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Just how bad is it? Audio mixer with headphone amp.
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paulca:
Hang on.  The pre-amps have active gain.  That does not mean they are putting out 30vpp.  In fact the NE5532 would clip at around 27vpp.  The values used are not my choice I borrowed them from a known design.  They can provide (from memory) -100db to +17db.  A sinewave input set to 500mV peak to peak at max gain comes out at something like 10Vpp.  I will redo the gain tests with the scope at the weekend to get actual figures, but nothing is pushing it anywhere near 30Vpp.

The +17db will only be used when the input signal is too low and I want to lift it to an acceptable level so it can be mixed correctly with another stronger signal.

The plan is (quite 'how you doing') to scope the preamps and mark unity on the pots.  Then I might mark -6, -3, +3, +6 db.  If I'm feeling board I might do more.  They will nominally remain at 0db.

I hope to select values for the summer/mix amp to produce 0db unity.

What I am hoping to achieve is that the final master volume control (either a dual pot or another active gain control) will also sit at 0db.  The headphone amp will have fixed gain so I need to pick values that when everything else is at 0db the headphones will be "comfortably loud".  If I go for a passive master volume it should be "very loud" so I do not need to lift the inputs to satisfy output level.  That would be the master pot at the stops.  If I go for an active gain on the master I will have the ability to "move the fader above 0db" to drive the output to +17db.  In that case the 0db headphone level should be the middle ground, comfortable listening level and the extra gain used for higher listening levels.

Does that make sense?  It's hard to explain.

It's not quite a desk layout as it would have 0db max on the master faders (usually) and an independent headphone level pot which may have positive gain.  In my case there is only a headphone output and so the master and headphone level are the same thing.

As to then connecting the headphone output to another device's line in, I obviously need to consider impedance and not push the headphone output too far as to drive too much current into that receiving device.  Aren't line level inputs "meant" to have an impedance of 100K?
Bassman59:

--- Quote from: Audioguru on March 01, 2018, 09:03:34 pm ---Then what will protect your hearing and headphones if the headphones amplifier has enough power to blow your head off?
--- End quote ---

Reduced supply rails for the output driver would work, but if the amplifier has to drive some esoteric high-ish impedance headphones, that won't really work. Nor, I suspect, would any other passively-implemented trick (the diode, whatever). Paul might wish to consider using one of the THAT VCA/detector chips to put a limiter in his amplifier.

That said: no analog mixing console I can think of has ever come with a built-in limiter on the headphone amp circuit. You could take the console's control-room/cue outputs, run them through a stereo limiter which then drives a headphone amplifier. Glue the headphone amp's volume knob in place, so the only volume control is the console's cue level. This prevents turning up the level after the limiter. And then be prepared to futz with the limiter so it truly does clamp only what's "too loud." (Might as well glue in place the limiter's makeup gain knob.) Adding a limiter feature to a digital console is a "small matter of software," but I can't remember if I've run across a console with such a feature.

Professional in-ear monitor systems have built-in limiters in the belt pack. Of course they have to be set appropriately.

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.

BrianHG:
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.

Better be safe than sorry.

Same with the potential of blowing a pair of 'expensive' headphones which were designed only to receive 2.6vp-p, or, maybe even as far as a deafening 5vp-p for short durations.
paulca:

--- 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.
Bassman59:

--- Quote from: paulca on March 01, 2018, 09:42:41 pm ---Hang on.  The pre-amps have active gain.  That does not mean they are putting out 30vpp.  In fact the NE5532 would clip at around 27vpp.  The values used are not my choice I borrowed them from a known design.
--- End quote ---

Clip level and gain are unrelated. You clip when your output signal bumps up against the supply rails (or depending on the op-amp, a volt or two below the rail). Input clipping level depends on the attenuation presented to the input signal before the amplifier.


--- Quote ---As to then connecting the headphone output to another device's line in, I obviously need to consider impedance and not push the headphone output too far as to drive too much current into that receiving device.  Aren't line level inputs "meant" to have an impedance of 100K?
--- End quote ---

Standard line-level input impedance is 10k or thereabouts. It's high enough to be a bridging impedance to a low-Z driver; most output stages put a small build-out resistor (say, less than 100 ohms) between the driver and the jack for line-drive stability and output-short protection. This means you can also parallel several inputs on one output without loading down the driver. Any higher input impedance incurs a noise penalty.

Driving a line-level input from a headphone amplifier output is fine. You can't destroy the input unless the driver-side power-supply rails are higher than the input side and the input receiver device doesn't have internal clamps.
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