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Old 24th April 2007, 12:25 PM   #21
hajame is offline hajame  Hong Kong
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Quote:
Originally posted by ppa
the ymmv's LC network (0,7 uH and 10 ohm) is good , this is the typical value in several amp, I would add also to the LC network a polyester or polypropylene 100nF capacitor in series to a 10 ohm 5W resistance between the output and the out ground to avoid local oscillation on the mosfet final stage.
Consider you that without the LC network the amp can oscillate,
even if the RC net it's added.



ppa

You are right! The amp still oscilates with RC network. And I have tried simply a 5 ohm in series with the output. This really makes the muddy frequency range clear. Soon I will try the LC network you suggested.
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Old 24th April 2007, 12:48 PM   #22
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Quote:
Originally posted by hajame

You are right! The amp still oscilates with RC network. And I have tried simply a 5 ohm in series with the output. This really makes the muddy frequency range clear. Soon I will try the LC network you suggested.
The series resistance is raising the Q of your speaker system --
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Old 24th April 2007, 10:35 PM   #23
ppa is offline ppa  Italy
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I have seen the amp and servo schematics , thanks to the high amp's speed the LC network's inductance should be smaller than 0,7uH. You should try L=0,5 ( 8 turns of enamel wire on a 1/2 inch dowel ). Consider you than if the inductance is smaller the sound is better.
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Old 27th April 2007, 08:41 AM   #24
hajame is offline hajame  Hong Kong
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Quote:
Originally posted by ppa
I have seen the amp and servo schematics , thanks to the high amp's speed the LC network's inductance should be smaller than 0,7uH. You should try L=0,5 ( 8 turns of enamel wire on a 1/2 inch dowel ). Consider you than if the inductance is smaller the sound is better.
ppa.
I've made an inductor with 12-turn enamel wire on an 1-inch dowel. I donno its inductance but I think it should be more than 0.7uH in order to make the effect more significant. But it takes time to observe the effect as I don't actually know the new speakers' tone. The photo below shows my work.
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Old 27th April 2007, 08:47 AM   #25
hajame is offline hajame  Hong Kong
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Quote:
Originally posted by Graham Maynard
Just a possibility that the crossover in your new loudspeaker has a high Q or capacitive crossover circuit without series resistors on the section involved. Do you have capacitive LS leads ?

You could try a 0.22 ohms 3 to 5 Watt wirewound in series with your amplifier output terminals (not ground as well).

Cheers ............ Graham.

A series 0.22ohm works significantly but the sound becomes tense. I am trying ppa's method snad hope it can solve the problem without changing the sound too much. Thanks anyway!
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Old 27th April 2007, 09:17 AM   #26
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Where 0.22 ohms in series with a loudspeaker can make considerable reproduction difference, the amplifier circuit itself is fundamentally flawed.
The amplifier is reacting to loudspeaker impedance.
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Old 27th April 2007, 12:09 PM   #27
hajame is offline hajame  Hong Kong
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Quote:
Originally posted by Graham Maynard
Where 0.22 ohms in series with a loudspeaker can make considerable reproduction difference, the amplifier circuit itself is fundamentally flawed.
The amplifier is reacting to loudspeaker impedance.
With the inductor network, the speakers still give me boomy 250Hz. 0.22 ohm completely cleans the sound up and I am now convinced that it is the right sound the speakers should give. The problem's solved!

Thanks, all!
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Old 9th May 2007, 12:01 PM   #28
hajame is offline hajame  Hong Kong
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Oh~
I've just noticed that I had made a very silly mistake for a long time!! I had swapped the hot outputs of the two channels of my power amp with the ground connections remained correct!!! I think the loudspeaker's return current had to go through the other channel's ground and then to the preamp,where the two channels' grounds merge, and then finally to the correct channel's power amp's power supply. This is such a long path and may interfere with the preamp's ground, right? Maybe the oscilation problem was caused by my fault but not the amp itself.

After correction, the sound is much more pleasent. Mid bass becomes thicker and treble is not as harsh as before.

Graham, I am going to remove the 0.22ohm to see if the sound will get muddy but I have soldered it inside the enclosure and it's quite troublesome to remove it. If problems don't exist, is it better to remove it for the sake of sound quality?
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