Amp humming with AC adapter plugged in.

Replies to This Discussion

Permalink Reply by C. B. Gitty (Ben) on April 3, 2012 at 12:42pm

Dan, I have been doing some experiments with my amp boards and am having the same trouble when running it off an AC adapter. I haven't found the solution yet. Glad you posted this though, maybe some electrical engineer out there will know!

Permalink Reply by Dan Sleep on April 3, 2012 at 2:18pm

I did find mention of using a "regulated" AC adapter, but I am not sure how I would know if the ones I am trying are regulated or not.

 

 

Permalink Reply by C. B. Gitty (Ben) on April 3, 2012 at 2:22pm

That is the direction my research has been heading too. Pretty sure all of the test adapters I got for testing are unregulated... I have to dig through my spare electronics bits bucket to see if I have an old 9-12V adapter in the 1000mA range to see if it makes a difference.

Permalink Reply by C. B. Gitty (Ben) on April 3, 2012 at 2:22pm

My best guess at this point is that if it has a little cylindrical-shaped thinger just behind the small plug where you plug it into the amp, it may be regulated. I know I have seen a lot of wall warts that have this, but all of the ones I have been using for testing do not. Worth a try.

Permalink Reply by Dan Sleep on April 3, 2012 at 2:47pm

OK, found one on eBay that says Regulated right on it. This may be the problem. It appears an un-regulated power supply provides varying amounts of juice which can cause the hum. I just need to confirm the minimum mah rating that the amp requires.

 

Permalink Reply by Ellwood T. Bear on April 3, 2012 at 4:57pm

Most digital amps use DC (i.e. 9 volt battery), if you are using an AC power supply you will get the humming/buzzing. Because  it is AC in and AC out, you want AC in and DC out..

Check the label it should read Input Voltage 110v AC output voltage 9 or 12 volts DC. If you still are getting the humming/buzzing then you will need add a filter.  For more information do a search for "Power Supply Filters".

Hope this helps.

Permalink Reply by C. B. Gitty (Ben) on April 3, 2012 at 5:42pm

The power supplies I have been testing with are definitely DC. I have a couple of regulated DC options on the way too Dan, though I don't know that 300mA will be quite enough to drive the amp circuit quite as hard as you'd want. 9V batters tend to vary in range from about 750mA to ~1200mA for the high-energy lithium batteries, according to wikipedia. I am aiming for 9V 1000mA regulated at the moment.

Permalink Reply by Ellwood T. Bear on April 4, 2012 at 11:33am

9v at 1000 ma is equal to 1 Amp that should power about anything you want.

Just don't cross the + and -  when plugged in to the amp, could blow the circuit. Always double check your wiring when building your amps.

Permalink Reply by eric gittins on April 11, 2012 at 2:00am

it should be easy to tell if your power supply is regulated or not. 

with no load on the supply, a regulated supply should read very close to the rated voltage on a multimeter, a non regulated supply will read usually quite a bit higher than rated when it has no load.

as elwood says, make sure you have DC out, and other than that i would tripple check your polarity.  buzzing like this is usually caused by voltage on the earth rail.

if your supply isn't regulated, it is a pretty simple build to make a voltage regulator that should also help to smooth any ripple voltage.  i'll dig around for a schematic and post it or a link.  a regulator chip does drop the voltage a little though, so a 12VDC supply might only do 10VDC regulated.

e

Permalink Reply by eric gittins on April 11, 2012 at 2:18am

a couple of regulator links

http://www.circuitstoday.com/fixed-positive-voltage-regulators

http://www.circuitstoday.com/adjustable-voltage-regulators

and if your power supply is AC you'll need to rectify it first like this...

http://www.techlib.com/electronics/regulators.html

hope this helps.

e

Permalink Reply by Bob Lumpkin on April 13, 2012 at 6:36pm

I don't think it has anything to do with regulated or nonregulated. All that has more to do with keeping your voltage constant. This has more to do with filters. I don't remember all the terms, but when you go from 120ac to 12 vdc you get noise (for lack of a better term).Remember the old days when you would get ignition noise on your car radio & had to have a filter? Similar problem. It's not hard or expensive to fix, I just haven't had time to experiment with it yet. I'm sure the answer is on here somewhere. If you come up with a cheap fix, post it. I've built some 15watt/12VDC Amps that sound a lot better with eight AA batteries than plugged in to my $15 120vac to 12vdc adapters.

ps: I think the fat part on some of the cords is actually a filter. It may be as simple as some kind of cap. I used to know all this stuff before I got old & senile.

Permalink Reply by eric gittins on April 15, 2012 at 9:02am

kinda true bob, but not really seperate.

i think you are talking about ripple from the original AC current being present in the DC output and coming out as 50/60 htz hum or 100/120 htz hum depending on how it was rectified. yes, filtering of the DC will help this.

but the regulated supply thing isn't a bum-steer.

voltage regulators are quite good at filtering ripple currents on their own, and as regulated power supplies are usually better quality to start with, they probably have better filtering on board as well.

if space isn't an issue you can use a junked computer PSU to give you a massive 12v supply, 15-30amps!  i have a couple i use as benchtop powersupplies for testing stuff as they also have 3.3v and 5v rails.