[Ajit]

I see the wisdom in not using a variable resistor Jeffrey. Thank you!

 

[soundNERD]

I figured out what the popping is, but can't figure out how to fix it. When the battery is connected, the voltage jumps to about 1.5VDC, then slowly back down to about .25 to .5 volts. This causes the cone on the headphone to stay pushed out a bit, and when the volume is turned up, the cone hits into the plastic enclosure. I have a 1000uf 15V cap on the L and R output, and a 10uf 50V cap on the L and R input. I have already checked for polarity, and it is correct. Is there a way to prevent this voltage from the headphone? Also, one more thing, when power is disconnected, the voltage comes down to about -3VDC, and stays there (with no headphones attached, just a multimeter) That causes the headphone, when it is connected to pull in a lot, and produce a loud clicking noise. Does anybody know the possible cause for this voltage?

Other than that, it sounds great.

Thanks for all the help, Mike

 

[jeffreyj]

Quote:

Originally posted by Ajit I see the wisdom in not using a variable resistor Jeffrey. Thank you!

Oh, don't feel bad - using a variable resistor for the input resistor (or even for the feedback resistor?!) will work as a volume control, but it does so by changing the ratio of closed loop to open loop gain. Many parameters in voltage feedback op-amps depend on this ratio, not the least of which are bandwidth and phase margin (stability), which is why it's considered bad practice. I meant to put this explanation in the original post but for some reason I forgot... must be old age (31).

 

[jeffreyj]

Quote:

Originally posted by soundNERD I figured out what the popping is, but can't figure out how to fix it.... Is there a way to prevent this voltage from the headphone?

This is how a single-supply amplifier with capacitive coupling to the output will always behave. The reason is that when power is applied, the output immediately zooms up to 1/2 the supply voltage and this change in voltage over a change in time (aka - dV/dt) is coupled by the output capacitor as if it were a valid signal. The larger the output capacitor, the slower the dV/dt that will still be coupled by it. This is the flipside to having better bass response - it's just considering it in the time domain instead of the frequency domain.

The solution is to use the LM4881's shutdown pin to enable the amp, not by switching the power. According to the datasheet, it draws a maximum of 5uA in shutdown. NiMH batteries will self-discharge long before the 5uA drains them... it would take 30,000 hours (~3 years) to deplete a 9V at this rate. According to page 8 of the datasheet, you tie the shutdown pin to the positive supply voltage through a resistor (say 10k) to *disable* the chip, then ground the shutdown pin when you want to *enable* it. So, leave the power connected all of the time and instead switch the shutdown pin between open (off) and ground (on) to operate the amp.

 

[soundNERD]

Thanks for the suggestion, jeffreyj, and I will try that. The only thing though, is that I use a NiCd 9V with a 120ma current. Will that matter? I am really satisfied with the battery life, so that is good, and I see that if it can run for about 3 hours on a 120ma battery that hasn't been charged in months, keeping it in standby won't hurt. Do I need to connect the resistor to + supply, since disconnecting the negative will bring the voltage up to half supply? And wouldn't connecting the + supply to the gnd through a 10K resistor short ito out?

Anyway, any ideas on the voltage while operating? I would really like that popping to go away, and am sure that the caps will be charged after listening for a 1/2 hour at least. Do I need a cap on the negative connection to the headphone?

 

[soundNERD]

After reading my last post, I noticed that I said it can run for 3 hours. I meant to say that It has ran for 3 hours so far, which is about as long as I have tried it for. How much longer it will last I don't know.

 

[PeterR]

Quote:

Originally posted by soundNERD Thanks for the suggestion, jeffreyj, and I will try that. The only thing though, is that I use a NiCd 9V with a 120ma current. Will that matter?

That's actually 120mAh. 5µA=0.005mA, so you'd get 120mAh/0.005mA=24000h (that's a rather simplified calculation, as the capacity depends on discharge time, but the bottom line is it's A LOT).
Quote:

Do I need to connect the resistor to + supply, since disconnecting the negative will bring the voltage up to half supply?

Yes, tie it to the positive supply and no, you don't disconnect anything. Just as in the schematic you posted. Take your time to read through that datasheet, it's all in there.
Quote:

And wouldn't connecting the + supply to the gnd through a 10K resistor short ito out?

It won't 'short it out' because you connect it through a resistor. It will draw some additional and quite negligible current when the amp is powered up (9V/10kOhm=0.9mA).

Quote:

Anyway, any ideas on the voltage while operating? I would really like that popping to go away,

That's exactly why you should use the standby mode, to get rid of the popping...And don't forget the 1µF Cb.

Quote:

Do I need a cap on the negative connection to the headphone?

No, that goes to ground.

 

[soundNERD]

Thanks, Peter. I will try that

 

[soundNERD]

Well, the parts were a little too tight on the board to do that. I guess it isn't that bad. I know this is sort of the cheater way, but could I just reverse the wires connecting to the headphone speakers so the cone pulls in instead of out and then I won't get popping?

 

[PeterR]

Quote:

Originally posted by soundNERD Well, the parts were a little too tight on the board to do that.

You need to rewire the switch and one additional resistor...
Quote:

could I just reverse the wires connecting to the headphone speakers so the cone pulls in instead of out and then I won't get popping?

For one, you can't that easily 'reverse the wires' in a headphone amp, you'd have to rewire the headphones. Moreover it would do nothing to solve your popping problem.

 

[soundNERD]

Yes, I would re-wire the headphones. It wouldn't fix it because the speaker would be able to move further?

And I have no switch. The Standby pin is connected to a trace on the pcb that goes under the chip to gnd.

 

[PeterR]

Quote:

Originally posted by soundNERD Yes, I would re-wire the headphones. It wouldn't fix it because the speaker would be able to move further?

That's how a speaker works, the membrane will move back and forth about its resting position. The speaker moving inward will sound about the same, perhaps with an added click sound as hit hits the end of its mechanical travel...

Quote:

And I have no switch.

Then how are you planning to turn this thing on and off?

 

[soundNERD]

That is what causes the popping, the speaker reaching its maximum position at volumes where it shouldn't be.

I have a switch. It is connected in series with the 9V and the regulator. I dont have one for the standby pin.

 

[jeffreyj]

Quote:

Originally posted by soundNERD That is what causes the popping, the speaker reaching its maximum position at volumes where it shouldn't be. I have a switch. It is connected in series with the 9V and the regulator. I dont have one for the standby pin.

PeterR and I have been trying to tell you that this is the wrong way to turn the LM4881 on and off! Ground STANDBY to turn the amp on and leave it pulled up to the positive supply through a 10k resistor to turn it off and the popping will stop. The datasheet explains this, we explained it, yet you are still asking why the popping is occurring.

 

[PeterR]

Quote:

Originally posted by soundNERD I have a switch. It is connected in series with the 9V and the regulator. I dont have one for the standby pin.

The battery will always be connected to the circuit, so you don't need the switch there. Instead, use this switch to pull the standby pin to ground.