Electronics > Beginners
Just how bad is it? Audio mixer with headphone amp.
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paulca:
So the pre-amp board got finished.  Works a treat.  Through my existing setup I can push the headphone amp into distortion which is most likely the pair of 5532s in it limiting.  Sounds clean, but I haven't done much scoping for noise.

It is still current hungry which has me a little perlexed.  My first thought was the 100R outputs.  Which should be much higher.  However it is still current hungry when not driving a load.

I need to get a proper measure of the current as I'm only going by the PSU's consumption which is 30mA before connecting the circuit and 76mA when connected.  Driving a load with it barely moves the current consumption, maybe 1 or 2 mA.

The other thing it could be is ultrasonics making the opamps work hard, no idea.
BrianHG:
Any scope shots?  From input to output... Feed a 20Khz square wave at different gains and different source amplitudes to verify flat response.
paulca:
So nothing I have will generate anything but a slightly mangled sqaure wave over about 5kHz even at 5K it's barely square.  I tried both my phone and my PC.  The mickey mouse chinese sig gen kit I didn't even bother with as it's square output is unattenutated and about 10V peak to peak with a 5V virtual ground.

The main issue I seem to have is noise from the PSU.  Something around 80kHz but the scope is having trouble with locking onto it at times.

Images here some are worth a laugh, like my phone trying to produce 20kHz:
https://imgur.com/a/owSEF

Sample of PSU noise, straight off the negative power rail:


This was after putting 470uF caps and 100nF caps from both rails to common.

So as this appears on all op amp output I need to get rid of it somehow.  It's at such a high frequency it shouldn't bother the audible range, but it does bother me and it probably isn't helping!

Max gain, 2k sine:


Min gain, 2k sine:


Just noise on the output.  EDIT:  I fixed the none zero output, was a miss connection.  It now mutes at minimum gain... or near enough.

EDIT2:  Yes, it's annoying that the scope does not capture the right hand side with the time division read out and sample rate :(
BrianHG:
What do you have for your PC?  What's the sample rate set to in the 'Speaker Properties' on the 'Advanced Tab'.  Also, in the 'Enhancements Tab',  make sue the 'Disable all enhancements' is selected.

By default, if you never set this, usually it is set to the crummiest weakest setting.
paulca:

--- Quote from: BrianHG on February 27, 2018, 08:51:22 pm ---What do you have for your PC?  What's the sample rate set to in the 'Speaker Properties' on the 'Advanced Tab'.  Also, in the 'Enhancements Tab',  make sue the 'Disable all enhancements' is selected.

By default, if you never set this, usually it is set to the crummiest weakest setting.


--- End quote ---

The PC output is a Texas Instruments PCM2707 USB DAC, but I was trying to use a YouTube video  :-[  I might install audacity again at some point.  But I have a signal gen in the post, though it's an FY6600 so it remains to be seen if it will need modified before it's safe to use.

However the phone didn't do a very good job either.

On the PSU noise.  A quick google suggests that meerly decoupling caps will not really help and I need to "bypass" with LC circuits.  I'll do some more reading.
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