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Custom DAC/Amp/Amp Combo build (Odac/O2/252s)
Custom DAC/Amp/Amp Combo build (Odac/O2/252s)
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Old 28th December 2017, 10:25 PM   #1
TypodCrow is offline TypodCrow
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Lightbulb Custom DAC/Amp/Amp Combo build (Odac/O2/252s)

TLDR;

Taking my current Odac/Objective 2 + Stax SRM-252s setup and building it into a single chassis, while adding VU meters. Please skip to the Challenges section, because I do need some help with this!

Inspiration:

Stax SRM-212 Protype (Showcased at CES2016)
My imagination.

The actual description:

I'm a student. In my program there are many internships, for which I have to travel, often with my audio gear.

My current "Desktop" setup contains a Odac line out into an SRM-252s amp driving a set of L-700's. It's not perfect, a bigger amp can make a big diference on these, a better dac could make things better as well, but this is not the point of argument for this post. I'm happy with the sound (far more than just happy). When I test a new set of headphones, I use my Objective 2 and its 1/4in (6.3mm) out, but this is also driven by the Odac. These are my essential audio components, which I don't forsee upgrading for the next little while.

The current look:

On my desk, I have stacked the SMR-252s and the Odac/O2 combo stacked upon each other. This gives me 2 chassis, with different foot prints, with separate power supplies, connected by RCA's, not-so-cleanly stacked onto eachother. This is what I would like to clean up.

The new look:

- I'd like to have a single chassis (small ish form factor), containig the Odac + O2 + 252s.
- I'd also like to add a VU meter; analog, single meter that represents both channels (Left and Right).
- Finally, I'd like this to run off a single power supply if possible. This will also mean that the VU meter doesn't depend on power coming from a specific (252s or O2) power supply.
- Use a USB A input port for the Odac, instead of the mini-b that's currently on the board.

What this will clean up:

- No RCA required for my most frequented gear
- Single power supply is much easier to set up when plugging into a new location, only need one outlet
- Looks cleaner than the "stack"
- Satisfaction of a sudo-DIY project

Interface:

Front:
- Separate volume control for the 252s and the O2
- Pro Bias out (252s)
- 1/4in headphone out (O2)
- VU Meter (Left and Right channels combined)
- Power LED's for each amp (seperate, just going to use the ones that come with the amps

Rear:
- DAC USB
- RCA Line in (connects to Source switch)
- RCA Line out (connects to Sink switch)
- Power supply input(s) - see Challenges section
- Power switche(s) - see Power supply input(s) note
- VU meter switch (On/Off)
- O2 Gain switch (High/Low)
- Source switch (Choose between Odac and Line in)
- Sink switch (Choose between O2, 252s, and Line out)

Challenges (this is where I need all of your help please!):

Power supply:
O2 takes 14-20V AC, 200mA. 252s takes 12V DC, 400mA. VU meters take 12V DC, 150mA (probably less since I'd keep them fairly dim). My ideal was to get a 16V AC, 1A power adapter, and use some rectifier diodes + voltage regulator in parallel with the O2 to get DC 12V from it (to power VU meters + 252s, each of those also in parallel with each other). I recently realized that this won't work, since the rectifier will short out the O2. I need some alrernatives to this.


I've checked with multiple sources and have been assured that this will not cause a shorting issue. The AC to DC conversion to power the stax amp will be as follows:

AC input (~16V, ~1.5A) --> Rectifier (16V DC) --> switching buck converter (12.5-13V DC) --> linear regulator (12V DC) --> Stax amp

I've been told that the switching stage is necessary so that the 25% difference in voltage isn't just burned off by the linear regulator, but instead actually gives me usable current. This is a plus for me since I know the rectifier will be ineficcient, and I will be poweing the VU meter in parallel with the Stax amp using the 12V DC output of this circuit.

Power supply Ground loops:
If I use a single power supply, and a single chassis, I need to confirm what can/can't be grounded to diffeerent parts of the amp, and whether any of this will cause ground loop hum.

