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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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