Resistors vs.peltz Autoformer- Differences


I just got an email from Zu.
They offer resistors for their speakers that will lower/raise impedance so that amps will have an easier time driving them.
What is the difference between using a resistor and using an Autoformer, like the Paul Speltz? Both seem to be doing the same thing.
Bob
gdnrbob
Nuts, I can't edit the title.
Resistors vs. Paul Speltz Autoformers- Differences?
Bob
Post removed 
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

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?
@almarg Thanks Al.  Appreciate the education...
Resistors vs. Paul Speltz Autoformers- Differences?
Both band-aid fixes, get the right amp to do the job in the first place and you'll be sonically much better off.
If you can't bear to change that amp that can't do the job, then change the speakers so it can.

Cheers George 
Resistor.... Q of the woofer and of the system will change.... QES and QTS will raise.  The woofer will want a larger box or want to head towards a sealed box rather than ported.  I've never really like what resistors did to the sound of the bass.... Of course,  it does raise the overall system impedance, but there is no free lunch.  
Be aware that depending on other criteria,  it is possible that you could get a sucked out mid range or a rolled off top end.... But,  its an inexpensive and easy experiment.  
I hope this helps,
Tim
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


Thanks Almarg, et al.
@millercarbon,
Yes, I agree, getting the right equipment is the best approach. I was curious as to how either method worked, especially with regard to sound reproduction.
Given that Zu has signed off on the resistors, I would imagine they find any tonal differences to not be significant.

B
Given that Zu has signed off on the resistors, I would imagine they find any tonal differences to not be significant.
The only way I could see that working, is if the speaker was purposely over damped in it’s design, and all you do with the resistors is to ruin the damping factor (output impedance) of the amp so then bass is then becomes correctly damped (to suit the room maybe)? Whatever it is, it’s a wank, so are Autoformers, just get the right speaker or amp to do the job properly. 

Cheers George
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
Parallel connection?  Everything that I said is incorrect. Sorry,  I must have mis read.  From Al's 1st post,  I thought that this was a series connection.  A parallel connection will decrease the load seen by the amp and hurt the amps current capability.  
Good Luck,
Tim 
@almarg ,
Correct as always. I was on another thread with millercarbon and got everyone mixed up.
Bob
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.


atmasphere In the context of this thread, George’s comments don’t add up
Sorry but I have to assume that "you" didn’t read everything

almarg
While using a resistor "will lighten the load on the amp" 
Also, damping factor will be affected very differently in the two cases
gdnrbob They offer resistors for their speakers that will lower/raise impedance

"They offer resistors for their speakers to Lower/Raise the impedance of the speaker" as seen by the amp

To raise the impedance of the speaker it would have to be a series resistor

My answer was in conformation to al’s answer, with the assumption that there was a series resistor involved, hence damping factor (output impedance) would be effected.

gdnrbob
As for any of this, getting the right amp to do the job is better than any of these "band-aid fixes", one of which is expensive and is only an interim fix.
Or change the speaker so that "wrong amp" then becomes the "right amp".
I’ve seen too many of my customers go down this "band-aid" fix route to "make" things work when they naturally don’t, and it always ends in disappointment.

Cheers George
@georgehifi ,
As I said before, I agree.
I only posted this to get an idea of what is being done and the differences between the Zu resistor and the Spetz Autoformers in terms of how they are operating.
I don't know a lot about such things, and wrote some things that were improperly stated and only rectified when I provided a link to the Zu webpage.
My main concern is: what is the difference between adding a resistor to a speaker vs. the Speltz autoformer? 
Bob
@gdnrbob what's your amp BTW? OTLs by any chance
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.

No matter how you look at it, resistor "parallel", with the amp terminals makes the amp work harder, and have more distortion, run hotter, will go out of it’s Class-A bias earlier and sound maybe harder.

If the resistor is in series then you screw up the damping factor (raise the output impedance) of the amp, so the bass is less controlled and muddled, (unless the speaker was purposely over damped design which I doubt).

As for the Autoformers, just an "expensive band-aid fix" to solve the problem of a bad match between amp and speaker, just get the right amp or speaker to do the job properly.

Cheers George
@luisma, @atmasphere ,
I have no desire to add resistors or the autoformer to my speakers.
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?
B

@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
gdnrbob OP3,025 posts
01-31-2020 9:05am
@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
I understand that’s all "you" wanted to know, but it’s the smoke screen that’s given here about autoformers/resistors that are in series or parallel that causes me to post, as it gives "others" the wrong idea that the use of them is going to solve a problem without any negatives, which is wrong!!

As I said the best is to have the right amp or speaker for the right speaker or amp, without using band-aid fixes that add other problems..

Cheers George