Resistors vs.peltz Autoformer- Differences

@gdnrbob

Bob, I would summarize what I and some of the others have said as follows:

If the impedance of the speaker is too low to be a good match for the amp, and one desires to keep both of them, a pair of Zeros will almost certainly be a much better solution than raising the impedance seen by the amp by putting resistors in series with the speakers.

And if the impedance of the speaker is too high to be a good match for the amp, and one desires to keep both of them, reducing the impedance seen by the amp by putting resistors in parallel with the speakers will most likely be a better approach than using Zeros (connected in reverse), especially when cost is taken into account.

Best regards,
-- Al

I was just curious as to the differences between the two methods with regards to how it affects an amps performance, as well as the resulting sound performance of a speaker.
Does this make sense? I'm probably using the wrong words but hopefully you get my drift.
Both seem to allow the amp to perform 'better', but does the resultant speaker sound change with either method?

The ZERO is used to increase the load impedance on the amplifier. The reason this works is all amplifiers have audibly and measurably lower distortion (you can see it in their specs) when driving a higher impedance load. But if the speaker is already a higher impedance (greater than 8 ohms for example) there may be no need. The advantage of the ZERO is it has wider bandwidth than most amplifiers, being good out to past 1MHz, starting at 2Hz. I don't know of even a solid state amp that can go that high. It can do this without absorbing amplifier power as generally speaking all matching transformers are very efficient.


Now one common place something similar to the ZERO is used is in ESLs like the Martin Logans, that is to say there is a matching transformer built-in to such speakers. It is through this transformer that the MLs have such a low input impedance. In the old days ML used to make the CLS 1 which had a rather benign input impedance, allowing common tube amps to drive it easily. But it didn't work so well with solid state, which is why they changed their transformers to something that presented the amp with a much more difficult load. This was done simply because there are more solid state amps than tube amps made. Now one could make the argument- 'just get the right amp and get rid of the transformer' but in the case of ESLs this is likely a tube amp with some very high voltages since transistors aren't around that can survive the voltages needed for the ESL driven directly. So a device very similar to the ZERO is used instead.


In the case of the resistor paralleled with the load, this is something specific to ZU Audio only. If you have one of their single-driver speakers like the Druid, the system impedance is about 12 ohms, right in the middle between 8 and 16 ohms. This makes choosing the right output tap on your tube amp tricky (it doesn't bother us since our amps have no transformers and simply hook up directly). Some output transformers can ring, especially if the amplifier driving it has no feedback. So the loading resistor is handy to prevent that (resulting is smoother more detailed presentation), if using the 8 ohm tap. Since the speaker is so efficient the power loss is negligible.

@almarg , @atmasphere ,
Thanks, that is exactly the information I was looking for.
Sorry if I wasn't clear at the beginning.
Bob

Look all, to prove it to yourselves that Zero's are a band-aid fix, just put a pair on an amp and speaker that have NO problems with each other and hear what happens, SOUND QUALITY TAKES A BIG HIT!
We've done the experiment at our audio society meeting, and it was 100% unanimous, that the Autoformer is a baind-aid for a problem that should be fixed at the source with different speaker or amp.
Cheers George

@georgehifi .
That isn't what this thread is about.
I just wanted a clear explanation of what these 2 devices are doing.
Ralph and Almarg provided that. 
Bob