Multi-way speaker for tube amps: what does really matter?

[the Interceptor]

Occasional lurker here, now registered - please be gentle!

So here I am, trying to figure out how to build the perfect multi-way speaker for a tube amplifier.

The basics I know: a linear and preferably not-too-low impedance and a high efficiency. But which other parameters play a role? Voice coil inductance? Overall number of voice coils (and inductors in the crossover?) in the system? Stiffness of the driver's suspensions? Moving masses?

And how do these things mesh, which one of them is really important to how well a tube amp works with the speaker?

Looking forward to your answers!
the Interceptor

[Pano]

Hi Interceptor and welcome to the forum.

Of course you know there isn't any perfect speaker, not for tube or otherwise. I'm not being smart, it's important to keep that in mind. Everything, absolutely everything is going to be a series of trade-offs.

That said, here is my general (tho not perfect) guide to what works well with tube amps.

• High efficiency. Getting to 92dB/watt or better will certainly help.
• Low Qts woofers. Like 0.3 or below. Actually it's the tube amp that helps these drivers achieve better bass. They tend to be the most efficient woofers, too.
• Large size. You don't have to have it, but it helps. Generally the more efficient the speaker, the bigger it needs to be for deep bass. With a big solid state amp you can force bass out of a smaller enclosure.
• Single, fullrange driver. A lot of tube fans are very happy with fullrange drivers. It's a simple, elegant direction to look. There is an entire forum here dedicated to fullrange speakers.
• Benign impedance curve. Many tube fans seem to like an impedance curve that isn't too much of a roller-coaster ride. (I have not found it to be a problem)

That's my list.
I have not found voice coil inductance to be a big deal, as long as it's dealt with in the crossover. As for moving mass and stiffness, the more efficient drivers tend to have low mass for their size and are stiffer than average.

I'm sure there will be plenty of other suggestions.

[planet10]

Originally Posted by panomaniac

Benign impedance curve. Many tube fans seem to like an impedance curve that isn't too much of a roller-coaster ride. (I have not found it to be a problem)

This is, IMHO the most important one. This is because most tube amps tend to have higher output impedance. The impedance curve then directly affects the FR. And when considering this one needs to consider both magnitude & phase.

A reactive impedance changes the loadline to an ellipse.

Now, in general, a flat impedance will be of benefit to most amps.

Single, fullrange driver. A lot of tube fans are very happy with fullrange drivers. It's a simple, elegant direction to look. There is an entire forum here dedicated to fullrange speakers.

The FR speaker eliminates the large influence of the XO on the impedance response. This is the route i have gone. Some other big advantages too. With 9-octave+ FR systems now available the biggest downside becomes ability to play REALLY loud and dynamics at higher levels. A really good FR can clean up as far as downward dynamic range goes. This really shows up in a FRs ability to throw a really solid 3D soundstage.

How efficient you need depends of course on how much power you have. And needs to be tempered by the impedance. A high efficiency speaker with a wild impedance, will most often not be as effective as a less efficient speaker with a benign curve.

dave

[the Interceptor]

Thank you for the salutation and the answers so far. Allow me to to shed some light onto why I am asking, maybe this will spark some more discussion.

My idea is this: a tube amplifier more or less is bound to have a fairly low damping coefficient. Thus, it makes sense to engineer a speaker which is very simple to drive for the amplifier, with "simple" referring to neither resisting incoming currents too much, nor sending too much back by inducing a back electromotive force (BEMF). But how does one achieve that in a speaker?

Well, what I am thinking of is to reduce the number of voice coils in the system and to make sure that those which remain have an inductance (Le) as low as possible. Theoretically, this should minimize the complexity of the speaker in the eyes of the amp and thus make them a good team. However, I am not sure which role a factor like the strength of the electric motor (BxL) plays in this. Maybe reducing the Le makes no sense when the BxL is so high that the induced BEMF is higher than with a driver with a lot of voice coil inductance, but a low BxL.

Also, I have noticed that most of the speakers which are known to work best with tube amplifiers, like horns with Fostex drivers for example, have one thing in common: they are equipped with drivers with a low moving mass, but also a soft suspension. This path is being left further and further with modern drivers. Those mainly sport higher moving masses and harder suspensions. And it makes sense, too, because amplifier power costs next to nothing nowadays. Nonetheless, my understanding of the matter is that a weaker and "softer" tube amp will have a very hard time propelling such a driver, because the latter will "demand" a certain amount of power and will "lean into the amp" (produce a lot of BEMF) before it starts working properly.

On that basis, it seems to me that a driver for a multi-way system which is supposed to work well with a tube amplifier should be made in a way companies made them before wattage became largely available. And I'm not even talking about vintage drivers, just low moving masses, soft suspensions and averagely dimensioned electric motors. Such drivers have become unpopular because they need lots of air and will reach their mechanical limit quite quickly. But keeping tube amps in mind, maybe they were not so bad after all.

Anyway, I am not certain how true as well as important my observations really are, thus I am looking for the insight of those who have been looking deeper into this matter.