[soundNERD]

OK, sorry. I thought you were saying that only the power on/off popping would stop, not the popping due to high volumes and dc on the output. Does the regulator draw a current from the battery at all times?

 

[jeffreyj]

Yes, the regulator will draw current from the battery at all times... but why use a regulator at all??? NiCd or NiMH batteries have very low internal resistance so their terminal voltages won't vary much in response to load changes, while the LM4881 has a decent amount of PSRR, to boot, so the benefit of regulating the supply voltage is doubly dubious.

 

[soundNERD]

A plain 9V would fry the chip, and wouldn't a 7.2V rechargeable? I think the max input voltage is like 5.5VDC.

 

[Ajit]

It is rated at 5.5V. It runs nicely off 4 NiMH cells.

 

[jeffreyj]

Quote:

Originally posted by Ajit It is rated at 5.5V. It runs nicely off 4 NiMH cells.

Thank you, Ajit, for helping soundnerd see the light on this one

Mike - of course you shouldn't apply more voltage than what the chip can withstand!?! Ditch the regulator, yes, but ditch the 9V battery as well since the regulator is wasting around 40% of the 9V's measly capacity, anyway. Four AAA NiMH will take up about the same space as a single 9V while delivering a little more than 4x the run time.

 

[soundNERD]

Can I use 4 nicd cells? becuase then, I would be able to just use a transfomer connection to charge it without opening it?

 

[soundNERD]

OK, I decided on what im gonna do. I have a 3.6V @ 280mah Rechargeable NiCd from RadioShack. I will have it inside the amp, and there will be a jack to connect it to the power supply.

How much voltage is needed to charge it? Somewhere around 4VDC @ 100ma for about a 3hr charge time? I know rechargeables always need more voltage to charge. Would 4V work? Or something like 5V? Also, How do I connect it? In parallel or series? Anotherwords, + to + and - to - from the plugin to the battery, or + to - and - to +?

I know the supply voltage range is 2.7VDC to 5.5VDC. Can I get the same undistorted power from 3.6VDC as I did from 5VDC?

 

[soundNERD]

Well, I got the battery thing done and working. I used a 7VDC 30ma transformer, which runs through a 7805 regulator to charge the battery overnight. The regulatior is to prevent the chip from being fried if the amp is on while charging.

Anyway, two people I know have heard it, and have asked me to make one for them also. Rather than making 2 boards manually, i designed the boards on my pc. The problem, though, is that the software i have is unable to save since it is a demo version. I printed it to the acrobat distiller, so I have an acrobat file. I didn't get to print it again mirrord before windows crashed (suprise, suprise!!). I attached the gif file of the pcb layout. I have fireworks mx to edit it. There is no mirror button, though. There are flip horizontally and flip vertically buttons, though. Will those work to mirror it? if so, whch option do i select?

Thanks, Mike

 

[jeffreyj]

You charge Ni-Cd cells with a constant current, not a constant voltage. The universally usable charging rate is 1/10 of the mA/hr capacity ("C") for 14 hours. Some cells are capable of being charged at 1/2C for 3hrs (or even 1.5C for 1hr) but do not attempt to charge a cell at more than 1/2C unless it is specifically rated for rapid charging.

Mike, you really need to get yourself a good electronics book to cover these basics instead of relying on someone more knowledgeable at, eg., this forum to spoon-feed the answers to your questions. I highly recommend "The Art of Electronics" by Horowitz and Hill.

To mirror a board layout in image editing software you may flip along either axis - doesn't really matter which - but vertical seems more intuitive to me.

Of course, you could always pay for the program, Mike...

[edit: corrected error in "flip" comments)

 

[soundNERD]

Yeah, I am going to just buy it. It really is worth it.

Oh, and I don't think it is my fault there is no beginner forum here.

 

[aderici]

Hi, i've been reading the posts,

what about the bypass cap? why is it used in lm4881 designs what happens if we dont use it at all?

 

[PeterR]

Quote:

Originally Posted by aderici Hi, i've been reading the posts, what about the bypass cap? why is it used in lm4881 designs what happens if we dont use it at all?

Have you read the datasheet?
http://www.national.com/ds.cgi/LM/LM4881.pdf
If not, you might want to take a look at page 11 (Power Bupply Bypassing and Selection of Input and Output Capacitor Size, last pararaph).

 

[aderici]

"As with any power amplifer, proper supply bypassing is
critical for low noise performance and high power supply
rejection."

power supply rejection means rejecting AC that may come from power supply right??

i will only be using this amp with rechargeable batteries. So is there any probablity of some AC coming from rechargeable batteries?

i'll be glad to hear less technical explanation,

 

[amb]

Quote:

Originally Posted by aderici power supply rejection means rejecting AC that may come from power supply right?? i will only be using this amp with rechargeable batteries. So is there any probablity of some AC coming from rechargeable batteries?

Power supply rejection refers to the amp's immunity to any disturbance in the DC power rails. AC ripple is certainly one of those, other induced noise could possibly come into play, but most importantly the amp itself could cause voltage fluctuations due to varying current demands. Without bypass capacitors to absorb such disturbances the amp's performance will be compromised.

 

[PeterR]

Quote:

Originally Posted by aderici "As with any power amplifer, proper supply bypassing is critical for low noise performance and high power supply rejection." power supply rejection means rejecting AC that may come from power supply right?? i will only be using this amp with rechargeable batteries. So is there any probablity of some AC coming from rechargeable batteries? i'll be glad to hear less technical explanation,

As AMB already said, the problem is not only ripple from the supply itself. An ideal power supply would maintain the same voltage no matter what current you draw from it. The voltage of a real world power supply however will vary according to the current it has to deliver. That means if an amplifier is busy drawing current to supply music to your headphones, you'll suddenly find a copy of that music on your supply rails. Not good. That's why we use capacitors to lower the power supply impedance. At higher frequencies even the inductance of a circuit board trace can cause trouble, so in addition to the bulk caps in the power supply, you also place small bypass caps directly at the supply pins of an IC.
As far as the cap at the 'bypass' pin is concerned:
Quote:

Originally Posted by National Datasheet Besides minimizing the input and output capacitor sizes, careful consideration should be paid to the bypass capacitor value. Bypass capacitor Cb is the most critical component to minimize turn on pops since it determines how fast the LM4881 turns on. The slower the LM4881's output ramp to their quiecscent DC voltage (nominally 1/2 Vdd), the smaller the turn on pop. Thus choosing Cb equal to 1.0 uF along with a small value of Ci (in the range of 0.1 uF to 0.39 uF), the shutdown should be virtually clickless and popless. While the device will function properly, (no oscillations or motorboating), with Cb equal to 0.1 uF, the device will be much more susceptible to turn on clicks and pops. Thus a value of Cb equal to 0.1 uF or larger is recommended in all but the most cost sensitive designs.