[brj]

Dave, if you get a few spare moments, would you mind taking a stab at my questions posted toward the bottom of page 2 (4 posts up from the bottom)?  I wanted to make sure I was interpreting your earlier comments correctly.

Thanks!

[David Ellis]

Sorry for my very slow response to this.  Your question is quite substantial.  I delayed due to time, then got wrapped up in other stuff and... forgot about it.

First, I should have used the term "reactive" instead of "reflection" in my statement.  While I was considering what the amplifier endures due to the varied loudspeaker impedance load as "reflection", this is not accurate given the common use of electrical reflection.  Component video folks use the the term reflection to describe the phenomena during impedance mismatch, and I should not have used this term.  

1) Impedance is a combination of resistive (real component) and reactive (imaginary component) contributions

2) The reactance of a component varies with frequency

3) Resistive loads are easier for an amplifier to drive

4) Signal reflections occur when two components have mis-matched impedances (with an acoustical result generally described as "smearing" of the sound)

5) Variations in a speaker's impedance curve mean that mis-matches with the amp will almost certainly occur at particular frequencies - even if the amp is well matched to the nominal impedance of the speaker

These statements are correct.

I feel it's important to add that with 99% of amplifiers, the impedance variation is not a problem.  This is simply because they are over-engineered and over-built.  This is necessary to accomodate the common impedance swings extant in 99% of loudspeakers.  I suppose the term "over-built" could also be termed "appropriately-built", but what I wish to convey is the industry has grown to accept increasingly difficult loudspeaker loads, and passed the burden to the amplifier folks.  I believe this is true with respect to loudspeaker nominal impedance, loudspeaker impedance variations, and loudspeaker sensitivity.  The industry has changed dramatically from the simple tubes and high sensitivity speakers of the 1950s.  

Digressing....

Certainly a SET 300b amplifier is the incorect application in 99% of systems today, but given the right loudspeaker, there IS something very special about the SET 300b tube amp.

Digrressing further...

Those old Pilot 232 and Pilot 240 tube amps are very solid units.  With some fresh capacitors they can sound very nice.  There is a reason these pieces of rusty hardware continue to sell for $200+ on ebay.

Back on track...

Assuming that I have all of that correct, what I was trying to isolate from your statement above was whether a flat impedence curve necessarily means the speaker is primarily a resistive load and therefore easier to drive and a "bumpy" impedance curve means that a speaker is more reactive in nature and therefore harder to drive?

Yes, this is my understanding.  

I realize that impedance peaks at particular frequencies indicate regions of increased reactance, but I wasn't sure what you could assume about the overall ratio of resistive to reactive. In otherwords, could the impedence curve be perfectly flat, but still dominated by the reactive contribution?

I don't think so.  I believe that if the impedance curve is flat, it's flat.  It will be a more benign load.

Is it as simple as reduced current demand = less thermal compression = better dynamics? If so, I still need the connection to impedance...

There are 2 answers.

1.  Yes, Reduced current demand = less thermal compression = better dynamics.

2.  A flatter impedance load will be easier for the amplifier to drive, and the amplifier design can be simplified (i.e. less feedback and less parts).

Eventually I'll know if these 2 factors are truly significant     The theory certainly looks good, but not all things that look good on paper have an impact in the listening room.

[brj]

And.... I find another thread that I missed a response to!  Sorry for my tardy reply, but thank you for the clarifications, Dave!  Let me know when you find out if those 2 factors are significant!

[David Ellis]

Lately I have been working through a book titled, "Modern High-End Valve Amplifiers", but van der Veen.  It's a very solid study.  He comments on page 29, "It is useless to hope that the output transformer will work optimally over such wide ranges of impedances and frequencies.  A much better solution is to provide the amplifier with an output impedance that remains constant throughout the entire audio band."

It appears my initial thoughts are correct - in theory.

Dave

[hubert]

Hi Dave,

hope you had a nice family's Christmas day.
Did you progress in your 3-way project? Finalized your drivers' choice?

Regards,
 

[David Ellis]

... Christmas was very nice.  My boys (age 3 and 5), make the experience feel very complete.

... Still no woofers from TC sounds    .  I really can't do anything tangible until these arrive.

[Marbles]

David,

What size TC Sounds woofer, 10", 12"?  What size will the bass cabinet be?

Thanks

[David Ellis]

12"... @ 4' of cabinet.

Dave

[Marbles]

12"... @ 4' of cabinet.

Dave

My conversions might be a little fuzzy, but I think that is 6912 cubic inches interior dimensions. So 24" tall would be reasonable, 16" wide would work.  Does that leave 18" deep?  Add 1.5 inches in all directions for the cabinet and outside dimensions might end up at 19" wide and 21" deep.

Is that pretty close to what you are thinking about?

Sorry if this was covered earlier in the thread.

[David Ellis]

Your dimensions appear to yield the appropriate volume, but the intended cabinet will be a taller/narrower backward leaner housing all 3 drivers.  I think these look better than a 2-piece speaker with a big square bass cabinet.   In all of this, the project is moving veeeery slow.  Until woofers arrive from TC sounds for testing, it's only a concept.

