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Cheap, variable, dual rail supply for amp testing?
Cheap, variable, dual rail supply for amp testing?
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Old 11th September 2019, 12:58 PM   #1
Rallyfinnen is offline Rallyfinnen  Sweden
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Default Cheap, variable, dual rail supply for amp testing?

I've been 'hooked', and testing many different amp boards lately, but I'm missing some kind of variable stabilized supply for this, to be able to adjust the supply voltage to the specific amp, and maybe also measure distortion performance depending on supply voltage.

I think I saw XRK mention something about some cheap variable dcdc boards somewhere, but they were only single rail, and I don't know if they are isolated so they could be staggered to a dual supply.

Input voltage can be 240VAC or dual/single supply from some linear PSU I already have (a few). Output.. maybe +-12 to +-50V and min 5A..

It should however not involve making my own PCB's or similar, since I don't have the equipment for this. I prefer 'click and buy', hopefully from a European source (slow and expensive to get china-packets through customs here nowadays). Nice price is important too

Hope you have suggestions!?
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Old 11th September 2019, 06:53 PM   #2
ubergeeknz is offline ubergeeknz  New Zealand
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If you have a dual secondary transformer, you could use two DC-DC variable voltage/current modules off it. And they will be isolated (by virtue of isolated secondaries).
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Old 11th September 2019, 07:05 PM   #3
jvhb is offline jvhb  Europe
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I just use a good simple linear power supply (dual secondaries 200VA transformer and decent filtering capacitance) with a Variac in front. Its relatively cheap, very flexible, and lasts almost forever.

If you already have a linear PS, you just need a Variac. You can keep the voltage stable by adjusting the variac as you load the amp under test.
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Last edited by jvhb; 11th September 2019 at 07:09 PM.
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Old 12th September 2019, 12:00 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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You could probably get away with a +/-35 volt supply.
That would cover a lot of chip amps and a lot of discrete amps.
It obviously wont give full power on something that needs +/- 70 v rails but at least it can power it up enough for testing.
I have a test rig similar to that, an addition was speaker DC protect on output, just in case. The power, input and output are all on croc clips for easy connect.
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Old 12th September 2019, 05:33 AM   #5
Rallyfinnen is offline Rallyfinnen  Sweden
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Thank you for your replies!
I think I have some transformer where the secondaries can be split, I have to check that!
Do you have some suggestion on decent DC-DC's?

Variac would be nice, but kind of fall out of the cheap-category..

I have been using a +-37VDC supply, but testing class A usually means lover voltages. Also testing heavy bias on AB amps requires lowering the voltage. This is where the variable voltage would be nice!.
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Old 12th September 2019, 06:00 AM   #6
Rallyfinnen is offline Rallyfinnen  Sweden
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Looking at ebay, this seemed like a possible candidate:
DC-DC 4-40V 200W 8A to 1.25-36V Buck Converter Adjustable Step Down Power Module | eBay

or this:
300W 20A Constant Current Step Down Buck Voltage Converter Power Supply Module 6171285473767 | eBay

Last edited by Rallyfinnen; 12th September 2019 at 06:14 AM.
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Old 13th September 2019, 09:28 AM   #7
Mark Tillotson is online now Mark Tillotson
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Some sort of current limiting is essential for testing amplifier designs, otherwise 100's of watts are available to burn up a faulty circuit instantly. The cheapest way to do that is add some switchable series resistors to the outputs.


There must be some designs out there for variable voltage, variable current limit regulators?
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Old 13th September 2019, 09:57 AM   #8
ubergeeknz is offline ubergeeknz  New Zealand
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At a push, some well selected resistors will do the job for safe powering up of amplifiers.

I've got a 0-30v 0-3A linear limited constant voltage / constant current supply on my bench.

I bought it from AliExpress but the TL07(8?)1 chips were fake and burnt up. Once I swapped them for good ones and identified and replaced a bad zener diode... it worked fine. As a precaution, I replaced the transistor with a proper one too.

So the design at least, most of the components, and the PCB, are good. 0-30v 0-3A (it goes over 3A actually maybe it would do 5 on the right transformer). Needs a heatsink but with that it will cope with a dead short at full steam (100W ish) at least for as long as I was willing to try.

Paired with a digital voltage + current meter it does the trick and it was pretty cheap. A few sellers have them and you might get lucky on the opamps and transistor, but I'd count on replacing them, they're not exotic or expensive. I've got a second kit and a dual secondary transformer there to let me build a split supply when I get around to it. I ended up padding the current pot to get better control at lower currents.

0-30V 2mA-3A Adjustable DC Regulated Power Supply Diy Kit Short Circuit Current Limiting Protection Board Module For Arduino
AliExpress

For a little more money there are some rather good microcontroller "lab" supplies available prebuilt, the DPS series with ratings from 30V/3A and up. EEVBlog gave these a favorable review and there are others online.

Third, if you're made of money, or get lucky, pick up a proper lab / bench split supply.

I would not bother trying to use those small one-chip cc/CV modules for this, they're really only designed for battery charging with a protection circuit inline and well below "rating". Things like dead shorts or reverse voltage will make them (literally) blow up. I had one blow bits of ceramic out and burst violently into flames on my bench once after I "accidentally" it on a 500Wh E-Bike battery Cheap, variable, dual rail supply for amp testing? damn near crapped my pants
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Old 13th September 2019, 11:10 AM   #9
Rallyfinnen is offline Rallyfinnen  Sweden
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Thank you for your input!

I ordered two of the '300W-boards' in the link above. They seem to have an adjustable current limiter. Guess I have to wait and see how they perform..
Until now I have been quite aggressive connecting things, and lucky most of the time
I have access to bench supplies at work, so sometimes when there is a little bit more money or time involved, I tried them at work first. However, it seems not many bench supplies have high enough dual rail voltage + current.
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Old 13th September 2019, 11:15 AM   #10
ubergeeknz is offline ubergeeknz  New Zealand
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I hope they work out but they do look like they're designed for eg. Battery charging where there's a steady draw, and may not limit current instantly, in the way you would like. I'd test one into a dead short or a small value load resistor with a low current set and see what it actually does before trying to use it. My 2c.
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