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What are the culprits in a "cheapy" audio amplifier?

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Anonymous
a b ) Power supply
February 17, 2005 7:54:09 PM

Archived from groups: rec.audio.tech (More info?)

I would like to renovate this pair of powered multimedia speakers that
seems to have a real mean AC buzz. I'm guessing this is coming from
the power supply part of the "amplifier" section (if you can really
call it an amplifier ;)  ). What would be some obvious things to look
into reconfiguring to knock out the hum? (repositioning a transformer,
grounding something, adjusting where signal lines go?, etc...)

Yeah, of course it is not worth the effort really to bother, but I
would like to make this a sort of pet project that maybe I could learn
something out of.
Anonymous
a b ) Power supply
February 18, 2005 6:08:26 AM

Archived from groups: rec.audio.tech (More info?)

First, be sure it is not coming from the input.

Check the power supply filter capacitor. Some audio power ICs havs a SVR
capacitor that could be bad or a bad solder joint disconnected it. Some real
cheap speakers I had hissed and hummed. I opened them and found there was no
ouput boucherot (sp?) cell that provide high frequency stability. I know
some newer ICs don't call for it, but this IC specifically requires it.

John

"dangling entity" <randycat99@earthlink.net> wrote in message
news:1108688049.395204.123750@z14g2000cwz.googlegroups.com...
>I would like to renovate this pair of powered multimedia speakers that
> seems to have a real mean AC buzz. I'm guessing this is coming from
> the power supply part of the "amplifier" section (if you can really
> call it an amplifier ;)  ). What would be some obvious things to look
> into reconfiguring to knock out the hum? (repositioning a transformer,
> grounding something, adjusting where signal lines go?, etc...)
>
> Yeah, of course it is not worth the effort really to bother, but I
> would like to make this a sort of pet project that maybe I could learn
> something out of.
>
February 18, 2005 6:45:12 AM

Archived from groups: rec.audio.tech (More info?)

In article <1108688049.395204.123750@z14g2000cwz.googlegroups.com>,
"dangling entity" <randycat99@earthlink.net> wrote:

> I would like to renovate this pair of powered multimedia speakers that
> seems to have a real mean AC buzz. I'm guessing this is coming from
> the power supply part of the "amplifier" section (if you can really
> call it an amplifier ;)  ). What would be some obvious things to look
> into reconfiguring to knock out the hum? (repositioning a transformer,
> grounding something, adjusting where signal lines go?, etc...)
>
> Yeah, of course it is not worth the effort really to bother, but I
> would like to make this a sort of pet project that maybe I could learn
> something out of.
>

Disconnect the line level input, if it still does it then its the
powered speakers. If not, then your problem lies further upstream the
signal flow.

--
Cyrus

*coughcasaucedoprodigynetcough*
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Anonymous
a b ) Power supply
February 19, 2005 12:35:22 AM

Archived from groups: rec.audio.tech (More info?)

Also forgot to mention- do you think there is a possibility of
bypassing the internal power supply entirely and just wire in a DC
voltage tap from my computer's power supply? That's gotta be some
"clean" DC coming out from there, eh? Would this little audio
amplifier respond to 12 v, you think?
Anonymous
a b ) Power supply
February 19, 2005 12:37:53 AM

Archived from groups: rec.audio.tech (More info?)

Also forgot to mention- do you think there is a possibility of
bypassing the internal power supply entirely and just wire in a DC
voltage tap from my computer's power supply? That's gotta be some
"clean" DC coming out from there, eh? Would this little audio
amplifier respond to 12 v, you think?
Anonymous
a b ) Power supply
February 19, 2005 2:05:54 AM

Archived from groups: rec.audio.tech (More info?)

DE
Do they buzz even if each Volume control is set at min?
Assuming it is fitted with a mains trannie, see if AC to DC rectification is
alternate halfwave (2 diodes) off a centre-tapped secondary or full wave
(bridge rectifier) off a simple secondary. The cheapo alternate halfwave can
produce more buzz and is less productive of DC voltage.
This stage should be followed by a meaningful, chunky smoothing/reservoir
electrolytic cap
If enclosed in metal case, make sure it is properly grounded. Ensure all
screens and returns are soldered as close to the common rail of the cap/s as
feasible or possible.
Make sure all sensitive jack inputs are auto-shunted to "deck" if not in
use.
These built-to-a-price MM products usually give acceptable repro apart from
inherent hum due to "design" omissions or the "that'll do for 10 dollars"
attitude.
Jim
Anonymous
a b ) Power supply
February 19, 2005 8:55:15 AM

Archived from groups: rec.audio.tech (More info?)

dangling entity wrote:
> Also forgot to mention- do you think there is a possibility of
> bypassing the internal power supply entirely and just wire in a DC
> voltage tap from my computer's power supply? That's gotta be some
> "clean" DC coming out from there, eh? Would this little audio
> amplifier respond to 12 v, you think?

before going down that road, which is more complicated,open your
speakers and either check the power supply filter caps for ESR or
simply replace 'em. That usually cures hum (sounds like 50hz mains
ripple from your description)

regards, Ben
Anonymous
a b ) Power supply
February 19, 2005 2:15:01 PM

Archived from groups: rec.audio.tech (More info?)

