CS4397 dac

[leo]

10uf Wima should be fine, I personally wouldn't use an oscon for signal but feel free to try it

The standard output stage +/- should be mirrored, it isn't for some reason and this gives the DC offset.
If you look at resistors R8 right channel and R9 left channel for the inverted side they are 36k, the mirrored side to these for the non inverted R14 right channel and R17 left channel are 22k1.
Basically R8 and R9 should be 22k1 to match 22k1 for R14 and R17.
If you change those resistors to match, and then measure the output pins of the op-amp (pins1 and 7 to ground) the DC offset should be a couple of mV's instead of the hundreds of mV's in the standard circuit.
You may lose a bit of gain but it shouldn't be a problem.
BTW a thanks to Ray for pointing this out

theres also a cap across each of those resistors, they don't match either only thing here is that on the non inverted side they are 1N and the inverted side they are 56p, I'm not sure if having 1NF across the NFB resistor would sound worse than 56p, its something I'd need to play about with, these caps shouldn't effect the DC offset though, its mainly the resistors mentioned above.

This dac is weird

I'd still use a input coupling cap in the amp or pre just as a precaution

[iihay]

Thanks Leo for the excellent explanation. I'll check this out at the weekend and replace the resistors. 10uF WIMAs are no problem I have loads.

I'll probably stick an input coupling cap in the amp and see how it goes. Although I guess this depends on if the DC is variable at all as DC in the amp I'm using (SKA GB150's ) can be nulled so a few mvs should be ok. Got my newly built speakers at the end of this chain though

[leo]

No probs
The only thing is if those ceramic caps are left with the odd value it may sound a bit odd, if measured probably makes the signal look all over the place

Once the dac is sorted and your confident the DC offset is low enough you should be able to remove the coupling cap, not sure I'd risk it with this thing though tbh

[iihay]

No probs
The only thing is if those ceramic caps are left with the odd value it may sound a bit odd, if measured probably makes the signal look all over the place

Once the dac is sorted and your confident the DC offset is low enough you should be able to remove the coupling cap, not sure I'd risk it with this thing though tbh

lol, this dac is just for and cheap and fun way to get a dac for my transport. my main system will still be the modded arcam so I think I can risk it a bit Ultimately (i.e when not skint) I fancy something far better, twisted pear buffalo maybe.

Iain

[leo]

I have my eye on that ESS sabre dac too

[leo]

Had a couple of PM's from folks wanting to try the simple mods
I've added the below, hope its easier to understand

For the coupling cap mod simply bypass
C27,C28,C30,C31,C6,C54

Doing the simple coupling cap mod gives a 800mV DC offset on the output so you need to add a decent quality coupling cap on the outputs feeding the phono sockets to protect the vol pot in pre-amps etc, even if your amp uses input coupling caps before the circuitry this offset from the dac can still damage the plastic track vol pots and certainly won't do speakers much good if theres no input coupling caps at all

C1 and C2 can be around 1uf-10uf, its best to try different types.
R1 and R2 can be 22R-100R, this is just for a bit of loading for the op-amp, some op-amps like AD826 can drive heavy loads so lower values are fine, about 68R-75R should be fine in all cases



The pic below should be easy to understand, it simply bypasses the chip on the bottom of the board thats on the analogue output, this does similar job to muting transistors, the sound is better without it IMHO, just add blobs of solder onto the little pads next to the chip



If you want to improve the measurement of the output stage and also reduce the DC offset to very low mV's change the following parts

R8,R9,R14,R17 to 30k
C32,C33,C34,C35 to 150pf

Measure the output L to ground and R to ground at terminal marked J5 on the board, it should now read roughly 1mV
You should be able to get rid of external C1,C2 (added above) without any damage from DC offset, I'd personally leave in C1,2 as a precaution though

[leo]

User gabdx PM'd me, I got the following after trying to reply

gabdx has chosen not to receive private messages or may not be allowed to receive private messages. Therefore you may not send your message to him/her.

[iihay]

Thanks leo, nice explanation, reducing down the DC is top on my list. I have a small problem with mine in that I have managed to lift the pads off on the chip underneath on the left channel. Is there any other way to do do this mod or should I remove the blob from the right channel to switch back in the chip on both sides?

Also I got carried away with cap replacement and pulled out the 2 25V ones on the rhs of the socket. On checking I don't have any decent 25V caps and I binned the ones I took off The best I can manage at present is 16V, I assume these will not do?

Cheers

Iain

[leo]

Hi Iain,

Hmm, let me think, its possible to still do the bypass, how hard would it be for you to desolder that SMD chip and remove?

16v caps is fine, highest DC for this dac is 12v for the op-amp decoupling

Cheers
Leo

[iihay]

Hi Iain,

Hmm, let me think, its possible to still do the bypass, how hard would it be for you to desolder that SMD chip and remove?

16v caps is fine, highest DC for this dac is 12v for the op-amp decoupling

Cheers
Leo

Should be ok removing that if I pick up some more braid tomorrow, depends if there is much danger of removing more pads which are then needed! Will I need to link some when it's off?

Actually would cutting the legs be an option here or does the thing need to go back on after?

Cheers

Iain