Ariamella
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- Nov 14, 2018
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- #21
Around as powerful would be fine, I know P113 is capable of 2.2w into 32ohm so that was where I went with it, I just didn't want to downgrade on power. I'll give it a go though!I don't think P113 as-is is going to cut it if you want substantially more output power than the Atom... at 8.66 Vrms into 150 ohms and 5.66 Vrms into 32 ohms, that's not exactly a slouch to begin with. Basically, what you'd want is an output stage with gain running on higher supplies, regulated down to +/-15 V for the opamp (we're only talking a few mA, so 78L15/79L15 series parts would be easily sufficient). The SMPS modules you are using are also available in 24 V, that would be a starting point.
Whoops, good catch! Fixed.1. Q1/3 are using the wrong symbol, and Q2/4 are connected wrong entirely.
Fair, I can always go back to the BD139/140s the original design calls for.2. I am not sure whether the simple diode bias circuit is going to get you enough bias... generally you want to be running output transistors like these at 20-30 mA minimum, or else their fT will be competing with a sloth on valium. (Not good for stability. Generally, you want buffer fT >3 * opamp GBW / (noise) gain.) 8 A parts seem a tad oversized in general. I might try some 2SC5200/2SA1943... even the shady second-sourced ones ought to be robust enough for this application. Make sure you get some silpads for mounting, I think those should be adequate in a headphone amp and they're practical and reusable (I doubt there'll be enough heat dissipation to make mica washers worth it).
That would be, uhh, C3, C4 and C7, C8, right?3. The two 100 µF capacitors in series can be replaced by a single part across both with no substantial detriment, according to simulation.
If I'm understanding right, that would be between pins 2/6 and 7 for the capacitor? I did see a zobel network mentioned on the P113 page but I didn't think it was necessary. I'll add one.4. At lower gains, it is generally advisable to add a small capacitor from opamp output to inverting input, say 22-47 pF, to aid stability. I would consider adding a Zobel network on the output for the same reason. And speaking of gains, having a way of selecting at least two different ones would be good.
I actually changed the design before seeing this comment because I realized this myself and now the ground plane is only at the secondary side and the amplifier. Take a look at the GitHub! As for the insulation, that makes sense.5. Your secondary-side ground planes have no business being anywhere near mains input nodes - mask out at least 1-2 cm around these areas if you can. You have some reading up on electrical safety and creepage distances to do. What you want is more or less an IEC Class II device. Use heatshrink to cover solder joints, and when mains wiring has to be routed alongside case panels (e.g. for the power switch), use double-insulated mains cable. You always want two layers of insulation between mains and the entire secondary side (a decent distance in air also counts though).
Got it.6. If a PE connection is commonly available from outlets (I guess Hong Kong uses British plugs?), you can make use of that... use a Y2 rated safety capacitor, ca. 10 nF, to connect incoming PE to chassis ground. Route out a slot in the board underneath the cap, maybe 2-3 mm across and about 1 cm wide. PE can be treated like L and N otherwise. Depending on chassis available, consider an onboard IEC power connector.
No amplifier with unbalanced inputs should have audio ground and PE firmly connected, this is just asking for trouble with ground loops. The Atom is using a transformer plug pack for good reason.
Thank you for your advice by the way! This is my first time designing an amp like this so I'm bound to make mistakes.
Edit: modified the schematic! How is it now? Am I right in assuming that the chassis shouldn't be connected to PE at all?
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