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Just how bad is it? Audio mixer with headphone amp.
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Audioguru:
Why does each input use two opamps and many parts to have a fairly low input impedance, a gain of only 1.32 times and pass earthquake frequencies down to 0.17Hz?
I simplified it but it still has a fairly low input impedance, a gain of only 1.32 times but it cuts frequencies below 14Hz.
I would probably simply use a piece of wire. Here it is simplified: 
paulca:
On headphone output levels.

The headphone spec says, the max power is 1.6W at 1Khz.  Is 1Khz likely to be the worst case?

Anyway directly driving the 38 Ohm headphones 7Vrms gives me 1.3W output with 184mA.  Sounds awfully high and requires 20Vpp through 38Ohms.

I found this test report for the headphones:
http://reference-audio-analyzer.pro/en/report/hp/audio-technica-ath-m50x.php

Which gives their sensitivity as 114.6db / V and their Vrms needs for 96db as 0.09Vrms.  ~2.6Vpp

I had a read through this as well, on headphone outputs:
http://www.apexhifi.com/specs.html

I'd like have at least 110db, concert level, but 130db is the maximum.

Using the spreadsheet in the last link, for a sensitivity of 114.6db / Vrms, into 38 Ohms, aiming for 130db, it gives me 912mW, 5.89Vrms and 155mA around 16 Vpp!

Trying something a little lower gives:
110db gives 0.59Vrms - less than 2Vpp
120db gives 5.9Vrms (yes I see the pattern) - ~17Vpp

So it seems due to the log10 nature of things small increases above 110db result in large changes in voltage. So if I go too low I have quiet headphones, if I go to far I have a dangerous (to other equipment) potential voltage on my headphone out.

As BrianHG said, the line level 2.5-2.7Vpp sounds reasonable.... but do I want reasonable?  5Vpp sounds better, giving close to 120db.

Thus, if I keep things around 2.5Vpp from input gain, summer, the headphone amp needs a gain of 2 and the ability to drive at least 50mA.  That would be 5 5532s in parallel to be audiophile, 3 would probably work.  Or more likely one of those OPAs.
paulca:

--- Quote from: Audioguru on March 03, 2018, 05:58:24 pm ---Why does each input use two opamps and many parts to have a fairly low input impedance, a gain of only 1.32 times and pass earthquake frequencies down to 0.17Hz?
--- End quote ---

Because nearly every circuit you find with a search on "Active volume control" will have a buffer amp.  To answer that further I would have to go back and read the whole chapter on volume controls again.  The answer is most likely because someone far more knowledgeable about these things than I proposed it.

Also, on the gain, remember that R2 and R7 are the volume control!  They will go to 0R and 10k, or 10K and 0R.

Reading this however:
http://objectivesounds.co.uk/articles/active-volume-controls/

Suggests the circuit was designed to have high input impedance was due to accomodate valves.  I'm not sure of this, the link is an only source with 1 amp for an active volume control.

Their simplification to 1 amp does not resemble yours however.
BrianHG:

--- Quote from: paulca on March 03, 2018, 06:02:44 pm ---On headphone output levels.

As BrianHG said, the line level 2.5-2.7Vpp sounds reasonable.... but do I want reasonable?  5Vpp sounds better, giving close to 120db.

--- End quote ---

This was for line level.  Using this level out of my PC sound card, my headphones burn my ears, though, 5vp-p would sound better.  Again, if you have a series 32ohm resistor in the line, then. double this to 10vp-p with no load, or at the op-amp output.  Note that the damping factor at low frequencies in the headphones improves without that series resistance.  If you need to overshoot a club/dance scene, like a DJ listening preping up the next track to mix, double this voltage again giving you the +10db out.  This can potentially burn out some consumer grade headphones and make you go death.

Anyways, stick with OPA551 for the premo-grade quality headphone amp output.  5x NE5532s in parallel will just consume a ton of extra current on you power supply and be lower quality sound due to slight timing differences and routing between each parallel IC.  In fact, if you want to drive a killer line level out, add another 2 there.  Use 100 ohm in series from each spare auxiliary cloned parallel output to isolate each one instead of using multiple ones.
paulca:
The series out resistors, what are the pros and cons? 

I gather if I put a 40R or similar in series with the output I can just do the sums again for 40+38 Ohm load... ?

For the buffered line out, I should aim a bit higher for the series resistor, say a 2.2K since I have used them elsewhere... ?

I have no intention of making myself deaf... or death ;)  I just don't like people telling me how loud my headphones are allowed to go.  I want them to not be at 100% volume.  I want 100% volume to be too much.
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