[mfsoa]

I know that you are supposed to use the tap that corresponds to the impedence of your speaker (or the lowest impedence that your speaker presents).

And that if you use a tap that is too high, you lose "control" of your speaker and that can result in frequency response shifts relative to an ideal amp.

So what's the drawback to always using the 2 ohm tap for speakers that range from 4 to 8 ohms?

If someone could give me a little real-world lesson (like, you get more/less power at 8 ohms than 2; you get better/worse woofer damping; better/worse freq response etc) I'd appreciate it.

So begins my foray into vacuum tube amps!

Thanks for the help.

-Mike

[Steve]

Hi Mike,

A couple of quick thoughts about using a 2 ohm tap. Alot depends on if the amplifier design. If close to max power with the proper tap (some go for lower distortion and a few percent less output power in the design). OK, for a 2 ohm tap.

1) Distortion is lower and power output is down. Damping is greater as the output Z of the amp is lower.

2) Frequency response is usually lower using a 2 ohm tap. If lower depends on the OPT design, if the secondary uses multiple taps or just designed for the 2 ohm setup.

Hope this helps.

[ctviggen]

Why do they have multiple outputs? 

[mfsoa]

Here's what seems to be a good technical summary, but somewhat over my head

http://www.vac-amps.com/Technical_Monograph_90-9.htm

I was looking for a more real-world, layman's overview, along the lines of Steve's post.

This stuff must be second nature to so many of you, but it's still new to me.

Thanks, Steve!

-Mike

[zybar]

Mike,

I honestly don't know the technical details, I just simply try the different taps and go with the one that sounds best.

"Usually", one sounds much better than the others and one sounds mediocre.

Have you tried different taps yet with the VAC amp? 

Do you hear any difference?

George

[mfsoa]

I had it on 4 ohm all along and just briefly tried the 2 and 8, but could not reach any conclusion.

You are right, though, the proof is in the listening.

I haven't tried to biwire yet, but my VSAs seem to be perfect candidates for using the 4 ohm tap for the (nominal) 4 ohm woofer cabinet and the 8 for the (nominal) 8 ohm mid/tweeter.

-Mike

[art]

Because a transformer is an impedance transformation device. In order for the plate to see the right impedance (it is designed to work into), you have to match/"select" the right tap.

Pat

[Steve Eddy]

Because a transformer is an impedance transformation device. In order for the plate to see the right impedance (it is designed to work into), you have to match/"select" the right tap.

Bingo.

se

[Freo-1]

Pat hit the nail on the head. In practical terms, in order to obtain optimal  power transfer between the speaker and the amplifier, connect to the tap that corresponds to the speakers nominal impedance.  The actual impedance of the speaker varies considerably with the frequency  applied, most of which has to do with the speaker crossover. 

[Steve Eddy]

The actual impedance of the speaker varies considerably with the frequency  applied, most of which has to do with the speaker crossover.

Right. Though even a single driver will have a large impedance peak at resonance followed by a rising impedance due to voice coil inductance. So even there you have a somewhat narrow window where the driver's impedance is close to its rated nominal impedance.

se