Adding tubes to SS system-Tube Preamp Amp or Int?

Ok. Then what is considered a 'benign' impedance? Also, are there any other general rules I should be aware of? What about the sensitivity of the speakers vs the power of the tube amplifier?

I know there wont be any 'absolute' rules. But how about some places to start for someone that really doesnt know much about it.

There are many members who can better answer your question, and I hope they will. That being said, you would like your tube amplifier to see as flat an impedance curve as possible, preferably remaining above 6 ohms and definitely not dipping below 4 ohms. Keeping your efficiency above 90 hz or more, depending on your amp, will also help. These are very general guidelines.

I should also have said that every 3 db variance in a speaker's efficiency results in either a halving or doubling of the power required to maintain the same volume level.

Nnck,

All of the subject areas you touch on are complex, so, at best I can offer only some very general advice.

I would start with looking for speakers. By far, speakers define the basic sound of the system. Find what you like and then look for a compatible amplifier. Because of compatibility issues, certain choices of speaker will eliminate certain amplifiers as contenders, so it makes little sense to choose the amplifier first. This is particularly the case with tube amplifiers which tend to have more limited ranges of compatibility.

Speaker efficiency and impedance characteristics MAY determine what is compatible. A lower efficiency speaker will mean a greater need for power. A speaker with a low nominal impedance (e.g., a 4 ohm rated speaker) MAY mean it presents a difficult load (again a bigger issue with tube gear than solid state). But, such a single number hardly tells it all. I would suggest looking at a Stereophile magazine test report on speakers. One of the first charts shown shows how the impedance changes with frequency. Even an 8 ohm rated speaker will have certain frequencies where impedance dips well below that number. Not only is how low that points important, but WHERE that dip occurs. If it occurs at around 60-150 hz (it is a bigger deal than if it occurs elsewhere because there is a lot of musical energy in that frequency range. The graph also shows another plot of "phase angle." Because of the characteristic of the load (the speaker) delivery of power and current do not go up and down in lock step so they end up being out of phase with respect to one another. A high phase angle, combined with a low impedance at a particular frequency, would make for a "difficult" load. I know this sounds very complex, but, at least Stereophile technical reports do give a subjective analysis of how difficult a load is a particular speaker under test.

For all of the technical arguments, ultimately, it does come down to personal trial. Whether or not you will "need" high power or a solid state amp to handle a difficult load will depend on personal practice (how loud you play the system, how big is the listening room) and personal priorities (compared to comparably priced solid state amps, tube amps rarely do bass with as much impact or tightness, but whether that is important is purely subjective).

When trying out different amps with the speakers that you like, don't overlook combinations that may seem incompatible. A lot of people, including dealers, overestimate the amount of power that is needed for a particular speaker. Often, a lower powered amp can deliver adequate power while also delivering qualities that a higher powered amp cannot. I know a local dealer that sells mostly low-powered tube gear that has several high-powered solid state amps to use for comparisons. Often, customers comment that the low-powered tube amp sound more powerful than the solid state amps. That is not really the case, but, what the listener is hearing is how good the sound can be at LOWER volumes when the amp sounds good. I know that my system is moving in the right direction when I find myself listening at lower, not higher volume.

I generally don't like the sound of most higher-powered tube amps (more than about 60 watts or so). If I truly needed that kind of power I would concentrate my search on solid state amps. The one notable exception is OTL (output transformerless) design. I like the sound of OTL amps for both high and lower-powered use. So, if I needed more power I would look first at OTL amps, then solid state designs (OTLs put out a lot of heat and have other operational issues that may rule them out).

I think that in most home listening rooms, and if listening levels are not too high, there are very few speakers that truly demand an amp bigger than 60 watts. In short, don't rule out lower-powered tube amps. I find that many otherwise fine audiophile-approved speakers suffer from being somewhat lifeless and dull sounding (particularly in comparison with horn and single-driver systems). To compensate, a lot of people play their systems quite loud. In that case, one may profit by not increasing power, but going with tube amplification, which tends to sound more lively at any given output. This is particularly the case with OTL amps.

Tube power amp is "tubiest". You have to make sure your speakers are tube friendly for this to work optimally though.

I use a tube pre-amp. That introduced a healthy dose of positive tube attributes without sounding overtly "tubey". That is good because I want my music to sound natural, whatever that means, not like the technology used. I'm sure different tube pre-amps will sound significantly different from each other and not all present the same results, especially if compared by swapping into the same system.

If you use a tube pre-amp, impedance matching to an appropriate power amp is important to optimize results, especially if a SS power amp is used.

I also have and use both SS and tube DACs. The tube DAC can also provide an audible dose of tube like sound. It can vary to quite SS like to distinctly "tubey" depending on the specific tube used and teh rest of the system, so tube rolling can be done to tweak the sound significantly in some cases.

I've used different tube makes in the pre-amp and have heard little difference in the sound there to-date from one type to another, other than background noise perhaps which can vary greatly from tube to tube, depending on health and quality.

For a tube integrated, again the main thing is to make sure the speakers are tube friendly and that the amp has enough power to truly drive them well.

Also, expect with a tube amp that is well matched to speakers that less power may suffice to deliver good sound at higher volumes than with SS amps. The reason for this is that tube amps tend towards soft clipping while SS amps tend towards hard clipping.