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Source Modding

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sachi

Active Member
Joined
Aug 4, 2006
Messages
518
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Location
Portland, Oregon
This thread is carried over from another forum, but figured i'd copy and paste it here. Source being very important in the chain (a fact that i realized quite late) and with a few people starting on DIY projects with DAC builds i thought this might be helpful. Not gauranteeing that these will work for your DAC so YMMV.

Most sources have DACs with a compromised I/V stage which is usually followed by a recon filter and /or buffer.
These two stages are the most crucial as they are essentially the analog output stage on the "DAC" source.

It so happened that I had an opportunity to buy a Sonic Frontiers Assemblage DAC 2 which uses dual PCM1702 DAC chips with the famed PMD100 digital filter. It uses a CS8412 for the digital receiver.

The one I bought came with caddock resistors and multicap capacitors int he I/V and buffer sections.
This was offered as an upgrade option if I remember by pats connection back in the day. So for 130$ shipped I was mighty impressed.

These small DACs have a reputation as giant killers. They retailed at 400$ in the late 90s.

Anyway..here are the mods I did

1> Replaced all old electrolytic caps in the DAC and I/V section with newer Nichicon Muse caps
2> Replaced I/V opamp with THS4031 opamps (adapter + opamps cost 12$ for each channel. Adapter is high quality one used in high freq applications and cost 7$ each)
3> AD797 for recon filter+buffer section. (9$ each)
4> PPS Coupling caps between the rails and NPO ceramics between rail to ground for all opamps

5> Last but not least K grade PCM1702 DAC chips (25$ per channel). This gives lower Inter modulation distortion and lower THD.

The transformation has been amazing. Everything is a lot livelier..better soundstage. The bass has just completely transformed. Tight and fast with plenty of impact.

I am totally sold on the fact that the THS4031 opamp is the best opamp out there for I/V and the AD797 is superlative in its filter role.

Total cost of this pgrade was about 120$ bringing the total i have spent on this DAC to 250$.

I won't make comparisons but suffice it to say..this little DAC bats way high up in the order.

Some pics

Before


After






This DAC got sold and has been replaced by its bigger brother, The Assemblage DAC 2.7
The one i bought was a platinum version and was kitted out quite well with very little room for improvement that I could conjure up.

It came with OPA627s for the I/V and filter and OPA604s for the DC servo.

I replaced the I/V and filters with THS4031 and AD797s respectively.
The OPA604 was replaced with an OPA227 which had much lower DC iinput offset voltage, 30dB better PSRR, CMRR and THD specs. Replacing the DC servo alone made a big improvement in black levels. Sound got crisper too.

I then proceeded to add decoupling caps, with bigger 3.3uF Wima film caps across the rails for the I/V and filter. ). Also put 0.47uF NOS Siemens styrene caps across the DC servo opamp.

One last step I did was replace themulticap PPFMX polypropelene caps on the I/V and filter section with Multicap RTX and Relcap RTE polystyrenes.

I know people give a lot of crap to DACs with an opamp output stage, but honestly, this DAC has to be heard to believe how good it is.

I recently finished a Pass Labs D1 clone JFET I/V stage for another DAC using similar DAC chips(different filter, DF1704). The difference is soo huge that I couldn't bear listneing to the one with the JFET discrete I/V stage.
WIll psot pics of that amp tomorrow.

Pics of the Assemblage DAC 2.7 for now.

dacmod002.jpg


The result left me :yahoo: :eek:hyeah:

All IMHO of course :D
 
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sachi

Active Member
Joined
Aug 4, 2006
Messages
518
Points
28
Location
Portland, Oregon
Very complicated job & nice work from you.

thanks..the mods are easy enough for anyone who's comfortable using a soldering iron.

THe one thing missing on the 2.7 is a proper reclocking or in other words a secondary PLL and a VCXO. It doesn't have a secondary PLL and uses a traditional crystal oscillator. :sad:
The 3.1 includes these two which aid in significant Jitter reduction. Very very few DACs in the under 2500$ range employ this.

I am still only scratching the surface when it comes to DAC and output stages but have had the good fortune to listen to a lot of very good DACs to get some kind of reference to go with..
I have a bunch of opamps I want to roll on the output filter, including the OPA627, LME49710, OPA1611,etc. The AD797 brings crisp highs, an air around the instruments to the table but sounds a tad analytical.
 
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Kamal

Active Member
Joined
Mar 27, 2007
Messages
721
Points
43
Location
New Delhi
Sachi you seem to be in a "golden place" where easy availability of good quality used eqpt/& parts for modding is a given.
What kind of inputs/outputs does the SF DAC offer-this is to elicit some info for Viren's DAC project.
 

sachi

Active Member
Joined
Aug 4, 2006
Messages
518
Points
28
Location
Portland, Oregon
Sachi you seem to be in a "golden place" where easy availability of good quality used eqpt/& parts for modding is a given.
What kind of inputs/outputs does the SF DAC offer-this is to elicit some info for Viren's DAC project.
Well the input options on the DAC are all SE (the 3.0 and 3.1 versions are balanecd DACs) in the form of BNC coax, AES/EBU, Optical. Output is just a single set of RCAs.

I'd be very interested in reading about Viren's DAC project and would love to join in as well. I have a wish list of stuff I want in a DAC :p

1> WM8804/5 for the digital receiver
2> Async USB input
3> Custom DSP filter or PMD100 or PMD200 compatible (PMD200 might not be feasible at all)
4> High Precision VCXO for the clock (this could be used as a master clock and fed back to the transport)
5> Secondary PLL to address jitter
6> PCM1704-K grade DAC chips
7> Discrete or opamp based I/V with zero global feedback.
8> NPO COG ceramic capacitors for bypassing
9> MLCC where decoupling and large bypassing caps need to be used
10> Mills or PRPs in the analog section
11> Polystyrenes in the analog section for caps
12> A super jung regulator for the power supply rails at the bare minimum.



good job!! those premium parts seem to be all coming from P***y ;)

Enjoy modding as its addictive...... :yahoo:

Indeed! :cool:
 

sachi

Active Member
Joined
Aug 4, 2006
Messages
518
Points
28
Location
Portland, Oregon
As promised..just finished wiring it up..

Elektor Audio DAC 2000 with Pass Labs D1 clone JFET I/V output stage.
Only has a Coax input. Thinking of switching that to a BNC input.

dacjfetiv001.jpg


Simple face plate..just a power indicator.

dacjfetiv002.jpg
 

kaushik

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Mar 3, 2008
Messages
2,550
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113
Location
Bangalore
Serious diy activity .. we all love it.... !
Keep the pics flowing ,you seem to have pretty good history with DACs..
 

spirovious

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Apr 10, 2008
Messages
12,622
Points
113
Location
Mumbai
I love to see all DIY.You are just too good in that,I can't do much other than replacing circuits,lens etc.
 
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