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Old 28th August 2006, 10:33 AM   #101
sachi is offline sachi  United States
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My reworked 7 channel chip amp with a 3 way active crossover and 2 passive pres in the same chassis.
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Old 28th August 2006, 10:47 AM   #102
sachi is offline sachi  United States
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This is how it was before i blew one of the transformer....




The amp is now much better laid out than before and there are no hums or hiss noises.

There are also no heating issues.
----------------------------------------

Parts that were made redundant
Transformer for the xover was removed and extra windings were made on the new tranny.
---------------------------------

The parts that were blown were...

Transformer-1 (350VA)(replaced with a similar rated one)
Bridge rectifiers-2( replaced with International Rectifier ones)
Capacitors..6X4700uF(replaced with 2x13000uF)
One LM4780 board(mids amp module)
replaced both the mid amp modules with LM4732 amps.
----------------------------------------

The entire amp was cleaned out..stirpped off all components laid bare and re built from scratch..

The position of the crossover too was changed to place it in the center so as to allow easy access to the trimmer pots.

The height of the Xover PCB spacer was increased..the board is now 50mm from the base plate of the amp.

The layout could have been better..much better but for the fact that the mouting of the ICs did not facilitate a neater layout..
Ideally a single tranny with only one wire from the capacitors to the amp modules would have been desired(that was the idea actually).

Since i had already drilled holes for the ICs and redoing them involded too much work we elected to try to make the best of it.

I have two Lm4780s in bridged mode[lows]

Two LM4732s in bridged mode[mids]
and two LM3886s in Non inverted mode[highs].
and a single OPA541 for the center channel.

Previous amp had some oscillation problem when i played around with the gains on the xover...no such problem now .



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Old 28th August 2006, 01:46 PM   #103
luka is offline luka  Slovenia
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This is my car amp in ATX power supply case. Amp is LM3886
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Old 28th August 2006, 01:48 PM   #104
luka is offline luka  Slovenia
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...and...
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Old 28th August 2006, 01:49 PM   #105
luka is offline luka  Slovenia
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..and finally
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Old 28th August 2006, 01:59 PM   #106
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Quote:
Originally posted by luka
This is my car amp in ATX power supply case. Amp is LM3886
You might want to see if you can slip a piece of PCB material in between the toroid and the PCB as shielding.
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Old 28th August 2006, 02:03 PM   #107
luka is offline luka  Slovenia
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Now that is not possible, but why would I do that? Under the toroid is only gnd or air.
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Old 28th August 2006, 02:27 PM   #108
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All transformers radiate -- in the case of switching supplies the radiation is very pronounced and will seriously affect the performance of the amplifier -- any amplifier.

So you have a trade-off between shielding the space between the amp PCB and power supply on one hand, and switching noise, unpredictable offset voltage etc. on the other.

See Linear Technologies Application Note 70.
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Old 28th August 2006, 02:33 PM   #109
luka is offline luka  Slovenia
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does this cupper between trafo and pcb have to be grounded?
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Old 28th August 2006, 02:40 PM   #110
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No, not really --

If you can access an oscilloscope take a look at the output waveform with and without a piece of shielding -- on the Hafler DH200/220 mods thread I pointed out that inserting shielding between the transformer and the low-level circuit boards cut THD by an 0.001% -- in the case of the Hafler 220 which I was working on I tried to reduce the transformer radiation -- remember we are working with a 60 Hz rectified sine wave so the level of problems are much lower than the square wave from a switcher.

I am making a leap of judgement here, assuming that you are using an SG3525 switching regulator.
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