Switches (terminals):
I'm not sure if I need 3 terminal for each positon on my source/sink switches (Left, Right, Ground) or if I can get away with keeping all the signal grounds constantly hardwired together, and just switch the Left and Right channels to the correct source/sink. This shortcut seems like a bad approach to me, which is why I'm facing the next challenge.


Switches (form factor):
Not sure what source/sink switches to use. I would like to stick to toggle switches, but I can't find a 3p3t switch (Sink switch needs 3 options). I could go rotary, but then I can't find a DP4T or DP3T rotary switch. I haven't looked too much into this yet, but they should both (Source and sink switches) be the same form factor/actuation mechanism (Toggle, rotary, etc).


These have been resolved. I've looked at many audio switch boxes and have found that the significant majority of them have a common ground. For this reason, I'll be able to use a DPDT toggle switch for Source switching, and a DP3T toggle switch for Sink switching. Ground will be shared for all sources/sinks.

Chassis:
As always, finding a nice classy one with the propper air flow is difficult. Would like to avoid DIY-looking cases. The front panel will need to be at least 8.5" * 2" (aesthetics + the form factor of the VU meters). Top down view, I'm thinking in the 8.5" * 6-7" range. Suggestions welcome, please limit to metal chassis - the Stax amp gets fairly hot, as does the O2 under heavy load.


This chassis size can now be revisited as I move things around inside the amp. One of the VU meters I recieved in my kit was broken. Therefore I'll only be running a single VU meter that visualizes the summation of both audio channels.

Links:
SRM-252s: Earspeaker System: SRS-2170 (Basic system) - Export version
Objective 2: NwAvGuy: O2 Details
VU Meters: 2PCS Panel VU Meter Warm Back Light Audio Level Amp w/ VU Driver Board Kit | eBay

Please, any help you can provide is much appreciated! I will be bumping this with remaining/new challenges as they come up. I'd like to get this working in a month tops!

Last edited by TypodCrow; 4th January 2018 at 03:45 AM.
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Old 28th December 2017, 10:39 PM   #2
TypodCrow is offline TypodCrow
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Reserved: Build log, pictures, etc.
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Old 31st December 2017, 08:44 PM   #3
hpeter is offline hpeter  Slovakia
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hmm, a ebay xmos usb dac pcb could fit iside that stax chassis
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BLOG. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000
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Old 1st January 2018, 01:23 AM   #4
TypodCrow is offline TypodCrow
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Quote:
Originally Posted by hpeter View Post
hmm, a ebay xmos usb dac pcb could fit iside that stax chassis
Any chance you got a link? Been looking at a few.
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Old 3rd January 2018, 08:23 AM   #5
hpeter is offline hpeter  Slovakia
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Original XMOS+ PCM5102 + TDA1308 USB decoder board USB DAC 384KHZ/32bit | eBay
im ok with this
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BLOG. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000
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Old 3rd January 2018, 05:22 PM   #6
TypodCrow is offline TypodCrow
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Quote:
Originally Posted by hpeter View Post
The ad says this can be run off usb. I'm wondering how much performance cost there is to this.

Also, is there any chance you have the measurements for this DAC? or any reviews? I like the Odac because it's a reference DAC (flat frequency response, great phase, great THD).
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Old 4th January 2018, 12:56 PM   #7
hpeter is offline hpeter  Slovakia
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sorry i have nothing, soundwise i am fine with it
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BLOG. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000
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Old 8th January 2018, 03:00 PM   #8
bczupa is offline bczupa  Poland
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I would use custom made troid transformer with separate wire for each device and build a separate power supply section for each.

Take must be taken to avoid ground loops - separates are much more flexible in this regard.
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Old 8th January 2018, 04:28 PM   #9
TypodCrow is offline TypodCrow
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Quote:
Originally Posted by bczupa View Post
I would use custom made troid transformer with separate wire for each device and build a separate power supply section for each.

Take must be taken to avoid ground loops - separates are much more flexible in this regard.
Doesn't this limit me a little more? To add a new device into the amp chassis (another VU meter, etc), I would need to remind the transformer for a separate line, no?

How about I just wind one that gives me 16v ac and 13v ac outputs. And then for every 12vdc device I add, I just parallel another rectifier + voltage regulator from the 13v ac transformer output. Is this a better solution?
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