'noodle' dac (ebay special)
Oct 26, 2008 at 8:08 PM Post #91 of 180

mwofsi

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Quote:

Originally Posted by gurusan /img/forum/go_quote.gif
So basically i just need a transformer with only 5V secondaries?


Not quite, 5V ac rectified produces 7.1V dc. The ic regulator, the 7805 needs a dc feed of a max of 2.5V above it's output voltage to ensure that it works/regulates. So 6V secondaries, I believe.

Quote:

And my CMOY power supply is a DC wallwart 12V.


Depending on it's VA rating you could possibly use this to power both items.I think it gets a little complicated depending on how the dac power supply board is wired. I can't figure it out, you need someone who knows more.
Mind you some people prefer the sound of a cmoy run at higher voltages, say 24V! (me for instance)

Quote:

I would like to learn a bit about PSUs though, so if all I have to do is give the DAC a 5V source then how difficult would it be to build a PSU?


Read up on Tangents Tread power supply, what little I know I learned there. Also see regulator datasheets. For you, pretty simple, Maplin will have the parts if you don't mind higher prices and lesser quality capacitors.

Quote:

I just need to step down 220 to 5V, then rectify it into +5V and -5V, then feed to the DAC, correct?


No, see above! The dac only needs a single supply of +5V, no negative required.

Quote:

Or am I just better off buying a 5V dc wallwart


Again Tangent knows one hell of a lot on this subject. It's a possibility but it's likely to be a downward step. Power supply improvements are often a way of improving dac performance.
 
Oct 26, 2008 at 8:11 PM Post #93 of 180

mwofsi

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Quote:

Originally Posted by gurusan /img/forum/go_quote.gif
If 5V is all I need to power the DAC, then can I just steal it from a USB port?


Complicated, it's a poor quality power supply. Best avoided, though Alien dacs and the like do this.

EDIT: Your welcome.
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Oct 27, 2008 at 8:46 PM Post #97 of 180

mwofsi

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Quote:

Originally Posted by ls206 /img/forum/go_quote.gif
It's a shame there's literally no diy audio on ebay UK


One gets used to it. Don't think I've had anything of this size/cost fom HK, but I've gotten smaller items. Taiwan as well, also similar and larger items from America.

It's all usually quite painless. Usual precautions about checking ebay feedback or you could ask in the forum about specific sellers. You may get charged taxes/import duties on anything over £18.

EDIT: Returning faulty items could be a pain though.
 
Oct 27, 2008 at 9:00 PM Post #98 of 180

gurusan

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So i decided to go ahead and use the PSU that came with the DAC again...

The caps seem to be of decent quality but the +5V voltage regulator (all I need since I bypassed output stage) seems questionable. It's an L7805CV

THe product sheet says:
Load Regulation100mV
Line Regulation50mV

I was looking at the LM1086CT-5.0...and it says:

Load Regulation20mV
Line Regulation10mV

That has far less ripple so I would assume it would make for a reasonable upgrade since it only costs 80p?
 
Oct 27, 2008 at 10:48 PM Post #99 of 180

mwofsi

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Quote:

I was looking at the LM1086CT-5.0...and it says:


Well we're well out of my league here, but looking at the data sheet they're not
pin compatible. That is the input/output pins aren't in the same place.

Also, I think you'll find that the Ripple Rejection/Supply Voltage Rejection are more important, than it's absolute ability to regulate. The caps also play a role in maintaining steady voltage I believe and the absolute value is rarely important in audio.

You could salvage parts and air wire or use perfboard. Also are you sure that the
L7805 is dead?

The adjustable LM317 has the best Ripple Rejection I've seen but needs a few resistors, cap and diodes as in the tread. Similar kits/units are available from Tangent and ebay.

EDIT: Tangent agrees with you and writes that the LM1086 can offer an advantage but scroll down to "Choosing a Regulator" here: http://www.tangentsoft.net/elec/tread/pguide.html
 
Oct 28, 2008 at 7:13 AM Post #102 of 180

gurusan

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not yet. I'm waiting on my toroidal transformer...and thanks guys I will stick with the 7805.

But for future reference, is it better to have a lower ripple and supply rejection or higher?
 
Oct 28, 2008 at 7:18 AM Post #103 of 180

mwofsi

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gurusan, if you haven't already check your power supply by ensuring there's no short between the 12-0-12 inputs. If thats ok set up a resistor divider and connect a 9V batterry (less risk than using your 12V wallwart) to the supply. As long as the ground/0 position corresponds it won't matter which of the 12V inputs is +ve or -ve. Check for 5V at the output.

Sorry it took me so long to figure that out
confused_face.gif


(EDIT: whether you actually get the complete 5V out may depend on the rectifier set-up of the power supply)

Quote:

I got a OPA sun HDAM i am going to put in this thing xD


Should be interesting, only the Zero crowd seem to have been making much use of them.

EDIT: Quote:

But for future reference, is it better to have a lower ripple and supply rejection or higher?


Higher, as in more decibels. Always get confused about decibels, but dB is a ratio and 3dB increase is double what went before.
 
Oct 28, 2008 at 7:24 AM Post #104 of 180

error401

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Quote:

Originally Posted by gurusan /img/forum/go_quote.gif
But for future reference, is it better to have a lower ripple and supply rejection or higher?


The measurements given are how much the noise is attenuated by the regulator. More negative numbers (in dB) are better. 0dB means the noise goes through untouched. Greater than 0dB means it is amplified (??!?).
 
Oct 28, 2008 at 7:43 AM Post #105 of 180

mwofsi

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Quote:

Greater than 0dB means it is amplified


Oops, thankyou error401.

EDIT:If I knew what I was talking about I should have said 3dB decrease is double the ripple rejection that went before. Editted Edit: And ripple rejection can also be expressed in other figures, more being the aim.
 

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