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Old 24th October 2006, 09:28 AM   #1
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Default noise problems with an OptiMOS

Hi,

I have built a Slone's OptiMOS with SMD implementation of the low-power part on a double layer PCB 160 mm x 50 mm. Input transistors are BC850C / BC860C, the SMD version of 2N5551 is MMBT5551, driver transistors are 2SC3421 / 2SA1358, output is "standard" 2SK1058 / 2SJ160. Powered by +/- 50V. The output transistors are biased to approx. 40 mA without signal.

It sounds great - very good detail, powerful bass.

The problem with it is I get an audible noise when listening at lower signal levels. If the CD player is not playing, nothing is to be heard, but when it starts, at low volume levels, noise is to be heard as high as the signal. I am afraid my amp is ringing... but unfortunatelly I don't have an oscilloscope.

I would be glad to hear some ideas what could cause such a noise, and what could I try in order to find out what is happening.

Thanx!
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Old 24th October 2006, 09:31 AM   #2
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What kind of noise? Buzzing, whistling, white noise?
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Old 24th October 2006, 09:33 AM   #3
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I would call it white noise, like the noise of a casette.
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Old 24th October 2006, 09:54 AM   #4
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Short the amp inputs, and see if the noise is still there. I suspect however that the problem is with your CD player...
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Old 12th December 2006, 01:00 AM   #5
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I have built 3 dual monoblocks of the Optimos and they are all dead quiet. The only noise is the mechanical vibration of the big transformers. They hum from time to time.

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Bill
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Old 12th December 2006, 01:37 AM   #6
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Check grounding/wiring.
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Old 21st December 2006, 05:39 AM   #7
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Which book did G.R.S. publish the OptiMOS design in? I have his Tab Electronics book and his High Power Amplifiers book, and he doesn't give names to the amps, though in the final design of the latter book, he refers to it as an optimized MOSFET design. Is this one, or is it published in the book I don't have?

..Todd
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Old 16th August 2011, 08:18 PM   #8
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As all the friends suggested check all of that, plus you could lift your inputs ground by connecting a 10 Ω resistor from inputs ground to ground. Also instead of the resistor you can put the same way a thermistor like a keystone CL 60. It has exactly the same resistance and is for 6A. Thermistors also can be used as surge supressors if you connect it on the primary of the transformer.That is from the great Nelson Pass. Check also your wiring if it passes close to the transformer it picks up noise.I hope gave you an answer.
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Old 17th August 2011, 04:08 AM   #9
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Have u connected the shield on both sides of the interconnecting cable...

Gajanan Phadte
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Old 17th August 2011, 10:56 AM   #10
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Yes absolutely , otherwise there is no connection.
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