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Old 14th July 2011, 01:48 AM   #1
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Default Completed LM1875 Stereo build

Finished my chipamp kit using a 160v 18+ 18+ Avel torodial. The amp boards are bolted to an aluminum heatsink whick is in turn bolted to the base of the enclosure (thermal paste at both locations). Sounds great but I do have a "hiss" when no signal is being sent to the amp. I only have the AC inlet earth and the power supply board ground connected to the chassis ground. Should I also ground the RCA inputs to the chassis? The enclosure is from ebay and just barely fits everything.

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Old 14th July 2011, 05:09 AM   #2
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How much gain are you runnimg them at, can you post the schematic? Probably wouldn't hurt to route the input wires away from the speaker wires.

Mike
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Old 14th July 2011, 06:33 AM   #3
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Just don't run that amp at high volume for extended time, those sinks are a little small. Nice job though, how does the face look?
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Old 14th July 2011, 10:51 AM   #4
txg is offline txg  Germany
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the enclosure looks very interesting, can you tell something about the enclosure quality?
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Old 14th July 2011, 05:08 PM   #5
johnr66 is offline johnr66  United States
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You shouldn't hear any hiss unless you put your ear near the speaker. Better double check for HF oscillation. I agree that you need a larger heat sink. If the black part of the case is aluminum, I'd put the sink into good thermal contact using an insulator if V- is on the sink.

+/- 15 volts and 8 ohm load is as hard as I'd push it with that heat sink.

Don't connect the inputs to the case. They should be returned to their own small signal ground point.
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Old 14th July 2011, 07:44 PM   #6
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As far as I know Brian's boards do not have a low pass filter, something this chip realy needs.
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Old 14th July 2011, 08:47 PM   #7
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Yes the enclosure is aluminum and I bolted the heatsink to it with arctic silver so the case is also sinking. The chips are insulated from the sink and case (I will double check). Yeah the hiss can be heard from several feet away and sounds like running water or kind of "liquid"
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Old 14th July 2011, 10:35 PM   #8
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Ok so I'm an idiot. The cable I was using was causing the hiss. Swapped it out for another and it now has a very quiet hiss if u put your ear right near the speaker.
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Old 14th July 2011, 10:39 PM   #9
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by bobsagat666 View Post
Yes the enclosure is aluminum and I bolted the heatsink to it with arctic silver so the case is also sinking. The chips are insulated from the sink and case (I will double check). Yeah the hiss can be heard from several feet away and sounds like running water or kind of "liquid"
Your probably good with the heat sinks. Monitor the heat with use to be sure. With the supply voltage in use, don't use less than 8 ohm loads even with good sinking.

Without a volume pot on the input, it is like running the amp wide open with only the input bias resistor as the primary input impedance. The amp will likely pick up noise or self feedback causing oscillation. Try shorting the input to input ground to see if the noise stops. (I'm assuming the amp is not connected to anything.)

If this works, you can do a couple things. First make the input impedance is as low as possible for your application. If the amp is for portable music players, for example, you can solder a 1k ohm resistor across the input jacks to short out the weak signal of the noise. (Don't fiddle with the input bias resistor) The audio from the player won't be affected.

The engineers here will tell you to filter HF. This is the most correct answer, but may take some rework to that amp to be proper.
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Old 14th July 2011, 10:49 PM   #10
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The primary function of this amp is to power computer speakers so the input will be my iMac. With the current gain setting (R3 is 1k but I couldn't tell you the db) I am running the source (iMac) at very low volumes. Couldn't I just increase R3?
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