[ProCast99]

Hello:

I continue my search for a nice, clean pair of expressive speakers for my souped-up NS-30. I'm starting to notice that there is quite the variation in impedances out there. I've read what Blair has said "use what works best" - but in this case, I am thinking of purchasing some new speakers, so wonder if there are any tips out there for me to consider before I buy...

I have a few newbie-type questions:

1) I've been told tube amps tend to deal with high impedance better than low impedance. Is this consistent with others' understanding? If so, what does this mean to me in terms of speaker choice?

2) I'm wondering if the "8 ohm" and "4 ohm" settings on my NS-30 are - approximations? For example, could I run a 6-ohm set of speakers from either of the settings, depending on what sounded best?

3) Could I run something with higher impedance without risking damage to the amp? Is there anything in particular I need to be vigilant for/about?

4) are there simple mods that can be performed to adapt to different (i.e. higher) impedances?

5) I understand that the "feedback" knob on my amp is also related to speaker load - is this also something that can be used to adjust the amplifier's performance in the face of different resistances?

Many thanks in advance --

[FullRangeMan]

My 2 cents are:
As you know, use of hi Sensitivity speaker, hi impedance is mandatory for small tube amps.
Some options you may like to evaluate:
1) Zu Soul Superfly: tube friendly 16 ohms, 101dB, no crossover for the 10 inches fullrange driver, super tweeter enter in 17-18kHz til 25kHz, price 3,750. new at factory site etc

2) Klipsch RF83: 8 ohms, 100dB, out of the line, if used is avaliable is a nice match.

3) Klipsch RF63: same above but 99dB, seems out of the line too.

4) Klipsch CF4: model from 90 years, two 12inches woofers and a big horn, 102dB, 8 ohms.
Good luck

[milford3]

Tube amps either SET (single ended triode) or push pull amps like high impedance rated speakers. 
SET amps work best with high impedance speakers over 90 db's or better.  There are tons of speakers on the market that are rated with high impedance.  Zu speakers with play with just two watts with a SET amp.  My SET amp is rated at 6.5 watts and drives my Axiom M22v3's with ease. 

[DaveC113]

Also check out Omega, any of their line will work great. I use a ~6 WPC EL34 SET amp.

[gooberdude]

I've only ever used 8 ohm drivers on my tube amps, seems to be the preferred impedence. Paul Speltz makes autoformers though, in case you fall for 4 ohm speakers but need your amp to see 8 ohms.

Just my opinion, but the knob on the back of my souped up NS-15 is useless.  My ears only prefer the farthest setting with no feedback.  Anything else is muffled & 'less', regardless of the speaker system used.  If your system needs feedback start tweaking & consider acoustic room treatments.

[Niteshade]

With Niteshade amplifiers, the speaker impedance switch and feedback control are used to interface the speakers and amp. There is no wrong place to put either of them.

The impedance control works differently with different models, upgrades, etc. It is also vastly dependent on how your speakers operate. Some speakers require feedback while others do not.

I have found that the 4 ohm setting is perfect for 6 ohm speakers as well as many 8 ohm models due to impedance dips. The proper switch setting is where your system sounds the best.

There are no other mods for speaker interfacing other than what I provide now.

Speakers up to 16 ohms can be used with the impedance switch on 8 ohms. No damage will occur.

===================
Speakers:

It all depends on how you want your system to sound. I have several pairs and they're all different. The most balanced pair I have are made by Polk. The most expressive and clear are my Klipsch RF-83's. My favorite old school models are acoustic suspension KLH's. They're more 'muddy' than the other two, but have the ability to provide a concert-like presentation and do get you involved. When I say muddy, I mean the highs can be overran my the mids and lows at times. It's not a big issue.

My favorite sensitivity rating is 92db to 95db.

[borism]

Audiokinesis, an AC site sponsor, offers "expressive" - no/little dynamic compression - speakers with sensitivities in the mid-90s and relatively stable impedance around 8 ohm. They are very tube friendly. I bought the now discontinued Jazz Modules and am driving them with a 30W tube amplifier. Great speakers!

