OB with Alpair 12P and Eminence Beta 15

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ed
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#76 Re: OB with Alpair 12P and Eminence Beta 15

Post by ed »

awesome Max, awesome!

how big is the room?
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#77 Re: OB with Alpair 12P and Eminence Beta 15

Post by steve s »

Good news that it works ok max open baffles are a really good way of doing things
Nice we300b valves by the way...
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#78 Re: OB with Alpair 12P and Eminence Beta 15

Post by steve s »

Looking at your speaker placement Max, they look parallel to the wall and about 18" away
There will be resonances going on behind them
Toeing them in slightly and a couple of layers of old fashioned underlay or the likes pinned or stapled to the back of the baffle can make quite an improvement in my experence ... just my thoughts
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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#79 Re: OB with Alpair 12P and Eminence Beta 15

Post by Ali Tait »

What was the cap mod on the pse's Max?

Must drag mine out for a listen sometime.
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#80 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

ed wrote: Sun Aug 20, 2017 11:55 am awesome Max, awesome!

how big is the room?
Cheers Ed. Room is 4.5m x 9m, angled ceiling 2.8m high.
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#81 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

steve s wrote: Sun Aug 20, 2017 12:09 pm Good news that it works ok max open baffles are a really good way of doing things
Nice we300b valves by the way...
Mrs N bought me the 300Bs when they went back into production in the early noughties. I'm a very lucky man :)
They don't do everything well, but the midrange is lovely. In this application they are really hitting the spot.
steve s wrote: Sun Aug 20, 2017 12:21 pm Looking at your speaker placement Max, they look parallel to the wall and about 18" away
There will be resonances going on behind them
Toeing them in slightly and a couple of layers of old fashioned underlay or the likes pinned or stapled to the back of the baffle can make quite an improvement in my experence ... just my thoughts
Good thoughts, I will try some experiments. They are actually about 36" from the wall, and I was kind of thinking that the old speakers behind them would break thing up a bit. But I'll have a play...
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#82 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

Ali Tait wrote: Sun Aug 20, 2017 12:44 pm What was the cap mod on the pse's Max?

Must drag mine out for a listen sometime.
If you have the WAD circuit diagram to hand, on the 6AU6, C4 AC-couples the screen grid to the cathode, and the standard value was 0.47uF.
Morgan Jones's book contained a worked example of how to determine the size of the cap for a given LF roll off corner frequency, and using that we came up with a bigger value. Something like 2uF or 4uF I think. The bigger cap gave better bass. I think I even measured the difference back in the day.
I think using a voltage regulator would be better still, but the intention when I built this pair was to keep them close to stock because I planned to sell them at some point, and I figured whoever bought them would prefer it that way. Also, I wanted them to serve as a reference to compare with the other pair as I developed them.

It is possible that Andy Grove chose the value deliberately to give a 'benign' LF roll-off, avoiding transformer saturation etc. But the WAD transformers were pretty beefy and I haven't detected any downside to the mod. I think quite a few of us did it at the time.
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#83 Re: OB with Alpair 12P and Eminence Beta 15

Post by IslandPink »

Sounds like a good combination , with two 15" to drive each side. Nice !
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#84 Re: OB with Alpair 12P and Eminence Beta 15

Post by Ali Tait »

Thanks Max, will have a look.
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#85 Re: OB with Alpair 12P and Eminence Beta 15

Post by simon »

Excellent Max. There's just something about the sound of OBs.

I'm struggling to get my head round how they produce deep bass though. Doesn't the sound cancel around the relatively small baffle quite high up? Or is that what the miniDSP does, shape the bass somehow?
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#86 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

simon wrote: Sun Aug 20, 2017 11:48 pm Excellent Max. There's just something about the sound of OBs.

I'm struggling to get my head round how they produce deep bass though. Doesn't the sound cancel around the relatively small baffle quite high up? Or is that what the miniDSP does, shape the bass somehow?
Depends what you mean by deep I guess. At the moment I'm not using the miniDSP to do anything other than crossover and take out the 40hz room peak. Everything else is flat.
I am planning to take some measurements, but haven't yet resurrected the mic preamp.
The Basta simulations posted earlier do include the OB predicted roll off.

These baffles are 54cm wide and then the timbers each side are quite deep, 7.5 I think, so does that make them effectively about 70cm wide?
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#87 Re: OB with Alpair 12P and Eminence Beta 15

Post by simon »

Well, I suppose I'm trying to get my round how much influence the top driver has. Beneath it there's the baffle and floor, so a nice long "wavelength" to generate lower frequencies. (There appears to be a gap between its baffle and that of the 12P which presumably is like a "short circuit" though? Same for the bottom driver but above it?)

But the sides, and especially the top, the front wave will cancel the rear quite high up, won't it? If the top has, say, 50mm of baffle above the top of the driver (I'm making it up) and the baffle is 25mm thick then the path from front to back is 125mm. v = f.lambda, so f = 343 / .125 = ~2700Hz. So I wonder if the top driver has only moderate influence on lower bass?

I don't know, just trying to understand. I guess basta knows what's going on even if I don't!

Edit - I think you might have edited whilst I was replying?

My understanding is (and I wouldn't recommend anyone putting any money on it!) is that it's the edge distance from the driver. Where's James when you need him. Mark can probably confirm or otherwise.

If there is a gap between the sub baffles it might be worth putting some tape over it to see if it makes any difference? James' first baffle were cardboard so something simple should suffice to check.
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#88 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

I think the Basta baffle model is probably accurate, it's been around for a while and it's quite widely used.
Remember you don't get complete cancellation, it's a 6db/octave roll off. And the pressure wave is propagating from the driver in all directions, so only a very small part of the wave sees the shortest path. And the presence of the wings does have an effect. So although the roll off might start quite high, for much of the pressure wave it's not starting until much lower (eg the bits of the wave propagating towards the floor)

One of the benefits of the two woofers is that, for the same SPL, the excursion is less than for one woofer. In theory that lower excursion should mean less distortion.

I just realised that I'm also able to play with the relative level of the woofers and the Alpairs, so it may be that I have the woofers set a bit higher in level. So not shaping the LF as such, but maybe I have the woofers turned up a bit?
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#89 Re: OB with Alpair 12P and Eminence Beta 15

Post by IslandPink »

simon wrote: Tue Aug 22, 2017 10:15 pm Mark can probably confirm or otherwise.
Who is this 'Mark' you speak of ?
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#90 Re: OB with Alpair 12P and Eminence Beta 15

Post by simon »

IslandPink wrote: Tue Aug 22, 2017 11:19 pm Who is this 'Mark' you speak of ?
Why, the Mark with a 100-and-something page thread about speakers :-).

Okay thanks Max. It still doesn't reconcile in my mind, I guess I need to have a play with Basta to see just what effect baffle size has.
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