API 312 Output Impedance

[ilfungo]

Hi sorry for a stupid questions
API 312 use 2503 output transformer with 4 identical 150 ohm windigs.
one 150 ohm primary and 2 x150 ohm in series secondary.
Is the oputput 300 ohm?
If is correct why all use 600 ohm t pad or 1k log pot between + - on output ot archiviet constant 600 ohm?
Thanks!

 

[CJ]

inductance (essentially Z at a certain frequency) is a function of transformer coil turns squared.

when you series the two 150 ohm winds, you are doubling the turns that create the magnetic flux in the core,

so you are in effect, quadrupling the inductance.

since inductance is really resistance as a function of frequency, you are quadrupling your resistance to AC , thus, your Z (AC resistance) quadruples from 150 to 600,

hows that for a long winded answer to a question that could have been answered by "when you series the two windings, the Z gets multiplied by 2^2.

 

[ilfungo]

Clear
Thanks!!!

 

[Bo Deadly]

Note however that the output impedance of the API 312 is not 600 ohms. It is designed to be loaded by 600 ohms. Another way to say this might be that it's characteristic impedance is 600 ohms. If the output impedance was 600 ohms and you loaded it with 600 ohms, the level would drop by a factor of 2 (6dB). But it doesn't because the output impedance is actually much lower. If you tried to load it with lower, like say 100 ohms, you would start to strain the capabilities of the output transformer. It would be over damped and the bandwidth would change.

But of course none of this matters because nobody would load an API 312 output with less than 600 ohms. In the console it would be loaded with 600 which, again, is the impedance it is designed to work with. As a stand alone pre, it would probably be driving a line input which is almost always 10K.

 

[ilfungo]

Thanks!!!

 

[ruffrecords]

Classic class A Neve transformer output stages have a similar step up. They are designed to be able to drive 600 ohm loads but Neve clearly state in their literature that the actual output impedance is 75 ohms.

Cheers

Ian

 

[FIX]

As far as drive, the transformer is basically 300 per winding. 2 in series is 600, in parallel, 150. The 3rd was typically used as a second output, or when needing more gain. You can parallel 2 for the primary for a little different sound. They sound best driven with a 2520, 990s sound kind of odd...