DAC modifications

[elaar]

Hi all,

I recently bought a Cambridge Audio DacMagic 3 and felt like doing a few mods. The first area i'm attacking is the power supply, rectifying and regulation.

Basically, there's 3 transformers, each with 15-0-15, all 6 rails go through a discrete diode bridge (crude 1n4004s), have a 100nF decoupling cap between rail and 0v, 3300uF 25cap per rail, and then finally onto positive fixed regulators, all of the 78xx variety. The 6 diode bridges and general seperation show some proper thought into the design of this DAC but i feel it could do alot better.

Diodes: I was thinking of replacing these with Schottky ones such as 11DQ10's, however i don't know much about diodes, would ultra-fast soft recovery ones be better? or are these schottkys a bit old now? I've also been told to use 0.01mF-0.1mF snubbing ceramic caps across each diode to prevent the hard switching, is this only really for harsher diode bridges?

Capacitors: Current ones look pretty cheap with no name written on them, I was thinking of replacing them with something like Nichicon HEs 5600mF or United Chemicon KYs, both aren't known for their audiophile quality but are very good caps for the price (I need 10 of them!) and have impedances of 0.012 (at ...) This sound okay?

Is there anything i should think about with the ceramic decoupling caps? they're just general cheapo ceramic ones at the moment.

Lastly, not sure about this, but the regulators are all the 4% regulation type, and i could be slightly anal and replace them with the 2% ones, there are great deal of them though


I'm a beginner when it comes to power supplies, having read relatively few power supply tutorials and would greatly appreciate any advice you could offer.

Thanks,
Andy

[rudi]

Hi Andy

when modifying a DAC you have to be very carefull not to waste your time to much. most of them is pretty much up there and your wins would not be that much, specially the mods you want to do ( I Know of somebody that went for much better parts in a Jolida CD's Dac and it sounded worse)

but just to help a bit here. first check the quality of the transformers. you can have a huge win here, if they are of average quality then i would suggest going with a very well known make (you will have better cores) and also go for a higher VA rating. this might be your biggest win three X 80VA transformers will do the trick

Schottly diodes will give you more clarity but do not bother with the ultra fast types. keep to a standard one, and bypass them with a good 22nf multi layer ceramic cap

for the caps - do not make them bigger. it just makes them slower. and then use Panasonic FC's but keep to the values used. and keep the ceramic caps seeing that they are best for RF.

for the regulators use ON SEMI or SG THompson devices. they are a bit better.

I have modified several DACs and I have found that the above mentioned mods helps a bit, specially the transformers and the Panasonic FC but you can do far better replacing a single opamp.

[elaar]

Hi rudi,

Many thanks for the reply and advice.
I'm currently looking into the quality of the transformers in the DAC, but they look like they're reasonably good, "specially designed" and
are pretty large considering they're doing about 20-30VA each, i'm waiting from a reply from a friend who has already done some mods on this DAC to see what he thought of them.

You were right to tell me to not go larger with the capacitors, after talking to some people we came up with the idea of making a daughter board, I intend on going from 3300uF to 4000uF with 4x1000uF capacitors (for each rail) to keep the capacitors small and the slew rate as high as possible.
So the daughter board goes 4x1000uF, 1x 10uF and 1x1uF (bypass caps) in parallel for each rail, does that seem okay?


Do Panasonic FCs have a good reputation then? I started looking at those first but the Panasonic FMs seem to have a smaller impedance compared to the FMs, so was just wondering why they are mentioned so often.

I was going to leave other areas for another time I agree with you that it's worth sorting out those opamps, so I was going put in some 8-way sockets and try some opamps, I have some spare OPA627s which hopefully should sound great in this DAC.

Thanks again,
Andy

[rudi]

Hi Andy

Just check your transformers again. If they are E/I cores then I would doubt them very much. The chances of them using a good core aren’t that great. Remember that accountants dictate final assembly. So you really want the best cores you can get.

if you are using the caps after the regulators then you are going to run into problems. Specially the ones feeding the opamps. you want very small caps after the regulators. Around 33uf and then a bigger one. I tried using bigger caps on a preamp of mine (brain child of Carlos FM) and the bass were BIG and FAT if i used big caps after the regulators. but before them you can load it if you want.

I have found that the Panasonic FCs are very good and they aren't expensive either very good replacement if you are modding your equipment

The OPA67 is going to transform your system (depending on what is in there already) if you are running a dual opamp then you can try AD823 or AD826 they are old opamps but still up there with the best and they are very cheap

[spikeymikey]

OPA627APG4 (PDIP) samples are available for free from TI. Here

[sbrads]

I used a single RC snubber across each transformer secondary on my Dacmagic2 and could hear the improvement in clarity quite easily. It's all you need to stop the diodes' turn off causing leakage inductance/diode capacitance HF ringing. I prefer this method to using 4 caps per bridge which just lowers the HF ringing frequency. I used 8R2 + 47nF although optimising on a good scope would be best but it can be very difficult to trigger on the fuzzy part of the ripple waveform that causes the problems. Softer diodes would be the icing on the cake.