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ZakWolf

Wed Nov 03 2010, 05:48AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608

I have a TDA2009A 10 watt x 10 watt stereo amp. When the volume is turned down almost off i get a hum in the speakers and then a little before halfway it stops. Is this because i am under powering the speakers?

SPEAKERS

brand new
6-1/2 inch
40 RMS per pair 200 watts max
4 ohm (which is recommended for the TDA amp)

AMP---->

Dr. Slack

Wed Nov 03 2010, 08:05AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It's nothing to do with your speakers themselves, or underpowering them, and everything to do with the detail of the "0v" or ground wires which link the ground ends of your power supply and audio inputs.

I suspect you'll find that the power supply ripple current is flowing (via the power supply decoupling caps) across part of the ground wiring in series with the audio input. Look up "star grounding". Just because wires are connected together and called "ground" doesn't mean that they're at exactly 0v everywhere.

ZakWolf

Wed Nov 03 2010, 02:31PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

heres the schematic could you tell me where the star grounding gos ?

Mads Barnkob

Wed Nov 03 2010, 02:33PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Dr. Slack

Wed Nov 03 2010, 07:35PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Mads, that's a long artcile. Even knowing what I'm looking for, I'm not sure I can see the wood for the trees.

The basic rules to follow are that

a) every component that's connected to a ground symbol must have its own seperate wire to the same point. This point is the centre of the star ... and

b) there must be NO other connections between the ground points ... and

c) this applies to the system as a whole, not just the amplifier, you must also include the signal source as well

Each wire from the point goes to exactly one component, not two or more, even if it seems tidier or less wasteful of wire to do that, and even if the components do all share a ground symbol on the diagram. So the speaker return, the 1ohm snubber, the 18 ohm feedback are wired by 3 seperate single wires to the star point. And three more wires from the other channel. If the chip is soldered down to a board with traces to these components, then the board ground metal is the star ground, and you just have to find the most central part of to define as your star.

It can be difficult to satisfy condition b, especially when a mains powered amplifer is chassis grounded to saftey ground, and a mains powered signal source is also so connected. Then there is a seperate connection running through the safety ground. This is especially bad as the loop it forms is large area, and connected into the most sensitive part of the amplifier. The answer in professional equipment is to use balanced audio feeds which aren't related to ground. The answer in commercial equipment is to connect the safety ground only to the mains side of the transformer, and to isolate it from the audio and low voltage ground.

This isn't the only way to do it, but until you understand where and when you deviate from it, it's best to follow it absolutely.

The one exception I'd make to the "everything to one point rule" is the power supply decoupling cap and the -ve end of the PSU rectifier (NOT shown on your diagram!) It will work if done properly, but there are such large currents flowing in this net, that it's easy to get it wrong. So it's best to wire the power supply together as a unit, then take a wire pair from the C4 terminals to the pin 9 and the ground star point.

Arcstarter

Wed Nov 03 2010, 08:08PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Try alot more filtering :). Either that or a linear regulator.

ZakWolf

Thu Nov 04 2010, 01:02AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

I tried my other amp circuit and it works fine.

I have another question. I have those 40watt rms and 200 watt max speakers. when the volume is low they sound great but when i turn the volume up it sounds very distorted it has
tweeters/mini speakers built in. what is the problem

Dr. Slack

Thu Nov 04 2010, 08:11AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I have a 100w/channel amp and 50 w speakers. It sounds paradoxical that it's better and safer that way round. If I turn it up until it sounds rubbish, that's the speakers distorting, which is safe (as long as it's on music peaks and not a continuous sinewave test)

Big beefy speakers are likely to be inefficient, and so need lots of drive. Coupled with a puny amplifier, that means that as you turn it up, the first thing that sounds rubbish is the amplifier clipping. Clipping generates a lot of high frequencies, which will get through the crossover and fry your tweeters in an instant. They are fragile and tend to built to withstand only a tiny fraction of the main speaker's rated power.

So yes, underpowered speakers need care, but it has nothing to do with hum at low levels, only safety at high levels.

Thinking more about your hum problem - what is your audio source? If it's something like an MP3 player, are you controlling the volume by its volume control, or do you have volume pots right at the front of the amplifier to pot down a decent line level from the MP3? The former will be much more likely to suffer hum and other unwanted signals than the latter.

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