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The Objective2 (O2) Headphone Amp DIY Project
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Old 19th September 2011, 07:54 PM   #431
Alexium is offline Alexium  Ukraine
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Originally Posted by RocketScientist View Post
There are two amplifier sections in parallel.
Ouch, I've missed that
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Old 20th September 2011, 09:36 PM   #432
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FYI: At least one user has already reported a bad MOSFET (Q1) which was likely damaged by ESD (static). It's really important to follow the tips about handling and soldering the MOSFETs in the Circuit Board Construction section.

This points to one more advantage of the desktop board: No MOSFETs.
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Old 21st September 2011, 05:25 AM   #433
Alexium is offline Alexium  Ukraine
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I suppose one could eliminate Q1 and Q2 MOSFETs and replace it with a bridge, if battery operation is not intended?
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Old 21st September 2011, 06:20 AM   #434
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I suppose one could eliminate Q1 and Q2 MOSFETs and replace it with a bridge, if battery operation is not intended?
No, please read:

Circuit Board Construction

Circuit Description

Cautions

The power circuit is necessary to reduce power on/off transients even with AC-only power. I've tried to make that as clear as I can. It's even on the schematic.
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Old 21st September 2011, 06:29 AM   #435
Alexium is offline Alexium  Ukraine
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Oh yes, sorry, you did make it very clear., it's just my short memory. I really forgot about those transients.
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Old 21st September 2011, 11:15 PM   #436
counter culture is offline counter culture  United States
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The half wave supply in the O2 generates some fairly significant peak charging currents in the primary diodes, filter caps, PCB traces, etc. That's because the caps "droop" a lot further between charging pulses than in a full wave supply. While the regulators do a great job of removing the resulting ripple from the rails they do nothing for the electromagnetic and E-fields (EMI) the charging pulses generate.

The wall transformer already has a 5 - 10 ohm effective impedance limiting the peak current of the spikes. Trying to limit it further isn't practical but I'm open to suggestions if I'm missing something.
The majority of the radiation is probably taking place from the lead from the wallwart to the O2 itself, it's probably the most effective radiating antenna in the assembly, most of the other conductors carrying those currents are pretty small, and inside the box anyway. The pickup would then be primarily on the leads to the input. The fact that the problem is reduced by grounding the input to the box tends to support this, the RF probably stays on the outside of the box when this is done. You could take a few turns of the lead from the wallwart through a ferrite close to where it enters the box, which will reduce the RF getting onto the cable and radiating. A passthru ferrite on the PCB at the input might help too.
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Old 22nd September 2011, 12:19 AM   #437
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Thanks Counter Culture. I agree the power "cord" is part of the problem. But this isn't an RF problem. It's 60 hz electro magnetic radiation with a smaller 120 hz component. A ferrite will do nothing at power line frequencies.

The good news is grounding the enclosure, and getting rid of the cap on the AC input, has effectively eliminated the problem.
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Old 22nd September 2011, 02:00 AM   #438
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What is radio if not 'electro-magnetic radiation'?

Of course you do get induced hum, but this is is usually where the conductors are in close proximity and the field strength obeys an inverse cubic law, as opposed to inverse square, and you wouldn't fix that by grounding to the enclosure.

120Hz doesn't propagate much, but then it's a switching waveform, so the frequencies involved are related to the risetime, not just the pulse repetition frequency.

'Course I haven't seen what the effect is at the audio output.

It's a comparatively trivial current.

You have to wonder why the problem doesn't show up in 100W amplifiers driving speakers with much bigger PSU currents despite the full-wave rectification, but mostly they have the tx next to the diodes and caps, not much of a radiating antenna.

Still, if you've fixed the problem, who cares what the cause was...
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Old 22nd September 2011, 02:37 AM   #439
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@counter culture, I guess it's semantics, but most would consider 60 hz an audio frequency, not a radio frequency. The current waveform is a small portion of a distorted sine wave.

The caps in the O2 are without any charging current for something like 90% of the time. So all the energy is condensed into an unusually small portion of the AC waveform. The caps fall much further from cycle to cycle than in a full wave amp which makes the peak currents much higher.

If you go way back in this thread agdr did a simulation of the diode currents in the O2 power supply. They're surprisingly high. I've brought other amplifiers near the O2's power supply and they too pick up the EMI.

Your point about big power amps is valid but it can still be a problem there. Doug Self talks about what he calls "inductive distortion" and I've seen it in amps with poor layouts, etc. But it only shows up when they're driven hard not when they're idle with nothing playing.
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Old 22nd September 2011, 04:20 AM   #440
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My point precisely. Do you imagine I post without reading the thread?

It's not the absolute peak current, but the rate of rise of absolute magnitude of current that determines the level of emission and it's harmonic content. The pulses occur 120 times per second, but they contain harmonic energy at greater than 120Hz, this is why they propagate sufficiently to affect surrounding equipment. As I said; 120Hz does not propagate that well, or perhaps it's more useful to point out that a quarter-wave whip would be 625 kilometers high.

The power cord is the most effective antenna in which these currents travel, if you can keep them out of the cord you may be able to prevent them affecting devices in the surroundings as well as the amplifier itself, which would be no bad thing, we all have a responsibility to minimize radio noise in the environment.

I don't know that you can attenuate the content sufficiently with a ferrite, it may, as you say, be too low in frequency to be absorbed by the lowest frequency material available, but, on the other hand, whatever the frequencies, they ARE propagating, which tends to suggest that they fall in the range susceptible to absorbtion by ferrites, they have an audible effect on nearby equipment and this makes them EMI, by definition radio noise.
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