Need Inexpensive OB Project for 2 Watts or Less

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Hi, I posted my tall, perhaps ridiculous order over on The Audio Circle. I got lots of wonderful suggestions from wonderful people. I went there because they have a dedicated OB category. I decided to post the same thing over here, just because I feel like I know some of you (Scottmoose, Dmason, Planet 10, Godzilla, GM and others :) I have never hung out there much.

Here is where I am. Even though I was happy with my Voigt Pipes with a Fostex 166 (after getting help from Scottmoose) I felt like I could do better after those were under my belt. With the encouragement of those mentioned and others on this forum I built my BIB's with Fostex 208 sigma and T90A. Afterwards, I stopped lurking on the forums and mostly just listened to music whenever I have a free moment.

A friend helped me build a wonderful pair of 45 SET tube monoblocks recently, and then passed away soon after he sent them home with me to try. I still have both speakers mentioned above and they both sound WAY better then ever with the SET amplification. But, for some reason I got the bug to build speakers again. I particularly have become interested in open baffle speakers.

I started with a very crude (drywall!) JE Labs style OB and through on some drivers from an old Grundig console. WOW!!! :cool:

Next, I cut on old solid door in half and made another JE Labs style OB. This time I found much larger drivers from an old RCA console. Wow all over again!!! Bass, percussion (particularly tympani) even better. But, not as well balanced are natural sounding in the highs as the first OB experiment. I tried some driver mods out. A learning experience. Did both some good and bad. I have no idea just what these drivers (specs) are. I just had hope that they would work because the consoles they came out of were not sealed in the back.

Anyway, I want to try another diy OB that will be better than the first two. Honestly, I think I will have to wait to do the really "good" (expensive) stuff that I would really like to try (Lowther drivers etc.).

Prior to posting on Audio Circle, I was thinking along the lines of a better (materials) JE labs with a "better" driver. I had considered the Audio Nirvana 15" alnico as one candidate.

I am close to choosing the more economical Hestia project of the two Hestia OB projects that were suggested to me over at AC.

I am also thinking however, about Godzilla's various inexpensive drivers over an H frame with an Eminence 15A in it.

Apparently those doing two way OBs are bi amping. Unless it is advisable to employ my Cary in bass duties, using passive filtering and attenuation etc., it looks like I should get an inexpensive plate amp instead. Would 100 watts be enough? Better to use one 300W plate amp, or two 100W ones? Recommendations?

Obviously, the driver(s) powered by my 45 SET amp need to be pretty efficient. Is there an OB suitable driver out there that is sufficiently efficient, with a decent frequency response, that does not cost an arm and a leg?

Realizing that everything is a trade off, do you guys recommend an inexpensive two or three way, over spending the same amount of money on some one pair of drivers, and putting them in another JE Labs baffle?

I am anxious to build a pair of OB speakers that are meant to be more than a mere experiment, but will still need to keep the budget low for now. I would hope to improve on my last two experiments. I guess I just want to get the "biggest bang" for my buck. Maybe you guys can keep me from trying something that you have already tried that you didn't like so much.


Hi, you need big membranes and little magnets, meaning high Q.
The "game" consists in having the drivers "controlled" by the crossover but easily driven by a SE amp. Making the sub as a specialized way ( just lifts the amp from the hard job.
Is there an OB suitable driver out there that is sufficiently efficient, with a decent frequency response, that does not cost an arm and a leg?
Just see the offers on Eb...y
Many big woofers from cheap speakers of the 70's and 80's just look for big membrane and little magnet ( the price should be around 20-50 $ for the pair )
Or you can just pull out all the speakers ( even if mid and tweeters are pseudo-fake, I mean, just there with a resistor and a capacitor ) and mount them on a baffle.
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Low power, Inexpensive and Open Baffle. I hope you realize that these three things do not go together.
OB bass is very inefficient, unless you start with a very efficient (expensive) driver. Then it will be just moderately inefficient. ;)

Yes, you can use a plate amp on the bass section, that can work well. You need horns to use a 45 SET well.

You can have a look at this project for an OB speaker that would really rock with a little 45 SET.
But not small, not cheap, not easy. Efficient, tho.
The $95 Goldwood GW1858 18" Woofer has been used in an H-frame in several designs. The high Qts~1 generates acceptable dipole bass without equalization circuits or amplifier boost.

Open Baffle Woofer for less.. Goldwood GW-1858
High frequency ideas:
(1) The Eminence 12" coaxial BETA-12CX has Qts~0.48 and has been used on a flat dipole baffle. You can experiment with opening the rear of the compression driver for dipole tweeter. Most just accept monopole tweeter polar pattern.

(2) $130 Dayton PS220-8 8"+whizzer full range is a lower cost alternative to $210 Tang Band W8-1808

Working your way to BIG Horn speakers? Both dipoles and horns offer controlled directivity which removes some short delay off-wall reflections.


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(2) $130 Dayton PS220-8 8"+whizzer full range is a lower cost alternative to $210 Tang Band W8-1808
Interesting choice, thanks. I saw that Dayton prototypes at Parts Express a few years ago, but never got to hear them.

