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Old 19th May 2020, 08:30 PM   #61
IanAS is offline IanAS  England
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Quote:
Originally Posted by Alfanijs View Post
TDA1305 mentions that the output is capable of driving a line output directly. So I bypassed the analog buffer stage completely
Isn't there some filtering in with the op-amps?
Quote:
Originally Posted by Alfanijs View Post
This bypasses the jittery 1315.
Where did you join the IS so as to maintain the differential inputs for the 1305?
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Old 22nd May 2020, 10:16 PM   #62
Alfanijs is offline Alfanijs  Netherlands
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Quote:
Originally Posted by IanAS View Post
Isn't there some filtering in with the op-amps?
Not that I can see. The TDA1305T datasheet states clearly that only a 1st-order LP filter needs to be implemented:
"Connecting external capacitors CEXT1 and CEXT2 between FILTCL and VOL and between FILTCR and VOR respectively provides the required 1st order post filtering for the left and right channels (see Fig.1). The
combinations of RCONV1 with CEXT1 and RCONV2 with CEXT2 determine the 1st order fall-off frequencies."
The internal RCONV = 2k; with CEXT=1nF this gives a -3dB point of 79.6kHz. In the DACMagic implementation this is followed by a second passive 1st order RC filter (R=680, C=1nF) with a higher cut-off frequency of about 234kHz. This is where I stole my outputs from.
The opamp buffer does not appear to perform any additional filtering.


Quote:
Where did you join the IS so as to maintain the differential inputs for the 1305?
Directly after the 1315, before the combinational logic. Unfortunately, the 1315 does not tristate and always outputs a signal, so it had to be isolated. The tricky part was to locate where to do this on the PCB. After some headscratching it turned out that only 3 PCB traces needed to be cut, all on the top side. See attached pictures for my birdsnest test setup.

I added a SN74HC157 to beef up the signal level, and also use this to switch between the RbPi/HifiBerry and CD inputs. Works nicely; just let me know if you would like more details.
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File Type: jpg IMG_8442_s.jpg (678.6 KB, 56 views)
File Type: jpg IMG_8443_s_detail.jpg (118.5 KB, 66 views)
File Type: jpg IMG_8444_s.jpg (831.2 KB, 50 views)
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Old 25th May 2020, 08:09 AM   #63
Alfanijs is offline Alfanijs  Netherlands
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Default Passive output stage

Quote:
Originally Posted by Puffin View Post
I did it to a LiTe Dac Ah with some nice poly output caps and a cheapo board I got from ebay some years ago. I was going to try some output transformers but didn't have the money at the time. I didn't think that the mods were necessarily better, but different. I might give it another go on the DM.
Isn't the LiTe Ah a current-output DAC? This would require an I/V stage as well, not such a straightforward mod.


Would be interesting to compare experiences for the DACMagic!
I used 2.2uF WIMA MKPs and was pleasantly surprised.
Will expand to differential setup later, needing 4 caps; suggestions for better, reasonably priced output caps are welcome.

Last edited by Alfanijs; 25th May 2020 at 08:12 AM. Reason: typos
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Old 25th May 2020, 03:43 PM   #64
Alfanijs is offline Alfanijs  Netherlands
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Exclamation Op-amp filtering

Hi IanAs,
In reply to your question:
Quote:
Originally Posted by IanAS View Post
Isn't there some filtering in with the op-amps?
I said that "the opamp buffer does not appear to perform any additional filtering". This is not entirely correct; there is a small capacitor in the feedback loop which I missed...
Since I have been intending to learn Spice for quite some time, I took this opportunity to model the output stage and simulated it. The opamp does add what looks like another 1st-order LP filter, at a frequency close to the preceding passive RC LP filter. Not really important I guess, but wanted to mention it anyway.
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