HighVividity!

Quote:
I will be glad to sell you one big OT for SS.
It was from Ducane 100W amp.
Actually you can have the whole amp with $10+shipping
Thanks, but the shipping would cost 10 times the amp's price!


The most important:
I would like to know and learn how to design and calcultate an OT! I need Theory and Math (but not so much....)

Greets:

Tyimo
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Old 16th January 2007, 03:01 PM   #7
Tyimo is offline Tyimo  Hungary
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O.K. Here is something interesting to read!

http://www.geofex.com/Article_Folder...es/xformer.htm

Tyimo
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Old 16th January 2007, 06:38 PM   #8
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Hi Tyimo,

Siemens did a series of books originally called "Shaltbeispiele" or something like that, as they where written in German but there where also English translations. They contained a collection of solid state based circuits for audio, power supplies, radio receivers and many other things.

There where at least 6 books published in this series and I know that in the first one and maybe the 2nd one there where complete instructions on how to design driver and output transformers for transistor amps. It could be possible to search for one of these books or something similar in public library. There must also be many other books from the early 60ties with this information in them, contrary to modern belief not all technical knowledge is available on the internet!

I know that my father have the complete series of Siemens books but I am unsure where he have them stored now. If you contact me by mail I can try to locate the books and scan some pages.

Regards Hans
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Old 16th January 2007, 07:39 PM   #9
Zen Mod is offline Zen Mod  Serbia
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Old 17th January 2007, 05:03 AM   #10
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Hi,

One more thing, if you build output transformers for a SS amp, especially push-pull, you will encounter some problems that are different compared to when building a OT for a tube amp.

It is easier to get a wider frequency range due to the low impedances and the low transformation ratio but the low impedance and the low number of turns create a problem to achieve perfect balance between the 2 parts of the primary, I rememember that it was recommended to use a bifilar side by side winding for the 2 primaries, i.e. you wind the primary with a double wire.

Regards Hans
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