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Old 26th February 2009, 12:58 AM   #41
johnm is offline johnm  United Kingdom
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Think I misunderstood your previous instructions Kevin? My DAC also has the two 22k resistors in place. But I thought these had to be replaced by 36k items you said earlier? :

"The filter stage posted on lampizator.eu didn't look right to me and has just occurred to me that the reason there is a DC offset at the output of the first Op-Amp stage after removal of the coupling capacitors is that the value of R14 (22k on the circuit diagram) is incorrect. To achieve common mode rejection down to DC it needs to be the same value as R8 (36K). The 14.7K resistor is then no longer required."

If that's not the case then great - not a fan of de-soldering in general
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Old 26th February 2009, 08:19 AM   #42
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Hi John,

It was the fact that the schematic showed R8 to be 36k and R14 to be 22K that rang alarm bells. If these resistors are matched in value there's no problem. There's a bit more gain if they're higher values and, in fact, on my DAC they have increased C32 to 100pf accordingly(to keep the filter cutoff at about 41 KHz).

Best thing to do is to remove the caps and see if you have a DC offset.

Kevin
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Old 26th February 2009, 05:57 PM   #43
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Just got the LM4562s today, plugged the chip in replacing the NE5532 without any mod. Do observe a hugh difference between the two set of chips.

LM4562 - more bass, mid range vocal more forward. However, the LM4562 seems to have lost the transparent high though when compare to the NE5532!

Swapped between the two chip sets and listened to my favorite test CDs - still the same impression!

Being not a electronic handy as both Kevin and John, will have to see what their opinions are after having their mod experiment complete ;-)
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Old 26th February 2009, 06:20 PM   #44
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LM4562 take a few days if running in to sound their best. Before then they can sound cold and clinical. After that they sort of disappear and leave the rest of the DAC to do the 'talking'

If you like a bit more warmth then the OPA627s (2 on a Brown Dog adapter) are also fantastic.

- John
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Old 26th February 2009, 07:34 PM   #45
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OK I have just modded my 'BIG' Dac and...... all I can say is if you have one you MUST try this. Sitting here with a BIG grin on my face!

All I did was remove op-amp nearest the edge of the circuit board, and replaced the one nearest the center with a single LM4562.

I then replaced all the four caps nearest that chip (marked L+ / L- / R+ / R- ) with wire links.

I then soldered a wire from pin 1 of the op-amp to the left output socket, and pin 7 from LM4562 to right output socket.

So if you don't like this tweak it is easily reversable and no harm done... I somehow think you will keep it like this though

Seldom heard digital sound so good..... superb! No obvious problems with the lack of a muting circuit either thus far.

** NO resistors needed to be removed on my board, like Kevin's. But you may need to check your particular board as there are some variations **
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Old 26th February 2009, 08:24 PM   #46
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Addition: If you perform this mod check the DC offset. I didn't need to as my pre-amp performs DC blocking anyway, but it's worth checking.
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Old 26th February 2009, 10:19 PM   #47
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Glad to hear it worked.

Been listening to mine a bit more tonight and it sounds pretty sweet now.

Op Amp upgrade next, I think.

Kevin
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Old 26th February 2009, 10:29 PM   #48
johnm is offline johnm  United Kingdom
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Kevin, glad to hear you're enjoying yours. I'm looking forward to being able to wick the volume up tomorrow and give it a real workout

Just one thing, should I have taken the feed to the phonos from after that 1K resistor that you mentioned earlier, or is taking it directly from pins 1 and 7 ok? If so which of those two of the four caps is it best to take the output from please?

Thanks,

- John
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Old 26th February 2009, 10:36 PM   #49
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Ideally a little resistance would be a good idea in case the OpAmp takes exception to a difficult load and oscillates. 1K is probably a bit on the high side for that purpose anyway. A couple of hundred ohms would be better. Either way, if you're only driving a reasonably short length of coax it should be OK.

Kevin
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Old 27th February 2009, 04:23 AM   #50
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1).i sugest replacing first opamp with AD826 because of it's high slew rate and frequency response so critical in this position after the dac (it is a video opamp linear up to a few GHZ and has 350V/uS slew rate) just try it,you'll never switch back to LM4562...also AD826 can drive heavy loads and you can add just 22 ohm resistor on it's outputs directly to RCA socket without any coupling cap
2).bypassing the second opamp is a good idea
3).also i suggest removing all the feedback capacitors near the first opamp just like in the picture...they are big values and they have a cut off filter above 13khz so you'll experience briliant highs after taking them all out - and it WON'T oscillate without them because CS4398 has a digital filter inside filtering all the junk
4).also consider local decoupling of the +/-12v rails of that AD826 with one 10uF tantalum bypassed with 100n Film

Click the image to open in full size.

this is the frequency response after taking out all that capacitors that drive HF to ground and leave only that 100 pico green capacitor that is in the opamp's feedback (in the schematic it is written that it has 56pF value but in reality it is 100pF - it might be changed also to 56pF - it is for the opamp not to oscillate)

Click the image to open in full size.
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