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Old 20th October 2012, 02:34 AM   #1
Matanic is offline Matanic  United States
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Default My first Gainclone

Greetings All,

Been reading your forum for awhile and decided to get the soldering iron out and try a gainclone.

I had a Sony AVR that was sick but it had good rear channels, the parts list said the IC's where LM3875-S, more like SK3875 same I guess just in a 5 pin so I pressed on.

Minimal design, 3 resistors and the IC's and the PSU which is what I am wondering about.

The xformer from the AVR is giving me a PSU with -37.4 / 0 / +37.4 vdc and the amp clips at low volume, plays okay at higher volume but still wants to clip. The voltage never drops below 73.5 vdc.

Do I need to throttle the PSU down some, to say 65 vdc?

Your thoughts....

Matanic
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Old 20th October 2012, 03:18 AM   #2
Einric is offline Einric  United States
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The datasheet for LM3875 shows +-35V for an 8Ω load being pretty reasonable.
The SK3875 is only a 5.5W chip amp.
The LM3875 is a 56W chip amp.
TOTALLY different IC's.
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Old 20th October 2012, 03:47 AM   #3
Matanic is offline Matanic  United States
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Ya I read the same thing about the SK3875 however these are a LM3875 in as a 5 pin TO-220 case. Trust me, there is no way these are 5.5 watt chips, the 5.5 SK3875 has 14 pins...

The PDF for the LM3875 says anything above 70vdc (35-0-35) could induce a fit ... hence my question

Note: Operation is guaranteed up to 84V, however, distortion may be introduced
from the SPiKe Protection Circuitry when operating above 70V if
proper thermal considerations are not taken into account. Refer to the
Thermal Considerations section for more information. (See SPiKe Protection
Response)
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Old 20th October 2012, 04:04 AM   #4
Einric is offline Einric  United States
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OK, Good, it sounds to me like you are getting Spiked.
I'm sorry, you could try and add an 8Ω resistor in series to the + lead on the speakers just to give an easier load to the chips.
This will lower the load and current on the chips.
That should allow the chips to operate happily, if not...there are other problems.
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Old 20th October 2012, 12:14 PM   #5
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SK3875 datasheet pdf datenblatt - Integrated Circuit Systems - 5.8W AUDIO POWER AMP ::: ALLDATASHEET :::
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Old 20th October 2012, 12:25 PM   #6
AndrewT is offline AndrewT  Scotland
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Your ~+-37V or so is well inside the +-42Vdc (84V) stated in the specification.
The 8ohm speaker load is OK and according to specification.

Is the circuit correct?

Have you fitted DC blocking capacitors in the NFB loop and at the input?
Have you fitted an RF filter at the input?
Have you fitted the Cs caps (4 of them) at the power pins of the chip amp?
Have you fitted the Thiele Output Network?
Have you tested the chip amp for output noise and output offset?
Have you checked the chip amp for no oscillation?

What about the heatsink?
How hot does the plastic package of the chip amp get?
How hot does the heatsink next to the chip amp get?
__________________
regards Andrew T.

Last edited by AndrewT; 20th October 2012 at 12:27 PM.
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Old 20th October 2012, 04:04 PM   #7
Matanic is offline Matanic  United States
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Andrew thanks for push in the right direction. I will review what I have, fix what is wrong and report back.
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