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Chip Amps Amplifiers based on integrated circuits

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Old 27th October 2011, 11:54 AM   #11
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hm... just found something..... this chip don't have ways to adjust gain, and itself is very high(40dB)
Is there anyway to reduce the gain, thus i can make the input more louder, thus minimize pick up noise(hiss) ?
Does any different if i put attenuation VR at the board before input ? (compare to controlling gain for this purpose)
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Old 27th October 2011, 01:10 PM   #12
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An attenuator pot at the input will help if there is still room to turn the level on your netbook higher, because the noise isn't coming from the amp, it comes from the netbook itself. It sounds like your netbook has a noisy power adapter that makes its way into the audio circuits. Mine as well... The cure to that is usually buying a high quality external soundcard.
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Old 27th October 2011, 02:32 PM   #13
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haha, i have a nice external soundcard, but doesn't dare to try X.x

But i'm making the amplifier+speaker to give as a gift for my friend, so i hope to thoroughly solve this problem.
It seems that the AC adapter doesn't DIRECTLY input the noise, because when I connect direct to speaker unit, it doesn't cause problem. It probably because of something that in amplifier induce into it by affect of AC charger.... still thinking, probably oscillation, which won't happen alone with speaker only. (only happen with presence of amplifier)
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Old 27th October 2011, 07:57 PM   #14
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It's an ordinary ground loop, I bet. You don't happen to have amplifier ground connected to chassis ground, which in turn is connected to security earth? Even with a nominally floating amplifier, coupling over the mains transformer may be big enough to cause audible problems.

With as few pins as this chip has, you probably have no chance of setting gain externally.
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Old 27th October 2011, 08:48 PM   #15
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Test with a battery powered MP3 player.
If no noise persists during test then follow this:
Purchase a 6 amp 90w capacity 15vdc laptop power pack and use it for your TDA7240. They're inexpensive. Avoid all models with a 3 prong cord (cannot work for car chip amplifiers plus computer source)!!! Use only a model with a 2 prong cord. That will give your laptop the opportunity to be the ground, without fighting over it.

Here is one possible example.
AC Adapter PA2521U-3ACA 15V 6A 90W for Toshiba Laptop | eBay
Please notice the 2 prong mains cord!!
There is also an authentic Toshiba part with 15vdc 6a available on the used market, and there's the possibility that some ordinary broken/retired laptops may have serviceable power packs.


Your input load may be missing. That can make a terrible noise when source device isn't connected. I couldn't find the circuit equivalent schematic, so I'm not sure if the input load is inbuilt or not. However, it seems to need a 10k resistor between pins 1 and 3. It also seems that the cap at pin 1 is far too small--I would attempt 22uF//220uf//4.7nF or something similar. You'd like your roll-off about an octave or more below speaker capacity, to avoid any bass blocking within the frequency response capacity of your speaker. The datasheet example supports 3 inch speakers. So, double-check me on that--I haven't used this particular car chip.


But, I do use a car chip with my computer. It has been running on the laptop power pack (SMPS) for 3 years. Of the many car chips that I tested, only LA4628 (one of the best performers) is able to overload a 6A laptop power pack.

The old model high gain philips when not bridged is able to have supplementary feedback circuitry added to 2 of its 4 channels and can achieve the highest quality performance for music playback fidelity on those two channels at up to 6 watts output power. The other two channels (incompatible with external circuitry additions) may be driven by a single drv134 (or a good buffer chip with some resistors added for the job) and used to power an additional woofer to 20 watts.
That same chip is awfully dull and noisy when auto-bridged (simple bridge) per the datasheet. It is true of most car amplifiers. Auto-bridge just isn't that good. Balanced line drive feeding quality parts is a better performance, albeit a bit more soldering.

P.S.
Car alternators run the voltage up to 14.85 volts DC when the engine is on. A 15v dc power pack is suitable for any reasonably designed car amplifier chip; unfortunately, some automotive products are horribly dangerous, like LED refit bulbs, that cause fire if operated beyond 12v, or Tripath car chip that burns up if operated beyond 13.9v. Clearly, not all automotive products were designed by car owners. This type of thing usually uses power diode drop to bring down the voltage to safe levels.
However, your chip can tolerate the Real full automotive voltage range, according to its datasheet.
Its always good to double-check both datasheets and real product sample too.
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Old 27th October 2011, 09:13 PM   #16
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Quote:
Originally Posted by sgrossklass View Post
It's an ordinary ground loop, I bet. . . .
Almost certainly.
Its a car chip with no car.

