OB woofer (FAST) for tube amps

[dhieber]

I’m looking for an open baffle woofer for use with NFB tube amps with low damping factors. Since Martin uses solid state amps, his published simulations and actual builds do not reflect the results that can be expected when the speakers are driven by tube amps. So, I doubt the Alpha 15 would be a good driver choice. With Martin’s worksheets no longer available, I am somewhat in the dark concerning a good choice. Maybe the Beta 15 would work. Any actual experiences?

[Bob Brines]

Why would you want to drive the(sub)woofer with tubes?

Bob

[Nelson Pass]

The Beta 15 will work well enough with anything that has a little
damping factor. I have used it with DF=8 in open baffles,
and it works fine.

[Vix]

Hi,

I am using Beta 15 on a narrowish OB. I have not been so happy with its bass until I tried Graham's T-bass circuit:
'T'-bass drive for OB LF drivers.

and my speakers:

Visaton B200+Eminence Beta 15=OB!

On the downside, T-bass circuit needs a SS amp capable of driving very low impedance loads.
If you decide to go with Betas, then you better biamp, driving Betas via SS amp and T-bass, and a fullrange of your choice via tube amp.
Overall, I have the impression that there are many more people happy with Alphas on OB than Betas...

Good Luck,

Vix

[rjbond3rd]

Random newbie thoughts:

Martin has said (a few times) that there is no essential difference between tube and solid state -- tube is equivalent to SS + series resistance of some value, and thus tube is just less flexible because the value cannot be reduced (because it's built in).

Now he chose the Alpha's in part because of the high Q, which counters (or mates nicely with) the rolloff due to the baffle width. So if you are going to have more series resistance, which raises the Q, doesn't that just mean you merely have to compensate?

You could compensate in a few ways, e.g. less baffle, less boundary compensation, or "lose" the bass in whatever other way. What's your room like?

[MJK]

Originally posted by rjbond3rd
Now he chose the Alpha's in part because of the high Q, which counters (or mates nicely with) the rolloff due to the baffle width. So if you are going to have more series resistance, which raises the Q, doesn't that just mean you merely have to compensate?

You could compensate in a few ways, e.g. less baffle, less boundary compensation, or "lose" the bass in whatever other way.

Absolutely correct. The key to doing OB design is to trade-off the type of amp, the power required and resulting SPL/W/m, the effective driver Qts (including any series resistance), the baffle size and shape, and the crossover location and slope. This requires some engineering of the system to get the bass to work out right and to transition smoothly to the next driver up in the frequency range. Unfortunately the state of the art design method for OB speakers, as seen on the various DIY forums, seems to be a cut and try technique throwing different drivers, crossovers, baffles, and external filters together to try and get optimum bass without any up front engineering. That is not the way I do the OB design work but to each his own.

[chrisb]

Originally posted by Bob Brines
Why would you want to drive the(sub)woofer with tubes?

Bob

So in light of what Martin and Robert describe as methods available to resolve the issue of adjusting for the amp's damping factor in the total system design formula, the answer could be that the original poster likes the sound of the particular tube amps he has in mind - that is, enough to try it for himself.

No doubt there are many tubed amplifiers that fall flat on their faces when measured on the bench at their power/ bandwidth limits, but there could just as well be some that wouldn't in this particular application.

[MJK]

If I can interpret Bob's response a little bit. This is based on my experiences.

If you plan on using a full range driver, hence the question in this forum, for most of the frequency range it will probably come with an efficiency in the mid 90 dB/W/m. In that case you really need a OB bass design that can produce a mid 100 dB/W/m output. This means multiple very efficient woofers or an active woofer system with significant power for the bass drivers ...... hence a plate or SS amp.

A passive OB system that is also efficient and is consistent with a low watt tube amp would require some thought with respect to the bass driver(s) selection and the crossover. A SS or plate amp and active crossover makes a lot of sense combined with the lower watt tube amp driving only the full range driver, this combination is a much easier and more flexible design.

[hifimaker]

Another reason to consider SS for bass... Depending how low you are looking to drive a woofer, a lot of tube amp output transformers fall off after about 30hz. The OPT would have to be exceptional to drive 20-25hz without significant roll-off.

-David

[chrisb]

Lowther America at VSAC 2008 - not a small room and neither was the sound, but this is certainly a bit of a hair-shirt shock hazard method to do OB & Bass





OB panels (Lowther PM6A - Tone Tubby Alnico Guitar speaker @ 200Hz)





most fun tho was the amps





to be honest - I don't know about the XO and amp for the Hartley Sub





OK, clearly not a low budget DIY project, but it clearly indicates that if you can afford to throw financial and in this case safety considerations to the wind ....