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First, some background info: I used my ALTEC 756B/Jensen RP302 combination on JE Labs Style open baffles. To fill in the bottom 2 octaves, I used a powered sub-woofer. After about 10 years with this setup, I decided I wanted to make 2 changes. First, I wanted narrower baffles (the JE Labs are almost 3 feet wide). Second, I wanted integrated woofers (no more separate powered sub-woofer).
After weighing my options and not wanting to give up my ALTEC/Jensen driver combo, I decided to go with the Pure Audio Project (PAP) open baffles and woofers. PAP cut a custom mid-range/tweeter panel for my drivers. This worked exactly as I hoped it would - the PAP baffles are about half as wide as the JE Labs OBs and with two 15" woofers per side, no separate powered sub is needed.
Initially, I bi-amped the PAP woofers off a separate Crown DriveCore 2 amp with built-in DSP crossover. I used the system this way for about a year until recently when I got the idea to see how the system would sound running it full-range with my 8 watt Bottlehead Paramount 300B amps. In order to do this, I purchased a pair of 80hz 4 ohm 12db/octave low-pass filters from Parts Express.
The reason the filters are 4 ohms is because the two 8 ohm woofers per side are wired in parallel. OK, so how did it sound? Not so great at first! The low bass seemed to be missing in action. Then I remembered reading that 12db/octave filters are 180 degrees out-of-phase. I assumed this was causing significant cancellation between the PAP woofers and ALTEC 756Bs.
The simple fix was to wire the woofers out-of-phase (reverse the +/- input connections) and that made all the difference! Now the low bass was back in business. See the photos below for the system setup.
Question 1 - Does wiring the woofers out of phase make sense? Question 2 - What is my speaker system's total impedance? To summarize: each speaker has two 8 ohm woofers wired in parallel and low passed at 80hz with a 4 ohm filter, one 8 ohm ALTEC 756B run full-range directly off the amp and one 16 ohm Jensen RP302 tweeter. A .47uf cap is used to high pass the tweeter.
Edits: 04/18/17Follow Ups:
Another woofer option to consider might be the Hawthorne Audio Augie. The company was recently reported to have shut down, but they seem to be selling them on Amazon, so I'm not clear what the status is. Lots of good writeups about the Augies.
The "Augie" has an excellent Qts (.92) and Fs (27hz) for an open baffle. However, it's 8 ohms and paralleled would be 4 ohms (which is the problem I have with my current woofers). Also, it's sensitivity (88.7dB) is a little low for my requirements. Anyway, thanks for the suggestion!
You might want to consider a MiniDSP Dirac solution.
Hi Gerry if you mentioned the actual make and model of the two 15" woofers you used in your wonderful looking Pure Audio Project OB speakers I missed it somehow!
I'm listening to: Art by Cast
Thetubeguy1954 (Tom Scata)
Central Florida Audio Society -- SETriodes Group -- Space Coast Audio Society
Full-range/Wide-range Drivers --- Front & Back-Loaded Horns --- High Sensitivity Speakers
The OB-A15neo woofers are custom made by Eminence for Pure Audio Project (use the link below). They're a significant upgrade to the well-known Alpha-15A. The OB-A15neo has a very good Qts (.72) for open baffles, but I wish the Fs (40.4hz) was lower.
I would consider replacing them if I could find something better but that's not so easy. The Acoustic Elegance LO15 looks like a possibility with a Qts of .82 and Fs of 33.8hz but it costs almost twice as much as the OB-A15neo.
Very interesting setup. That doorway needs to be relocated several inches to the left! :)
Well done, very nice looking project. You might also try posting over at AudioCircle...Open Baffle Forum. Quite a bit of discussion on the Pure Audio Project over there.
Thanks Mick! Also for mentioning the Audio Circle OB forum. I didn't know or forgot about it. I took a look and there appears to be lots of interesting and active threads.
Here are a few thoughts I didn't mention in my original post because it was already a bit long. The reason I originally bi-amped the system is because the PAP Neodymium woofers are less sensitive than my Altec/Jensen drivers. With the Crown woofer amp, I was able to dial up the woofer level to match the other drivers. However, two items made me think the new setup could work.
The first is a quote from the woofer product design manager: "(this model) can play with any tube amp starting at 3.5w. They are very responsive 95/96db drivers!" The second is that I believe having 2 woofers per side increases the output somewhere between 3 and 6db.
Even though those factors should get the woofers up to the level I need, in practice I did lose some bass below 40hz. Of course we are comparing 8 watts vs. 525w for the Crown amp into 4 ohms. However, running the speakers full range using just the Bottlehead Paramount 300B amps sounds better!
My only concern now is the speaker system impedance. My Paramounts are upgraded with Magnequest BH-5 output transformers and only have 8 and 16 ohm taps. If my OB impedance is much lower than 8 ohms, I wonder what impact that has power or distortion? I know impedance calculations can be complicated and that it varies with frequency so that's why I asked the impedance question.
I think I'd follow Tre's lead for openers. Pretty complex issue, but I think the bottom line with most tube amps is that you'd like to stay above 4 ohms throughout the freq.range. I see where Spatial has recently introduced a 16 ohm "Triode" version to their Model 3 and 4. So you can see where this is trending. Tube amps in general simply favor a higher impedance stable load.
I would need to see the impedance curves for both the 15s and the 8" speakers.
Assuming the nominal impedances, 8 ohms in parallel with 4 ohms (from 80Hz down) is 2.67 ohms.
That's probably not the case. Again, you need to look at the impedance curves of the drivers you're using.
The 8" speaker might have a high impedance below 80Hz and the load seen by the tube amp might be around 4 ohms at those frequencies.
4 ohms connected to the 8 ohm tap will show the 300b one half the normal impedance and that will increase the power and increase the distortion.
Have Fun and Enjoy the Music
"Still Working the Problem"
Given that the impedance will be below 4 ohms, WHAT IF... I wired the woofers in series instead of parallel for 16 ohm impedance? Of course I would need to switch to an 8 or 16 ohm low pass filter.
Does series wiring of multiple woofers introduce its own set of issues? Thanks.
Thanks for the response Tre'! The woofer impedance curve is shown below. The Altec 756B bass/mid-range driver is actually 10". It's over 60 years old, so finding an impedance curve won't be so easy. FWIW, I have read that the Altec/WE cone speaker drivers had very flat impedance curves.
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