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Old 21st September 2012, 01:17 PM   #31
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The pair I modified belong to a friend so I'm going purely from memory. The problem with the original pair was a broad suckout in the upper bass leading to a thin sound. As I recall, Final's values for the resistor were off by a wide margin causing the impedance to drop far too low and probably resulted in a large part of the suckout we noticed subjectively and later measured. The customary values for the resistor in such a circuit are usually higher (1.25 times, IIRC) than the drive unit's DC resistance, so if I have interpreted the R/C circuit correctly as a zobel network it should be around 7.5ohms, assuming 6ohms for the drive unit.

I didn't know the parameters of the aluminium unit so I can't say if the the Peerless unit has similar specs. In any case I designed the new network from scratch.

Incidentally, we have since changed to, and settled on, a Scanspeak paper/carbon fibre which is superb, again with a new cross-over design. The blend with the panel is undetectable, due in large part I suspect to the quality of the Scanspeak driver.

Adjusting the voltage to the panels via adjustable AC power supplies has also been a useful fine-tuning device.

I hope some of this is of use.
Tim.
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Old 21st September 2012, 06:38 PM   #32
Lenin21 is offline Lenin21  United Kingdom
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Smile Tell me EVERYTHING....

Well, I'm determined to address the upper bass/lower mid suckout, so if you've developed an even better solution, I'm keen to implement it.

Could you share full details of the modifications? I'm desperate to retain the 0.3s magic, while addressing its weaknesses...
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Old 22nd September 2012, 09:02 AM   #33
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Lenin21,

I don't recall the inductor value but I will measure it when I visit my friend next week. Fortunately we mounted it externally so that we can swap air core for ferrite core at will. I'll also recalculate the resistor/capacitor values when I have located the specs of the exact Scanspeak drive unit we used.

Tim.
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Old 4th October 2012, 09:53 AM   #34
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Further to my last reply, I haven't been able to measure the inductors but the details of the changes I made are as follows.

New bass driver - Scanspeak 18W8545.00
Capacitor - 10uF
Resistor 6.8 Ohms

I recall the inductor value was very close to my calculated value of 2mH. I think I ended up at around 1.85mH after fine tuning by measurement and by ear, so I suggest starting at 2mH and tuning by ear, removing a few turns each time.

I also changed the treble section capacitor from 35uF to 40uF which enabled the use of Russian paper in oil types, 4 * 10uF in parallel (I also used one for the bass section capacitor). These sound a lot better in the treble than the bipolar electrolyic used by Final and there is just enough room to mount them beside the main board in the bass of the unit.

Tim.
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Old 9th October 2012, 08:30 PM   #35
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well i almost bought these for 800.00. glad i read this thread.
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Old 11th October 2012, 11:14 PM   #36
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Tim-after reading thid thread a couple of times and thinking i can probably get this pair for 600.00 do you think i should buy em and do the mods? im not rich but i could afford these. i have a luxman m-117 to drive them with. plus id really like to put these polks on the RR tracks behind my house and video it.
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Old 7th November 2012, 12:26 PM   #37
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Further to my last post, I have now measured the inductors and they are 2.15mH. With the other changes mentioned previously the integration between bass unit and panel is impressively smooth, even when you know where it is (usefully, I have perfect pitch so I can 'zone in' on particular a frequency range). Of course the quality of the Scanspeak's upper bass helps too.

vrusso123 It is up to you. With the mods mentioned they are certainly very good. If it helps I'm a professional musician and my friend who owns the modified pair is a piano tuner. He was so unhappy with them when he bought them that he was going to sell them but since the mods he loves them. I'm always impressed when I hear them but I use modified Quad 63s myself, which I prefer.
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