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Reference DAC Module - Discrete R-2R Sign Magnitude 24 bit 384 Khz
Reference DAC Module - Discrete R-2R Sign Magnitude 24 bit 384 Khz
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Old 8th April 2015, 10:28 AM   #2391
Dimdim is offline Dimdim  Greece
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Join Date: Sep 2008
Location: Athens
Reference DAC Module - Discrete R-2R Sign Magnitude 24 bit 384 Khz
Last night I took a few measurements of my DAM. The measurements were not intended to determine the absolute performance characteristics of the DAC, but rather to determine the influence of different grounding schemes on its performance.

After I had installed a pair of alpha 20 line stages I had measured my DAM's noise floor at -87dbs, which was suspect at best.
The installation of the α20s had coincided with me changing my plastic IEC with a metal one, that also provided filtering.
Metal meant that the main's GND would be connected to my chassis. You may remember that the Soekris does not have insulation on its mounting holes, so if you are using metal standoffs to mount it you are effectively connecting your signal ground to your chassis.

To make a long story short, isolating the DAM from the chassis improved the noise floor by a whopping ~23dbs. It also made significant difference in the dynamic range, THD, etc.

So, beware of your grounding (ok, this goes primarily to the "noobs" at audio DIY).

Then I decided to check something else out. I took measurements both with the serial port connected to my PC and without.

Having the serial port connected did at least some damage to most of the measured aspects. For example the noise floor went up by almost 14dbs. So I wouldn't recommend having the serial port connected to the PC at all times.

But you should keep in mind that the PC that took the measurements was the same one that was connected through its serial port, so that probably accounts for some part of the "damage". I'll come back with more data.
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Old 8th April 2015, 10:37 AM   #2392
bambadoo is offline bambadoo  Norway
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Super. Thanks for information. I have the serial connected the whole time between my rpi2 and the dam1021. I am unable to hear any difference in sound with the serial cable connected or not.
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Old 8th April 2015, 11:29 AM   #2393
spzzzzkt is offline spzzzzkt  Australia
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Location: Melbourne
Quote:
Originally Posted by bambadoo View Post
Super. Thanks for information. I have the serial connected the whole time between my rpi2 and the dam1021. I am unable to hear any difference in sound with the serial cable connected or not.
I think DimDim is overstating the issue, but I guess that depends on your setup.

I've attached two spectrum analysis I did a few weeks ago. Both are done with the TOSLINK connected but no signal, so the DAM is unlocked. Volume is set to V+00.

The main peak of noise with serial connected is at 80kHz, and I suspect this is related to the onboard switching power supply on the serial interface chip. There is a second peak at 40kHz, and third lower peak at around 46-8kHz. There are other minor differences close to the noise floor of my setup.

I have the transformer in an ABS box, with an umbilical with +/-7.5VAC running to the DAM.
The only grounding of the DAM is currently via the XLR jacks.

I'm using a USB->RS232 adapter, hooked to a MacBook Pro.

Setup is not calibrated, so I make no claims beyond relative differences between the two.
Attached Images
File Type: png K8_DAM_TOS_V0_nolock.png (103.5 KB, 757 views)
File Type: png K8_DAM_TOS_V0_nolock_SER.png (109.3 KB, 748 views)

Last edited by spzzzzkt; 8th April 2015 at 11:38 AM.
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Old 8th April 2015, 11:41 AM   #2394
Dimdim is offline Dimdim  Greece
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Reference DAC Module - Discrete R-2R Sign Magnitude 24 bit 384 Khz
Like I said, the testing was done with a common ground between the DAM and the PC doing both the testing & the serial connection, so that might have something to do with the problem. I will definitely do some more testing, including the influence that a serially connected Arduino may or may not have.
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Old 8th April 2015, 02:57 PM   #2395
ristar is offline ristar  Singapore
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With my setup there is a very audible noise when using a real PC serial port (in fact, a mouse scroll and all can be heard). With a USB->RS232 adapter, I don't get any noise.
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Old 8th April 2015, 03:31 PM   #2396
Dimdim is offline Dimdim  Greece
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Reference DAC Module - Discrete R-2R Sign Magnitude 24 bit 384 Khz
I forgot to mention that in my setup I'm using a USB to Serial adapter.
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Old 8th April 2015, 08:58 PM   #2397
uncola is offline uncola  United States
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Join Date: May 2012
hey guys I decided this will be my first DIY project other than cables(I made a few to learn to solder). Since I am a newb, should I wait til the firmware gets updated to remove the sample rate change clicks and power off pop, or is it easy to update the firmware? I saw someone posted a link to a usb to serial cable but I'm not sure how easy it is to update firmware.. do I just open the case, plug the little pin headers onto pins(looks to me like the pins for pc case hdd led, power led etc) and plug usb into my pc, then on the command line: DFU push coolfiltersandbugfixes.bin? Basically, is it safe for me to order the pcb now? It's going to be a basic build, probably the 0.05 resistor version.

edit: this is the serial to usb cable.. http://www.ebay.com/itm/310676792112...%3AMEBIDX%3AIT
I don't see any pins on the dac pcb that they'd go on.. is that something I have to install on the pcb and solder?

Last edited by uncola; 8th April 2015 at 09:01 PM.
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Old 9th April 2015, 05:15 AM   #2398
robertrowett is offline robertrowett  United States
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Quote:
Originally Posted by uncola View Post
hey guys I decided this will be my first DIY project other than cables(I made a few to learn to solder). Since I am a newb, should I wait til the firmware gets updated to remove the sample rate change clicks and power off pop, or is it easy to update the firmware? I saw someone posted a link to a usb to serial cable but I'm not sure how easy it is to update firmware.. do I just open the case, plug the little pin headers onto pins(looks to me like the pins for pc case hdd led, power led etc) and plug usb into my pc, then on the command line: DFU push coolfiltersandbugfixes.bin? Basically, is it safe for me to order the pcb now? It's going to be a basic build, probably the 0.05 resistor version.

edit: this is the serial to usb cable.. USB to RS232 TTL UART PL2303HX Auto Converter USB to com Cable Adapter Module | eBay
I don't see any pins on the dac pcb that they'd go on.. is that something I have to install on the pcb and solder?
Go here and read very carefully. https://hifiduino.wordpress.com/2015...c-users-guide/
This was my 2nd DIY project and 1st dac project. I wish the hifiduino site was available when I started! Be patient and follow instructions exactly. Buy the parts that others have mentioned even if you need to order them.
Have fun!
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Old 9th April 2015, 06:54 AM   #2399
uncola is offline uncola  United States
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Join Date: May 2012
thanks robertrowett. hehe yes I'm reading that page and I've read about casework etc for a while, was planning on making a speaker amp but I'm more into headphones so I think this dac would be more rewarding.. also smaller and easier
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Old 9th April 2015, 07:01 AM   #2400
ristar is offline ristar  Singapore
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Join Date: Mar 2013
Just a note that the linked USB to serial cable may not work - do verify before buying. The DAM1021 requires real RS232 signals not RS232 TTL level signals... if it's RS232 TTL you'll need a TTL to RS232 converter in addition to it... best to look at the implementation thread for verified working cables.

I personally think the DAM1021 is easy to do for a basic setup (transformer... digital inputs of choice... case work and pot). Maximising it and tweaking it more will require a lot more work (changing capacitors to polymer ones; arduino; choice of dc power supply; output stage and dual-mono once the firmware is updated.. etc.) - depending on the extent you're willing to do so for the results.

At the end of the day, remember to look through all the issues people have already encountered so you don't get any nasty surprises.
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