Small Linkwitz dipoles

Rip Van Woofer

Rip Van Woofer

Audioholic General
<font color='#000000'>Just a couple of days ago I finished my first electronics DIY project since building Dynakits back in the 70's: the Siegfried Linkwitz - designed dipole minimonitors PMT1.

After the holidays I'll edit this post w/links to pix and babble more about them, but for now I can say -- for a small room like mine, they're splendid! Very transparent, amazing soundstaging, good dynamics. Main drawback is they're bass shy, but he has a line-level EQ circuit design to compensate for that. I'll build it later. For  now, the passive sub from my previous Cambridge Soundworks Ensemble 3 set (a 6.5" driver in a bandpass enclosure) is handling the bass -- wired in reverse polarity from the monitors! Fills in the bottom without boominess and works surprisingly well.

Merry Christmas and Happy New Years to all!

[Jan. 1] Completed the line-level 6db/octave bass EQ circuit he suggests to correct for the natural dipole roll-off. It made the subwoofer unneccessary. Very clean, natural bass! Gave it a workout today with piano (Evgeny Kissin beating a Steinway into submission) and orchestral material. Even before the EQ they were, and are, very non-fatiguing. If you have a small room and want to DIY speakers for it, I can recommend these unreservedly. The only caveat is that you need space to allow for a couple of feet or so between the speakers & rear wall.

[Dec. 28] Below is an email I sent to Siegfried Linkwitz w/my first impressions, a question, and his reply.

I will have a page on my Website soon with construction photos & notes.

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Siegfried;

Thought I’d give more feedback on the PMT1s, offer one mild criticism, and ask a dumb question or two. Replies welcome, of course.

Let me preface my impressions of the speakers by saying that, for now, all I can offer is subjective opinions based on listening to music, and that I am fully aware of the limitations and pitfalls therein. I’ve ordered your test CD and am building the test mic to help me optimize them for my room (or the other way ‘round).

Further, since my means have always been fairly modest I have no experience with “high end” speakers beyond brief listening in stores. Frankly, many have left me wondering whether they were worth the price. The diploles are replacing a Cambrige Soundworks New Ensemble III sub/sat system (a very good speaker for the price, IMO). Before that it was original large Advents bought new in the early 70’s. However, as a one time musician (percussion) and frequent symphony concertgoer I know what live, unamplified music is supposed to sound like.

I said before that they were “splendid”. That opinion hasn’t changed. Everything you said about dipoles in general and the PMT1s in particular are there: open, uncolored sound, excellent reproduction of ambience. The only coloration I *think* I hear is on solo piano, a slightly “dark” sound that I tentatively attribute to the treble rolloff and/or the broad hump in the upper bass since I’m not using the EQ yet. The stereo imaging is superb. It gave me shivers to hear a solo clarinet right about where the treble knob is on my preamp! Soundstaging (so beloved of audiophiles) is realistically large but not exaggerated.

Your many caveats about their limited output had me concerned as I was building them, but appropriately used in a small room like mine there’s no problem. They have more than enough dynamic range to clearly render complex orchestral textures in things like a Mahler symphony, as loud as I can comfortably listen, without becoming ragged and compressed; my ears would say “uncle” before the speakers did. And they handle rock and roll fine, too. And salsa. And jazz...

I haven’t yet built the line level EQ but will very soon. Meanwhile, I’ve hit on a surprisingly effective improvised solution for the bass. I’m using the sub from my previous speakers (a 6” driver in a bandpass enclosure) wired in reverse polarity from the dipoles. Wired “correctly” the bass was boomy. I tried reversing the polarity just on a whim. Magic! I cannot hear any obvious peaks, and things like tympani and bass drum sound natural (I should know). Go figure. I’m looking forward to measuring that setup to see if the facts bear out my perceptions!

Bottom line: in the right setting they can give any number of $1k and up small monitors a real run for the money. I can live with these a long time. But if I have a good year and can talk the Mrs. Into it, a pair of Orions would be just the thing for our living room!

My one criticism concerns your slight roll-off of the high treble for “spectral balance” due to their limited bass. I see your point but surely few builders will use them without either the EQ circuit (which, by your measurements, puts them in a league with just about any good conventional small monitor), or a subwoofer, or both. So why worry about balance? My “built in” treble already exhibits Linkwitz-Riley behavior starting at about 15kHz! I don’t need any more rolloff! I’d rather start with the flattest response I can get in a speaker and adjust the treble on my preamp or receiver for “spectral balance”. For now, I’ve goosed the treble a bit with the knob. But I’d rather see you publlsh a crossover circuit without the rolloff as well as the original design. Explain the pros and cons and let the builder choose.

Since I’m ignorant of crossover design (reading about it makes the eyes glaze), could you tell me how to undo the rolloff, if it is merely a matter of removing or changing the value of a resistor or two?

Dumb question: About that mic [a DIY testing mic elsewhere on his site]: In the photo I can see one resistor, but your circuit shows two resistors and a cap. Are they both in the box housing, too? And, will it overload the typical PC or Mac sound card? That’s how I plan to use it, along with a shareware Mac spectrum analyzer/oscilloscope program.

Another dumb question. Even though I have a small room and probably can’t place a dipole woofer far enough from the rear wall (I figure 2-and-a-half or 3 feet max) I’m itching to try one anyway. Fool’s errand, or worth a shot?

Thanks again. If you wish to post any of this in the reviews area of your site, feel free.

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Hello Bruce,

Thank you for sharing your impressions.
For your and other people's benefit I have added the xo that will give a flat frequency response, http://www.linkwitzlab.com/proto.htm#PMT1 .

To your questions, there is a 10k resistor and a 2.2uF cap in the mic box. The other 10k in the schematic represents the input impedance of the preamp or sound card. I do not know if the mic overloads your sound card. There is no danger, but if it does, use the described attenuator.

The rear opening of a dipole woofer should have at least 3 feet of open space behind it.

Cheers,

Siegfried Linkwitz</font>
 
Rip Van Woofer

Rip Van Woofer

Audioholic General
<font color='#000000'>I just put up a page with pix and brief notes.

Dipole Speaker Page

I'll add more notes and such later...actually, I really need to work on the whole site!</font>
 

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