In the same price range I'm very fond of the Sears 8" fullrange. A very good driver that works well on OB. I'd want more than two watts to drive it, but that might be OK for some folks.
I saw that Dayton prototypes at Parts Express a few years ago, but never got to hear them.

According to several owners, the Dayton PS220 sounds better than it measures, and it measures better at Klippel Labs than the SCARY SPLs on Dayton datasheet. A contour filter network (2.7mH parallel with 13 ohms ) looks good on Xsim. Voice Coil Magazine measurements also look better than Dayton's. It is useable down to 200Hz in an open baffle.

Klippel Non-Linear Test Results Large Signal Identification
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Thanks for the link. I got to see the Dayton drivers about 2 hours after they returned from the Kippel test - and they still had the marks on them. The Dayton crew were pretty stoked about the results. :up: But I never did get to hear them!

The Seas fullrange is also well behaved on OB and the rising on-axis response seen in the graphs averages out with the reflected sound from the rear of the driver. Works well that way. A small inductor can be use to shape the upper response as need, just like with the Dayton.

I suppose that with a powered sub, either of these would be just about efficient enough for a 45 SET amp in a small space.
I am planning my own OB speakers for my SET amp mainly as well.

There will be 3 drivers, including one Ribbon or AMT, a wide-ranger and a large woofer for sub (it could go to two large woofers per channel too).

I may do some form of slot-loading on a frame for the woofers.

I am planning to make it bi-amped per channel with a simple passive cross-over for the full-range to the tweeter, dealt with by the SET Tube amp.

The woofers will be crossed-over on the computer (or other) and amplified separately, probably by a plate amp or my existing SS amps.
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I understand the sentimental reason to want to use the 2W SE tube amp your friend made for you. OB is the wrong choice speaker. You should make a good back loaded horn speaker with a sensitive driver. Something like the Zen Acousta by Kohu. A Lowther, PS220-8, etc.


Btw, if you really like SE class A, don't limit yourself to tubes. Nelson Pass' ACA is a 5W SE class A amp that can be built for under $30 and sounds great. There are also lots of class A amp projects in the SS forum that are simple and inexpensive - but would cost a fortune of you were to buy a commercially made version of it.
YashN, That is my set up now. Ribbon tweeter and big mid range with passive crossover, two 15" woofers per side actively crossed with a minidsp. The bass needs lots of membrane and wattage to keep up with even a small amp driving the mids/highs.

Very cool. Which woofers did you use and how did you arrange each pair?

Forgot to ask: any pics? :D
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Hi xrk971, Those Zen Acousta speakers look really cool! I wish I could listen to them (and others) in my room to see what each one does or does not do!

I wondered if you could elaborate a little more about why "a good back loaded horn" is the best thing to pair with my 45 SET amps?

My room is small (16 x 14 x 8 feet). I normally don't listen real loud. I do have a taste for a large range of music. I do appreciate good dynamics.

I listened to some tape today through the 14 x 9 inch vintage RCA oval drivers on a JE Labs type baffle. Later, I listened to my Fostex 208 sigma BIB's (which you probably know are back loaded horns, albeit the most simple horns on the planet. There are things I both prefer and don't like as much about each. I would say the upper end of my horns is more refined in all but the very bottom end. On the other hand, I like the bass of the OB sound a lot. No box. I also like the having the depth of image I get with the OB's. I think my first two pair of diy speakers excell in acoustic music. The experimental OB's seem to rock more.

What am I trying to accomplish? Well, when I first built my Voigt Pipes with the Fostex 166's, or my BIB's with the 208 sigma's, the shoutiness bugged me. As they aged, as I tweaked them and my room and especially with the SET tubes; that was no longer a concern with them. What remained to work on, was boominess in the bass. I feel like I even have boominess or excessive bass with the single driver the OB's. I think it is mostly my room, though I have tried to treat it a lot. The treatments have helped a lot. Besides being a small room, I think it may be due to a large opening into the dining room to my right side from my listening position.

My first OB experiment with a vintage 9 x 5 inch Grundig oval driver was better balanced, but not quite efficient as I really needed. With the somewhat boomy and thick sounding large RCA driver, I have plenty of volume, dynamics and a very addictive ability to reproduce a drum kit. The best I can get to date in my room!

I am not ready to invest in Lowther drivers or real elaborate OB's. Nor am I ready to build a really sophisticated back loaded horn like your Zen Acousta. Although I really am curious as to where it would get me. I am a country boy, so I don't get a chance to listen to as many types of speakers or systems as I would like to. I guess what I am seeking is something that would give me some of what I liked about both of my OB experiments, without the things I didn't like as much. The real sound of a drum kit that that I get with the large vintage RCA driver in OB which plays more loud than I need (and yes, with the 45 SET amps). The better balance throughout the spectrum that I got with the smaller vintage Grundig drivers, even though I could have used a little more efficiency.