[djbnh]

Just my opinion, but the knob on the back of my souped up NS-15 is useless.  My ears only prefer the farthest setting with no feedback.  Anything else is muffled & 'less', regardless of the speaker system used.

Ditto re: NS-40.

[richidoo]

For tube amps, flat speaker impedance is more important than high impedance. 4ohms is OK if it's mostly flat. Dipping below 4ohms is not great idea for a budget tube amp, but it's not the end of the world. Tube amps can handle high impedance loads because they have no shortage of voltage. SS amps can run out of voltage at high volumes on high impedance speakers. Speakers used to be 16 ohms when there was nothing but tube amps available to power them. Now that SS amps rule, they prefer a lower impedance because they are better at making current than voltage.

The higher the speaker impedance, the higher the damping ratio will be with a given amp, allowing the amplifier to control the drivers better, giving better bass response (punch and slam,) also cleaner midrange tone. SS amps usually have output impedance of <.01ohms so damping ratio is usually always very high, making bass sound good. But tube amps usually have real output impedance of 1.5ohms on the lowest tap. So a 4 ohm speaker is not a high damping ratio, making bass sound muddy (or warm)

Tubes don't deal with impedance, they rely on transformers to connect to a real world mechanical load requiring current.

Feedback lowers the output impedance of the amplifier, giving more control of the mechanical load, and sounds similar to lower impedance taps on the output transformer. The higher impedance taps usually sound better, so you will want speakers that enable using the highest tap available. More life and better transient response from the higher tap.

[ProCast99]

Thanks everyone --

This is perfect - I now feel like I sort of know what I am doing! Bring on the salesmen!

[FullRangeMan]

Niteshade/Richidoo: good posts, very instrutive, thanks.

[Poultrygeist]

I have a friend with several tube amps with both 8 and 16 ohm taps. Before I sold my Zu's I tried them with the optimum 16 ohm taps yet could tell no difference between them and the 8 ohm taps.

[FullRangeMan]

I think the difference is the tube life span.

[Quiet Earth]

I don't think so, since the tube sees the transformer primary, and the transformer secondary sees the speaker.

There's an easy, non scientific way to do this. Play a song through your 8 ohm taps. Listen carefully and then pause the music. Now, without touching your volume control or turning anything off, switch the positive speaker leads to the 4 ohm taps. Play the song again. Whichever tap seems the loudest, or has the most control, drive, or force, or has the most powerful bass is the right tap to use. Why? This is the tap that is transferring the most power to the speaker at a given input level.

[Ericus Rex]

Roger Modjeski has talked some about this in the Music Reference circle.  He says 'light loading' the amp (using the 4 ohm tap for 8 ohm speakers, or using 16 ohm speakers with an 8 ohm amp) will extend tube life.  However, you won't get the same power.  He says if you don't get clipping while light-loading and listening at your normal listening levels then by all means light load.  Check out his circle for more info.

[FullRangeMan]

Well, remembered: Usually the power with 16 ohms speaker is greater than 8 ohms.

[Quiet Earth]

Light loading - I think that is either not good information or possibly taken out of context. The idea of using a tap that is twice what you think your speaker's impedance is just because you are hung up on "impedance" seems to miss the point of why the transformer is there in the first place.

Maximum power transfer makes more sense to me. I think that passing all of the power generated from the tube into the load (your speaker) will make the tube run cooler and last longer than wasting power by trying to fool the transformer by using the wrong tap on purpose. If that is what you mean . . . .

[Quiet Earth]

Well, remembered: Usually the power with 16 ohms speaker is greater than 8 ohms.

Hogwash. 

[Ericus Rex]

Roger probably knows more about tubes than all the rest of us on this circle combined.  Why not go to his circle and read his comments directly?

[FullRangeMan]

Hogwash. 

Well, remembered: Usually the power with 16 ohms speaker is greater than 8 ohms.
It is no my opinion, as a fact all tube amps had more power avaliable in 16 ohms than in 8 or 4(Ohm law), but is not alot of watts.
Blair would inform us how much it is for his amp.