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Carrie USB-Powered Headphone Amplifier - Page 2

post #16 of 889
I would be happy to try you multiplier out too.
Have you tried line and load transient testing?
post #17 of 889
Okay okay you're right, I'm a victim of playing around and overengineering instead of actually thinking about the problem :P

The pi filter has a much more linear response and it'll do better with less tuning and less parts. Watch out for a resonant peak somewhere in the audio frequency range though.

Edit: In the spirit of overengineering, what about a gyrator instead :P
post #18 of 889
Quote:
Originally Posted by dsavitsk View Post
For an easier boost converter, take a look at TI's DCP020509 and DCP020507. A little more expensive than some other options, but they have DIP pins, and a pair can be used to make a +/- supply.
the dcv family is the newer replacement for the dcp.

both are unregulated - but will good line regulation (1%).

if you want to make them regulated, you can apply feedback on the SYNCin pin and turn them into gated regulator - as those chips turn on in 2us, it should be no problem.
post #19 of 889
the regulated version of the dcv family is the dcr family but it outputs just 3.3v or 5v so they are of no use here.
post #20 of 889
Quote:
I haven't simmed the load response, but it should be fine, and most of the load is probably going to be static anyway.
you will need a capacitor on the output to provide the needed transient current and to help the multiplier when the input voltage is low.

the ac performance will worsen once you factor in realistic esr /esl.
post #21 of 889
Thread Starter 
Quote:
Originally Posted by dsavitsk View Post
I would go 2 channel as, frankly, the third doesn't get you much but it does draw a bunch more current.
You know what, I honestly didn't even consider that. I envisioned it as some sort of BantamDAC + Mini^3 with a modified power supply, but since current is a bit tight, I can stand dropping a channel.

I'm going to need help figuring out how to do the current draw calculations. I really don't know where to start. Each chip is going to sink some sort of quiescent current as well as operating current, then there's the current going into the likely low-Z headphones. Preliminarily, let's try the AD8397 and see what kind of numbers we get.

If things work out with rds' power supply, we'll have 12V to work with. The datasheet says max current draw at a 10V supply is 12 mA/Amp (milliamps per Ampere? What does that mean?). The PCM2702 will take at max 30 mA for the digital section and 25 mA from the analog section. That means 67 mA so far, and rds gave us 150 mA of play. That means there's a healthy 83 mA to play with, yes? Plus, one of the features of the AD8397 is that it's capable of delivering 310 mA into a 16 ohm load. Should that worry me? What am I missing from my calculations?

Quote:
Originally Posted by Mellow Mushroom View Post
why not use battery power instead?
Part of it is because I don't want to have to worry about charging it all the time, and part of it is because it will be taking the signal from the BantamDAC. It would be some kind of headache trying to fit some sort of battery in there, figuring some sort of charging method, blah blah etc. I'd rather just have one wire in and one wire out, one for the DAC, and one for the headphones. It's simpler and cleaner that way, I think.
post #22 of 889
Quote:
Originally Posted by joneeboi View Post
If things work out with rds' power supply, we'll have 12V to work with. The datasheet says max current draw at a 10V supply is 12 mA/Amp (milliamps per Ampere? What does that mean?). The PCM2702 will take at max 30 mA for the digital section and 25 mA from the analog section. That means 67 mA so far, and rds gave us 150 mA of play. That means there's a healthy 83 mA to play with, yes? Plus, one of the features of the AD8397 is that it's capable of delivering 310 mA into a 16 ohm load. Should that worry me? What am I missing from my calculations?
I'm not sure how rds is planning to get 150mA at +/- 12V out of USB. That's 150 x (30/5) = 900mA drawn from USB (max 500mA). And the converter he's planning to use is only 2W, while 0.150 x 30 = 4.5W. Realistically you'll probably get closer to 75mA at the +/-12V rails, and that's with nothing on 5V.

You need to calculate all the currents with respect to 5V. When you boost the voltage on the output side, you also boost the current on the input side. This comes from the power formula, P=IV. If you increase V but maintain I, power increases proportionally. Since the input of the device needs to provide the same power but at a lower voltage, I increases proportionally on the input. For linear regulators you can assume that the current drawn from the unregulated rail is the same as the current drawn from the regulated side (and the power difference is burned in the regulator).

