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| Just how bad is it? Audio mixer with headphone amp. |
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| Audioguru:
When you have too many level adjust pots then one or two of them always end up in the wrong position. Everything you said about gain and noise applies to a microphone mixer that you are not making. A line level mixer does not need gain here, there and everywhere and makes inaudible noise. |
| paulca:
--- Quote from: Audioguru on February 13, 2018, 12:32:56 pm ---When you have too many level adjust pots then one or two of them always end up in the wrong position. Everything you said about gain and noise applies to a microphone mixer that you are not making. A line level mixer does not need gain here, there and everywhere and makes inaudible noise. --- End quote --- So every audio mixer is wrong. Every sound engineer friend I have is wrong. --- Quote ---Set the master fader to the bottom of the sweet spot region (e.g. -5 dB). Set the channel faders for each input in the sweet spot, usually at the 0 dB point. Unmute the inputs and dial up a rough mix using the preamp input gains as if they were faders. Let the preamp gains fall where they may. Now, forget about the preamp gain knobs and focus on mixing the show using the faders. All the faders are now near zero dB for maximum mixing finesse. --- End quote --- http://www.fohonline.com/current-issue/74-tech-feature/10445-back-to-basics-gain-structure.html I can find hundreds of references if you want. Gain is added at the input. Always is. With your approach, if one input changes, all inputs need to change to balance the output. This might be fine in my case where I might have one or two inputs active at a time, but if you have a 24 channel desk and one line feed from a keyboard is low it would be completely ludicrous to alter the mix of the other 23 channels to rebalance the mix. |
| paulca:
The other consideration is noise. Every gain stage amplifies noise. If you attenuate a signal you do not lower the inherent SNR of the equipment. Attenuate it enough you will lower it into the noise. When you then amplify that signal you amplify the noise as well, therefore you have just cut your SNR dramatically. So the approach of attenuate then amplify is backwards and ruins you signal to noise ratio. You want to lift your signals away from the noise ONCE then attenuate them to the desired level. Running a desk with the faders above 0db suggests a bad mix, for that reason. This master mix output is not gained again until it goes to the power amps or the recording equipment. In a recording studio the line and mic inputs will be gained only once. (excluding guitar pre-amps, amp heads, active mics, electronic instrument line level output amps). |
| Audioguru:
--- Quote from: paulca on February 13, 2018, 12:52:43 pm ---With your approach, if one input changes, all inputs need to change to balance the output. This might be fine in my case where I might have one or two inputs active at a time, but if you have a 24 channel desk and one line feed from a keyboard is low it would be completely ludicrous to alter the mix of the other 23 channels to rebalance the mix. --- End quote --- Nope. When the mix opamp has a gain of 10 times then most input level controls are set to halfway on their logarithmic level controls. If one input is low then simply turn up its input level control without touching the other input level controls. The boost of up to 10 times is 20dB which is plenty. Audio opamps are low noise and are used with a gain of 200 times or more for microphones. With a gain of only 10 times in the mix opamp then its output noise level is 20 times less (-26dB). Inaudible. |
| dmills:
Dangerous Audioguru... In a virtual earth summing stage the noise gain is proportional to the number of inputs, so while not a problem with a small number of inputs the summing amp on a large desk can very easily become a noise problem, to the point that actually breaking the desk down into a tree of summing stages can actually result in lower total system noise. I would note that with even halfway respectable opamps (Which the 5532 easily are), operating in a suitable impedance environment, the noise from the RESISTORS usually exceeds the noise from the opamps. I highly recommend that the OP obtain a copy of "Small signal audio design" by Douglas Self for more then they ever wanted to know about how to design audio doings. Regards, Dan. |
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