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RE: AA100 OPT imp...

Posted by burwoodc (A) on January 24, 2015 at 23:33:57

In Reply to: AA100 OPT imp... posted by Steve O on January 24, 2015 at 21:23:59:

Thanks for the info Steve-o

here is a quote from the thread made by Dave below,

I have conducted numerous tests on both of the SA-100's OP transformers from the unit I have here, and can say without a doubt that Sams has done it again. In particular, the transformers on the SA-100 are in fact 7800 ohm units (7831.47 ohms to be exact) AS MEASURED USING THE FULL SECONDARY winding. This is the ONLY way to measure the primary impedance of a given OPT, as it is the full secondary winding that forms the basis of a transformer's design to begin with. The 4 & 8 ohm taps then are simply just that -- taps on the full 16 ohm winding configuration. The 16 ohm winding is NOT a derivative of the 4 or 8 ohm windings. Therefore, it is always the full secondary winding that is used to determine primary impedance.

Because of the multiple windings and highly interleaved winding process used in high quality OPTs, the 8 ohm tap cannot always be placed at exactly 8 ohms, but is a close approximation. This usually results in the 8 ohm tap reflecting a slightly lower impedance, and the Fisher transformers used in the SA-100 are no exception. In this case, the 8 Ohm tap reflects 6961.3 ohms from an 8 ohm load, which may be the tap that Sams used to generate their data from -- which in any event is wrong for the reasons I have indicated. Either way, their data is "a bit off" regarding these transformers.

Using the 4 ohm tap for measurement usually produces far less error, as the 4 ohm tap simply uses 50 % of the windings used to make up the full 16 ohm configuration. Therefore, any error present then is usually quite small. In this case, the Fisher SA-100 transformers reflect 7901.23 ohms back to the output tube plates with a 4 ohm load placed on the 4 ohm tap. This is within 1% of the impedance reflected from the full 16 ohm winding with a 16 ohm load applied, while the 8 ohm tap only reflects 88.9% of the full secondary impedance.

In any event, since the primary impedance of the X-101 transformers is very close to this impedance, they would in fact work from an impedance standpoint, although with the LF limitations previously noted. And (as I know you know!), the NFB network would require some alterations as well.

If you are interested in building your own clone of the SA-100, I would recommend that you use the transformers I did, and have recommended to others to use -- who have now used them with success as well. My transformers came from a Heath AA-100 amplifier, and have proved to be every bit the equal of the original Fisher transformers -- in nearly every way. First, their impedance is so close as to be insignificantly different. Second, their lam stack is IDENTICAL in size to that of the SA-100 transformers (although they mount up differently). Third, they come from a pentode based 7591 amplifier rated for 25 watts RMS per channel, and finally, the measured performance of my clone shows them in fact to perform at least at the level that the SA-100's transformers will.

As an added bonus, if you follow the basics of my clone version, you also have all the NFB issues for these transformers worked out for you, eliminating that element of concern.

Once EFB II is installed in the SA-100 and its performance is improved, then we'll know for sure. But every indication is that whatever the SA-100's transformers could do, the Heath (Stancor) transformers can do just as well.

Edits: 01/25/15

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