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2nd August 2012, 01:05 AM  #1 
diyAudio Member
Join Date: Jun 2009

SANSUI AU70 or 1000A???
Hello everyone.
I have a pair of SANSUI output transformers labeled “AU70”. I am not certain which amplifier they came from. As far as size and dimensions, my OPTs are comparable to other OPTs that can deliver 3035W. First guess would be the Sansui AU70 integrated amp, which uses 7189’s in a PP configuration for the final stage. However, I estimated the transformers’ primary impedance to be 6,300 ohms, which is somewhat low for the 7189’s. On the other hand, 6,300 ohms is suitable for a final stage using 7591’s in PP as used in the Sansui 1000A amp. Could anyone, please, clarify the mystery and put an end to my dilemma? I would like to build an amp and use these OPTs, but I am not sure what tubes to use. Thank you in advance for your help and advice. 
2nd August 2012, 03:08 AM  #2 
diyAudio Moderator

as long as you know the transformer specs you can use them in any project you desire....
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the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus.... 
2nd August 2012, 03:50 AM  #3 
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Join Date: Mar 2008


2nd August 2012, 04:10 PM  #4 
diyAudio Member
Join Date: Jun 2009

@ Tony:
You are absolutely correct. Unfortunately I do not have all the transformer specs. I used a rather empirical method to measure the primary impedance, which is about 6,300 ohm. I have no idea about the power handling capabilities of these transformers. This is why I need to know which amp they came from. I would really hate to fry a pair of perfectly good transformers. @Korey: AU70 is indeed the obvious first guess. However, as I said before, the 6,300 ohm impedance is rather low for the 7189 tubes used in the AU70 (data sheet indicates a plate to plate resistance of 8,000 ohm, even 10,000 ohm in class AB1 – triode mode). On the other hand, the data sheet for the 7951 tubes used in the 1000A indicates a plate to plate load of 6,600  therefore my dilemma. Perhaps someone on this forum has one of these amps and is kind enough to check for me the label on the output transformers?............. It would be greatly appreciated! Thank you. 
2nd August 2012, 08:11 PM  #5 
diyAudio Member
Join Date: Nov 2005

I have output transformers from the Sansui 1000A receiver and their transformer number is 4105234 and sometimes they are labeld 1000A and sometimes both numbers are labeled on the end bell. So most likely your transformers are from the AU70 integrated amp which uses 7189 tubes. Although according to this schematic the impedance should be 8K.
Does your irons shape like these? Too bad I couldn't find a picture showing the transformer number... http://images02.olx.com.my/ui/7/65/0...1286515715.jpg 
2nd August 2012, 09:14 PM  #6 
diyAudio Member
Join Date: May 2011
Location: Little Rock

We all easily lose sight of the fact that the real working load of an amplifier is a loudspeaker, something very, very far from a resistor. Variations of 25%, that seem so important to us when designing, are *nothing* compared to the real world use. You certainly cannot hurt your transformer by a slight mismatch  if so, they'd all be dead already.
If you want to use them with 6BQ5/EL84's and are concerned with linearity, just keep working voltages down and idling current up, connect as triodes. Not tons of power output, maybe +7 or +8dBW, but clean. All good fortune, Chris 
2nd August 2012, 11:16 PM  #7 
diyAudio Member
Join Date: Jan 2008
Location: Eureka, CA

What is your empirical method to estimate the primary impedance? I.e. what is the turns ratio and how did you determine it?
Thanks! Last edited by Michael Koster; 2nd August 2012 at 11:38 PM. 
2nd August 2012, 11:59 PM  #8 
diyAudio Member
Join Date: Jun 2009

@directdriver
Thank you very much for your effort and the detailed information. The schematic you attached shows the OPT impedance as 8K, just as the 7189 data sheet. Judging by the labeling of you 1000A transformers that shows “1000A”, it appears that mine are indeed from an AU70. I have to get to the bottom of this impedance discrepancy. Could my transformers be shorted? Who knows!!! I bought them as a set including the mains transformer, which was definitely shorted. Thanks again, directdriver!!! @ Chris Hornbeck I understand perfectly what you are saying and you are correct. The idea was to figure out if I can get 3035W out of these babies instead of just 1517W. Thanks for your input. @Michael Koster The method to estimate the primary impedance consisted of multiplying the impedance of each speaker tap with the turn ratio squared (turn ratio different for each of the 4, 8, and 16 ohm taps). Because of measurement inaccuracies, the calculated impedance values ranged between 6,150 and 6,600 ohm, the average being 6,300. 
3rd August 2012, 12:09 AM  #9 
diyAudio Member
Join Date: Jun 2009

By the way, I determined the turn ratio by applying 15, 10, and 5 volts and measuring the voltage on each speaker tap. I did a total of 9 tests, good enough for estimating purposes.

3rd August 2012, 01:09 AM  #10  
diyAudio Member
Join Date: Jan 2008
Location: Eureka, CA

Quote:
What I'm saying is that low voltage unloaded measurements at certain frequencies could introduce gross voltage errors. Even the simple winding DCR can account for 10% difference in actual turns ratio vs. measured voltage ratio. If the 8K ohms specified includes say 10% DCR (primary plus reflected secondary), and you measure without a load, then your impedance ratio could be off by something like 20%. Add to this a little overshoot due to leakage inductance at low voltage and you may have explained your discrepancy. In fact, if your most representative measurement was the highest, 6600 ohms, and your DCR component causes a 10% voltage error in an unloaded measurement, you have exactly 8K... Assuming the 8K is measured with the rated load. Last edited by Michael Koster; 3rd August 2012 at 01:19 AM. 

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