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Old 8th October 2018, 10:08 PM   #61
wrenchone is offline wrenchone  United States
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Distortion measurements still haven't happened, but the headphone amp made a trip to Burning Amp 2018, powered by a bench supply. I used my AKG 240 Studio headphones for the demo. The final version of the amp will be powered by a hefty 19V laptop adapter, with a discrete DC-DC converter to provide the negative supplies. I only need 5V RMS, 7V peak to provide earsplitting volume (0.5W RMS), so a supply rail diet will make heat sinking much more simple.

After some initial hiccups, the DC-DC converter is humming along nicely on my bench.
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Old 16th October 2018, 12:40 PM   #62
sgrossklass is offline sgrossklass  Germany
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Originally Posted by wrenchone View Post
The final version of the amp will be powered by a hefty 19V laptop adapter, with a discrete DC-DC converter to provide the negative supplies.
Uh-Oh. Ever heard of a can of worms?

You may have all kinds of fun with ground loops and EMC there, depending on how the notebook power supply is built (they vary widely) and how good your DC/DC layout is. The CD player you mentioned earlier seems to be an IEC Class I device (why do people do this in consumer audio? WHY?), don't be surprised if you encounter trouble with unwanted noises. (Same as an ordinary PC basically.)

Assuming you can afford it in terms of space, a conventional split secondary transformer supply (preferably with a shield winding if beyond 20-30 VA) followed by some capacitance multipliers would be a lot less problematic and more robust. Efficiency actually is not that bad given a good-quality transformer, maybe some Schottky rectifiers and a moderate (but still adequate) dropout on the cap multipliers. Nothing beats a conventional transformer in terms of isolation from mains and PE.

Last edited by sgrossklass; 16th October 2018 at 12:45 PM.
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Old 16th October 2018, 01:13 PM   #63
Mark Johnson is offline Mark Johnson  United States
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Discrete Headphone Amp.
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Originally Posted by sgrossklass View Post
Nothing beats a conventional transformer in terms of isolation from mains and PE.
Batteries do a pretty good job too. When I listen to my battery powered O2 headphone amplifier, there is no mains hum at all. Even when I crank the volume control to maximum.
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Old 17th October 2018, 01:38 AM   #64
wrenchone is offline wrenchone  United States
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I've only had 38 years of experience designing SMPS, so I guess I know nothing about layout or ground loops or anything of that nature...
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Old 17th October 2018, 11:06 AM   #65
sgrossklass is offline sgrossklass  Germany
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OK, so we'll assume that your DC/DC should be fine then - but how well do you know the 19V supply (which I assume is an off-the-shelf part)? What you want is a mains filter going to PE (I assume "hefty" means >75 W?), a transformer with shield winding, and not an awful lot more than maybe a kOhm and something nF-ish between PE and secondary-side ground. Copious amounts of common-mode chokes never hurt, but I guess you know that...

I still don't know how much I'd trust something like this as an audio supply, but it should at least work OK.
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