[Marbles]

Your dimensions appear to yield the appropriate volume, but the intended cabinet will be a taller/narrower backward leaner housing all 3 drivers.  I think these look better than a 2-piece speaker with a big square bass cabinet.   In all of this, the project is moving veeeery slow.  Until woofers arrive from TC sounds for testing, it's only a concept.

It sounds like it will be a nice speaker.  I'm pretty sure I saw a prototype of a cabinet that matches that description that either you posted here, or you posted on your website.  This would have been a year or two ago.

Edit:  Yup, found it...

[EProvenzano]

Hi Marbles,

I believe that is the first conceptual cabinet for Dave's 3 way.
The faces you see in the picture are a flat fronted, trapezoid baffle.  In recent posts Dave alludes to a completely different design using a leaning front baffle.

alla Usher AC10



I can't wait to see it myself  

[David Ellis]

Correct,

The initial thought was to use a trapezoid with a flat baffle.  I learned subsequently that a leaning baffle makes the crossover & impedance more favorable.  The variables just work better this way.  

Using a mechanical means to advance the woofer phase makes the electrical proposition better.  

I don't think I am going to attempt a rounded back cabinet though.  It'll look more like something from Avalon or Wilmslow.





Dave

[Marbles]

Very nice looking.  Best of luck with this speaker.

I hope TC Sounds gets moving, so you can proceed.

[hubert]

IMHO,

Aesthetically spoken, I guess the main issue is about the front firing 12". In that way, I have to say that Avalon's designs, with their cutted angles, are an adequated response, allowing to perceive less width of the global shape; straight cutted lines work very well in this matter and furthermore are modern design principles notably used in cars' aesthetics, at least in Europe (BMW, Ford-europe, Renault...)
But these shapes could be viewed as "not soft enough" for someones.
Dave and others, any suggestions/links to commercial designs about how to reduce the visual width ?

 

[David Ellis]

Aesthetically spoken, I guess the main issue is about the front firing 12".

Exactly

When I discussed the woofer with TC Sounds, we agreed the very best approach for high sensitivity would be a 15" woofer.  We also agreed that it would look way too huge and nobody ( or nobody's wife) would buy a speaker with a 15" front-facing woofer.

There are decent looking 3 speakers with big cabinets that look acceptable IMO:  Avalon, Kharma, and Usher.  I really don't know of any others.  Many years ago JBL made a trapezoid (the initial look), but I really need a rearward leaning speaker.  I am open to suggestions in this regard.

[hubert]

I really need a rearward leaning speaker. I am open to suggestions in this regard.

I guess that for many technical/economical reasons , you don't want to go to a side firing woofer...

So, my better suggestion is to visually reduce the width by creating a black-painted surface, englobing the tweeter and mid, and widening towards the woofer; all the other parts could be clear veenered . All this, in conjonction with a few cutted angles.
IMHO, difficult shapes to build aren't necessary in this matter.
"Easy to build" for you and your customers probably is one of your goal, even if not the main, isn't it?
I guess you are thinking of slanted front and rear baffles, with horizontal top and bottom...but a simple parallelepipedic shape could be used if you use a little stand to lean the enclosure backwards; in such a way, no need of different angles than 90° to build the enclosure.
 

[David Ellis]

I guess that for many technical/economical reasons  , you don't want to go to a side firing woofer...

Well not for those reason.  I have heard a few side-firing commercial speakers and thought the side-firing woofer didn't work very well.  Front firing woofers seem to sound a slightly better to my ears.

So, my better suggestion is to visually reduce the width by creating a black-painted surface, englobing the tweeter and mid, and widening towards the woofer; all the other parts could be clear veenered . All this, in conjonction with a few cutted angles.

So... kinda' like the bigger Kharma speaker.

As you offer considerations/ideas, please do keep in mind the cabinet will be 4 cubic feet internally.  This is quite large, and the midrange chamber/tunnel must be considered too.

I guess you are thinking of slanted front and rear baffles, with horizontal top and bottom...

Yes.

but a simple parallelepipedic shape could be used if you use a little stand to lean the enclosure backwards; in such a way, no need of different angles than 90° to build the enclosure.

I thought of this, but the appearance is slightly... cheezy/cheap.  In an "economical" speaker a single spike in the front to create a backwards lean would be fine.  For a better looking speaker, the simple spike is less than desirable - according to my wife.  Some complexity in the construction process is acceptable  for aesthetics.  However, a round back for the enclosure is not acceptable.  This is way too much work.  Also, my wife really doesn't prefer the appearance of a rounded back cabinet.

[TomW16]

Hey Dave,

I am really loving the concept of this new 3-way speaker.  I hope that TC Sounds sends you those woofers soon so that further R&D can continue.  

All the best with the design and thanks for the update.

Take care,

Tom

[David Ellis]

Soon....

This is very optimistic, but I conveyed that I'd rather have the woofers "good" than arrive soon.

I have learned that ya' can get stuff good, or cheap, or fast.  But, ya' can never get all three.  I am prone towards the "good" end of the spectrum.