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Old 7th October 2007, 08:06 PM   #1
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Default Split Rails off 5V USB

Hi Guys,

I'm toying with the idea of making a PCM2702 USB DAC and would really like this thing to drive headphones properly (with reasonable quality). But, that means (to me at least) having proper split rail supplies for the HP amp (which is going to be a TPA6120). Also, I want it completely portable being powered off the USB bus. I know there are some consessions I'd have to make (filtering the noisy USB bus), but should be doable?

I've found this one. It's the TPS65130 from TI. It seems to have all the right attributes, but TI is not making any claims as to it's noise and ripple performance.

Am I going about this the wrong way? This part is small and would be a PITA to mount (but doable). Any other suggestions/parts you guys can think of or am I going about this completely wrong? I was thinking of separate + and - step up converters, but it doesn't give me warm fuzzies going this way. There also don't seem to be a lot of inverting regulators around.

Your thoughts would be highly appreciated!

Cheers

Gert
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Old 7th October 2007, 08:29 PM   #2
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Sorry I'm not educated enough to give you the reply your looking for, but thank you for showing me this device.

I built an ultra compact portable headphone amp and used a MAX629 boost converter and a rail splitter. This would have worked much better, but I never came across it when I was searching for a suitable device.

The switching frequency is plenty high enough, which is good for audio, not so good for circuit layout. I think with proper filtering this would work out very well (but what do I know )

To bad it's in such a DIY unfriendly package.
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Old 7th October 2007, 08:39 PM   #3
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Default Re: Split Rails off 5V USB

Quote:
Originally posted by gbyleveldt
Hi Guys,

I'm toying with the idea of making a PCM2702 USB DAC and would really like this thing to drive headphones properly (with reasonable quality). But, that means (to me at least) having proper split rail supplies for the HP amp (which is going to be a TPA6120). Also, I want it completely portable being powered off the USB bus. I know there are some consessions I'd have to make (filtering the noisy USB bus), but should be doable?

I've found this one. It's the TPS65130 from TI. It seems to have all the right attributes, but TI is not making any claims as to it's noise and ripple performance.

Am I going about this the wrong way? This part is small and would be a PITA to mount (but doable). Any other suggestions/parts you guys can think of or am I going about this completely wrong? I was thinking of separate + and - step up converters, but it doesn't give me warm fuzzies going this way. There also don't seem to be a lot of inverting regulators around.

Your thoughts would be highly appreciated!

Cheers

Gert
You might take a look at dsavitsk's HPDAC project. It's pretty similar to what you're trying to do; PCM2707 USB receiver to a TDA NOS DAC into a headphone amp powered by a TI TPS61040 that generates about 10V, which is then split by the generation of a virtual ground, all bus powered. It apparently sounds quite good, though I built one and couldn't get it working properly.

Using two separate converters is probably not as good an approach from a sound quality perspective. You've now got two significant sources of noise instead of one, and it's not common-mode either. I think the virtual ground approach makes more sense, as long as you don't plan on connecting this to an external amp.

I've done this myself as well, using the TI DCP02 series converters. These are galvanic, unregulated devices (ie. miniature transformers after an oscillator) but they're very easy to use. DIP packaging and the only external required parts are some decoupling caps. They're available in a variety of fixed voltages, and even in a 2x15V version that (since they're galvanic) can be wired to produce +/-15V referenced to 'real' ground. I'd recommend a pi filter followed by a regulator after these though; they're very noisy (like any DC-DC), and of course unregulated. They're pretty foolproof though, so this might be a good option, if you can live with the 2W maximum power output.
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Old 7th October 2007, 09:04 PM   #4
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Default Re: Re: Split Rails off 5V USB

Hi Error, thanks for the reply!

Quote:
Originally posted by error401

I think the virtual ground approach makes more sense, as long as you don't plan on connecting this to an external amp.
That what I wanted to do too, but canned the idea for that reason. It might go on an amp or headphones, so it has to be split rail.

Quote:
Originally posted by error401

I've done this myself as well, using the TI DCP02 series converters. These are galvanic, unregulated devices (ie. miniature transformers after an oscillator) but they're very easy to use. DIP packaging and the only external required parts are some decoupling caps. They're available in a variety of fixed voltages, and even in a 2x15V version that (since they're galvanic) can be wired to produce +/-15V referenced to 'real' ground. I'd recommend a pi filter followed by a regulator after these though; they're very noisy (like any DC-DC), and of course unregulated. They're pretty foolproof though, so this might be a good option, if you can live with the 2W maximum power output.
Ha, thanks for those, I'll look a little into it, but I see you can't get samples and getting them in small quantities in South Africa is not going to happen. They do look really nice though - I'll try and hunt them down here.

Cheers guys!
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Old 9th October 2007, 03:06 AM   #5
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There's a bunch of different manufacturers of DC-DC converters, if the TI ones are hard to get. If you're desperate, check out Mouser and Digikey; maybe they'll ship overseas. If RS in the UK will ship to Canada, they should ship to South Africa.

I'd consider using linear regulators after the DC-DC converter.
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