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PostPosted: Sat Dec 04, 2010 12:15 am 
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Joined: Thu Nov 25, 2010 10:56 pm
Posts: 3
Location: Germany
Dear all,

from time to time I've been looking in this forum and now. .. here comes my first post.
Since a couple of weeks I have a pair of this little full range desktop speakers: http://www.markaudio.com/plan/alpair-5-desktop-tl

They show a good soundstage and astonishing detail level. Much better than the bulky BR I used before. But the drawback is that they rather sound thin to me which may be fatiguing after a while. This is mostly due to room acoustics and listening position, high speaker fs and the (to me formerly unknown) "full range sound". I decided to keep them because... they look so cool :D

So I want to give them help by some simple electronics. I am going to test attached preamp. It is to cover three issues: Baffle step correction, notch filter on speaker (and badly coincidenting room) resonance, and a little bass boost.

Essentially it is a fet diffamp with BSC (R10, C4) and bass boost (R6, R7, C1, C2) in the feedback network. The output stage is an EF fed by a bootstrapped double T filter for achieving a fairly narrow notch at the speaker resonance at about 130 Hz.

Do you see any flaws in this? I'll apreciate any comments, especially on the miller compensation cap. May it be omitted since the OLG is rather low?

Cheers,
hassan


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PostPosted: Sat Dec 04, 2010 7:48 pm 
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Joined: Tue Dec 14, 2004 12:26 pm
Posts: 4806
Location: Lisbon, Portugal
It sounds "thin" to you because it lacks BSC.
That speaker needs 3 to 4db of BSC.
It gets tiring to listen to, yes.
You're on the right path implementing BSC, make the speaker sound more linear, neutral and you may not need the bass boost.
I would not use bass boost right away, only if really needed - it may cause problems to that driver.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein


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PostPosted: Sun Dec 05, 2010 7:40 pm 
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Joined: Thu Nov 25, 2010 10:56 pm
Posts: 3
Location: Germany
Hi Carlos,

now I have breadboarded this circuit. It sounds quite good already. Still very "warm", but much of the nastiness is gone and most of the detail finesse is preserved. The bass boost is still active and it seems that it does the trick together with the BSC.

I don't worry about excessive lower end signals: Bass is boosted only down to the frequency the enclosure is tuned to (ca 50Hz). Going further down, "subsonic" levels are reduced by C1 and C3. Apart from this, I neither listen at high volumes nor have more input power than this little speaker can handle.

Nevertheless I will try your advice: Cut bass boost and strengthen BSC which at the moment plays a minor role in the circuit.

Cheers,
hassan


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PostPosted: Wed Dec 15, 2010 9:45 pm 
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Joined: Thu Nov 25, 2010 10:56 pm
Posts: 3
Location: Germany
Hi all,

I have been listening a while to this circuit now. I did some minor changes but the overall impression remains quite the same: BSC helps, bass boost helps, mid range is always very present and imaging from good to stunning.
For those curious of the circuit's performance, I've added distortion spectrum (measured) and frequency response (simulated).

Cheers,
Hassan


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