GC's and noise

pjpoes · 9th September 2005, 03:54 PM

I have heard various people talk about things such as picking up rf and other forms of noise, and had some questions about that.

First, what sorts of things should I do to keep that minimized. I do pick up radio sometimes, but very little.

I dont have hum like an obvious ground loop, but a very faint amount does exist. Nothing you can hear from the listening chair, or even a foot away from the speaker, but it does exist. Is this something you live with, or can little tweaking here and there fix that.

How about hiss, I have a bit of hiss. Again, not enough to be heard easily from my listening chair, and not enough to hear over the noise floor of most music I have, but it does exist. Can this be reduced, or is this also just a common issue with this design.

These amps are noisier than my modified acurus amps, and I dont believe the mods made the noise floor much lower, a little maybe.

jackinnj · 9th September 2005, 04:14 PM

RFI picked up on the input nodes is easily reduced by placing a 200pF cap across the Vin+ and Vin-. I have also used 150pF to ground on each pin in other amps.

you can also get RFI through the power supply if it is not properly decoupled AND your power supply can generate RFI through the junction capacitance of the rectifier diodes and inductance of the transformer secondary. (This is why I don't like those designs which put the diodes in close proximity to the amplifier chip.) This type of RFI can be in the tens of kHz to a few hundred kHz -- it is possible that it is a cause of unexplainable DC offset in some designs (you'll have to go to Analog Devices website for the explanation.)

pjpoes · 9th September 2005, 05:49 PM

do you know what I have to do to measure dc offset. I want to say I read that you just put an 8ohm resistor across the output leads and measure dc voltage across that. Is that correct. Do I have to short the input or anything.

h_andree · 10th September 2005, 11:01 PM

I had some faint his and a slight hum. Connecting the
ground of all power supply together and connecting it
to the ground pin of the power reduced it from 3 mV to
0.3mV (Multimeter).

Harry

pjpoes · 11th September 2005, 08:31 PM

is that with measuring the dc offset? Did you do as I had said, is that infact how its done. I have an 8ohm wirewound sandcast resistor I could use to measure that.

jackinnj · 11th September 2005, 09:14 PM

all the opamp tests you'll ever need, the first one should help you out.
http://www.intersil.com/data/an/an551.pdf

9th September 2005, 03:54 PM	#1
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	GC's and noise I have heard various people talk about things such as picking up rf and other forms of noise, and had some questions about that. First, what sorts of things should I do to keep that minimized. I do pick up radio sometimes, but very little. I dont have hum like an obvious ground loop, but a very faint amount does exist. Nothing you can hear from the listening chair, or even a foot away from the speaker, but it does exist. Is this something you live with, or can little tweaking here and there fix that. How about hiss, I have a bit of hiss. Again, not enough to be heard easily from my listening chair, and not enough to hear over the noise floor of most music I have, but it does exist. Can this be reduced, or is this also just a common issue with this design. These amps are noisier than my modified acurus amps, and I dont believe the mods made the noise floor much lower, a little maybe.

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9th September 2005, 04:14 PM	#2
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	RFI picked up on the input nodes is easily reduced by placing a 200pF cap across the Vin+ and Vin-. I have also used 150pF to ground on each pin in other amps. you can also get RFI through the power supply if it is not properly decoupled AND your power supply can generate RFI through the junction capacitance of the rectifier diodes and inductance of the transformer secondary. (This is why I don't like those designs which put the diodes in close proximity to the amplifier chip.) This type of RFI can be in the tens of kHz to a few hundred kHz -- it is possible that it is a cause of unexplainable DC offset in some designs (you'll have to go to Analog Devices website for the explanation.)

9th September 2005, 05:49 PM	#3
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	do you know what I have to do to measure dc offset. I want to say I read that you just put an 8ohm resistor across the output leads and measure dc voltage across that. Is that correct. Do I have to short the input or anything.

10th September 2005, 11:01 PM	#4
h_andree diyAudio Member Join Date: Apr 2003 Location: New York	I had some faint his and a slight hum. Connecting the ground of all power supply together and connecting it to the ground pin of the power reduced it from 3 mV to 0.3mV (Multimeter). Harry

11th September 2005, 08:31 PM	#5
pjpoes diyAudio Member Join Date: Jan 2005 Location: New York	is that with measuring the dc offset? Did you do as I had said, is that infact how its done. I have an 8ohm wirewound sandcast resistor I could use to measure that.

11th September 2005, 09:14 PM	#6
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	all the opamp tests you'll ever need, the first one should help you out. http://www.intersil.com/data/an/an551.pdf

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