Custom DAC/Amp/Amp Combo build (Odac/O2/252s)
Dec 28, 2017 at 4:22 PM Thread Starter Post #1 of 2

TypodCrowd

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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!
 
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