OB bass. Can anyone clarify this please.

Bumpy · « **on:** 8 Oct 2020, 07:02 pm »

We all know about compromised bass output due to cancellation of front wave by rear. But what do we experience in reality.

1. Very little movement of the bass driver.
2. Bass driver thrashing about but little perceived output.

NeilBlanchard · « **Reply #1 on:** 8 Oct 2020, 11:26 pm »

I posted in the wrong thread - can this be deleted, please?

MJK · « **Reply #2 on:** 9 Oct 2020, 02:02 am »

With 15" and 18" woofers I get #1.

ooheadsoo · « **Reply #3 on:** 9 Oct 2020, 04:20 am »

At the moderate volumes I listen at, my 4 10" woofers barely move with program material.

Openly Baffled · « **Reply #4 on:** 9 Oct 2020, 08:46 am »

There is a mathematical relationship between cone area, x-max, frequency and SPL as a function of effective baffle width.
Anyone who claims they have 95dB+ SPL OB bass at 30Hz and can "hardly see the cone move" is telling porkies.
(Or they have 4 x 21" woofers!)
You can calculate the Infinite baffle SPL and the discount by 6dB/octave below the baffle Fc.

Bumpy · « **Reply #5 on:** 9 Oct 2020, 09:58 am »

OK, I asked the question as, at normal listening levels, my 15" Eminence Alphas barely move and I have to touch the cone to confirm they are actually playing. Like many people with OBs more of their wonderful bass would be excellent.

The consensus seems to be that you need additional movement of the bass cone to elevate volume. So how do you achieve that?

1. Have a bass driver that is equal or more sensitive than the midrange (mine are both about 95dB)
2. Enhance the output of the bass driver with an enclosure (box does not find favour with me)
3. Reduce cancellation of front wave by the rear wave using side panels, U or H frames ( I'm OK with experimenting here)
4. Replace my 1.8W 45 triode amp with one that will drive the bass (not easy to find in a triode amp).
5. Get 16 ohm bass drivers which are easier to drive??? (not sure if, electronically, this is sensible)
6. The 'nuclear option' of going active on the bass driver with separate active low pass filter, additional amps with plenty of watts and of course its own volume control to balance it with the midrange.

Any views on these options

MJK · « **Reply #6 on:** 9 Oct 2020, 11:47 am »

Quote

Have a bass driver that is equal or more sensitive than the midrange (mine are both about 95dB)

If you are using a passive, or no crossover, you really need the bass driver to be more efficient than the other drivers.

Quote

Reduce cancellation of front wave by the rear wave using side panels, U or H frames ( I'm OK with experimenting here)

Going to a U or H frame will push the bass lower at the cost of reducing the SPL/w/m from the woofer. You might also consider adding a second Alpha 15A woofer wired in parallel to each channel to up the output.

Quote

Replace my 1.8W 45 triode amp with one that will drive the bass (not easy to find in a triode amp)

This is probably your biggest problem. A low wattage tube amp is not compatible with a passive crossover OB speaker system. Ditch the tube amp and get a higher powered SS amp that will tolerate the changing speaker impedance and provide more power.

Quote

The 'nuclear option' of going active on the bass driver with separate active low pass filter, additional amps with plenty of watts and of course its own volume control to balance it with the midrange

That will also work.

Consider modeling your OB speaker in the BASTA program (http://www.tolvan.com/index.php?page=/basta/basta.php) to get a feel for what you are getting and what trade-offs can be made.

Digi-G · « **Reply #7 on:** 9 Oct 2020, 01:23 pm »

Big bass drivers eat up a lot of power. My SVS subwoofer has 1000 watts, which might sound like overkill, but it's not.

I would suggest a dedicated solid state amp for the OB bass. Something with a volume control would allow you to balance the bass with your main speakers.

In regards to #2 Bass thrashing about... this sounds like subsonic noise, like from a turntable. Your original post is kind of cryptic though, so I'm not sure what you're getting at.

lokie · « **Reply #8 on:** 9 Oct 2020, 01:58 pm »

Interesting discussion with excellent info..
Thanks.

opnly bafld · « **Reply #9 on:** 9 Oct 2020, 02:34 pm »

Quote from: Bumpy on 9 Oct 2020, 09:58 am

5. The 'nuclear option' of going active on the bass driver with separate active low pass filter, additional amps with plenty of watts and of course its own volume control to balance it with the midrange.

