# How to understand the data sheet for this 'universal' SE output transformer?

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#### ericj

I've got a Stancor A-3849 universal 10w single ended output transformer here. The box alleges 1k-10k impedance.

I have the instruction sheet, but i can't make any sense of how to use it?

Plate and B+ terminals are clearly marked on one side, and there are six terminals on the other. I understand the concept that the impedance is set by which secondary terminals get hooked up to the speaker - but I don't understand how to apply that concept based on the matrix presented by the manufacturer.

Here's a picture of the sheet. Apologies that i haven't managed to slap it on a scanner yet to get a clearer image:

#### MrCurwen

What primary : secondary impedances are you aiming at?

#### MrCurwen

If let's say 5k : 4 ohm, a good fit might be lugs 1 and 5, for 5k : 3.6 ohms.

#### artosalo

I don't see any confusion.
As an example, 2000 ohms primary impedance is formed when 6 ohms load is connected between lugs 1 and 6.
Obvious 4000 ohms primary impedance is formed is the lugs are same and secondary load is doubled to 12 ohms. Etc.

#### ericj

In the short term, about 3.5-4k to 8 ohms.

But I'd like to understand what I'm looking at, so i can apply that knowledge again in the future. I don't know that I'll permanently install this transformer in the circuit that I'm currently looking at.

#### artosalo

4267 ohms is got when 8 ohms is connected between lugs 3 and 6.
And 3555 ohms when 8 ohms is connected between lugs 2 and 6.

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#### ericj

So the main part of the grid there is the voice coil impedance?

#### ballpencil

Choose your target primary impedance from the uppermost row then move down the column until you see the closest number to your speaker impedance and move left to find which lug combination to connect the speaker to.

Thanks.

#### famousmockingbird

So the main part of the grid there is the voice coil impedance?

Yes

#### JMFahey

To see it from another side (actually means the same) , a transformer does not "have an impedance" by itself, but converts/translates a speaker impedance into a higher impedance seen by the plate.

What they actually offer is a fixed primary, and a tapped secondary with 6 taps, which offer different *voltage* ratios and *impedance* ratios.

They might very well just print the voltage ratios and let you do the Math, or print just a couple values.

They were incredibly honest (or suffer OCD ) and offered a full table covering all possibilities, even the most impractical ones , such as .09 ohms or 0.43 or 21 ohms

Personally I prefer extra data to pick from and not having to guess "mystery data".

Those weird values are what your calculator will show if you actually do the Math, tap by tap; it's good that they show all at once and you pick closest possible.

Fine, since load impedance is not that critical; 20% upor down is completely inaudible.

So keep that datasheet, even photocopy it as a backup ... and if you lose it, no big deal, you'll have to feed primary, say, 40V to 120V , measure voltage at secondaries and recalculate it yourself

#### ericj

Yeah, i understand that.

As for being complete, the box advertises 1k to 10k, but the table only goes to 7k. So it might be nice if they stated the ratios.

I had no idea that sub-ohm voice coil impedances were a common thing at any point? Certainly not in the distant past - I know car audio guys like low-Z speakers.

#### jjman

You can double the figures in the 5000 column to get 10000. Or type into excel and extrapolate with a ruler. Or add the 10000 column in excel. Some of the possible connections are off the chart literally and so to speak.

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#### ericj

Thanks. i wondered if that was the case.

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