I wonder if I will be happy with a dedicated bass driver (that I can dial in) with a separate amp doing bass duties. That very exciting realistic sounding percussion. Or, if it will prove to be too much (boomy) for my room/taste. Perhaps a large driver on a smaller baffle is the answer? I may experiment with cutting my current baffle down. Or, would I be happy with just something like an Altec 755 driver on another JE Labs baffle? I wonder about this because as much as my T90A supertweeters do for both my BIB's and the two OB's, I can hear a loss of "integrity" whenever I use more than the one driver. I could not hear it before I got these amps, but now I can. So, I have my supertweeters on a shelf.

Sorry for sort of thinking out loud. Any are invited to respond. Although I already have been given a black hole's worth of designs to consider.

PS. Thanks for the heads up on the inexpensive amps. It would be fun to try them. But yes, I was lucky to know Paul Weitzel (Tube Research Labs). I have not been able to compare these to other SET amps. I do know that they do deserve to mate with a worthy speaker. I am not in a position to match them with the quality that they do deserve. But, I am anxious to try something that hopefully will be a worthwhile improvement.


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A good back loaded horn (BLH) speaker is very efficient at creating bass from the rear membrane energy so that you get bass out of a small full range driver. The Zen Acousta is not a "flat" response speaker as it uses Lowther and BLH's tend to not be flat. However, they generate a big warm rich sound. They sound very nice and this is an extremely large horn. There are other BLH designs, like the FHXL, FH3, BIB, and many from Scottmoose (look at Woden Design), and HM's website. When you don't have a lot of watts, a high sensitivity driver plus a BLH can actually get loud. 1watt into a 97dB Lowther is uncomfortably loud. The horn will get you bass down to about 55 to 60Hz at circa the same SPL as the upper range.

Inexpensive and very good sounding, low distortion amp designs abound in the Solid State forum.

Take for example this FX8 MOSFET amp I made, it's 75w/ch (4ohm) 50w (8ohm) and uses the very linear and well regarded 2SK1058/2SJ162 MOSFETs. This amp was built for about $30 total parts including custom PCB's and heatsink (not kidding) if you source components from China. It's perhaps my best sounding amp, and I have a lot of amps. There is a variant called the FH9 that uses lower cost vertical FETs that also sounds excellent.


You do need a dual rail 35v power supply though - but once you have that, many amps can use the same supply.

More info in this thread:

Most of Apex's designs sound very good and are easy to build. He says the FX8 is his best sounding "simple" amp.
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You did ask for a OB for 2 Watts. You have a little amp?!
Never build a complementary-pp-design. You will not, NEVER, get a clean sound - but a sound to drive a rumbling roaring wooden horn - the most misunderstand. My mind.-)
To build any inexpensive OB you build an inexpensive OB. Use cheap drivers - Qts are not as important as the build in -, use not crossovers - better to brush parts of cones with damping glue -, clamp!!! and decouple!!! the drivers to avoid, to counter the wooden (flat, grey, curtained, diffuse) sound - NOBODY does do, because NOBODY does KNOW,-! Use no lace to connect, use, as example, solid core 0,6 mm, not bigger diameter!!! Use not expensive connectors - do not use connectors,-!
You will have got an inexpensive OB, a very very very good, clean! sounding OB!

I do talk about fullrange - only,-!
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Ahha It Worked!

Thanks for the responses. Nice to tap into the vast wealth of diversity in opinions. Xrk971, the ss amp thing is way cool. Maybe you should sell those things for good profit.

Cumbb, I agree with your sometimes less is more concept.

I wanted to post a picture to help clarify what I am working with. This should show my small (14 x 16 x 8 foot) room. It should show the large diffuser I built which I believe is a factor in my liking my experimental OB's as well as all other speakers. My two speaker projects can be seen (the Voigt Pipes & BIB's). My OB Experiment with the vintage driver from an RCA console. Last but not least, my 45 tube SET amps.

I made friends with Paul Weitzel (Mr. tuberesearchlabes - TRL). He deciced to design a 45 tube circuit out of kindness to me and I think intellectual curiosity. He would work on one monoblock and then let me clone his work in the other one, though we lived an hour and a half apart. The whole four years he was dying of undiagnosed pancreatic cancer as well as building his orders for his Dude preamps and VERY LARGE amps ordered by his customers. He wanted to tweak them more, but sent both home with me to try. He immediately begin working on a preamp to go with them which he intended to market at a fraction of the price of the Dude. It would be very simple, but he expected it to be really good. His undiagnosed condition (he was not one to go to the doctor) seemed to cycle. I expected him to recover the last time he got really sick, but he never did. The memory of him lives on as the amps play music. They make both my pipes and BIB's sound WAY better than they ever did before. I started experimenting with the OB's after I got the SET amps. I intend to keep them!


"trap circuits"-?

My mind: Remove the wooden material from the amps and the stands, basics. Wooden material is THE cleanness-killer.:down:
To cover the big, tall fronts of speakers with textile, as to see, is necessary.:up:
Use much thinner rca-wirings. 0,2 or so. To get a more focussed and clearer and "more black" sound.
My minds,-)

Would you try some DY-1256U,-? Or Beyma 12GA50,-?
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