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RioTubes
2008-01-12, 10:53
Plan to DIY a linear psu for my SB2. I'd appreciate feedback. I take the signals out the SB2 BEFORE they reach the DAC and analog section.

1. I do NOT need to power: 2 clocks, dac, analog section, spdif, headphones

2. I DO need to power everything up and including xilinx (I tap off the signals after the xilinx but before the dac at the resistors between these 2 chips). Also need to power the display.

Per diagram below, can I get away with a single 5V external psu to feed the internal 3.3V for Input/logic and 1.2V for CPU core?

The schematic has a "dual switcher" before the 3.3V and 1.2V regs. How do I defeat this switcher?

Finally, what is the VFD in the schematic (right middle)?

Thanks!

randytsuch
2008-01-14, 13:12
I think the VFD is the display, because the switcher is supposed to power the display.

So, if you defeat the switcher, then you will not have a display anymore.

And, I think you still need the two clocks, one to support 44.1 Khz and one for 48 Khz music. From what I remember reading, the clocks are fed into the FPGA.

You might consider upgrading the crystal to a tent clock, or something like that. That is what I plan to do. I would power the clock from an external regulator.

One of these days, I would like to modify my SB, to take everything but the display off of the switcher, and power them from a cleaner, external PS, but have not had time to work on that much.

Randy

RioTubes
2008-01-14, 19:14
Thanks Randy!

Yes, based on your remark about VFD I googled it and it stands for Vacuum Florescent Display. I think "EF" stands for emission factor.

I reread an old post by Vinnie Rossi of Redwine Audio and he used a 12V SLA Battery regulated via LT1086CT-5 to supply 5V main input voltage. He used 12V directly off the battery to replace the internal 14V switching supply. He went on to say that he believes one should be using at least a 12Vdc, 1000mA linear regulated power supply. He added an umbilical with a switchcraft 3 pin plug to input 5V, 12V and GND to SB2. He also increased the filtering on the input of the SB2. He concluded by saying that he does not believe that the SB draws more than 1.5A so a rating around this would be adequate. So this is what I plan to do.

As far as the clocks, I removed both. I feed the SB with a 11.2896mHz Tent XO located in my dac. The Tent clock is fed by a Tent Shunt Reg. I don't use the dac or analog stage in the SB. I just feed the signals into my dac and synchronously reclock them.

Thanks!

randytsuch
2008-01-14, 23:05
Thanks Randy!

As far as the clocks, I removed both. I feed the SB with a 11.2896mHz Tent XO located in my dac. The Tent clock is fed by a Tent Shunt Reg. I don't use the dac or analog stage in the SB. I just feed the signals into my dac and synchronously reclock them.

Thanks!

That is what I was thinking about trying, a tent clock powerered by his shunt reg.

How does that sound, compared to the stock clock?

I have one SB3 modded by Vinnie, which uses the internal DAC, and a SB2 that I use as a transport. I am working on a project with the SB2 to recase it, and have the DAC (a twisted pair Opus) in the same case.

Randy

RioTubes
2008-01-15, 14:04
That is what I was thinking about trying, a tent clock powerered by his shunt reg.

How does that sound, compared to the stock clock?


Randy

Of course, it must be repeatedly said that the SB2 is a wonderful device, especially for the price. That said, my external Dac fed by an OK transport was quite a bit better. I assume that a stock SB2 would be on par or better than the traditional transport, so I chalk this up to the external Dac being better than the dac/analog stage of the SB2. Now the external Dac fed by the modified SB2 (used solely as a transport) is an entirely different animal. Very high resolution, low noise floor to where it picks up the subtlest of details. The best combo I have experienced in my system.

randytsuch
2008-01-19, 13:29
Hi Rio,
I was looking at the Sb2 last night, and figured out some things that you may find helpful

The switcher and the multiplier that generate thh 11VAC and 55VDC are on the display card. They feed 5VDC through the ribbon cable to the display card, then send back about 15 VDC through the ribbon cable. From there, it goes to around a 100 ohm (I think) resistor, and the "Big Ass" cap. This powers the two regs that generate 9VDC and 5VDC.

So, you could generate 5VDC, and modify the SB2 to use this 5VDC to just power the display card. This should help isolate the display power, and it's switching supplies from the rest of the SB2.

There is a Linear Tech LT1940 dual step down, switching voltage regulator, in the area behind the ethernet connector, which generates the 3.3 and 1.2 VDC. It is a 16 pin, SOIC part. One of these days, I may try to generate the 3.3 volts with a linear regulator, but that's not in my immediate plans.

Randy

RioTubes
2008-01-24, 09:25
That's helpful Randy. I was able to read your post and follow the schematic in post #1 above. Makes sense. I guess this means that I could have 4 separate feeds through an umbilical from a 6V SLA Battery:

1) 3.3V for IO, logic (could use an LT1121 reg for this)
2) 1.2V for CPU core (regulator for this?)
3) 5V to power the switcher/multiplier that feeds display
4) ground.

I've never worked with 1.2V so a bit clueless on what type of reg to use for that. One run could be raw unregulated to feed the switcher for the display. I wonder if this will have any positive effect since the switcher that feeds the display will probably contaminate the other two through ground?

randytsuch
2008-01-28, 23:52
That's helpful Randy. I was able to read your post and follow the schematic in post #1 above. Makes sense. I guess this means that I could have 4 separate feeds through an umbilical from a 6V SLA Battery:

1) 3.3V for IO, logic (could use an LT1121 reg for this)
2) 1.2V for CPU core (regulator for this?)
3) 5V to power the switcher/multiplier that feeds display
4) ground.

I've never worked with 1.2V so a bit clueless on what type of reg to use for that. One run could be raw unregulated to feed the switcher for the display. I wonder if this will have any positive effect since the switcher that feeds the display will probably contaminate the other two through ground?

I would be careful with the 1.2V for the CPU. I am not sure about the CPU in the SB, but in general I think parts that run on low voltages like this require tight tolerance on the voltage. If it was +/-5%, that is not much in volts, only 60mv.
You may just want to provide 5 volts to the SB, and let the dual regulator provide the 1.2 volts. That's my plan.

As far as the display, I think you can add a ferrite bead between the two grounds, to help filter the noise. I have not played with beads myself, but am going to buy some to throw in my SB, to see if it reduces the noise.

Randy

RioTubes
2008-01-29, 20:14
Thanks Randy for your ongoing feedback. I'm going to proceed per your suggestion on the 5V supply. Thanks!