Dual WM8740 DAC question

skyra

Member
2017-11-15 9:45 pm
Hello Everyone!
I stumbled upon a chinese DAC board, that have a few strange anomalies...
The circuit in question is the ominous "Dual WM8740 DAC" discussed in numerous other topics.
When arrived, I noticed that several parts on the board have actually rather different values mounted than what is printed on the PCB.
For example: 680 pF instead of the marked 6.8nF, 4.7nF in place of 3.9nF, 100pF instead of 1 nF, also some filter caps with different values in the power supply part.
My question: Is this just the usual chinese sloppiness, or maybe (?) due to some improvement in the circuit? (doubt) I know it is an outdated design, but still doesn't sound too bad even in this form. Anyone with similar experience?:rolleyes:
 

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miro1360

Member
2010-03-20 3:20 pm
things you should do for a better sound:
- bypass coupling signal capacitors (if any)
- remove the strange opamp filter and use only an RC filter
- use a better opamp
ask the seller for a schematic, then I can mark there what components to remove and what to short

for your original question:
the other capacitor values only change the resulting filter shape, the filter is probably a sallen-key type ... why have they changed the values? they used what was in stock
 

skyra

Member
2017-11-15 9:45 pm
Thanks for the advice, I already did the changes You've mentioned. (filter caps, +bypass, rect. diodes, opamps). There are no coupling caps in the signal path (thankfully:)) Also replaced the suspect parts with the factory recommended values (Wolfson's dual differential output schematic). Furthermore changed the DAC-s internal filter to slow rolloff, (leg 27 to +Vdd) which resulted in a smoother sound. For the money this is a surprisingly nice performer...
 

diyiggy

Member
2019-01-16 12:22 am
Hi,


Most of the time the dac chip if genuine (not always with cheap chineese boards) is not the culpritt. Layout of the boards, active and passive parts are. Power supply regulation is often made with cheapest choice as the crystal. Sometimes if not always (as to see with the low price).

Passive and active parts are not always genuine or simplier than their marking according my experience which is a little old now (things imprved since ?... not sure!). Sometimes the vendor is ok (often when a factory name is involved)


But such boards are very good to learn to tweak and jump in the hifi diy.


No better advice than miro1360 : you need the shematic often the vendor provides by email. Then check if the values on the shematic and some active parts are the same than the physical board. Without, one is in the dark.


If you can desolder, you should play with some cheap reliable aop not asking for too much sota layout by adding a DIL8 adaptator for swaping different DIL8 operational amplifiers -aka the filter described above. That also means to no spend too much monney in relation to the board cost.

First you have to understand- we don't know about your exact knowledge yet?- if the oaps are single or double model . For example the double OPA2864 gives often good results. It doesn't say the passive parts value are ok with suxh swap but such experiment are good to understand and learn.
If you are not beginner forgett what I said and just know most of time it's not so the dac chip quality you mainly hear wjth those boards (another boards with the same dac chip may give better or worst result.
 

miro1360

Member
2010-03-20 3:20 pm
For the money this is a surprisingly nice performer...

I agree, the WM8740 is nice sounding delta-sigma ... I don't think it will be fake (can be recycled, if it works I wouldn't worry about it, the worse case would be an recycled R-2R in SMD package (for example PCM1704 - never buy this from ebay, they recycle it with unregulated hot-air and they degrade or destroy it completely))
the next very important note: avoid the opamps from ebay (use only trusted sources) ... opamps from ebay are cheap ones remarked to a higher class

as diyiggy mentioned, you can improve the clock (which is critical for delta-sigma) ... but if you're already happy with the sound, just try different opamps and enjoy it ;)
 

phase

Member
2004-10-04 11:59 pm
Might want to see if there is another option for the usb input, pcm27xx aren’t the greatest.

As for the output filters, you’re better off to trace it out and make a schematic in order to see what you have. Chances are it’s the Wolfson circuit from the data sheet or a copy of an Arcam circuit even.
I wouldn’t bother running straight out of the 8740.