When an amp puts out 90 watts....


into 16, 8, or 4 ohms, what does that tell me about the amp? I am use to seeing an amp's output increase, sometimes even doubling, when going from say 8 ohms into 4 ohms.

The amp in question is the Copland CTA 506.
finsup
Its probably a tube amp.
That its power supply is capable of sourcing the increased current necessary to provide constant power across decreasing impedances.
As Zd says, and I just confirmed, it's a tube amp. This is how tubes amps typically spec. Nothing to get worried about. And by the way, only a very few solid state designs with massive power supplies are able to double power into halved impedances. This is NOT the norm.
"This is NOT the norm."

It's the norm for some people.
You might want to read more about this:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

An amplifier does not have to double power to sound musical and to also have a lot of authority.
Yes, I knew it was a tube amp but did not know that was characteristic of all tube amps. I just finished reading a thread with posts by Atmasphere and Almarg and others about amp design and was reading a review about the Copeland amp. The statement about it putting out the same power confused me with all the other info I had just tried to absorb.

A lot of what they wrote about was way over my head but rather than just plead ignorance and let it go at that, I have decided to try to educate myself about amps, and specifically, matching amps to speakers.

I think part of the reason I don't like my system as much as I think I should be able to given the components, is that there may be a amp / speaker mismatch although I am experimenting with room placement and will be bringing in a different source soon enough.

Ralph, that link keeps appearing in other threads so I guess it is about time I dove in...Thanks.
You should have just said so. List your whole system and some basics on your room. I'm sure someone will be able to point you in the right direction. As far as room placement goes, thats a great place to start. You'll learn a lot just doing that alone.
Rrog: It is the "norm" for me and, because it matters in my system, it's why I have a Krell 400cx driving my Thiel CS6s. My point was that the vast majority of amps do no comply with the doubling of power by the halving of impedance from the incrementals of 8 to 4 to 2 Ohms. Only a handful of manufacturers install power supplies robust enough to do this.
If you start with a reasonably powered amp and know the speaker impedence and efficiency are also reasonable, and act reasonably, you don't need to spend another moment on this power issue. Copland stuff is well regarded and I suggest you just set the whole system up and avoid the avalanche of opinions you could subject yourself to regarding room treatment, tiny wooden speaker wire bridges, gigantic power regenerators, fuse direction, and hairstyle tips...however, MY opinions are to be cherished as True Art and a gift to the reader...I'm a giver.
You should have just said so. List your whole system and some basics on your room. I'm sure someone will be able to point you in the right direction. As far as room placement goes, thats a great place to start. You'll learn a lot just doing that alone.

Maybe I will start another thread later but really, my reason for asking stems from reading a couple of different amp threads. There was a lot of discussion about impedance, slew rates, etc. and some calling out others for wrong information, etc. I guess I just want to have enough information to be dangerous like some of you other guys. :)

But seriously, I could not remember any information about amps delivering the same power into different loads and that is why I asked. Now I know why that is the case.

If you start with a reasonably powered amp and know the speaker impedence and efficiency are also reasonable, and act reasonably, you don't need to spend another moment on this power issue. Copland stuff is well regarded and I suggest you just set the whole system up and avoid the avalanche of opinions you could subject yourself to regarding room treatment, tiny wooden speaker wire bridges, gigantic power regenerators, fuse direction, and hairstyle tips...however, MY opinions are to be cherished as True Art and a gift to the reader...I'm a giver.

Wolf, I am a true connoisseur of your nuggets of wisdom...
Do NOT share that with your therapist.
Not only is consistent power output typical of tube amps, but also McIntosh amps that use autoformers have a stable power into all impedences.
Not meaning to hijack my own thread but since Mjcmt brings up McIntosh and autoformers, I bought some Speltz speaker cables and an IC from him some years ago. I recall he was also offering something called a Zero Box.

