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Hum from σ25 w/wall wart

sigma25, sigma26, sigma24

Hum from σ25 w/wall wart

Postby funch » April 22nd, 2018, 7:10 pm

I'm using a Sigma 25 to power one of these: http://www.diyaudio.com/forums/attachme ... ltages.png which was designed to be powered by two 9V batteries.

The S25 is in the same enclosure as the amp, and is powered by an 18VAC/1A wall wart. I'm getting an ever so slight amount of hum, which I have determined to be from the
power supply. I was wondering if anyone here has also experienced hum when powering a Sigma 25 with a wall wart. Thanks.
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Re: Hum from σ25 w/wall wart

Postby amb » April 22nd, 2018, 9:04 pm

Do you have an oscilloscope? Disconnect the amp and check the σ25's output to look for ripple. If you can see ripple, note its amplitude and frequency, then we could determine whether the problem is with the σ25 itself (i.e., the 78xx regulator). Otherwise, the problem is elsewhere. Does the hum persist when the source is disconnected? Does it vary in level with the volume (assuming you have a volume pot at the input of the amp)?
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Re: Hum from σ25 w/wall wart

Postby funch » April 22nd, 2018, 9:58 pm

Thanks for the quick reply. No oscilloscope here. The hum is at the same level regardless of the volume setting. I will try another reg and see if that makes a difference.
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Re: Hum from σ25 w/wall wart

Postby amb » April 22nd, 2018, 10:15 pm

Also possible ground loop. That's why I asked if the hum persists with the source disconnected (and volume set to minimum). What source are you using? Also are you sure the wallwart is AC output rather than DC? Does it have a 2-prong AC plug or 3-prong?
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Re: Hum from σ25 w/wall wart

Postby funch » April 23rd, 2018, 2:11 pm

I was afraid it might be a ground loop, hence my OP. The hum is there regardless of source connection. The wall wart is a 2-prong, and plugging it in either way makes no difference.
It also doesn't matter if it's plugged directly into the wall or into my PS Audio Power Plant. I've even tried using a different outlet on a different circuit and still get the hum.

While the hum is not loud, it's still detectable during quiet passages.
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Hum from σ25 w/wall wart

Postby amb » April 23rd, 2018, 2:22 pm

From what you described it doesn’t seem like a ground loop. Once you disconnected the source, and the fact that the AC is 2-prong, rules out any loop.

Maybe the amp circuit has such poor PSRR that it’s picking up some noise from the supply rail. Or maybe the voltage regulator isn’t regulating. What’s the measured input DC voltage into the 78xx input pin? What’s the output voltage? Are you using a 7815 or some other voltage?
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Re: Hum from σ25 w/wall wart

Postby funch » April 23rd, 2018, 3:08 pm

Unloaded, the wall wart outputs 20.70VAC. I'm using a 7818 reg, and output is 17.88 VDC.
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Re: Hum from σ25 w/wall wart

Postby amb » April 23rd, 2018, 4:17 pm

What’s the loaded voltage at the regulator’s input?
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Re: Hum from σ25 w/wall wart

Postby funch » April 23rd, 2018, 7:32 pm

20.10 VAC
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Re: Hum from σ25 w/wall wart

Postby amb » April 23rd, 2018, 7:35 pm

Ok, but what I was asking for is the post-rectification DC voltage (at the 7818’s pin 1).
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