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Old 29th March 2005, 04:51 PM   #1
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Location: Rochester, MN
Default Clipping GC?

I just got done fixing my GC (thanks to all you who helped), and now it can output at much higher volumes. I had R1 and Rnfb reversed, so, 1x signal amplification, D'oh!!!

However, there seems to be a different problem:

Speakers are 91 db sensitive, 8ohm , MTM, two 5.25 drivers, 2 mids and one dome tweeter, rated at 100W
rectified voltage is around 34 volts, 330va supply (Avel Toroid)
Measured SPL from about 1 meter peaks around 93-94 db in room, and then the distortion will occur.

It really is promient on a punchy midrange/lower midrange notes. I turn it down as soon as I detect it though, never prolonged.

Before this distortion though, it sure does sound nice!

I have no reason to believe that my speakers are affected, but, its not out of the realm of possibility.
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Old 29th March 2005, 06:29 PM   #2
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You only say the trafo you used.
Can you describe the complete PSU?
What caps?

Also, 3-way speakers, multiple drivers, probably "complex" crossovers... your amp needs a zobel.
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Old 29th March 2005, 06:32 PM   #3
joensd is offline joensd  Germany
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Quote:
What caps?
That was my first thought as well.
~50W should give plenty o´ beats with 91dB speakers.

Realize then snubberize.
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Old 29th March 2005, 06:36 PM   #4
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Quote:
Originally posted by joensd

That was my first thought as well.
~50W should give plenty o´ beats with 91dB speakers.

Realize then snubberize.
Indeed.
But 91db "quoted" sensitivity only says that they can play loud.
Loud doesn't always mean good.
It has to be LOUD AND CLEAR. (c) tbla.
They can still be a tough load for an amp, if they have big impedance dips in the bass.
Very common these days...
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Old 29th March 2005, 06:52 PM   #5
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Sorry, I'm using the basic lm3875 from www.chipamp.com, 1500 uF 50v Panasonic FC Capacitors.

Here are the speakers:

Atlantic Technology 371 LR
Specifications
Manufacturer's Suggested Retail Price $1,499.00
Type Sealed-box, 3-way D'Appolito
Frequency response 80Hz - 20kHz +/- 3dB
Driver complement 1'' silk dome tweeter, two 3.5'' midrange, two 6.5'' woofer
Nominal impedance 8 ohms
Sensitivity 91dB
Recommended amplifier power 100 - 200W RMS
Cabinet finishes -
Dimensions 7.5''W x 23.65'' x 11.375
Weight 27 lbs.
THX certification Yes

I have the parts to make the Zobel network, it came with the kit.
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Old 29th March 2005, 06:55 PM   #6
joensd is offline joensd  Germany
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If the speakers have an impedance dip of let´s say 2Ohms it could indeed be the Spike protection kicking in causing distortion.
(let alone an 8Ohm speaker shouldn´t have a dip of 2Ohms; but well manufacturers quotes...)

I´d try more capacitance first (if you use 1000u for example).
Stick another 4700u or more on each rail and listen.
If it´s not that try to get an impedance graph of your speakers.
And then, well probably put another chip in parallel...
(but I don´t think you´ll have to)
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Old 29th March 2005, 09:08 PM   #7
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Quote:
Originally posted by drfrink24
Sorry, I'm using the basic lm3875 from www.chipamp.com, 1500 uF 50v Panasonic FC Capacitors.

Here are the speakers:

Atlantic Technology 371 LR
Specifications
Manufacturer's Suggested Retail Price $1,499.00
Type Sealed-box, 3-way D'Appolito
Frequency response 80Hz - 20kHz +/- 3dB
Driver complement 1'' silk dome tweeter, two 3.5'' midrange, two 6.5'' woofer
Nominal impedance 8 ohms
Sensitivity 91dB
Recommended amplifier power 100 - 200W RMS
Cabinet finishes -
Dimensions 7.5''W x 23.65'' x 11.375
Weight 27 lbs.
THX certification Yes

I have the parts to make the Zobel network, it came with the kit.

Just looking at this I say: naturally you have a bass as loose as a mad cow.
You need to go high capacitance PSU, snubberized.
And you must use the zobel.
Also, what speaker cabels are you using?
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Old 29th March 2005, 09:19 PM   #8
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Actually, I have the snubberized PCB, but, very little components for it.

For speakercable, its just some stranded monster cable I had lying around. This is a bedroom system.

I could try the Zobel stuff, would be easy to implement.
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Old 29th March 2005, 09:21 PM   #9
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Quote:
Originally posted by drfrink24
For speakercable, its just some stranded monster cable I had lying around.
Stranded?
You need the zobel.
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Old 29th March 2005, 09:36 PM   #10
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in the MTM configuration the load impedance is nominally 4 ohms, so the output is current limited -- this will show up as a flag on the National Semiconductor interactive design tool:
http://www.national.com/appinfo/audi...gn_Guide15.xls

try limiting the supply voltage to +/- 26V, you are really cooking the chip with a 34V supply and a 4R load.

the heat sink should have a thermal impedance of 2.6 or less.

i don't know why a zobel would aid the situation -- it is really quite a lot less complex.
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