Resistors vs.peltz Autoformer- Differences

Hi Bob,

While using a resistor will lighten the load on the amp it will also waste a lot of the power the amp is putting out. For example if a 4 ohm resistor is placed in series with a 4 ohm speaker (that is really 4 ohms) half of the power supplied by the amp will be converted into heat by the resistor, as opposed to being supplied to the speaker. The power lost in an autoformer, on the other hand, will be minimal.

Also, damping factor will be affected very differently in the two cases, and in favor of the autoformer. In the 4 ohm example I just cited, the damping factor seen by the speaker will be less than 1 if a resistor is used. But if an autoformer is used to provide the same increase in the load impedance seen by the amp the damping factor seen by the speaker will correspond to the damping factor of the particular amp (for a 4 ohm load) multiplied by two.

Best regards,
-- Al

This cues a related question I've been mulling for quite some time.

Assuming the impedance increase added by the autoformer doesn't put the load outside of the amp's desired impedance range, what are the downsides of adding one?  

That is, why don't we just automatically bump up all of our speakers to around 16 ohms, and then remove the autoformer if a special case amp shows up that doesn't like that high of impedance (e.g., some of Nelson Pass's SIT amps generate more distortion at higher impedances and prefer the 4-8 ohm range)?

Thanks.

Hi Chris @cal3713 ,

I guess the most major downside of doing that would be that most solid state amps can’t provide much more than half as much power into 16 ohms as into 8 ohms.

And in the case of tube amps having 4 ohm and 8 ohm output taps I would expect that presenting the 8 ohm tap with a 16 ohm load would result in some degree of compromise to both maximum power capability and distortion performance.

Also, a less frequent concern may be that the increase in damping factor I referred to earlier (and the corresponding reduction in the output impedance presented to the speaker) may result in some speakers being overdamped, resulting in compromised bass performance and/or other adverse effects on tonal balance.  Of course, it's also possible that the increased damping factor/reduced output impedance could be beneficial in those respects in many cases, depending on the particular speaker and amp.

Best regards,
-- Al

Here's the page:
https://www.zuaudio.com/low-voltage/2020/1/24/loading-resistors
It looks like this is only for a few of their speakers.
Bob

@gdnrbob

Hi Bob,

Thanks for providing the link. Zero autoformers are normally used to increase the impedance seen by the amplifier, so I had assumed that your reference to using resistors as an alternative referred to inserting a resistor in series with the speaker, which would raise the impedance presented to the amp. But I see in the Zu writeups that what they are referring to is placing a resistor in parallel with the speaker, to **reduce** the high impedance some of their speakers would present to the amp.

A Zero autoformer can be connected "backwards" to accomplish the same reduction in the impedance seen by the amp, but the need to do that would only arise in uncommon situations. And in those situations I would expect that the large cost difference between the two approaches would make resistors the preferable alternative.

Best regards,
-- Al