Yes, looks like a single rail supply from ur commentary. The trannie's
location too MAY be a culprit. Check that the large filter/reservoir cap
Negative side goes to Screen of audio jack/s.
Change it - you could even double it to E2200/16V - and measure the DC
volts across it. If about 11-14V, and STILL buzzing, I reckon as a trial you
could run *detached* amp section off the PC's smoothed supply.
Only wastes about 6 Watts, I guess, so a 200-250W PSU would tolerate it. C
But then you would have a 0V which is grounded permanently via the PC's
3-wire AC cord - I imagine previously it was *floating*, since the MM amps
are likely to have a 2-wire AC input.
Jim
Anonymous
a b ) Power supply
February 19, 2005 9:00:08 PM

Archived from groups: rec.audio.tech (More info?)

The AC wires directly into the speaker housing (no wall wart). There
isn't even a printing anywhere stating AC input/DC output specs. I
guess this thing is a real "piece of work", eh? Upon closer
examination, there is an area that was previously covered under a
sticker that says 9V DC, fwiw. Weird, eh?
Anonymous
a b ) Power supply
February 19, 2005 9:10:08 PM

Archived from groups: rec.audio.tech (More info?)

By ESR, I take it to mean "electrical series resistance"? By measuring
it, I guess it would be a very low value I would be looking for, no?
(how low is low, btw?) How do I distinguish a low ESR cap, or does
that quality just come inherently by simply using a high-quality
capacitor?
Anonymous
a b ) Power supply
February 19, 2005 10:19:35 PM

Archived from groups: rec.audio.tech (More info?)

Lots of good info here from everybody, btw. I appreciate everybody for
posting in!
Anonymous
a b ) Power supply
February 21, 2005 10:56:41 PM

Archived from groups: rec.audio.tech (More info?)

Is Xicon a good enough brand for reasonable quality capacitor? Is the
sky the limit for capacitor size or is the 2200 uF a quite adequate
amount of overkill? I can get a 4700 uF, 16v electrolytic cap, just as
well.

Would you believe Radio Shack doesn't have a 2200 uF electrolytic
radial cap in their catalog? :l
Anonymous
a b ) Power supply
February 22, 2005 10:36:17 AM

Archived from groups: rec.audio.tech (More info?)

"dangling entity" <randycat99@earthlink.net> wrote in message
news:1109044601.011081.95850@f14g2000cwb.googlegroups.com
> Is Xicon a good enough brand for reasonable quality capacitor? Is the
> sky the limit for capacitor size or is the 2200 uF a quite adequate
> amount of overkill? I can get a 4700 uF, 16v electrolytic cap, just
> as well.
>
> Would you believe Radio Shack doesn't have a 2200 uF electrolytic
> radial cap in their catalog? :l

Take an axial lead cap, insulate the minus wire and bend it around the body
of the cap to suit.
Anonymous
a b ) Power supply
February 22, 2005 3:33:20 PM

Archived from groups: rec.audio.tech (More info?)

Try 1 x 2200uF, or improve smoothing with 2 x 2200uF in // or 1 x 4700uF.
The larger you make it, the more filtered the noise, and the better it will
"withstand" AF dynamic surge-current demands.
If you have to use a radial, you can always use an axial cap and bend one
wire over (may have to extend it by jointing and soldering a bit of solid
wire and sleeve the lot; a larger diameter may force you to crank this wire
under base) to reach the board holes. Observe polarity symbols!
Jim.

"dangling entity" <randycat99@earthlink.net> wrote in message
news:1109044601.011081.95850@f14g2000cwb.googlegroups.com...
> Is Xicon a good enough brand for reasonable quality capacitor? Is the
> sky the limit for capacitor size or is the 2200 uF a quite adequate
> amount of overkill? I can get a 4700 uF, 16v electrolytic cap, just as
> well.
>
> Would you believe Radio Shack doesn't have a 2200 uF electrolytic
> radial cap in their catalog? :l
>
Anonymous
a b ) Power supply
February 23, 2005 10:34:53 PM

Archived from groups: rec.audio.tech (More info?)

OK, will do...

I'll post back with the results, when I have done the deed.
Anonymous
a b ) Power supply
February 25, 2005 3:47:17 PM

Archived from groups: rec.audio.tech (More info?)

Perhaps you should 'Shun the Shack' and buy from regular electronic
vendors. Radio Shack is a malignant cancer on the ass of society, that
I will say.
Anonymous
a b ) Power supply
March 7, 2005 1:49:14 AM

Archived from groups: rec.audio.tech (More info?)

Here's the update- I replaced the 1000 uF radial with a 2200 uF axial,
and it seems to do the trick! Yippeee! The buzz is still there, but
much lower. I have to put my ear right up to the speaker to hear it,
so I think that is much more manageable than the way it was before.

So I guess this lil project is a success. I have to thank all of you
for posting in and pointing me in the right direction. Next step-
swing up the voltage rails to 100 v or so to get a couple hundred
watts... :D  Just kidding!
Anonymous
a b ) Power supply
March 7, 2005 1:59:38 AM

Archived from groups: rec.audio.tech (More info?)

As a sidenote, it kinda makes sense why they picked the capacitor they
did for this product. The dinky one they chose fits on the board and
slides into the enclosure nicely. The mondo 2200 uF monster of volts
does *not* and required a bit of "persuasion" to get it to fit nice.
It does stand-out as the "piece da resistance" out of all of the other
components on the board.
!