| Electronics > Beginners |
| Just how bad is it? Audio mixer with headphone amp. |
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| paulca:
--- Quote from: BrianHG on March 01, 2018, 10:04:19 pm ---Hmmmm, 3 inputs. 0db gain through 1 input if you set volume to max. That's 2.6vp-p. Say all 3 inputs hit a bass drum at full volume, since you are mixing, that's now 7.8vp-p. Let's say you want a little gain at each input, like many Windows apps have low volume, like a lot of youtube recordings, say you allow for +6db gain peak on each input. That's potential occasional 15vp-p on your line out. Now, consumer amps, which have 50k input impedance, are expecting 2.6vp-p, maybe they can withstand 5vp-p before you might damage their input devices, lets be even more generous and say 10vp-p. Sending them 15vp-p might blow their input, and it only takes a single pulse. Of, if said amp feeds that through, amplified to the speakers, what will happen there. --- End quote --- There are a lot of "says" there. Which could all be countered by "why". Why would you add 6db gain to an already loud signal and why would you add 6db to all channels? Anyway, assuming it was by mistake, the input gains where all turned up full and all 3 inputs kicked off at the same time. Maybe, taking another page from (very) basic mixers, a "clip" light on the output of the mix amp. A basic red LED which comes on at some fixed Vpp. It could just be a mosfet or similar with it's gate circuit set to trigger the LED at, say 5Vpp. When that starts flashing with the beat I know my mix is just a little hot, if it lights up permanently it's too hot. Of course that could be complimented with similar lights on each input. I could even go as far as having two lights. A green set to come on at (whatever amounts to 0db line level) and a red set to come on at "too much". At some point I'm going to have to bring Moscow out. Must, Should, Could, Won't this time. |
| Bassman59:
--- Quote from: paulca on March 01, 2018, 10:04:54 pm --- --- Quote from: Bassman59 on March 01, 2018, 09:56:55 pm ---The reality is that a live-sound or studio mix person learns really early on that before you hit the solo/PFL button, your hand is on the cue volume knob and it's turned down. It's really the only way to prevent damage to ears, headphones and studio monitors. I know a guy who used a stereo volume pedal when mixing monitors -- it let him keep his hands on the channels he was adjusting while giving him quick access to his cue-wedge volume. --- End quote --- My brother's old spirit folio had a 1Khz test tone on the master. You can imagine how much fun that was if you hit it accidentally though a 2K PA. It wasn't much fun on headphones either. I believe in the end he super glued it OFF. --- End quote --- My iPod had a series of 0 dBFS test tones on it, until the time the random shuffle sent a tone through the PA. That was hilarious. Actually, the worst control on a mixing console is the "Solo-In-Place" button. It's so dangerous that on some high-end consoles it was put under a shield that you had to lift before you could press the button, kinda like the button that launches the nukes or a History Eraser Button. What does it do, you ask? Normally, your headphone/cue bus gets its signal from the main mix bus. (Some consoles let you select the default source.) When you press the Solo/Cue/PFL ("pre-fader listen") button, the cue bus gets its signal from the selected channel, so you can listen to that signal in isolation. It's incredibly useful. You can cue up effects before you put them into the mix, you can listen to one instrument while the whole band is going, all that. When you enable the Solo-In-Place mode, the cue bus drives the main outputs, replacing your mix with whatever you've cued up. Imagine that during a show, you go to cue up the kick drum in the cans, and SIP is enabled, and now all your audience hears is kick drum (and stage wash). Yikes! So the button gets a special cover, and the cover gets taped down, and even though it's a useful feature during sound check, nobody ever uses SIP. |
| BrianHG:
No, I don't mean using the leds as indicators.... I mean using the LEDs as zener diodes. Take the output of you line out op-amp, feed it through the series 100 ohm resistor to prevent potential oscillation from the line out cable. And tie 2 red 2.1v type LEDs in parallel, 1 forward, 1 backward, to GND. The led window up to around 1.5v, or 3vp-p, has a current in the picoamp level and capacitance under 10pf. This roasts most zener diodes, as their reverse bias current is in the high microamps and at low voltages, they have capacitance upward of 500pf. Then as the voltage goes up, the red led slowly begins to illuminate and the current softly rises to around 10ma at around 1.8v, 3.6vp-p, around 2v, 4vp-p at 20ma, past the drive range of your op-amp. Your aren't supposed to see these, they should be SMD Leds hidden on you pcb just before the output jack. They will give you a clean unhindered 3vp-p output, and for louder signals, they will smoothly curve limit the output to 4vp-p. This is a cheap clean mechanism to voltage limit your output. Normal clipping diodes wont have the rounding effect, if you string enough signal diodes in series to limit you output to 3vp-p, it will flatten there just like an op-amp reaching it's supply rail limit. If you use 2 zener diodes in series, their capacitance changes with voltage across each diode and the low voltage ones have high capacitance, though, at 20khz, I doubt they will affect the sound much. As for their current curve as they approach their zener voltage, they will sit somewhere inbetween the sharp small silicon diodes and the soft leds. You can use Green Leds for even a softer regulation knee and you get a limiter which begins to cap at around 5vp-p. Good for +4dbu support, with still a little extra range support. (I use Green Leds on my Subwoofer project to prevent shooting your woofer out of it's case...) |
| Bassman59:
--- Quote from: BrianHG on March 01, 2018, 10:31:46 pm ---You can use Green Leds for even a softer regulation knee and you get a limiter which begins to cap at around 5vp-p. Good for +4dbu support, with still a little extra range support. (I use Green Leds on my Subwoofer project to prevent shooting your woofer out of it's case...) --- End quote --- It’s more fun when the subwoofers catch on fire! |
| paulca:
So I spent an hour with LTSpice and hacked out a two channel version of the amp. It all goes well until I get to the split for MASTER and AUX outputs. The both output the signal okay, but no matter what I do with the feedback resistor on the master op amp it reads exactly the same as the Aux output. I've tried with and with ground references on the + input. I've tried with a single resistor on the mix amp output and individual resistors on the output amps. Something fishy is going on. Any ideas? EDIT: Never mind, it was the master amp that was broken, R17 (Rf) is useless without a resistor to ground to actually divide the feedback. Doh! |
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