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Old 18th December 2013, 04:59 AM   #1
diyAudio Member
Join Date: Feb 2013
Location: Auckland
Default cisco power supply, grounding and the Millett starving student

Hello all,

I've recently completed populating the PCB for my 12AU7 SSMH, and now I'm ready to put it into an enclosure (assuming that it all works okay)

I purchased a 48v switch mode supply off Trademe (basically Ebay for kiwis) which was listed as "delta electronics". Little did I know, it turns out that it's the same Cisco branded supply that's been reported as having issues with a floating ground output that goes above ground potential in certain operating conditions, and can fry sources connected to the SS.

This is the supply I have:

I was considering what I could do to make this supply safe to use with the SSMH, and my Skeletondac which I'm currently using as my primary source, and thought, it would be pretty easy to just tie the output ground to the ground pin of the mains outlet. Would there be any problem doing this? I assume this would eliminate any issues with transients showing up on the ground rail of my amp, but could it interfere with the switching circuitry? I have little experience with switchers, beyond building a few circuits using the 34063...

I was already considering opening up the supply to augment the filter caps and add in some inductor based filtering to clean up the supply as much as possible. So grounding the output at the same time wouldn't be any hassle for me.

If anyone could shed some light on whether my idea is workable or not I'd really appreciate it. I don't really want to buy another supply for now, as I paid a decent amount for this one, and don't mind opening it up to tweak it a little bit if it's possible to do.

Thanks for reading this!

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Old 18th December 2013, 11:52 AM   #2
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Join Date: Jan 2013
Location: Sydney
"floating ground output that goes above ground potential in certain operating conditions??"

What exactly you mean? Looks like you have one ground, then positive output, which is +48 and then another GROUND which, under certain conditions, would become floating? Looking at the picture I can tell you that there are only two wires at the end. PLUS and MINUS.

I used to design PWM power supply and can tell you that it is impossible. PWM controller collects its information ( in our case it is output voltage feedback directly from the output (+ and GND), sometimes before or after inductive element, to make close loop feedback control simple and easy to operate). By saying that, PWM controller constantly monitors its +48 DC output (it is single output power supply, so it monitors only one single channel) and, if there is something wrong (again, it depends on particular design) it must trigger an alarm.

About making it better. It is really difficult to implement. Output voltage has broad variety of harmonics, that is a result to be a PWM power supply. And normally, inside PWM power supply we have ONE huge inductor, and few little ones.

The big one is calculated to have contentious current (again, it depends on design) and have nothing to do with filtering output voltage.

The little ones are put in place to filter high frequency noise, put in place to filter output voltage. This is where we can play with elements.

But my opinion, if you need clean +48 DC output, go for AC transformer and bridge+capacitor. No digital noise at all.
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Old 18th December 2013, 12:48 PM   #3
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Join Date: Feb 2013
Location: Auckland
Ahhhhh, my brain was a bit too switched off when I made that post I think. I'll try and explain the situation more accurately...

What I understand is the negative output on the power supply is normally ground referenced, but there's a chance in some circumstances for it to have a transient show up on it and damage devices on the input. I assume because they are ground referenced (ie. usb dac) and it pumps current into the dac chip if its not isolated by a buffer or similar.

My understanding of the issue isn't too good, mainly because I haven't been able to find a whole lot of info on it, at least explained in a manner that makes sense to me.

Here's a couple ideas by others, in case my understanding of the issue is flawed so you can check it out yourself:

Originally Posted by
Note on sources for the SSMH ...
To those of you contemplating sources to use with the Starving Student, be advised when attempting to use the Alien DAC or the BantamDAC. There is a unique charge effect that may potentially develop on the RCA jacks of the Starving Student - PCB version or point-to-point. This may happen if a signal is present while the Starving Student is powered up and the signal is disconnected. Or, it may occur when the Starving Student is powered up by itself but not connected to a source. The resulting discharge when attempting to connect an Alien or Bantam DAC may fry the output of the DAC chip. Please note that this phenomenon is unique to the Starving Student and has not been known to occur with any other amplifier. Neither has it ever been reported that other sources have had issues with the Starving Student.
Originally Posted by (by dsavitsk)
My best guess: there is a small amount of capacitance on the Cisco PS between output ground and safety ground. If this gets charged up, it seems to be enough to toast some chips -- particularly the one in the Bantam -- if the output of the DAC touches the Starving Student ground before touching signal input.
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Old 6th February 2014, 06:20 PM   #4
maurycy is offline maurycy  United States
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Join Date: Dec 2005
Location: NJ
Just curious if you were able to find a fix for the problem? I have Cisco PS and SSMH boards on the way and I would not want to fry my source once I get everything up and running. The SSMH power up sequence already asks to wait before plugging in headphones and unplugging them before turning the amp off.
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Old 6th February 2014, 10:33 PM   #5
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Join Date: Jun 2012
I have build a lot of SS amplifiers, and I can say I have never seen this happen, but I have not been testing with the two DAC's that are mentioned, my usual sources have never had a problem.
The SS is a great starter DIY amp just enjoy it :-)
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Old 7th February 2014, 05:38 PM   #6
maurycy is offline maurycy  United States
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Join Date: Dec 2005
Location: NJ
Fred, thanks for replying. For now, I have a cheap Chinese DAC which if I lose, it will not be the end of the world. But once I get the setup up and running, I will definitely upgrade to a better DAC. Hopefully, the SS will play nice with whatever I have.

I was just checking with OP if maybe there is a way to modify the Cisco power supply to reduce the possibility of frying the DACs.
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