DDF
100+ Head-Fier
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- Aug 11, 2006
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I received my Dac Ah this week and, after correcting the output resistors to 148 ohms (for the +6V supply) and tweaking the offset pot for 3.3V dc at the combined dac output to center the dynamic range, I have to say I was very disappointed. It sounded somewhat distorted and the bass would lose control. Compared to my bog stock MSB Link Dac, the Dac Ah was a real dog.
I took a few measurements to see if something was amiss. Since the bass was out of control, THD at 100Hz was measured first with the unit warmed up (to allow the dc levels to settle at the dac o/p’s):
http://www3.sympatico.ca/dalfarra/Da...h_Warm_THD.gif
then after 20 minutes in the freezer (!):
http://www3.sympatico.ca/dalfarra/Da...h_Cold_THD.gif
Distortion is definitely higher when cold, but nothing that would explain the bad sound, and the THD performance when warm only slightly exceeded the TDA1543 dac spec. For a 6V supplied version, I wouldn’t put heat sinks on the dacs. The files also show there’s a bit of power supply 60Hz, and probably more importantly, 3rd harmonic 180 Hz present, but its pretty far down.
Not shown are the IM measures, averaging about 0.1%. Also of note is that the THD spectrum is rich in 7th harmonic, which can sound edgy.
For reference, here’s the THD of the test file:
http://www3.sympatico.ca/dalfarra/DacAh/100HzDigi.gif
and of the Link Dac (pretty much at the limit of the test file).
http://www3.sympatico.ca/dalfarra/Da...nk_DAC_THD.gif
So, I dug back inside the Dac Ah and turfed the output op amp with the passive mod well described by Kim in another thread. I’m not fond of the dac ah’s o/p stage: it uses a non-inverting op amp with no dc block on the input. Effectively, the +ve and –ve leads are imbalanced by the dac dc o/p, 3.3V. Even the TDA spec sheet shows the better implementation, an inverting op amp with the +ve lead biased up by Vref. My unit was also shipped with AD847 high speed op amps. The original cct called for 5532s, and the 847s are much more particular about layout. For all I know the 847s may be oscillating (no high speed scope at home or I would have checked).
Instead of depopulating the op amp, I connected its output to ground through a 2.4kohm resistor (the 847 likes to see >1kohm), and ran a 3.3 uF good quality poly cap off the dac o/p to the RCA jack. This is going to be used with a Govibe 5, which has an input impedance of 8.75kohm at full volume, and 9.5kohm at min. The 3.3uF was the min size, to avoid premature bass roll-off. Here’s a little excel spreadsheet to show what I mean:
http://www3.sympatico.ca/dalfarra/Da...y_Response.xls
I hooked it back up and instantly: BANG! THIS is what everyone’s been talking about! Before the passive mod, the drums on Kyuss’ “Thong Song” sounded like they were about to explode into tinder, and the strings of the guitar on the Beasts of Bourbon “Not Gonna Try No More” sounded like they were scraping against the fret board. Honestly, pre passive mod, the Dac Ah is a door stop (well, at least mine was). Now: not bad! Mids are forward, but that’s good and the high’s cleaned right up. The thing’s still a bass monster, the DT770 of dacs
but it’ll make a decent dac for work.
Here’s a final THD measure after the passive mod:
http://www3.sympatico.ca/dalfarra/Da...iveMod_THD.gif
The picture doesn’t come close to doing the change justice, but what does stick out is that the nasty 7th has decreased significantly, as has all the “hash” up at the top end. I don’t know if this explains the difference, but it’s a good thing. The pictures also show that about 6 dB output is lost with the passive mod.
In the end, I think I’ll be happy with the dac ah, but out of the box it’s fatally flawed:
-wrong dac output resistors so output clips
-wrong reference voltage bias so it clips asymmetrically (+ve peaks don’t clip at the same levels as –ve peaks)
-very high speed op amps were used in a design proven in with low speed ones, without adding any required decoupling caps on the power pins
-it uses a flat out dumb output stage design
But after fixing all this, it’s a great way to spend $105. The bass is still a bit silly (exaggerated and not that tight), but nothing eq can’t fix.
