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Old 29th May 2007, 05:17 AM   #1
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Default Hiss in my P3a Amp

Hello!

I just finished a channel on my P3a amp and I get a loud hiss. Just to clarify, when I say hiss, I mean the sound of an FM tuner that is not on a station, or the sound of air between your teeth. The "sss" sound in the letter ess.

I only hear this when I take the input from my PC sound card. I don't hear it when I input something like a portable MP3 player. Knowing this, I thought there was a ground loop, so I disconnected the input shield from ground. This didn't help. I even tried disconnecting the star ground from the chassis (earthed). didn't work.

I'm using an Avel transformer: Parrallel input and Series output (Red and Orange as CT). Dual Power supply. I'm pretty sure I have the transformer wired right, but please make suggestions if you think otherwise.

I'm using ESP's Dual Power Supply design: http://sound.westhost.com/project04.htm

Please let me know if you have any suggestions as to what this might be. or how I can fix it.

Thanks!
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Old 29th May 2007, 11:11 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

Should simply mean your soundcard output is noisy, nothing to do with the amplifier.
Possibly you need to attenuate the PC output and therefore up the PC output level.

/sreten.
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Old 29th May 2007, 11:31 AM   #3
Did it Himself
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Agree with sreten. Most onboard sound is poor quality, especially laptops.
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Old 29th May 2007, 01:15 PM   #4
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Oh no. I mean this FM sound is LOUD. - I can hear it from the kitchen loud, even when the PC volume is turned all the way down. I used to run the soundcard to a receiver and it worked just fine.

I don't think I need a preamp do I?

Thanks,
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Old 29th May 2007, 01:32 PM   #5
Did it Himself
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A pre-amp will just amplify the hiss even more.

Did you use an ESP board for your amp? It's possibly it's oscillating if it's your own design.
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Old 29th May 2007, 01:40 PM   #6
anatech is offline anatech  Canada
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Hi blap0220,
richie00boy has a good point there. Also consider the amount of noise coming out of your sound card. Some don't properly filter the output also. This sounds like a lot of ultrasonic stuff going on.

-Chris
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Old 29th May 2007, 07:27 PM   #7
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Yaa. I'm using one of the ESP circuit boards. Tried to build exactly to spec. One thing I noticed was that I'm only using 4700uF for the power supply filter instead of 10000uF. Could this be my problem?
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Old 29th May 2007, 07:44 PM   #8
Keld is offline Keld  Sweden
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Quote:
Originally posted by blap0220
Yaa. I'm using one of the ESP circuit boards. Tried to build exactly to spec. One thing I noticed was that I'm only using 4700uF for the power supply filter instead of 10000uF. Could this be my problem?
No!

I suspect your soundcard or computer they can spitt out a LOT noise. are the computer properly connected to protective earth?

Try with another source like a CD or DVD.
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Old 29th May 2007, 11:31 PM   #9
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If my computer is causing this noise, how come my old yamaha receiver didn't have this noise? What's different? this is not a ground hum. This is the white noise you would hear when you can't find a radio station.

Thanks!
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Old 30th May 2007, 12:37 AM   #10
anatech is offline anatech  Canada
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Hi blap0220,
Quote:
If my computer is causing this noise, how come my old yamaha receiver didn't have this noise?
'cause.

Each system has it's unique characteristics and susceptibilities. Most commercial products are engineered for maximum immunity to noise like this. It's entirely possible for another brand to make noise like crazy. Just accept the fact that computers are known to be noisy little fellas. They even came out with spread spectrum clocking to reduce the noise power at any one frequency to help get around noise emission codes(electrical emi).

If your amplifier has a high enough bandwidth, it may react easier to this type of noise. Why not try an RC network on your input right at the jack to limit the bandwidth of your input signal? That may cure it.

-Chris
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