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Baffled by a switch mode power supply problem.

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Posted on March 29, 2021 at 10:28:12
mg16
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Posts: 462
Joined: October 18, 2001
Contributor
  Since:
December 0, 0000
I have the entry level Stax 252S energizer/amp and L300 headphone system.
Sounds great, but I would like to replace the el-cheapo 12 vdc wall wart that powers the 252s energizer. Also would like to eliminate having to use the 120vac to 100vac transformer, that now converts my USA mains voltage to Japanese 100vac mains voltage for the wall wart. I have built numerous regulated power supply's over the years, and put together a very nice supply using the schematic for the positive LT1085CT regulator circuit. It works great, supplying a constant 12.4vdc under about triple the load it will be required to deliver. Unfortunately, when feeding the 252S energizer, it won't even light up or supply any power to it. Plug in wall wart back in, and everything works fine. Recheck my diy supply, and it works fine under load on the test bench. I am guessing that the 252S amps switching power supply,(that coverts the 12vdc to the required 580v for the circuit), is screwing somehow with the LT1085CT voltage regulator chip.
The wall wart is marked 12vdc at 500ma output, and reads 22vdc no load on my Fluke meter.
Is it not possible to feed a switching power supply with a regulated voltage? Do you just feed the amp with 22 vdc unregulated, and have the SMPS pull down the voltage under load?
Don't want to damage the energizer/amp, but things should not be this hard I would think.
Anyone have any experience with this problem?
Thanks for reading this.
MG16

 

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That is weird. Polarity reversed maybe? (nt), posted on March 29, 2021 at 10:56:27
Jonesy
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nt


"I know just enough to get into trouble. But not enough to get out of it."




 

RE: That is weird. Polarity reversed maybe? (nt), posted on March 29, 2021 at 11:32:19
mg16
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Posts: 462
Joined: October 18, 2001
Contributor
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December 0, 0000
No, I made sure . I used the same connector, and it is marked on the wall wart as center pin negative, outer contact positive. Wired it that way and double checked with my meter.

 

RE: That is weird. Polarity reversed maybe? (nt), posted on March 29, 2021 at 15:42:19
Duster
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Location: Pacific Northwest
Joined: August 25, 2002
Is it possible to feed the circuit with the voltage regulator disabled, so it approximates the output of the wall wart, but still a better sounding option than using the stock wall wart, mg16?

 

RE: That is weird. Polarity reversed maybe? (nt), posted on March 30, 2021 at 08:35:35
mg16
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Posts: 462
Joined: October 18, 2001
Contributor
  Since:
December 0, 0000
I guess I will try to see how it sounds unregulated.
Thanks,
MG16

 

RE: Baffled by a switch mode power supply problem., posted on March 30, 2021 at 10:21:40
jedrider
Audiophile

Posts: 13640
Location: No. California
Joined: December 26, 2003
As the power source says 12 vdc but measures 22 vdc, it appears that the internal power supply actually expects > 12 vdc as a valid input. Usually, you want to feed a regulator about 2 volts over what it is expected to deliver. I'm not an expert, but like you, I have experimented a lot. I notice that some equipment is just very particular to what voltage and amperage you deliver and just doesn't operate at any lower voltage or amperage.

 

Or... existing adapter is non SMPS?, posted on March 31, 2021 at 12:28:44
Jonesy
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Posts: 2657
Location: Winnipeg, Manitoba
Joined: September 1, 2005
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  Since:
March 1, 2018
... and it's a linear power supply you need to use with the Stax unit?

I came across an article where someone is using a car battery for the 12v DC. They mention the adapter included with the Stax unit is "non" SMPS.

Cheers!

Jonesy



"I know just enough to get into trouble. But not enough to get out of it."




 

I think Jedrider has it right..., posted on April 9, 2021 at 18:19:19
Lew
Audiophile

Posts: 10578
Location: Bethesda, Maryland
Joined: December 11, 2000
The thing wants to see some voltage above 12V at its input, probably because it has its own internal regulators that drop some voltage. If you don't feed it enough to trigger, it's dead. I had the same or a similar experience with a linear supply I built for a solid state phono stage. The preamp had markings on its PS input socket stating the min and max DC voltages it wanted to be fed, so I knew what my problem was in that case.

 

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