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’m not sure if everyone is keeping in mind the distinction between series connection of a resistor and the parallel connection that is being suggested by Zu for some of their high impedance speakers. The effects of the two kinds of connections are of course completely different.

For one thing, the parallel connection being suggested by Zu will actually improve the damping factor, at least a little bit, rather than degrading it. The reason for that is that with a resistor connected across the speaker terrminals the impedance "looking back" from the speaker will be the parallel combination of that resistance and the output impedance of the amp. And the impedance of that parallel combination will be lower than the output impedance of the amp.

Also, Bob, I think you meant to address George in your previous post, not Millercarbon who has not participated in this thread thus far.  An understandable mistake, though, given the latter's posting frequency :-)

Best regards,
-- Al

Aside from the load impedance, wouldn’t adding a resistor in series with speaker driver affect the high frequency response of the driver, particularly in case of a single driver or a tweeter?

@kalali

Yes, adding a resistor in series will dramatically change the output impedance that is presented to the speaker. That can and in most cases will have major tonal consequences in various parts of the spectrum, depending in part on how the speaker’s impedance varies as a function of frequency.

Best regards,
-- Al

@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

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

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