Resistors vs.peltz Autoformer- Differences

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.

My main concern is: what is the difference between adding a resistor to a speaker vs. the Speltz autoformer?

@gdnrbob   In your case would the resistor be placed in parallel (across the speaker or amplifier terminals)? If yes, then obviously the resistor is a lot cheaper. Since this is something ZU is recommending for use with their speakers, I would not regard it as a 'band aid' fix as George suggests. ZUs are easy to drive and you don't need an amplifier with high 'current'; in fact they seem to sound better when the amp has a fairly high output impedance.


You'd have to try the ZEROs to see if its worthwhile, but since this is a method suggested by ZU I don't think the ZEROs are worth consideration.

In the context of this thread, George's comments don't add up. I have to assume that he didn't read everything here including the link.

The use of paralleled resistors by ZU has nothing to do with the speaker being overdamped. Its has to do with the fact that the drivers are typically 10-12 ohms which is an impedance that isn't matched well by some output transformers. In some cases this makes a difference with the amp, in other cases not so much, but the resistors are there to push the impedance down to a point where the 8 ohm tap (in some cases) will be appropriate. Our amps work fine with the ZU speakers and no resistors (or ZEROs) needed.

Depending on the number of drivers used in the speakers the resistors and taps on the amp will vary so as per usual, simply check with the manufacturers involved.