So here's what you've got: AD8397 draws 15mA max when powered from 24V, and you've got two of them, so 30mA. Reflect that onto 5V and you get ~180mA from the USB bus. PCM2702 takes 30mA on 3.3V and 25mA on 5V (max) which you can just add, so you're up to 235mA. I wouldn't worry too much about dynamic currents, the capacitors in the supply will source/sink most of that current, and the average load current is going to be very small. I think it can be safely ignored.
post #23 of 889
Thread Starter 
Was it +/- 12V? I thought he meant 12V. Also, I only have one AD8397 since it's dual channel, yah? With one less opamp, that halves the current from 180mA to 90 mA plus the contribution from the DAC, which yields 145mA.

15mA * (30/5) = 90mA
90mA + 55mA = 145mA

I'm a little confused about that (30/5) conversion you're doing though. There aren't any units next to the 30, but I think we can safely assume you mean 30V. And if that's the case, why isn't it 24V? Is it because he's boosting it to 30 and shaving it down to 24?

edit: Hm, it seems I just missed the fact that I had 1337 posts. Shucks.
post #24 of 889
Quote:
Originally Posted by joneeboi View Post
Was it +/- 12V? I thought he meant 12V. Also, I only have one AD8397 since it's dual channel, yah? With one less opamp, that halves the current from 180mA to 90 mA plus the contribution from the DAC, which yields 145mA.
If it's the same USB power supply I'm thinking of, he plans +/-12V.

The Iq is per amplifier on a dual. That's what the mA/Amp means.

Quote:
I'm a little confused about that (30/5) conversion you're doing though. There aren't any units next to the 30, but I think we can safely assume you mean 30V. And if that's the case, why isn't it 24V? Is it because he's boosting it to 30 and shaving it down to 24?
Yea I thought that might've been missed. Your assumption is correct, basically the linear regulator that gets it down to 12V is going to draw approximately the same current as it's loaded with from the 15V supplies, so it's easier to just use the input voltage for the math. If you prefer, you could calculate the load at 24V and then factor in the efficiency of the LDO, but you'd end up at about the same result.
post #25 of 889
you also need to factor in a) the idle current of the dc-dc convertor (30ma for dcv010512, from a 5v source), b) its inefficiency (<85%, lower for low load situations), and total watt output (<1w for dcv010512).

so your headroom is tightening quickly.
post #26 of 889
Quote:
Originally Posted by rds View Post
I definitely wouldn't use such a supply for audio.
Any reason why you would not use something like a charge pump to create an negative rail?

There is the switching noise, but IIRC they are much more efficient than the the boost convertors people seem to be wanting to use. As well you can get them to have high switching frequency (i.e MAX889T) to try and keep the noise out of the audio band.

the other thing that it does is simplify the ground. USB ground will be ground all the way through the circuit as opposed to having one ground through the DAC section and then one virtual ground in the Headphone section.

I think ultimately you could build a Mini^3 + BantamDAC USB powered in one box this way pretty easily.
post #27 of 889
Quote:
If it's the same USB power supply I'm thinking of, he plans +/-12V.
No I don't plan +/- 12V. There's no point in trying to get that kind of voltage off a usb. The current would be useless as you pointed out.
I'm using one output of the DCP020515DP. I want a single 12V supply.
That means we can draw 148 mA at 15V and 89% efficiency. The ldo is rated at 150mA as well. I estimated the upper limit and 150mA for this reason. Maybe I'm pushing it. But for sure 140mA is reasonable.
It does require the builder to think about current draw if they're getting close to the 140mA mark, but as an example you could even build a 12V PPAv2 with the buffers biased at 25mA (near the max) and you'd be OK.
So it will be pretty robust in terms of powering headphone amps.
post #28 of 889
Quote:
Originally Posted by cobaltmute View Post
Any reason why you would not use something like a charge pump to create an negative rail?
they have next to nothing in terms of current capability, and high output impedance, two things critical to a high quality audio amp.
post #29 of 889
200ma is not sufficient for this?
post #30 of 889
Quote:
they have next to nothing in terms of current capability, and high output impedance, two things critical to a high quality audio amp.
Also consider that the really crappy power only applies to one rail. So you have one rail that is pretty robust (the usb) and one that is garbage. If you use that kind of asymmetrical power for an op-amp you can bet the results will be less than awesome.
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