Transformers - Impedance vs. Turns Ratio

[guitarrock04]

In my quest to build both an LA2A and an 1176 on the cheap, I've got most everything acquired, minus transformers. I would like to use some of the stuff that I have on hand, if at all possible, so I wanted to run this by everyone for some clarification.

LUNDAHL LL5402 Output 1176 - 2 + 2 : 1 + 1

...is what I have specified as the OT. On the datasheet from Lundahls website, no impedances are given. I have a nos Thordarson TR195 on the shelf, and I'm wondering if it wouldn't be an acceptable substitution. It has 100ohm primary with 100ohmCT secondary.

Any thoughts?

I understand the concept of turns ratio. That's pretty straitforward. I'm still trying to grasp the difference between a 100:100 vs. a 15k:15k transformer though. As I understand it, impedance is the measure of frequency dependant resistance. I.e., resistance at 60hz is different than 20khz. Right? So is it just that the different impedance ratios will have different frequency responses?

Thanks in advance.

 

[PRR]

> As I understand it, impedance is the measure of frequency dependant resistance. I.e., resistance at 60hz is different than 20khz. Right?

Not really.

RESISTANCE is -constant- at all frequencies and all voltages and temperature and whatever.

"Impedance" is used loosely to mean: like a resistance, but not constant.

> I'm still trying to grasp the difference between a 100:100 vs. a 15k:15k transformer

It is largely about frequency. The main parameter is Inductance. Pure Inductance is an impedance which rises in direct proportion to frequency. The inductance has major influence on transformer bass response.

Is it an audio transformer or a radio transformer? If audio, is it cheap stuff with 150Hz-5KHz response or good stuff with 50Hz-15KHz or great stuff with 20Hz-20KHz response?

Assuming the claim is 15K at 50Hz, the transformer's impedance is roughly:

1K5 at 5Hz
15K at 50Hz
150K at 500Hz

Unloaded impedance proportional to frequency.

Loaded in 15K resistor (ADC input), it is

1K5 at 5Hz
7K5 at 50Hz
13K5 at 500Hz
...i.e., "about 15K over the usable range".

What if you load it in 100 ohms? Then it would seem to give deeper bass.

There is no free lunch. There is copper resistance. For audio iron it is usually 5%-10% of nominal impedance. So a "15K" winding "could" be loaded with 100 ohms, but with 750-1500 ohms of copper loss, the useful output in the 100 ohms is mighty small. You can cheat a little. You can't cheat a lot without giving up something.

> On the datasheet from Lundahls website

When asking questions, you are liable to get more response if you post the link so lazy (or busy?) folks don't have to go hunting for data you already have.

And while that sheet is a bit obfuscated, when wired primaries in parallel and secondaries in series, it is essentially 600:600, although they claim best (fantastic) performance when worked from -15 ohms into >800 ohms.

> Thordarson TR195 ... 100ohm primary with 100ohmCT secondary.

I dunno if this is for the LA2a or the 1176, but neither is really intended to drive 100 ohms. And Thordarson made everything, from super wide-band stuff to cheap walkie-talkie parts, so I can't guess (and am too darn lazy to hunt for data). If it is supposed to do 100 ohms at 50Hz, and you used a driver intended for 600 ohm loads, and only drive it above 300Hz, it may work very well. But try to put some bass in it, with a driver intended for 600 ohms, and the amp will feel strained, probably distort all to heck.