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Old 14th March 2009, 07:06 PM   #131
Mush is offline Mush  United States
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Thanks Lux54.
I've avoided using anode follower as last stage because of impedance considerations. Wouldn't an anode follower dc coupled to a cathode follower or even an SRPP be better for driving signals across interconnects?

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Old 14th March 2009, 09:41 PM   #132
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Quote:
Originally posted by Segran


You get a schematic from the guy selling the DAC if you ask for it - at least I did, after buying it!

Unfortunately the schematic is a bit "schetchy" and some component values don't match what's actually put in the PCB.

I haven't asked for the PCB layout, but if I was selling this DAC and trying to make a living from it I wouldn't give away the layout!

For the price, together with a publicity design's being gaining, publishing basic PCB layout would be great marketing move. I wasn't really after the gerber files - just to see if the PCB layout in general is any good. Not making them available (schematics and PCB layout) at least in their basic block-diagram form, could be good enough reason for suspicion...

Boky
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Old 14th March 2009, 09:52 PM   #133
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Quote:
Originally posted by luxury54
if it is not much trouble for you to build the tube output stage then don't bother with any of these operational amplifiers and analog stage modding...
i used this schematic after trying various cathode followers and stuff...this will wipe the floor in a comparison with any of yours or your friend's expensive cd-players

Click the image to open in full size.

for good results you must use 6,3v stabilised DC on the filament (not AC),and all grounds connected in a star fashion
(DAC Gnd + pin9 on tube socket + the "-" of the filament + the "-" of anode supply + RCA out Gnd)
also remember to take signal directly from L- and R- of the DAC since this schematic inverts the phase
Since you have a soft spot for tube output, give fast switching soft recovery diodes a go, in particular in DC filament supply...

Boky
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Old 14th March 2009, 11:18 PM   #134
Segran is offline Segran  Sweden
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Quote:
Not making them available (schematics and PCB layout) at least in their basic block-diagram form, could be good enough reason for suspicion...
To tell the truth I haven't asked for the layout. Might be worth while to as Lawrence for the silkscreen print, showing the components.

There are labels for the resistors as well - under the components... And showing the resistance value, not the number...

Quote:
i think these chinese guys used the lowest quality solder they put their hands on
I can agree that the solder used is bad, I used the same method to clean the holes (removing components, filling fith new good solder and sucking the holes clean).

Quote:
Warning to those that are thinking about this board.
If you don't feel up to modding this board for what ever reason, have a look at this:http://www.beis.de/Elektronik/ADDA24QS/ADDA24QS.html
The DA24QS as a kit is twice the cost of the BIG DAC. You can buy bare PCB's if you want as well. It even comes in a balanced output version. I haven't tried this DAC myself, but it seems to be very well engineered. Component values for the analog filter looks a bit strange but does the job. The only problem I can see is a 0,4-0,5 VDC offset on the output of the first OP, but changing the components according to AN-84 will solve that (same as on BIG DAC). All components are thru hole (except the chips from Crystal and the PLD) so modding would be easy. You still need to be "semi-pro" with the soldering iron as the Crystal chip's are TSSOP footprint.

I'm happy with the BIG DAC after the mod's I've put in, so I'll stick with it. At least for now. And for the price of it, I regard it as good value.
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Old 15th March 2009, 11:31 AM   #135
legarem is offline legarem  Canada
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Quote:
Originally posted by Segran


I can agree that the solder used is bad, I used the same method to clean the holes (removing components, filling fith new good solder and sucking the holes clean).