Quote:
Originally Posted by sgrossklass View Post
With as few pins as this chip has, you probably have no chance of setting gain externally.
That's right.
Fortunately, a computer can push that amplifier almost all the way to potential even without increased gain. Unfortunately, the majority of computers will clip themselves at beyond half blast at the sound card. It may be worth a try to make a small preamp for the computer.

A fun little Jfet op-amp or the MooseFet project may be fun to try out. I like my MooseFet, but will admit that its a lossy triode sound processor specializing in straightening out the harmonics of MP4, Itune, HD Radio, AAC file types, and unnecessary harmonic distortion for MP3 or CD. But, not getting a headache when listening to Mp4 and HD radio is really just priceless. The gain is 6.

See the Class A NE5534 project at Decibel Dungeon: Building a buffered Gainclone chip amp.
This is NOT as complex as it looks--what you're seeing is both the preamp and its power supply built onto the same board. Have a look at those two sections individually and then it begins to look simple.

And heck, I even like JRC4580's, two of them run with their outputs paralleled (stereo parallel preamp), because it sounds expensive and costs only a few cents. That is same as the high current output jack of M-audio Revolution 5.1, a sound card that doesn't clip itself. I don't see any reason why the same chips can't be added to any sound card as an external preamp. JRC4560's work identically. JRC4558's do not.
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Old 28th October 2011, 01:33 AM   #17
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hm... then we can mostly conclude that because of it design for CAR AUDIO, it is very susceptible to groundloop (because there are hardly groundloop in car ?) I was going to try the method called "loop breaker circuit", see if it fits ?
something like that, which sounds good ? (isolation transformer is better, but very costly if sufficient for audio bandwidth )

I'm using 22uF for pin 1 capacitor (feedback), so shouldn't have problem. Cannot adjust gain means that the internal hiss will be loud all the time ? I don't like that....

hm..... Thinking of using attenuate VR instead of gain (we can't adjust gain...), thus minimize the hiss and buzz from input only... but better than none.

Putting 10k load between pin 1 and 3 (feedback & input pin) will get rid of load missing noise ? (its annoying) I don't have exact 10k, could I use 12k, 13k, etc ?
I personally don't like batteries, cause it will degrade over small period of time, compare to power supply. Using for house speaker.
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Old 28th October 2011, 03:41 AM   #18
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Quote:
Originally Posted by danielwritesbac View Post
Your input load may be missing. That can make a terrible noise when source device isn't connected. I couldn't find the circuit equivalent schematic, so I'm not sure if the input load is inbuilt or not. However, it seems to need a 10k resistor between pins 1 and 3. It also seems that the cap at pin 1 is far too small--I would attempt 22uF//220uf//4.7nF or something similar. You'd like your roll-off about an octave or more below speaker capacity, to avoid any bass blocking within the frequency response capacity of your speaker.
tried using 11k resistor across pin 1 and 3, the 'input load missing' noise really disappear, thanks alot ! but still alot of other noise. Could explain to me how this is work ?
About the bass blocking, maybe it will kill my speaker, so i prefer less bass, haha
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Old 28th October 2011, 04:21 AM   #19
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yay, I have achieve improvement of the noise, recorded and attached link.
The configuration is using a attenuation VR (volume control) between computer output and amplifier input.
Before using this, my computer must configured volume to 5% for normal audio, 15%-20% for booming. above those will deaf my ear.
But now I'm using 100% of computer volume, and control using VR, the noise is significantly reduce ! (difference between 5% and 100% noise)
The record sound is connected to AC charger all the time, just changing VR knob.
With music, its not obvious but audible, obvious when quiet transition. (heard as microwave oven sound) at high volume, the noise doesn't hears like motorboating with music on, just like distortion.

Is the control of knob with no music playing, but connected to computer. with low volume and high volume, the noise is different pattern and sound.

Although it has been better, hope to eliminate it (from root cause).

Extra : using safety ground direct connect to input ground doesn't change anything.

Last edited by guitar89; 28th October 2011 at 04:31 AM.
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Old 28th October 2011, 07:51 AM   #20
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Quote:
Originally Posted by guitar89 View Post
. . . I was going to try the method called "loop breaker circuit", see if it fits ?. . .
That is generally correct for grounded computer audio source to grounded amplifier--they are fighting over the ground.
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