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| Phono cable and common mode noise filter? |
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| Marco:
--- Quote from: C on May 31, 2018, 08:45:26 pm ---Good differential amps --- End quote --- It's not going to make much difference for audio ... his circuit is good nuff. Just use it with twisted pair cable instead of coax. |
| Richard Crowley:
--- Quote from: FriedMule on May 31, 2018, 02:59:29 pm ---The original circuit is on this page: http://sound.whsites.net/project51.htm --- End quote --- The original circuit is correct. But you have made an incorrect connection where there was no connection in the original. The wire from GNDREF to the 5K pot does NOT connect to the negative signal node from R9 to R11 and the V- input of the op-amp. In the original circuit this is simply a "crossover" where the horizontal and vertical paths DO NOT connect together. But you have added a "connection dot" which grounds the negative signal completely. This totally kills any differential properties of the circuit. By now, you must have understood that your two unbalanced signals from your unidentified source do NOT represent a balanced source in any way, shape, or form. They are the Left Channel unbalanced output, and the Right Channel unbalanced output. If you combine them together in a differential input circuit such as you have show, you will end up with a monaural audio signal that consists of the Left Channel MINUS the Right Channel. This will sound horrid and will be useless for any practical application. You can add "pseudo-balanced" outputs to a source with only unbalanced outputs by measuring the output impedance and creating a matching impedance for the "negative side" of a balanced circuit. After measurement of the output impedance (which isn't really that difficult, even with minimal test gear), it is simply a matter of a balanced connector (TRS or XLR) and a 2-cent resistor to ground. Many pieces of pro audio gear use this scheme and although I find it slightly sleazy, it is quite effective and I can't argue with that. However you said that you are trying to reduce 30KHz interference noise? That hardly requires (or would be effectively reduced) by using a traditional balanced/differential connection scheme. We use balanced connections mostly to reduce lower-frequency noise (like 50-60Hz mains power hum and overtone buzz). Filtering out 30 KHz would be much more easily done with conventional, simple low-pass filters. I have to wonder how a 30KHz noise could really even be affecting any normal audio path as it is well above the high-frequency pass-band of even "hhigh-fidelity" audio (i.e. 20KHz) |
| David Hess:
--- Quote from: C on May 31, 2018, 08:45:26 pm ---Good differential amps are differential in & differential output. TI calls these Fully differential amps --- End quote --- If Texas Instruments had not confused instrumentation and difference amplifiers with differential amplifiers, then they would not have needed a fourth catagory. |
| David Hess:
For what it is worth, if you are suffering from ground loop induced noise, then running the single ended signal with or without a shield (1) into a high impedance input difference or instrumentation amplifier at the receiver can improve the situation considerably. The instrumentation amplifier does not care if the source is single ended or differential. There are better ways to handle this but it requires no modification to the source. This assumes however that there is a separate ground connection between the source and input to limit common mode excursions which will usually be the case when grounded power cords are used on both sides. (1) A lot of cheap audio and video cables just have twisted pair instead of coaxial construction. |
| FriedMule:
--- Quote from: Richard Crowley on June 01, 2018, 03:10:21 am --- --- Quote from: FriedMule on May 31, 2018, 02:59:29 pm ---The original circuit is on this page: http://sound.whsites.net/project51.htm --- End quote --- The original circuit is correct. But you have made an incorrect connection where there was no connection in the original. The wire from GNDREF to the 5K pot does NOT connect to the negative signal node from R9 to R11 and the V- input of the op-amp. In the original circuit this is simply a "crossover" where the horizontal and vertical paths DO NOT connect together. But you have added a "connection dot" which grounds the negative signal completely. This totally kills any differential properties of the circuit. --- End quote --- The changes did I make after this comment: --- Quote from: T3sl4co1l on May 31, 2018, 02:18:28 am ---Yes, but, you still want a lowish impedance ground on the shield, otherwise the common mode (AC, RF) voltage can be ridiculous. For the same reason, you may want common or normal mode filtering, rather than just differential. Basically just split C1 in half (two 440pF, well, use the next common value 470pF), to ground, one from each side. Tim --- End quote --- --- Quote from: Richard Crowley on June 01, 2018, 03:10:21 am ---By now, you must have understood that your two unbalanced signals from your unidentified source do NOT represent a balanced source in any way, shape, or form. They are the Left Channel unbalanced output, and the Right Channel unbalanced output. If you combine them together in a differential input circuit such as you have show, you will end up with a monaural audio signal that consists of the Left Channel MINUS the Right Channel. This will sound horrid and will be useless for any practical application. --- End quote --- To a start, I did not know if it would work so I asked here as my first question and got this answer: --- Quote from: T3sl4co1l on May 31, 2018, 02:18:28 am ---Yes, but, you still want a lowish impedance ground on the shield, otherwise the common mode (AC, RF) voltage can be ridiculous. For the same reason, you may want common or normal mode filtering, rather than just differential. Basically just split C1 in half (two 440pF, well, use the next common value 470pF), to ground, one from each side. Tim --- End quote --- --- Quote from: Richard Crowley on June 01, 2018, 03:10:21 am ---You can add "pseudo-balanced" outputs to a source with only unbalanced outputs by measuring the output impedance and creating a matching impedance for the "negative side" of a balanced circuit. After measurement of the output impedance (which isn't really that difficult, even with minimal test gear), it is simply a matter of a balanced connector (TRS or XLR) and a 2-cent resistor to ground. Many pieces of pro audio gear use this scheme and although I find it slightly sleazy, it is quite effective and I can't argue with that. --- End quote --- could you please explain it more, maybe with a small shematic? --- Quote from: Richard Crowley on June 01, 2018, 03:10:21 am ---However you said that you are trying to reduce 30KHz interference noise? That hardly requires (or would be effectively reduced) by using a traditional balanced/differential connection scheme. We use balanced connections mostly to reduce lower-frequency noise (like 50-60Hz mains power hum and overtone buzz). Filtering out 30 KHz would be much more easily done with conventional, simple low-pass filters. I have to wonder how a 30KHz noise could really even be affecting any normal audio path as it is well above the high-frequency pass-band of even "hhigh-fidelity" audio (i.e. 20KHz) --- End quote --- I was trying to filter as you write, but got the answer that is was impossible, that I only could filter HF. |
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