Yes. The bass is the first thing I noticed.
I recently bought a used Mk 1 DAC which I plan to heavily modify. Iāve read through this entire thread to glean as much info as possible, so I really appreciate all the work folks have shared, and particularly the info shared by Ted Smith.
I will likely build a new chassis to provide enough room for the mods I am going to make, and to improve shielding and vibration control.
The mods I have in mind are as follows:
New 12V DC supply for the analog board. This will be a custom build with an R-core transformer, tuned snubber circuit, active rectification, cLC raw DC filter with a fairly large choke, feeding a discrete regulator. The regulator design is based on a circuit by tombo56 on DIYAudio. This supply will be able to source a few amps with very low noise (<5uV).
Upgraded (or possibly new) supply for the various 5V supplies, adding tuned snubber circuits on the transformer secondaries, replacing the rectifiers with fast-recovery schottky devices, beefing up the filtering, and replacing the regulators with lower-noise options.
Replace the AC input connector - probably Furutech.
Bypass the Op-amp based secondary regulators on the analog board for the output stage and use Sparkos 9V regulators (one for each channel). This will increase the Op Amp power supply voltage from 7V to 9V.
Replace the Vocm resistor dividers with an active circuit (one for each channel) using a TI REF5045 voltage reference buffered with a Analog Devices LT6202 opamp. The current R5/R6 resistors will be removed. This will set the Vocm at 4.5V (half of the power supply voltage.
Replace the AD8132 op amps with AD8139. The AD8139 has lower noise and lower offset voltage. After making this swap, Iāll see what the DC offset voltage is and come up with something to reduce this if itās too high.
I may also experiment with adjusting the feedback circuit to increase the gain a bit.
Replace the output transformers with APS v2 transformers.
Remove the circuitry after the transformers and connect the secondaries directly to high-quality panel mount XLR connectors (removing the PCB mounted XLR and RCA connectors).
Remove the filter chokes on the XMOS USB input IC so that this is completely disabled.
I have a couple other projects Iām trying to finish before starting to tackle this, but I have already designed the circuit boards for the 12V supply and the active Vocm circuits. But it will probably be mid to late summer before I can hear what this sounds like.
Iād appreciate any comments or feedback on these ideas.
Great plans! Thanks for sharing!
You probably know, but I will still reiterate. 5V supplies are on the stock PSU, which not only serves as a power supply but also a hub connecting the processor/display board and the input board. I think you will have to keep it because of the ribbon cables connecting the boards have certain specs (most likely, impedance is the most important one). @tedsmith could clarify this.
@jaytor you can disable XMOS USB chip by removing L1 and L2 - the ācleanupā inductors on power rails right before the chipās power pins.
Thanks, yes Iām aware of the additional functions of the existing power supply board. I can replicate the connections to the SD card reader, display controller, and digital input board on a newly designed pcb, but Iām not sure the benefits would be worth it (making the modifications a bit cleaner and saving some space). Once I get the DAC apart, Iāll look at it more carefully and decide.
Thanks, yes thatās what I meant by
Are you going to order a PCB from tombo56 and populate it with the devices he recommends? Or are you going to completely design your own board?
I designed my own board. I wanted to add a good snubber circuit that I could tune to the characteristics of the transformer secondary, and use choke filtering to reduce current spikes. I also incorporated a bridge/resistor/cap circuit to connect output ground to chassis ground.
The board size is also optimized to be the same width as the transformer and choke to reduce any wasted space in the chassis.
Also, the heatsink that tombo56 used for his PCB design is no longer available, so I designed the PCB around some heatsinks I had on hand.
But otherwise, the regulator circuit itself is pretty much the same as tombo56 designed it.
I couldnāt agree with you more. Best sounding DAC I have heard. Bravo to Ted Smith & APS.
We need more members like you here. Seem you know what you are doing.
Thank you. I spent the early part of my career doing digital electronics (computer graphics, video, and super-computers) but hadnāt really done much analog design work since college (last 70s). Iāve been an audiophile since before college, but once I started my career, I didnāt have the time to build my own gear.
About six years ago, I decided to try building a Firstwatt clone (F5 Turbo monoblocks) and got a real kick out of it. Since them Iāve built a much of stuff (four pair of speakers, two pair of subs, four line stages, three stereo amps, four pair of monoblock amps, and a streamer - some tube, some SS).
I donāt have nearly the knowledge and experience as some of the DIYAudio contributors, but I have enough of a background to pick stuff up fairly quickly, so my builds have gotten more sophisticated as Iāve learned.
Iām currently using a Denafrips Terminator Plus. The DAC is the last component in my system that isnāt DIY. Iām hoping that with the mods I have planned that I will be satisfied replacing my Denafrips with the modified DirectStream.
Hereās an example of one of my latest builds - a two-chassis DHT linestage using the EML-20A mesh plate tube. This is fully my own design and I love the way it sounds.
I just retired at the beginning of this month, so I have more time to play with my electronics and listen to music. Yeah!!
Pretty!
Nice!
I had a Denifrips Terminator Plus 12th connected to my Denafrips Hermes 12th. The APS Directstream was the winner. The Terminator was sold. I still use the Hermes.
Beautiful work!
Weāre all can learn from jaytor on how to do proper modifications. True CUSTOM !
Yes we can.
Please tone it down everyone.
Have you been able to listen to a comparison with PGM?
We had a group gathering of five individuals, four of whom are experienced audiophiles. We used his Airlense external streamer to ensure fairness. We compared the new PMG, the APS LESSP, and a MK2 with APS transformers. All DACs were matched for level and used the same cables and preamp. In the end, we all agreed that the sound between the PMG and APS LESSP was so similar that we had a hard time discerning any significant difference though there were some. The MK2 was just not as clear and revealing. The APS LESSP appeared to have a slightly more concentrated, deeper pinpoint imaging, while the PMG exhibited a somewhat broader stage. Both of them sound incredibly amazing and very clear with nice tight extended bass. The PMG seems to be less forgiving on some not so good recordings. I am extremely pleased with the performance of the LESSP, and we all liked them both a lot. I have no desire to upgrade, especially considering the price tag. The APS LESSP is a bargain. My friend was pleasantly surprised by the sound quality. He mentioned that there is a new firmware available, which he plans to install this weekend. In our next meeting, we will compare the PMG with its built-in streamer and updated firmware to the APS LESSP with my custom streamer.
What is this?