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Old 14th June 2010, 01:04 PM   #1
elaar is offline elaar  United Kingdom
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Join Date: Oct 2005
Default PSU design/upgrade for a Cambridge Audio S700 Isomagic DAC

Hi Guys,

I'm trying to resurrect my old Cambridge Audio Dac, the PSU has seen better days and it's a lovely DAC so I thought I would make a new one for it. I was hoping someone could look over my calculations to see if this would work?

For those that don't know, the DAC uses a power brick with a umbilical cord. The original PSU uses a small 15v ferrite core transformer (rated at about 25VA), with 2200uf filter caps, going through a diode bridge and then along the umbilical cord where each section of the DAC has it's own regulator (12v, -12 or 5v, about 12 in total).

Where my potential troubles start is that I would like to use a 12v * 2 toroidal transformer rated at 50VA total, I believe I can *just* get away with this, but wanted to make sure, this is what I have planned:

12v * 2 going through MUR820s (due to their voltage drop of about 0.6v at 1A)
(12v x 1.414) - 0.6v = 16.35v, factoring in the 13% regulation and with no load it will reach about 18.5v. I will be using 3300uF Pan FC caps and as each line can supply 2A and the DAC uses more like 1A i'm assuming I will be able to get a bit higher voltage than the 16.35.

I would then like to use regulators in the PSU, I want to use LM338 series, I know they are overkill with them being 5A, but they seem to have one of the lowest dropout voltages at this power level, with the datasheet showing about 1.75v at 1A (and I have quite a few spare). I was hoping to set the output voltage at about 14.5-15v, maybe nearer 14.5 as 14.5+1.75 is very close to 16.35.

On the DAC itself, the main regulators in question are L7812s, the datasheet for those says a minimum voltage of 14.5v is required, the datasheet graphs show at 1A you can get away with 14.2v.

Ideally I would use a 15v transformer, but I have so many of them I would like to use them up before ordering yet even more. Is anyone else able to tell me if these margins are acceptable?

I was also thinking of changing the main regulators on the DAC from L7812/7912 to something like TL780-12, as the worst case regulation figures (min/max) seem to be about 5x better, or is that a bit overkill?

Many thanks in advance for any advice, hopefully my messy calculations make sense.
Cheers
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Old 27th August 2010, 10:20 AM   #2
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Quote:
Originally Posted by elaar View Post
Hi Guys,

I'm trying to resurrect my old Cambridge Audio Dac, the PSU has seen better days and it's a lovely DAC so I thought I would make a new one for it. I was hoping someone could look over my calculations to see if this would work?

For those that don't know, the DAC uses a power brick with a umbilical cord. The original PSU uses a small 15v ferrite core transformer (rated at about 25VA), with 2200uf filter caps, going through a diode bridge and then along the umbilical cord where each section of the DAC has it's own regulator (12v, -12 or 5v, about 12 in total).

Where my potential troubles start is that I would like to use a 12v * 2 toroidal transformer rated at 50VA total, I believe I can *just* get away with this, but wanted to make sure, this is what I have planned:

12v * 2 going through MUR820s (due to their voltage drop of about 0.6v at 1A)
(12v x 1.414) - 0.6v = 16.35v, factoring in the 13% regulation and with no load it will reach about 18.5v. I will be using 3300uF Pan FC caps and as each line can supply 2A and the DAC uses more like 1A i'm assuming I will be able to get a bit higher voltage than the 16.35.

I would then like to use regulators in the PSU, I want to use LM338 series, I know they are overkill with them being 5A, but they seem to have one of the lowest dropout voltages at this power level, with the datasheet showing about 1.75v at 1A (and I have quite a few spare). I was hoping to set the output voltage at about 14.5-15v, maybe nearer 14.5 as 14.5+1.75 is very close to 16.35.

On the DAC itself, the main regulators in question are L7812s, the datasheet for those says a minimum voltage of 14.5v is required, the datasheet graphs show at 1A you can get away with 14.2v.

Ideally I would use a 15v transformer, but I have so many of them I would like to use them up before ordering yet even more. Is anyone else able to tell me if these margins are acceptable?

I was also thinking of changing the main regulators on the DAC from L7812/7912 to something like TL780-12, as the worst case regulation figures (min/max) seem to be about 5x better, or is that a bit overkill?

Many thanks in advance for any advice, hopefully my messy calculations make sense.
Cheers
did you ever make any progress on this?

