Is there any way to run the Mojo off of USB power?
I really like the Mojo, it's the best-sounding DAC I've ever used, but the fact that it runs on battery and I have to charge it is annoying to me because I really only listen to music sitting at my desk. If I charge it all the way and then run it with the USB cable plugged in, it is still running off the battery?
There is quite a bit of info in the FAQ in post #3
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If you fully charge Mojo then use it in a desktop it will not switch off; the power dissipation that the charger uses in matching the current drawn by Mojo is negligible. You are only at risk when charging and using at red ....
Just to give you some numbers - fully charged and matching Mojo's current draw the power dissipation is 107 mW for the charger circuit. That will increase running temperature by less than 1 deg C. But at flashing red it is 910 mW for the power dissipation in the charger.
Now I could fix this by using a switcher based charger rather than a linear one - but these inject too much RF noise onto the battery. This would impair sound quality, and Mojo's design goals was that plugging in the charger would have no significant change in SQ - which would not happen if I used a switcher based charger. I am not prepared to damage SQ as to me this is the most important aspect just for a tiny improvement in usability.
Rob
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To understand it better, let's assume Mojo is off and charging.
Now the charger has two modes of normal operation - constant current, which is set to 330 mA, and constant voltage which is set to 8.200 V. Now when the non charging battery battery voltage is less than 8.200 V, then the charger supplies a constant current. But when the non charging battery voltage gets close to 8.200 V, then the charger switches mode to constant voltage at 8.200 V. The current that is charging the battery then slowly falls from the initial 330mA, to zero - its in the trickle charge mode now. Eventually, the non charging battery voltage hits exactly 8.200 V, the charger is in constant voltage mode of 8.200 V, no current now flows into the battery, and the charger switches off automatically. When the battery voltage falls to 8.0 volts, then the charger will return to charging. Tip - if you want to force the charger to top up Mojo's battery to 8.200 V then removing the charge USB, wait 5 seconds, reattach, and the charger will top it up to 8.200 V.
Now imagine that Mojo is on at the same time as it is charging. In this case, the battery will continue to charge until it gets to 8.200 V, and the charger is set to voltage mode and gives 8.200 V too; so no current flows into or out of the battery; but Mojo itself is drawing 180 mA of DC current, and this will simply come from the charger - so the charger will supply the needed 180 mA for Mojo. It will do this for ever, and it won't switch off. This is intended, as it means that the battery is effectively not being used to supply the bulk of the current, won't charge or discharge, is held at a safe level, and will operate like this for a very long time.
Now we have been talking about DC currents, and this is indeed the vast bulk of the current. But what about dynamic currents and noise? Because the output impedance of the battery is much lower than the charger, then the noise of the charger is reduced; also dynamic currents still comes from the battery. So running in this mode ensures the best of both worlds - low RF and audio band noise from the battery, large dynamic currents available, and low PSU impedance too - but without the worry of the battery wearing out from charge and discharge cycles.
I hope this clarifies.
Rob
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when the Mojo is being used and being charged especially when driving lower impedances there is a net drain on the battery. This means that the charging circuit does not quite provide enough power to power the Dac and amp circuitry and keep charge the battery at the same level this is because we had to limit the amount of charge over a given time due to thermal constraints. Our charging time with the Mojo switched off is usually four or max five hours this is a little inconvenient but when we compare this to other Dac amps that need up to a full twenty four hours to charge we feel that we didn't do such a bad job.