bcarso
Well-known member
I have been working on an amplifier with a discrete component input stage. I did some noise estimates and figured that the design would be adequate for the application.
When samples began arriving I found a ton of excess low frequency noise. After many red herrings it was traced to excess noise in surface-mount 0805 resistors. A breadboard with axial parts was 25dB (!) quieter in a roughly 5Hz-400Hz noise bandwidth, compared to substituting an SM part in what was the most sensitive point in the circuit.
I then took four pieces of the SM part and connected them in series-parallel to get the same value but cut the current density through and voltage drop across each in half. The noise dropped by 6 dB. By the way, the power dissipation is small (about 7.5mW for a 1/10W part).
These resistors are standard 1% Ruthenium oxide parts from R*hm. I tried some alternate parts in 1206 that were just as bad. I have Yaeg* and P*nasonic parts on the way to test.
I wanted to share this experience and solicit comments, especially if any of you have had similar problems. The datasheets I've looked at so far for SM resistors have no information on an excess noise index.
This effect may account in part for the perception that many surface-mount boards in products have problems with sound quality. Even if there is little or no d.c. running through the parts, you will get excess noise riding on the signal when there is a voltage drop across the parts.
When samples began arriving I found a ton of excess low frequency noise. After many red herrings it was traced to excess noise in surface-mount 0805 resistors. A breadboard with axial parts was 25dB (!) quieter in a roughly 5Hz-400Hz noise bandwidth, compared to substituting an SM part in what was the most sensitive point in the circuit.
I then took four pieces of the SM part and connected them in series-parallel to get the same value but cut the current density through and voltage drop across each in half. The noise dropped by 6 dB. By the way, the power dissipation is small (about 7.5mW for a 1/10W part).
These resistors are standard 1% Ruthenium oxide parts from R*hm. I tried some alternate parts in 1206 that were just as bad. I have Yaeg* and P*nasonic parts on the way to test.
I wanted to share this experience and solicit comments, especially if any of you have had similar problems. The datasheets I've looked at so far for SM resistors have no information on an excess noise index.
This effect may account in part for the perception that many surface-mount boards in products have problems with sound quality. Even if there is little or no d.c. running through the parts, you will get excess noise riding on the signal when there is a voltage drop across the parts.