I too am planning to build a new PSU from scratch, but since I'll be ordering all the components, I was thinking of a Dacmagic style set up with three separate transformers - 2 for analogue, 1 for digital. But I'm a bit of a novice, so any thoughts on this welcome.
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Old 19th October 2010, 07:49 AM   #3
Louagie is offline Louagie  Belgium
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Hi guys,

I need some help.
I've gotten an Isomagic (same as above) but there's probably something wrong with the PSU. I'm looking for some kind of schematic of the PSU (DIn-connector, what is used what not).
I have no knowledge of electronics (and poor one of English by the way ), but somebody is helping me out to get the PSU fixed.
Or is there a place on the web where I can find more info?

thanx,
Nico
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Old 22nd October 2010, 04:58 AM   #4
Louagie is offline Louagie  Belgium
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No one ?
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Old 25th November 2010, 05:47 PM   #5
algoart is offline algoart  United Kingdom
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Join Date: Sep 2008
Location: Kent
Hi, I hope this thread picks up as I'm hoping to upgrade the PS on mine. This topic was handled and then died :-(
Try this:
cambridge isodac s700 mods/upgrades

Good luck,
Alistair
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Old 25th November 2010, 06:26 PM   #6
elaar is offline elaar  United Kingdom
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Join Date: Oct 2005
Hi messymanco, sorry for the delay in replying.
I did make the replacement PSU as described and it works very nicely.
Although it had very tight margins, the DAC draws very little power and voltage measurements show that everything performed as expected.

Louagie, you don't really need a schematic, it's the most basic a power supply could be. You have the main transformer, that has 2 x 15v secondaries, it goes through a bridge rectifier made up from 4 discrete diodes, and has a 2200uF capacitor on each rail, exactly like Basic Symmetric Power Supply | Circuit Diagram
but it isn't centre tapped.

Algoart, I remember checking that thread, I followed alot of the recommendations, including replacing the main caps inside the DAC with panasonic FS ones.

I never touched the regulators inside the DAC itself in the end, maybe that's one for the future, but it sounds nice enough as it is.

I have included some pics, the psu looks a tad ugly inside due to alterations I kept making on the design.

I can tell you guys the exact components I used if that helps.
Cheers,
Andy

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Old 26th November 2010, 10:09 AM   #7
algoart is offline algoart  United Kingdom
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Location: Kent
Hi Elaar,
Thanks for getting back to us. PSU looks pretty hefty! Great pics too!
You went with your 12V transformer as initially planned?
Do you know how much power the dac consumes?
The caps you replaced, are they the large blue ones at the front of the pcb (back - in the pic)?
I'm fortunate enough to have some people who could help me with the PSU build (as I have only a rudimentary grasp of this stuff), so I'd like to ask you for your parts list which could help things on the way.

Did you ever read any more on the intriguing idea of running one Isomagic per chanel? Overkill perhaps, but the idea appeals to me (due to me being a sucker for that kind of thing anyway!!). Going to have to start digging!

My digital sources are a PC and an Audio Analogue Paganini CDP. I run the PC through the Isomagic to an Accuphase E202 into 10" Tannoys in 120L boxes.
I've tried Arcam BB, Musical Fidelity X-Dac and I really prefer the sound from the Isomagic. Sheer pleasure!

Happy listening!
Alistair
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Old 30th November 2010, 08:30 AM   #8
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Default Cambridge Audio S700 Resurrection/upgrade

Hi folks,

Just stumbled on this thread, having recently dug out my S700. Before finding this forum, I actually ordered some parts for my DAC, as it spent years always 'on' then years not used. The PSU caps have done their work & must be tired.

That said, it still works & after a couple of hours running in the background, it started to sounded nice again!

Will post a report, piccys & parts supplier once the work's complete.

Mark
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Old 2nd December 2010, 02:18 PM   #9
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Join Date: Nov 2010
Default Cambridge Audio S700 Resurrection/upgrade

... would anyone believe me if I told 'em, it's snowing(?)

Parts arrived, fitted, ran for a few hours & sounds better than it did new! I bought 6x Nichicon Fine Gold 2200uF 25V (NFG-880) & 8x Elna Cerafine 47uF 25V (ELNAC-085) from hificollective.co.uk (Part numbers in brackets).

Phoned the order through, was promptly answered/ordered, in-stock & delivered despite the weather, within 4 days.

Was it worth the 20.79 with postage, VAT etc? In short, yes, very much so.

I could build a better PSU as seen previously, this would make a huge difference, I speak from experience. I could line the casework with sticky-back bitumen pads, I could re-wire the unit with pure silver wire, replace all the fixed voltage regulators with super ones from Audiocom International, replace the HDCD LED with a blue one & mount the house on Sorbothane feet ... for me, the re-cap will do nicely, especially as the original 2200uF's in the original PSU had started to leak, as had one or two 47uF caps.

All in all, a good hour or so of fun playing & many years of listening to come.

Mark
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File Type: jpg S700.jpg (741.6 KB, 134 views)
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Old 13th February 2012, 09:33 PM   #10
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Join Date: Sep 2010
Location: Riga
Hi,

got me a IsoDac without PSU. I built a PSU with several +/- 15V terminals, wired it to DAC .... now I know why I was able to get it for free. :-)
Analog stage negative regulator L7912 is toasted and it's output is short circuit'ed to the analog ground. I suppose one of the IV stage opamps is dead, but as I do not have schematics, it would be necessary to remove all the elements connected to the negative analog rail one by one... To make log story short, if anyone can PM me with hint where to find schematics for CA IsoDAC it would be appreciated.
In case you wonder - I already asked a local dealer representing CA a while ago and they had not the schematics, and, of course, I was not able to get any response from Cambridge Audio at all.
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