I
I was asking me if this solder was really bad or if it was because the parts were soldered with ROHS solder that doesn't contain any lead.
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Old 15th March 2009, 12:05 PM   #136
legarem is offline legarem  Canada
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Quote:
Originally posted by luxury54


Hi,how do you like the sound characteristic of AD826?

it is a very fast opamp with 350V/uS slew rate compared to only 20 of LM4562 ,and in this place after DAC it is imperative to have such a good slew rate and wide frequency capability, that's why also tubes sound great because they have a very high slew rate never masking the microdetails (it is a video opamp linear up to a few GHZ,so is perfect in audio and will not oscillate easily)
i don't like transformers,i have the feeling they are more mellower and laid back than tubes kind of compressing and leaving only macrodinamycs ,masking the small micro details, it's about lazy rise times,so another slew rate thing...
I don't want to offend you but it seems that 'theory' is really more important for you than anything else.

First

A very fast op amp doesn't gives ANY garantee of good sound. Again perhaps on paper. Try a LM7162 and give me your results. Video op amps were NOT designed for audio. BTW LM4562 doesn't have bass and don't play well if they are not broken in.

Tubes have a very high slew rate ? Not sure, really not for most of them

The slew rate is measured by using a square wave and calculating the number of volts that the square wave rises vertically on the Y axis, over the amount of time it takes for this voltage rise to occur, on the X axis. If the slew rate is high, the sound will tend to be "crisp" with a bit of "edge" to it, while a low slew rate gives a sound that has less of these characteristics to it.

Look at the square waves of tube gear and transistor gear and tell me which ones 'often' have better squares waves. Musicality is still another thing but we can't measure this parameter


So you don't like transformers

Again your feeling like you wrote is based on what ? Mediocre vintage transformers that rolls off highs ? Come on TRY real things so you could change your minds. Lundahl transformers (and probably some others) on my dac doesn't sound slow, laid back etc like your description.

Like I wrote on a previous message Heat your soldering station, experiment and listen instead of reading specs.
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Old 15th March 2009, 12:34 PM   #137
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dear legarem,in this place the opamp is only a sumator between the + and - alternances of the DAC amplifying only the differences between alternances and masking all that is in common (any kind of digital or supply noise) so this is a must to have a high slew rate here for being able to follow and reproduce all the low level details...
you don't think that tubes have a high slew rate?
then what about electrons heated up by the filament that have the capability of the speed of light in vacuum,because it is really vacuum inside
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Old 15th March 2009, 08:58 PM   #138
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Right, a couple of photos I took while updating the filter components:

Click the image to open in full size.

Filter area with components removed. The four blue 1nf capacitors are correct for the new design so they stay. The rest go.

Note that all four resistors on the left hand side in each channel labelled 22k or 36k (22k fitted in all cases) are changed to 13.7k. The two resistors labelled 6k8 in each channel are replaced by 3.32k resistors. The caps near these two resistors labelled 56p and 102 are repaced by 220pf. The resistors marked 1k are in series with the first Op Amp stage outputs. I replaced these with 75 ohm resistors. The two missing caps on the right hand side, where the yellow wire is attached, used to sit between the two stages. I'm now taking the output from here and have junked the second Op Amp stage.

Note also that the 4 electrolytics on the left hand side between the DAC and Op Amp stage are gone.

Click the image to open in full size.

New filter components in place. This photo was timestamped 1 hour and 40 minutes after the previous one. That's how much fun I had soldering in the new components!

Click the image to open in full size.

Some power supply decoupling under the Op Amp and one of the 220pf caps had to go on the underside.

Click the image to open in full size.

The four decoupling capacitors were bypassed by a solder bridge on the underside of the board.

I hope this is helpful for anyone making these changes. I will try to draw a schematic of this part of the board and also annotate one of these photos so we can see what components are where.

Kevin
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Old 15th March 2009, 11:46 PM   #139
johnm is offline johnm  United Kingdom
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Superb work Kevin - thanks a bundle for those pictures

For the two power supply decoupling caps did you just scrape away some of the circuit board protective layer and solder the ground ends to that? It appears you've done that in the picture?

Cheers,

- John
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Old 16th March 2009, 12:05 AM   #140
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Yep, exactly that. There's a big area of ground plane under the chip.

Kevin
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