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Why do modern PP OPTs have 16 Ohm taps?


2003-02-11 9:02 am
It seems like almost all standard PP OPTs from different manufacturer have 16 ohm taps, even though there are hardly any modern speakers that are nominally rated at 16 ohm. Is there any other reason for this?

Adding extra 16 Ohm winding is not "that" expensive, if you think about relationship between impedance and winding turn. This is true for cheap design.

OPT manufacturer is not amplifier manufacturer and not speaker manufacturer. You want flexibility/usability with your OPT so you want to provide variable taps.

16 Ohm tap doesn't mean that your speaker must be 16 Ohm to be optimally connected. If your amp works best with low impedance, may be you can hook up low impedance speaker to high impedance tap (of high impedance primary), to get the correct impedance ratio.

In case of expensive OPT (silver winding for example), may be you don't want to waste taps. Because flexibility is very costly. Amplifier designer will prefer an exact primary impedance (that will give optimum amplifier performance) at average high end speaker impedance (secondary impedance), i.e. 5 Ohm.


2008-12-18 2:29 am
If the manufacturer is happy to leave some secondary copper unused then any ratio is possible. It does create interleaving problems, though, as a P-P OPT should be balanced.

The only real problem is insertion loss; if the transformer is well made balance is not an issue if there is an appropriate simmetry in the winding geometry. Actually all economical OPT's with muti-secondaries are made like this. Regardless of sectioning and stratification of each secondary section, at the end you get two terminations and intermediate taps. In this case, typically, there can be one intermediate tap at 4 ohm if the full secondary is 8 or there can be 2 taps at 4 and 8 ohms if the full secondary is 16. In the latter case, using 8 ohm, for example, you can use the 16 ohms termination for loop feedback.
When you use just the intermediate tap the efficiency of the OPT is lower (i.e. higher insertion loss).

Transformers that use all secondary turns are better, for the same geometry and materials, but they trade off the possibility of multi-secondary unless one re-wires the entire bunch of secondaries for the desired impedance. Basically one secondary impedance per time. In this case however, regadless of the number of sections, it is impossible (by definition) to have exact 4 and 8 ohms secondaries. If one of the two is perfect the other is a bit higher or lower than the nominal value depending on the geometry.
useful if you have speakers with bi-amp terminals and you want either section to be +3db from the other. If you find you do not have enough low end, put the HF section on the 8 ohm tap and the LF section on the 16 ohm tap and the end result will be that the LF section will be 3db louder than the HF section.
I thought 16ohm PRO/guitar speakers were made for paralel connecting
Pro 16 ohm speakers are made for parallel connection and load sharing, they are usually very high power to begin with; most famous Guitar 16 ohms ones are low power, as in 15W to 25W, and to achieve a specific sound "signature".
As to transformer taps, original Marshalls were 16 ohms only and much more modern ones, such as 90's on JCM900, hav all 3 taps (4/8/16) but impedance selector uses only 8/16 ohms ones, go figure.
*If* you want 4 ohms, you must open the amp , grab your trusty soldering iron, and hardwire *that* tap to the switch.
And the standard 1960 4x12" cabinet is *supposed* to be used set to 16 ohms.
With such powerful sponsors, no mystery awkward 16 ohms impedance continues alive and kicking.


diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
It seems like almost all standard PP OPTs from different manufacturer have 16 ohm taps, even though there are hardly any modern speakers that are nominally rated at 16 ohm. Is there any other reason for this?

you'll never know when it might come handy, however, in my latest OPT build for a push-pull 6C33 power amp, i used 2,4, and 8 ohm taps......no 16ohm tap....
Just because the amp is modern doesn't mean everybody has to use modern speakers. Many audiophiles still prefer vintage speakers and drivers that are typically 16Ω. If you're an amp maker, don't you want to maximize your customer base?

I have vintage drivers so I am glad the 16Ω tap is still available on modern amps.