[Jeroen]

Quote:

Originally Posted by nleahcim It sounds, though, like having only 5V available is going to be quite the limiting factor, so without adding an additional power source (batteries or wallwart) - maybe a chip like the MAX743 might be a good idea? Is 24V a good supply voltage to work with?

If you're going for an opamp based amp, it is within the working range for most opamps (e.g. the popular ad8620, OPA2134, etc.)

Quote:

Originally Posted by nleahcim And so if I do end up using something like the 743 - any reccomendations for a small amp that doesn't draw *too* much power?

Assuming opamp based, a Cmoy or mint can be made so that they don't draw to much power from the supply. I'm using a mint with ad8620 opamp (not biased in class a), buf634 buffers (low bandwidth mode), no tle2426 (the regulator ground is connected to the amp ground.) Gain will depend on your headphones, but since I'm using mine with hd580 and hd25's I placed a switchable resistor on the output to eliminate hum on the hd25's (I'm using a gain of around 5.)

Quote:

Originally Posted by nleahcim edit: if I'm reading correctly - jamont's solution was to get an external power supply right? I'd really prefer not to do that...

I believe so. It's bad enough needing power for my notebook sometimes, I wouldn't want to need it for my amp too.

 

[13405]

You could, although a little expensive use a DC to DC converter

something like this:

http://uk.farnell.com/jsp/endecaSear...U=253340&N=401

that one will give out +/- 15V from a 5V input @ up to 3W. costs about £16/$30 but im sure you could find them cheaper in the states/elsewhere. They come in all shapes and sizes so there will be one for what your needs are.

 

[13405]

Also, if you are worried about the peak to peak noise value of 75mV - I have seen ones with much lower than this. However with a little effort this can be almost eliminated. Use an ultra low noise voltage reference, then create a dual tracking op-amp based supply circuit from this.

With careful use of capacitors with this should give a low noise supply. If you want something simpler, look into the LT1086 reg - very low noise, and easy to implement.

 

[Jeroen]

Quote:

Originally Posted by Oli You could, although a little expensive use a DC to DC converter something like this: http://uk.farnell.com/jsp/endecaSear...U=253340&N=401 that one will give out +/- 15V from a 5V input @ up to 3W. costs about £16/$30 but im sure you could find them cheaper in the states/elsewhere. They come in all shapes and sizes so there will be one for what your needs are.

Is this unit a lot better in some respect? From what I can understand, this thing does basically the same as a max743? The max743 can put out 125mA at 24V = 3W (p=v*i right?) So it's about the same power. At 5V this will be 600mA (plus the inefficiency of the reg =~ 700mA), well above the official specs of usb. I know most pc's put out more, but we're talking about a device for a laptop so I don’t think you want to draw a lot more than that.

Quote:

If you want something simpler, look into the LT1086 reg - very low noise, and easy to implement.

Do you mean this LT1086? I believe we are looking for a step-up dual-voltage reg for this application (or something to do the same job), but this is a single drop-out reg. Or am I missing something? (could well be the case) Remember that the input-ground of the amp is going to sit at usb-gnd, so dual supplies will be very handy (otherwise you'll have to up the input to halfway rail voltage, which is probably worse imho.) Also, 5V single supply limits your choice of opamps quite a bit.

Quote:

Use an ultra low noise voltage reference, then create a dual tracking op-amp based supply circuit from this.

Never tried that, but it sounds impressive

Seriously, wouldn't that be rather inefficient (as in mA’s) for this project?

 

[jamont]

Quote:

Originally Posted by nleahcim edit: if I'm reading correctly - jamont's solution was to get an external power supply right?

Yes, that's correct.

 

[13405]

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