x-over for tube amp

[REALFLEO]

Hi all,

I'm pretty new to audio and I have a few questions.

First, I am looking to build a tube amp. I read a lot and I understood that I should build loudspeaker with good sensitivity first. >90DB

First question:
I don't understand how the sensitivity of drivers are related to the overall sensitivity of the loudspeaker. For exemple, if I have a 92 dB tweeter and a 90 dB woofer. Is it possible to have a box with a 88 sensitivity ? Is it the loudspeaker construction that affect it ?

Second question:
Is it real that first order crossovers are better with tube amp because it passes both frequency and phase unchanged ? I looking for a 2 way.

Third question:
Do you have good websites or books to learn theory like that ?

Thanks,

FLEO

[stokessd]

First, there are good performing tube amps and not-so good performing tube amps. A push-pull tube amp is much closer to the perfect voltage source ideal than single ended amps. Push-pull tube amps also tend to produce much high powers than SET's.

What that means to you is that if you build a push-pull amp, you don't have to be so concerned with speaker impedance and efficiency as with a SET.

My personal belief is that SET's have no place in high performance audio.


If you have a pair of drivers that are say 92 and 90 dB efficient, they are probably not flat in frequency response, and are really only that efficient at one point. A passive crossover is subtractitive, in that you can’t add response, you can only take it away (there are exceptions in limited regions, but they are rare in audio crossovers). So to flatten the response of a driver your crossover is taking away energy in different regions (essentially stealing efficiency). You’ve got to knock down your most efficient of the two drivers to match the less efficient one. Plus there’s baffle step which will eat 6 dB of efficiency in the bass region. To completely compensate for that (not that you should), you’ll knock 6 dB off the efficiency across the spectrum.

In contrast an active crossover can accomplish all of those shaping tasks and feed an amplifier attached directly to the driver with the shaped signal allowing the full efficiency of the driver to be used. The downside is complexity and more amplifiers needed.

I’m not a fan of first order (electrical) crossovers, it’s really hard to make a good sounding speaker with them. You’ve got to have exceptionally smooth and low distortion drivers well outside their driven range.

Even if you build a fairly inefficient speaker in the mid 80’s range, you are still only talking about a couple watts to drive it to normal listening levels. A 15 watt EL84 amp or a 35 watt EL34 amp should drive any 2-way you build nicely. IF you want to do serious hearing damage, you could use a 60 watt KT88 amp.

As far as good books, Vance Dickison’s (sp?) loudspeaker cookbook is good, so is art of electronics for the basics of electronics.


Sheldon

PS keep in mind that these are MY opinions and not gospel.

[REALFLEO]

First, thanks for the reply.

When you are saying that the drivers may prolly not be flat in frequency response, is the point where they reach 90dB on the driver response graph ? What is the peaks that drivers can have at their resonance frequency. Is it the same ?

What kind of crossover do you suggest me 2th order. I'm asking that because I read that I sould use simpler crossover with a tube amp.

Ho do we produce a frequency response graph ?

Thanks for the books suggestion !

FLEO

[v-bro]

There is a peak visible in the "impedance graph" of any dynamic driver.
This peak is mostly nothing to worry about, to most it is just a means of knowing how the driver will affect the amplifier. Or in other words how great the load is. The impedance curve is also necessary to be able to do quite precise calculations on the values of the filter.

Peaks in the "frequency response graph" are what designers of loudspeakers aim at first though when designing a filter circuit.

A second order filter will just "eat" away more of the curve of each driver. Where both drivers (partly) produce same frequencies they will mostly amplify the response in this part of the frequency curve. I think the advice to use only a 1st order filter is just ment for single ended (triode) tube amps, like the small 16W 300b tube amps for instance. A pentode high-power tube amp should have no problem with higher order filters.

A frequency response graph can be measured pretty low-tech with a microphone on your computer soundcard and some software that can be downloaded somewhere (I don't know, I've read it in someones post). I use a MLSSA system with simulated "dead-room/semi dead-room" function...
The best way is still in a real dead-room (or semi dead-room...) and real dedicated test equipment...

[REALFLEO]

Thanks, I'll read about it ...

[REALFLEO]

ok... I'm now wondering what is the best driver cone materials for a 2 way 6.5"

Is it true that paper cone offers flatten response ?

Thanks,

FLEO

[v-bro]

Mostly yes, paper cones are among the "softer" materials. Generally the harder materials tend to show nasty behavior in the upper regions.
This is due to cone breakup, materials like aluminum, kevlar and carbon fibre break up at higher freqs and show peaks and high distortion in that area.

To my opinion paper cones sound nicer overall...

[REALFLEO]

by the way, what is the best size for a 2way woofer ... 5", 6.5" or 8"

I tought 6.5" was better to reach any type of freq...

Is there a big difference for the volume needed between 5" and 6.5" ?

Thanks again ;-)

FLEO

[v-bro]

It totally depends on the design, a larger cone can produce excellent mids when x-max is compromised.

Volume is totally dependent on thiele and small parameters of the driver, I know of a 10" woofer that can produce very low freqs. in a 15 liter cabinet.... Generally closed cabinets can be kept smaller, but there are other types of enclosure designs (compound f.e.) that allow even smaller dimensions.

Designing a loudspeaker is much like cooking, ask a cook how hard it is to make all the different dishes, he'll say it's not so hard.
For a cooking newbie there is quite a lot to learn, oven temperatures, usage of herbs, ideal pans, what knives to use, how to make a souffle that wont implode etc.etc.etc.

Not a bad idea to read a book first....

[lndm]

Originally posted by stokessd My personal belief is that SET's have no place in high performance audio.

I'd like to put in a vote for the single ended amp.

I like a low order crossover but it's not always practical. Second order filters are probably the most common starting point and you should probably build these at first until you develop the skill to decide for yourself.

I think that a low order crossover conceptually mates well with the idea of a valve amp, but my preference is just as much when using solid-state amps.

Originally posted by v-bro
I think the advice to use only a 1st order filter is just ment for single ended (triode) tube amps, like the small 16W 300b tube amps for instance. A pentode high-power tube amp should have no problem with higher order filters.

How is this so?