I'll go and search on this topic and see what I can find, but does anyone have any comments on Speltz's Zero Box and under what circumstances using them might be of benefit. Again, I am unhappy with my current speakers: I don't know if it is the amp or the room, or I just don't like the speakers but I do think there might be a mismatch in my system.

System:
Speakers: PBN Montana XP
Ampilifer: Butler Audio 5150 (currently in use in 2-channel mode only)
Pre-amp: Lector Zoe
Source: Raysonic CD-128
"Again, I am unhappy with my current speakers: I don't know if it is the amp or the room, or I just don't like the speakers but I do think there might be a mismatch in my system."

What is it that you don't like about your system and what do you expect from an upgrade?
Fin,
I think that is indicative of sn amp with higher output impedance, usually but not always an attribute of tube amps. An amp like that will probably perform best into higher impedance speakers, ie 16ohms, which is not common. Its a special breed that needs a special mate. My recollection is that pbn is probablyof the more typical ilk, impedance of 8 ohms or less. Probably not the ideal pairing, but workable.
I have seen some pnm impedance measures in stereophile that show wide impedance flucuations from 4 to 16 ohms at various frequencies. Far frm the constant high impedance that an amp like that would probably do best with.
The OP's Butler Audio amp is not a full tube design. I believe it is solid state with a tube input stage. Therefore it will not necessarily need a speaker with a flat impedance.

Finsup, It sounds like you are not happy with the sound at present. There are allot of reasons why that could be. If you could explain more about what you're not liking and maybe some details about your room dimensions maybe someone can give you some suggestions.
I have seen some pnm impedance measures in stereophile that show wide impedance flucuations from 4 to 16 ohms at various frequencies. Far frm the constant high impedance that an amp like that would probably do best with.

That depends on the amp. If it is OK with 4 ohms and has a 10db of feedback or more, its probably not an issue. Also, Sometimes you can get by without the feedback if the higher impedances also occur at higher frequencies. A ZOBEL network might just take care of the extra output.
I have heard from an amp designer that he has abandoned the Zobel because the Zobel network can knock down power output into the higher frequencies and diminish extension. If the speaker efficency is near 96 plus, less of an issue. Have you experienced this? Jallen
ZOBEL networks always have to be set up for a particular situation. Sometimes a choke needs to be in the network to prevent High Frequency rolloff.
Ralph, re your post about 4-16 ohms at various frequencies not being a problem if the tube amp has 10db or high NF seems correct. As you know, I posted links to Soundstage and Stereophile bench tests of the ARC VS-115 and Ref 150 tube amps. Both amps use similar amounts of NF (about 12 to 14db) and have similar output impedances of about 1 ohm off the 8 ohm tap. The output FR of both amps was about +/- 1 db over the audible FR spectrum when driving a simulated speaker load that had similar impedance fluctuations as staed above. Atkinson said the output FR might even be tighter off the 4 ohm tap where output impedance was 50 to 60 percent of the 8 ohm tap output impedance.
Bifwynne, Our amps don't use any feedback at all, yet they can work quite well with speakers that have some pretty wide impedance variation. It all has to do with the intention of the designer of the speaker. ZOBEL networks are one way you can get a zero feedback amp to work with a speaker that is expecting the amp to have a much lower 'output impedance'.
Got it Ralph. But outside of the few exceptions like your OTL type amps, a pure "Power Paradigm" amp that doesn't use NF and that has a somewhat high output impedance will likely choke on speakers that were designed to be driven by low output impedance high current SS amps. Especially if the speakers have wacky impedance and phase angle curves.

You and Al used a jackhammer to get through my thick skull to get me understand the basics of these various interactions. And . . . thanks to the Soundstage and Stereophile bench test reports on the ARC VS-115 and Ref 150, I finally got to see numbers wrapped around the theory.

Folks who are interested in tube amps should keep this stuff mind. Not sure how Zobel networks works, but for folks who are interested in tube amps like yours, or other zero feedback tube amps, it might be worthwhile for those folks to get up to speed on that EE variation.

Regards,

Bruce
Please, instuction for Bias setting, thanks