PERSPEX QUASAR ver 2.0 prototype build

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chris661
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#76 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

I must admit, Mark, that they were disappointing to listen to as well.
I do wonder how the manufacturer arrived at the curves they display, and whether these units could be considered faulty in comparison. At the time, Colin noted that the updated version of the driver has a different whizzer, so it's likely they're at least a little aware of the problem with these drivers.

The Supravox unit measured rather well in comparison: a fairly smooth rolloff above 1kHz, which could easily be integrated with a decent compression driver or small-ish full-range unit. Probably only needs a highpass filter to get something good: the ultimate in simplicity.

I'm looking around to see if anyone has done a 3-4" driver optimised for use above 500Hz - ie, high efficiency, low Xmax.

Chris
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#77 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by simon »

Did you measure the bottom end of the Supravoxes Chris? I'm curious what you found if so. I wonder if they roll off a little higher than I'd like, but it could be the H frame I'm using rather than the driver.
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rowuk
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#78 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by rowuk »

The Fostex Sigma drivers that I tried looked VERY similar. More problematic for me was the fact that like with most high BL drivers, they seem to choke the life out of reproduction.

I can only imagine that they were designed for a 500 Hz front loaded horn to even the response out.
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#79 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by steve s »

rowuk wrote: Thu Aug 27, 2020 5:29 pm like with most high BL drivers, they seem to choke the life out of reproduction.

Really.. please explain
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chris661
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#80 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

simon wrote: Thu Aug 27, 2020 2:40 pm Did you measure the bottom end of the Supravoxes Chris? I'm curious what you found if so. I wonder if they roll off a little higher than I'd like, but it could be the H frame I'm using rather than the driver.
Yeah, did some measurements of the Supravox drivers, too. They're very well-behaved at the top end. With regards to the LF extension, with the mic close to the cone it had a gentle rolloff below 70Hz. At a greater distance, the response becomes choppy (room interaction), and the dipole losses seem to come in hard. Below 70Hz falls off a cliff.

If you like, I'll label up the REW file and upload it. PM me and I'll send you the link to my Google Drive.

Cheers,
Chris
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#81 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by simon »

Thanks Chris, PM sent.

It wouldn't surprise me if mine are rolling off closer to 70Hz than 40Hz though I've never measured them. I think James designed the Quasars with cardioid response to help the bass, but might be wrong
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#82 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

From what I've seen, a U-frame alone isn't enough to make a cardioid response with a null at 180-degrees. The successful implementations of a cardioid response feature either additional active drivers delayed to absorb the rear wave, or a cabinet that's designed to "leak" just the right amount towards the sides and rear, so that sound heading that way from the front of the cone is cancelled.

It's difficult stuff to pull off, and I'm not convinced that this design can achieve a cardioid pattern over a useful frequency range.

Replying to your PM now.

Chris
vinylnvalves
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#83 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by vinylnvalves »

Just for information, another speaker implementation utilising the same FR drivers -http://www.modelpromo.nl/The%20Liiondia ... peaker.htm. Again as with all these “ boutique “ speakers no measurements..

The response curves for the FR drivers look typical from my experience of Lowther and Supravox whizzer cone drivers. The measurements even at a few centimetres can be affected by the room, I have some traces of a horn measurement I did In the house and outside with the mic in the horn, they were different, not much - but different. I have found the MLS instead of a sine wave is better for the higher frequencies for in room measurements.
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#84 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

... Not quite the same full-range units.
Those are essentially the V2 version, and the whizzer profile is quite different. Note the folded back edge.

My bet is that helps to damp/redistribute the resonances, hopefully avoiding the large spike these originals have.

These drivers still appear to have their uses, but IMO as a midbass rather than a full-range unit.

Chris
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#85 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by vinylnvalves »

Yes they generally make nice midrange drivers if you chop off the whizzer cone... :wink:
chris661
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#86 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

Well, they're not mine to chop, but I did note that they sounded considerably better behind the speakers. Measurements confirmed the big peak had gone, although the 3kHz dip was still present.

The design goal here was a 2-way, though, which IMO rules out these particular full-range drivers for mid-high use.

Chris
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#87 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by JamesD »

Quick comment on the bottom end measurement - My measurement is -3dB at 41Hz and Nicks using the Emm Beta 15 was 38Hz for -3dB (From memory). There are a some critical items for obtaining this bass measurement - one is the series connection of the two drivers lowering the effective Fs of the bass driver - the Supervox 285GMF having Fs of 47Hz without doing this - i.e. trading sensitivity for bass extension. two is ensuring a good air seal in the driver mounting and in the baffle construction. Three is ensuring a good air seal between the bottom of the baffle and the floor - this is really important - the extension below 80Hz to 40Hz is dependent on the virtual reflected baffle below the floor so the baffle has to seal to the floor for this to be effective since it in effect doubles the baffle height... Bottom end measurement also depends on a room that can support the measurement unless short full bandwidth frequency squawks are used - Chris knows all this so I'm sure he used appropriate techniques.

With regard to the cardioid shape of the response - one should not expect the sort of null that a cardioid microphone achieves but 6-9dB is possible and this is sufficient to enable the baffle to work close to a rear wall or corner without the rear reflected wave & its room resonance over powering the front wave and its room resonances
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#88 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

Hi James,

I'm struggling: how would the series crossover drop the resonance of the Supravox?
Your numbers align quite well with my close-mic'd measurements, but the in-room response was much more choppy as would be expected.

Attached is the Supravox-only close-up vs the 2-way speaker in-room on the listening axis.

Chris
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#89 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by JamesD »

Quite right to query the Fs bit as I was being sloppy - it has a similar effect on the system response but doesn't actually change fs - it changes Qes by increasing Re - the relevant equation for that is

Qes = 2 * Pi * fs * Mms * Re / (BL) ^2

and adding series resistance increases Re that changes Qes and the change of Qes leads to a lower fc (love using fc to describe an open baffle system) as a trade off against system efficiency - the driver efficiency doesn't change but more of the amplifier output is lost across the series resistance than before as system Re. Note - that this doesn't boost the low end either but it shelves the mid bass and upwards down to match the bass roll off at a lower point - this is a similar effect to adding mass to the cone - that would directly affect Fs and reduce driver efficiency...

Double note is that this is a first order effect and there are other second order and higher effects to take into account - no such thing as a free lunch and all loudspeakers are a compromise even before we use then in a room...

Hope that wriggles around my sloppy use of terms... :oops:
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#90 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by JamesD »

Forgot to say that the orange curve looks familiar :D Thank you for showing it here. What do you think caused the two-way curve to roll off earlier? Some cancellation effect in the room from driver interaction or room mode?

The GMF285 is a lovely sounding driver in its own right and makes a great conventional two way with the Supravox tweeter that can be used from 2KHz - it doesn't work quite so well as a conventional two way in the Quasar as the gap between the two drivers becomes noticeable and distracting... Having said that I must try the small sonido with the GMFs in my Quasar once the baffles are out of store...
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