Infinitely variable ohm load. Is box rise obsolete?

[Wicks]

Impedance matching transformers have been used for decades.

They're real.

It is a way of manipulating your amp load...used a lot back in the day...with 10W amplifiers.

But I'd worry using them at the power ratings of today's class D amplifiers.

With thousands of watts, a few percent of inefficiency is a LOT of heat.

Auto transformers or variacs also use brushes inside them to float over the transformer coils. There's going to be some resistance at that junction.

I would suppose the larger variacs are better built but I'd watch for heat.

The main drawback is probly cost. A Recone is got to be much cheaper then a variac that can handle the appropriate power.

You'll also want to at least double the power rating of that transformer, for reliability and proper power handling.

I've ran transformers at their power ratings and they get HOT...

That said.....I love your ideas Snow! Go for it and try it out. Just be careful of heat and high output voltages. Boosting voltages from already high power rails can be dangerous. Especially with transformers with large exposed terminals.

[SnowDrifter]

I was waiting for you to chime in Jamie. Appreciate your input, as always

[Wicks]

I was waiting for you to chime in Jamie. Appreciate your input, as always

Back at 'cha!

I love a good SnowDrifter post to get people thinking.

Not enough of those lately...

[P4killer_]

The part about if the speaker blows the amp will see a short is a good one. Im going to school for this stuff and my friend is a EE, but its been awhile since I looked into this stuff..

But given ac the transformer itself will have some added impedance to the circuit. And as others have said, im not sure the actual speaker impedance will change like you want. At the end of the day the transformer will present its own load to the amp.

Now im getting mixed up. Ive been in logic and embedded systems for a long time and maybe im mis remembering. But the jist of what im trying to say it that I dont think it works how youve described.

I need to go jog my memory.

[Wicks]

http://en.m.wikipedia.org/wiki/Impedance_matching

I would also verify that the transformer can support the frequency response needs.

You might get some filtering through it...

I've used transformers at 60Hz but not lower...

[_paralyzed_]

What is downstream in an AC circuit?

After the power source.

Impedance matching transformers have been used for decades.

They're real.

It is a way of manipulating your amp load...used a lot back in the day...with 10W amplifiers.

But I'd worry using them at the power ratings of today's class D amplifiers.

With thousands of watts, a few percent of inefficiency is a LOT of heat.

Auto transformers or variacs also use brushes inside them to float over the transformer coils. There's going to be some resistance at that junction.

I would suppose the larger variacs are better built but I'd watch for heat.

The main drawback is probly cost. A Recone is got to be much cheaper then a variac that can handle the appropriate power.

You'll also want to at least double the power rating of that transformer, for reliability and proper power handling.

I've ran transformers at their power ratings and they get HOT...

That said.....I love your ideas Snow! Go for it and try it out. Just be careful of heat and high output voltages. Boosting voltages from already high power rails can be dangerous. Especially with transformers with large exposed terminals.

Yes they have been used in amps, but they are used to match the load, not change the load. A good example of how they are used in amps is a JL Audio amp that will put out it's rated 1,000 watts at 4,3,2 or 1 ohms. It changes the output, not the load.

[P4killer_]

I see now. Much of what I found gives a recommended speaker impedance with an output listed. Which is what I was thinking like the transformer will obviously effect the total Z of the ckt. But they more or less solve that for you.

So obviously there is gonna be heat etc. But have you considered thd? What I saw was rated at 5% at 60hz and .1% at 1k. Idk how that effects things but typical thd of a decent amp is .01% no?

[_paralyzed_]

http://en.m.wikipedia.org/wiki/Impedance_matching

I would also verify that the transformer can support the frequency response needs.

You might get some filtering through it...

I've used transformers at 60Hz but not lower...

That article is about impedance matching, like showing a one ohm load to a one ohm amp. But that is changed by using different speakers. It is not an article about variable ohm loads.

[_paralyzed_]

Read more of wicks article. Now I don't know what to believe!

Awesome stuff Snow Drifter!

[Wicks]

When in doubt...hit the whiteboard.

Here is a scenario where you have an Amp with 100V of output (left) with a variac in between your 4ohm load (right).

You want less output at the load so you "turn down" your variac.

Variac primary has twice the turns as the secondary. Therefore the voltage out of the secondary is half the primary.

So we get 50V to the load.

50V at 4ohms will give us around 625W.

I'm keeping this simple with just a resistive load, to look at the basic math.

That gives us a load current of 12.5A.

So with the transformer turns-ratio, that gives us 6.25A in the primary.

Primary power = 6.25A * 100V = 625W. So we have 625W in the primary and 625W at the load. Power is conserved and the numbers check out.

*In real life there will be some efficiency loss so there will actually be less power in the load but again, we'll keep this simple.

So the Amp outputs 100V and 6.25A. Therefore the Amp "sees" a 16ohm load.

OK,

Now time to boost

Now we have 1.5x the turns on the secondary so the voltage rises by 1.5x.

With 100V of input we get 150V of output.

150V over our same 4ohm load will give 5,625W.

150V over 4ohms = 37.5A.

A turns-ratio of 1.5 will give a primary current of 56.25A.

Therefore power in the primary is 100V * 56.25A = 5,625W (same as load). Numbers match.

So the Amp outputs 100V and 56.25A. Therefore the Amp "sees" a 1.78ohm load.

All with the same Amp and load, just turning the knob on the variac, you give the Amp a variable load.