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Old 17th February 2003, 12:35 AM   #21
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I took it in to the lab and looked at the outputs. Everything seems perfect and in-phase. I am not sure what the problem is. Maybe it dosnt like my speaker cables or somthing. I will try another set of cables, and try another set of RCA cables from my CD player. We'll see if that straightens things out.

-have a good one.

-Paul Hilgeman
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Old 17th February 2003, 12:55 PM   #22
JohnG is offline JohnG  United States
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Could you be asking more of the LM3875 than it can deliver? This chip is really designed to work into an 8 ohm load, and if you take a careful look at the datasheet, you can see that it does not do well into 4 ohm loads (this is from memory, so I don't have specifics). Since you have an MTM, you probably have a 4 ohm (approximately) load over much the bass and midrange. As I recall, you can run into problems with current limiting. If this is the case, try the LM3886.

Just something to check into.

John
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Old 17th February 2003, 01:31 PM   #23
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Paul

Do you still have the "old" amp you mentioned ? Is the problem gone when you connect the speakers back to this one?

If the problem persists then I have a suspicion:

Did you ever take apart one of the speakers (I sometimes do such things out of curiosity) ?
If you, for instance, unplugged the woofers (or are they soldered ?)and wired them the wrong way again, then the described effects can happen.
Bass will only be in-phase when one of the speaker cables is wired out of phase. But then the tweeters will be out of phase which isn't contributing to good imaging at all (apart from the fact that you will have a bump or a dip in the crossover area).

Regards

Charles
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Old 17th February 2003, 02:01 PM   #24
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Nope, didn't do that either. I am an experienced speaker DIYer, but not an experienced amplifier DIYer.

Thanks,
-Paul
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Old 17th February 2003, 02:02 PM   #25
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Question Power Supply

Paul and John,

Could this be a power supply issue? At what are the voltage rails operating? If this IC is run between 25 V and 27 V it is a beast. It actually produces more power into a 4-ohm load than into 8-ohms. I think that It would pretty much rock on the Dayton III's.

If it is not the power supply, It sounds like a possible capacitance issue.

Later,
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Old 17th February 2003, 05:25 PM   #26
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Maybe that is my problem, I am using a 25-0-25 transformer, which would give me about 35 volts on each rail.

How can I lower this?

-Paul
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Old 17th February 2003, 05:44 PM   #27
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Default Voltage should do fine

Hi Poul.

Itīs a shame your amp doesnīt work
The voltage you are using should be fine, I have used a 29-0-29v trafo and driven 4ohm MTMīs without any problems, other than the chip getting hot.

Why dont you try to hardwire everything together instead of using a pcb ?

I never could get a pcb to work a good .

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Kim
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Old 17th February 2003, 07:58 PM   #28
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Default Impedance swing?

Paul,

I am far from an expert, but the DIII's impedance plot is more than 8 ohms from about 50-80Hz. I wonder if this extra high impedance peak is lowering the developed power of the amp significantly. Just an educated guess from an uneducated member!

S.
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Old 20th February 2003, 02:32 AM   #29
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Question Any word?

Paul,

Did you find and correct the problem? Just wondering.

Later,
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Old 20th February 2003, 02:50 AM   #30
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From my limited experience, it almost sounds like the speakers have a common path to ground that has too much impedance.

If you have ever lost the ground connection to your headphones you will know what I mean.

I used to build with board like this until I went beyond AF.
I use circuit board material and cut the traces i need with a dremmel tool...results in very good ground planes.
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