Adapting SMPS adaptors
Those 12v, 2A SMPS adapters (also sp. 'adaptor') are ubiquitous ... e.g., used for external hard-drives.
I've got several lying around and want to adapt them to a decent-quality 9-10v adapter for audio use (specifically, headphone amp that normally runs on 9v batts.).
I have a Tangent TREAD reg., which is based on a single LM317, and pretty much goes off the datasheet. I thought putting TREAD after the 12v, 2A SMPS would give me okay pwr (haven't built anything yet!). But Tangent says this is not a good idea because:
EDIT: Some putatively better (Medical-grade) SMPS adaptors are avail. from, e.g., Mascot.no, ELPAC, etc:
Dunno how good they are WRT noise, ripple, isolation, etc.
NOTE: Tangent's TREAD is only avail on Archive.org
Query: How about a tracking pre-reg -- it's based on TWO cascaded LM317s?
Reply: That won't change the bandwidth of the LM317, any more than cascading two filters does. All it can do is increase attenuation somewhat.
Study the ripple rejection vs frequency graph in the LM317 datasheet. Then realize that switching power supply noise is typically in the 10 kHz to 1+ MHz range where that graph is dropping off most steeply.
1. Check to see if the PSU is mains isolated.
2. Have a look at the output to see if you can see what kind of regulation feedback is used.
Both of these can be achieved by opening it up.
1. Check to see that there is an isolation transformer between the mains and the low voltage side. If there isn't then this is probably too dangerous to use for anything other than a commercially tested double insulated laptop.
2. If the low voltage output has a pair of resistors across it with the centre point going back to the SMPS controller, then you probably have a shunt feedback circuit that can be modified.
If the original PSU was 12V then reducing the upper resistor by 20% will reduce the output by 20%. I wouldn't recommend any more than this as the PSU may not have the PWM variation required to run that low, but 15-20% should be OK as a well designed PSU will correct for load and mains fluctuations within that range.
It may WELL require additional filtering as previously muted.
You could just use CRCRC and let the resistors drop a few volts.
Having said all that the amp probably would run at 12V quite happily, you might need to change a few caps if they have been specified at 10V.
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Is there Black Box-approach way ... other than X-ray, CAT, PET or MRI? E.g., DMM test?
If you can't open it up, I would go for the CRCRCRCRC (get the jist) route.
Measure the current that your amp needs and let the R's drop the extra voltage.
But, as I said, It'll probably work fine at 12V anyway.
I like the CRCRCRCRC approach ...i.e, some kind of universal/generic/one-size-fits-all "post-filter". You know, like something those audiophile companies market for an arm-and-leg ;)
Current needed at amp is no more than 60mA -- it's a headphone amp after all.
I'm not worried about voltage. If 12v is too little for effective LM317-based regulating ( at needed o/p 10v), I can always go for one of Linear's LDOs.
So CRCRCRC with 10R resistors and 1000uF 16V caps should work perfectly.
The R's only need to be 1/6 watt so they are tiny.
This would also serve to filter the output of the SMPS.
You might even be better with smaller 100uF or 220uF caps.
If you want a regulated supply then sadly the 12V SMPS does not have the headroom (ie extra volts) that most linear regulators need to work correctly.
There are some low dropout regulators but, I really dont think you need them in this application.
Most amps work brilliantly with good CRC or CLC supplies. My experiments with the Pass B1 showed virtually no improvement between a good CRC supply and a full blown Salas Shunt Reg.
This will insite the purists I know. But if you are a purist then you will need a dedicated PSU.
I'm surprised there is not a MODERN solution to this issue: A MONOLITHIC high-speed linear regulator -- or another type reg -- with minimal addit components (caps, resistors) that one simply sticks after the SMPS adaptor. Even something like a high-bandwidth fixed-voltage monolithic -- like hypothetical high-bandwidth 7809 -- would work. Maybe it does ... but it has not been brought to my attention??
The LM317 runs quite a bit warmer than the LM337. Sound is okay, for the 1st 15min, but then sounds "tired". The lethargy does not worsen any further, but I'm at a lot as to the root cause. I've replaced about all components but I'm all outta ideas. Maybe the SMPS you suggested will fare better??
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