Dave
I took a few measurements to see if something was amiss. Since the bass was out of control, THD at 100Hz was measured first with the unit warmed up (to allow the dc levels to settle at the dac o/p’s):
http://www3.sympatico.ca/dalfarra/Da...h_Warm_THD.gif
then after 20 minutes in the freezer (!):
http://www3.sympatico.ca/dalfarra/Da...h_Cold_THD.gif
Distortion is definitely higher when cold, but nothing that would explain the bad sound, and the THD performance when warm only slightly exceeded the TDA1543 dac spec. For a 6V supplied version, I wouldn’t put heat sinks on the dacs. The files also show there’s a bit of power supply 60Hz, and probably more importantly, 3rd harmonic 180 Hz present, but its pretty far down.
Not shown are the IM measures, averaging about 0.1%. Also of note is that the THD spectrum is rich in 7th harmonic, which can sound edgy.
For reference, here’s the THD of the test file:
http://www3.sympatico.ca/dalfarra/DacAh/100HzDigi.gif
and of the Link Dac (pretty much at the limit of the test file).
http://www3.sympatico.ca/dalfarra/Da...nk_DAC_THD.gif
So, I dug back inside the Dac Ah and turfed the output op amp with the passive mod well described by Kim in another thread. I’m not fond of the dac ah’s o/p stage: it uses a non-inverting op amp with no dc block on the input. Effectively, the +ve and –ve leads are imbalanced by the dac dc o/p, 3.3V. Even the TDA spec sheet shows the better implementation, an inverting op amp with the +ve lead biased up by Vref. My unit was also shipped with AD847 high speed op amps. The original cct called for 5532s, and the 847s are much more particular about layout. For all I know the 847s may be oscillating (no high speed scope at home or I would have checked).
Instead of depopulating the op amp, I connected its output to ground through a 2.4kohm resistor (the 847 likes to see >1kohm), and ran a 3.3 uF good quality poly cap off the dac o/p to the RCA jack. This is going to be used with a Govibe 5, which has an input impedance of 8.75kohm at full volume, and 9.5kohm at min. The 3.3uF was the min size, to avoid premature bass roll-off. Here’s a little excel spreadsheet to show what I mean:
http://www3.sympatico.ca/dalfarra/Da...y_Response.xls
I hooked it back up and instantly: BANG! THIS is what everyone’s been talking about! Before the passive mod, the drums on Kyuss’ “Thong Song” sounded like they were about to explode into tinder, and the strings of the guitar on the Beasts of Bourbon “Not Gonna Try No More” sounded like they were scraping against the fret board. Honestly, pre passive mod, the Dac Ah is a door stop (well, at least mine was). Now: not bad! Mids are forward, but that’s good and the high’s cleaned right up. The thing’s still a bass monster, the DT770 of dacs
Here’s a final THD measure after the passive mod:
http://www3.sympatico.ca/dalfarra/Da...iveMod_THD.gif
The picture doesn’t come close to doing the change justice, but what does stick out is that the nasty 7th has decreased significantly, as has all the “hash” up at the top end. I don’t know if this explains the difference, but it’s a good thing. The pictures also show that about 6 dB output is lost with the passive mod.
In the end, I think I’ll be happy with the dac ah, but out of the box it’s fatally flawed:
-wrong dac output resistors so output clips
-wrong reference voltage bias so it clips asymmetrically (+ve peaks don’t clip at the same levels as –ve peaks)
-very high speed op amps were used in a design proven in with low speed ones, without adding any required decoupling caps on the power pins
-it uses a flat out dumb output stage design
But after fixing all this, it’s a great way to spend $105. The bass is still a bit silly (exaggerated and not that tight), but nothing eq can’t fix.
Dave