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Old 10th May 2009, 09:31 PM   #1
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Question Multi-tap OT

Ok.I'm in an inqusitive mood this afternoon...or so it would seem. When using an OT with multiple impedance taps,it seems to me it would be best to use the highest impedance possible (provided, of course, it matches your speaker) to use as much winding as possible. Right/wrong???
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Old 10th May 2009, 09:41 PM   #2
kevinkr is offline kevinkr  United States
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Yes, that actually makes sense, mainly because the unused portion of the winding represents a design compromise that resulted in somewhat higher leakage inductance and stray capacitances than one without the taps. Oddly enough I have measured significant differences in transformer performance with cheaper opts like the A-471 (Dyna ST-70) where the 16 ohm tap clearly had less phase shift, and more extended response into a 16 ohm resistive load than was the case with the 8 ohm tap and an 8 ohm load. (Also true of 4 ohm tap, and 4 ohm load)

Jack Elliano at Electra-print considers this to be a serious enough issue that he actively discourages the use of tapped secondaries if at all possible, and if not he will actually specify the frequency response for each tap or winding combination possible - from that you can also see this is the case. (All windings in always perform a bit better.)

FWIW I agree, and an astute observation.
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Old 10th May 2009, 11:41 PM   #3
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Thanks, Kevin...I guess I have more neurons working than I thought !!!
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Old 11th May 2009, 07:17 AM   #4
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Interesting post by Kevin!

From what I have read I understood that transformers performance is also related to the ratio between primary and secondary: the lower the ratio, the better the performance. As such a 16 ohm secondary would also be better, as it implies a lower turns ratio than 8 or 4 ohms (given the primary impedance is kept constant)

Related to the first question in the thread... I always wondered how good the solution presented by Sowter is? Their units have four windings in the secondary that can be configured for different impedances, keeping all windings used. To me it seems the perfect solution, but I don't see it often (only Sowter, so far).

http://www.sowter.co.uk/push-pull-ou...ansformers.php

(the configurations can be seen at the end of the page, left)
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Old 12th May 2009, 10:50 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by ErikdeBest
Interesting post by Kevin!

From what I have read I understood that transformers performance is also related to the ratio between primary and secondary: the lower the ratio, the better the performance. As such a 16 ohm secondary would also be better, as it implies a lower turns ratio than 8 or 4 ohms (given the primary impedance is kept constant)

Related to the first question in the thread... I always wondered how good the solution presented by Sowter is? Their units have four windings in the secondary that can be configured for different impedances, keeping all windings used. To me it seems the perfect solution, but I don't see it often (only Sowter, so far).

http://www.sowter.co.uk/push-pull-ou...ansformers.php

(the configurations can be seen at the end of the page, left)
Hi Erik,
What you state is also true and starts to become a pretty significant issue around 3K or so.. Oddly enough trading off interwinding capacitance against leakage inductance is a pretty common strategy in UL and pentode OPTS, but for triodes low leakage inductance is pretty critical and the trade offs generally need to go in the opposite direction.

The difference between 8/4 , and 16/8 is only 1.414, but the difference between 16 and 4 is 2:1 which is why having even multiple tapped secondary windings wired in parallel may reduce leakage inductance more than the most care and attention to winding geometries with the conventional 4, 8, 16 ohm series connected winding where half of it is not used when driving a 4 ohm load for example.

Actually the Sowter solution is a fairly good one, and is somewhat common here in the USA with vendors like Electra-Print, a very few Hammonds and some OEMs doing this as a matter of course. Most vendors here would prefer to sell you a transformer with a single secondary for the reason that the whole winding is always used and makes the best use of copper, coupling, and bobbin window area all other factors being the same. (Less leakage inductance than would be the case with a tapped secondary.)
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