OB project, Jame's Quasars copy

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thomas
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#1 OB project, Jame's Quasars copy

Post by thomas »

I cobbled together these err beasts over the New Year, seeing as I had some old wardrobe doors to dispose of.
They are more or less James's Quasars, but a bit wider to accommodate the Eminence Beta 15s (thanks Phil for the epay heads up) and a pair of Sidcup's finest, two fairly modernish PM6C.
This is my first ever speaker build! And I don't know what I'm doing whatsoever... but they seem to sound quite good, nice present-but-tight bottom end (missus),reasonably balanced midrange (albeit with some of that unavoidable Lowther 'forwardness') and if there's a lack of top treble then I can't notice it....they are also pretty efficient, my 801a amp seems very happy with them.
The phase plugs were an attempt to calm down the midrange peak, but don't really.
Incidentally I also have some old EX3's but I haven't dared drop those in yet (screech alert!)
I remember hearing the original TP1's many years ago and I reckon the Lowther drive unit needs a front horn (or be seriously off axis) as well as a rear horn, but I digress....also my woodworking skills were stretched to the limit with these OBs...
The drive units are in series, with a 30uf cap bypassing the bass driver. I haven't gotten around to ordering any inductors yet.
Aesthetically they are quite err challenging!
I was wondering if folk knew of any idiot's guide for measuring frequency response?
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pre65
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#2 Re: OB project, Jame's Quasars copy

Post by pre65 »

Bravo Thomas. :)

That is some phase plug, did you make it ? :lol:
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#3 Re: OB project, Jame's Quasars copy

Post by SimonC »

thomas wrote: Wed Jan 10, 2018 11:09 am Aesthetically they are quite err challenging!
Not quite as aesthetically challenging as that rug though...

Good work on the speakers our Thomas, nice to see you venturing into the world of speaker building :)

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#4 Re: OB project, Jame's Quasars copy

Post by Mike H »

Excellent! Image


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#5 Re: OB project, Jame's Quasars copy

Post by simon »

Bit harsh on the rug I reckon :lol:

Excellent Thomas! We need more threads like this - so I can try to convince the missus it's not just me that's mad. I'm guessing you painted them though - the wardrobes weren't that colour?

I found mine responded well to tuning the crossover. I got a nice improvement limiting bass to the wide bander, I cut around 150Hz IIRC. Similarly, I was originally running the bass helper without any filter and limiting the bass to below the wide bander really helped the coherence. No surprises.
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#6 Re: OB project, Jame's Quasars copy

Post by Mike H »

I assumed the colour is the original dark stained finish?
 
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#7 Re: OB project, Jame's Quasars copy

Post by thomas »

SimonC wrote: Wed Jan 10, 2018 12:43 pm
thomas wrote: Wed Jan 10, 2018 11:09 am Aesthetically they are quite err challenging!
Not quite as aesthetically challenging as that rug though...

Good work on the speakers our Thomas, nice to see you venturing into the world of speaker building :)

Simon C

Ooooh! Meow!

Thanks chaps.

The phase plugs I picked up many years ago, I can't remember where...they're ceramic. I'm a bit of a magpie....

Yes the wardrobe doors were that dark colour! Not my choice, Ikea jobbies, so a swap with nicer oak ones left me with these bits to play with. Nasty 15mm chipboard, I had to hoover my patio after I cut them out.

I'm assuming the cap bypass across the bass driver is a bog standard filter which with an assumed 8r gives an f-3db of about 600Hz or so. I was going to start clipping in extra capacitance to drop this, once I have got to grips with how they sound now. James quotes 100uf for his AER/supravox drivers.
I was going to get a pair of 1.5mH inductors to put across the PM6Cs but that was just before xmas and hifi collective never got back to me for some reason.
I can see this whole thing is going to take me a fair bit of trial and error....
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#8 Re: OB project, Jame's Quasars copy

Post by simon »

Ah, okay. They look black on my phone. ("No Johnny" - a Fast Show reference there for the kids!)

My crossover bits weren't cheap. Big chokes with low DCR are surprisingly expensive...
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#9 Re: OB project, Jame's Quasars copy

Post by Nick »

simon wrote: Wed Jan 10, 2018 4:11 pm Ah, okay. They look black on my phone. ("No Johnny" - a Fast Show reference there for the kids!)
Sorry to depress you, but the "kids" have never head of the Fast Show :-(
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#10 Re: OB project, Jame's Quasars copy

Post by Ali Tait »

Shoddy!
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Dave the bass
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#11 Re: OB project, Jame's Quasars copy

Post by Dave the bass »

Ali Tait wrote: Wed Jan 10, 2018 4:46 pmShoddy!
Dang! Beat me to it!

Top Ikea Conversion work there MrT, that phase plug though.....
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#12 Re: OB project, Jame's Quasars copy

Post by IslandPink »

Oooh.. ebonised, is it ?
Classy ! & good effort, always nice to see a new speaker.

PM6C sounds like a good choice, I had a look at them myself - only 92.5db/w , so around the same as where I am at the moment with 2x FX120.
Given this is an 8R (ish) driver the 'proper' Quasar-style crossover for this will be similar to what I tried for the Fostex FF225WK which has similar efficiency, size , Re and Qts.
If you get an 8.2H choke then it should be close to this :
Quasar_FF225_cross.JPG
You need the big choke and cap values to put the crossover around 150 to 200Hz.
You should be able to get a pretty good result using a laminated E-I core choke ( talk to Colin ) , that will help keep the cost down a lot compared to a big air-core like the ones I have and James used. I'd recommend you use at least 6.8mH though, otherwise you'll have the crossover too far into the midrange, the Eminence with the deep side-wings down there needs to be rolled-off fairly early to avoid colouration.
With the proper crossover, you should get a lot of improvement from where you are now. You will be able to fine-tune the cap value by ear, just checking upper bass tone and overall dynamics. It was fairly easy to judge with the Supravox and FF225WK.

Oh yes, the padding down of the bass driver will be required as it'll otherwise be too efficient relative to the PM6C. You might want to start with 3.3R to the Eminence.
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thomas
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#13 Re: OB project, Jame's Quasars copy

Post by thomas »

Thanks for the pointers there Mark. Looks like with the variables to play around with this could take me quite a while with trial and error.
Obviously I can clip in capacitance to my hearts content, but is it worth buying a handful of cheap(ish) inductors and trying adding them incrementally in series and only splashing out on an air core when I'm happy with the set up?!
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Ray P
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#14 Re: OB project, Jame's Quasars copy

Post by Ray P »

Goodo Thomas...
Sorry, I couldn't resist!
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Nick
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#15 Re: OB project, Jame's Quasars copy

Post by Nick »

Quick experiment. One half of the primary of a 30va Vigotronix VTX-146-030-215 toroid is just under 8H at 100Hz.Not very constant with frequency, its only 2H at 1kHz, but might be worth a play.
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