This is very easy and inexpensive to do with a couple of speaker level input plate amplifiers.

opnly bafld · « **Reply #10 on:** 9 Oct 2020, 02:41 pm »

Quote from: Digi-G on 9 Oct 2020, 01:23 pm

Big bass drivers eat up a lot of power. My SVS subwoofer has 1000 watts, which might sound like overkill, but it's not.

A low sensitivity driver in a box with eq to make it play low eats up a lot of power, for Bumpy with OB Alphas, 100 watts may be overkill.

Early B. · « **Reply #11 on:** 9 Oct 2020, 03:10 pm »

Quote from: opnly bafld on 9 Oct 2020, 02:34 pm

This is very easy and inexpensive to do with a couple of speaker level input plate amplifiers.

Just don't expect great sound. Most plate amps are cheaply made and sound accordingly.

Bumpy · « **Reply #12 on:** 9 Oct 2020, 03:16 pm »

Quote from: Early B. on 9 Oct 2020, 03:10 pm

Just don't expect great sound. Most plate amps are cheaply made and sound accordingly.

I crossover at 100Hz so hope amp quality is not that important at those lower ranges.

Bumpy · « **Reply #13 on:** 9 Oct 2020, 03:18 pm »

No comment yet on the switch to a higher impedance bass unit such as 16 ohm or even 32 ohm. Will the bass be louder and will the valve amp benefit.

Bumpy · « **Reply #14 on:** 9 Oct 2020, 03:28 pm »

Quote from: Digi-G on 9 Oct 2020, 01:23 pm

Big bass drivers eat up a lot of power. My SVS subwoofer has 1000 watts, which might sound like overkill, but it's not.

I would suggest a dedicated solid state amp for the OB bass. Something with a volume control would allow you to balance the bass with your main speakers.

In regards to #2 Bass thrashing about... this sounds like subsonic noise, like from a turntable. Your original post is kind of cryptic though, so I'm not sure what you're getting at.

Regarding the bass driver thrashing about. This is certainly not what mine does, but I see many videos of speakers doing just this and knowing my drivers have a good x max I was just wondering if that was how it was meant to be.

opnly bafld · « **Reply #15 on:** 9 Oct 2020, 04:21 pm »

Quote from: Bumpy on 9 Oct 2020, 03:18 pm

No comment yet on the switch to a higher impedance bass unit such as 16 ohm or even 32 ohm. Will the bass be louder and will the valve amp benefit.

No
The amplifier will benefit from not having the load of the extra bass drivers.

Bumpy · « **Reply #16 on:** 9 Oct 2020, 04:32 pm »

Quote from: opnly bafld on 9 Oct 2020, 04:21 pm

No
The amplifier will benefit from not having the load of the extra bass drivers.

Thanks, was about to bid on a pair of 32ohm speakers

ooheadsoo · « **Reply #17 on:** 9 Oct 2020, 09:46 pm »

I agree with MJK, you might want to use a more powerful solid state amp. I'm using an amp that delivers 30w into 8 ohms and have no problems achieving pleasing bass spl. Loud enough where you can't carry a conversation at the same time. This isn't much power by modern solid state standards.

Hasn't MJK published many public domain OB designs?

Bumpy · « **Reply #18 on:** 10 Oct 2020, 07:14 am »

My next task is to try a vintage Rotel amp 35 wpc running the entire OB. This should give me a reasonable handle on the bass that might be achieved in the current configuration. I am expecting the midrange to 'collapse'.

Bumpy · « **Reply #19 on:** 10 Oct 2020, 07:18 am »

Does this animal exist?

An active crossover capable of creating a 100Hz low pass filter, but with high level inputs (speaker output from main amp) and line level output (to separate power amp)

So essentially its a subwoofer but without the built in amplifier, allowing me to use a better quality amp.