Audio amp IC for battery power that doesn't pop when powered up

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cstratton
 
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Audio amp IC for battery power that doesn't pop when powered up

Post by cstratton »

Being more of a digital guy it's been a while since I shopped for audio amp ic's, so I'm wondering if anyone has a recommendation for an audio amp suitable for driving a small (maybe 1.125") speaker in a practical-joke type of gizmo.

The device will be battery powered, and I hope to keep all but a tiny corner of the MCU powered down until it wakes up in response to a momentary switch - the intent being to have it last on 3 AA or even AAA cells for weeks or longer of non-usage. This puts the constraint on the audio amp that it must be able to be powered on from a no-consumption state (either by a built in enable or an external FET) without making a click/pop sound. Since I only need mono output (but do have two DAC's available) using a "stereo" part in a bridged configuration might be a possible solution, since that would remove the need for an output capacitor and it's audible charging.

Any suggestions for a good, cheap modern amp chip? I notice the arduino audio shield seems to be using a TL072 op-amp.

Incidentally, it would be fun to make the project arduino compatible, but the BOM just adds up to too much compared to the limited value of what this is intended to do (and standard boards are too big to fit, even as a prototype). At the moment I'm looking at the STM32F100 family, as they seem to be half the cost of an ATMEGA 328 and include the DAC and resonator and it looks like the cheapy eval board will *just* fit with some persuasion. But I still have to dig through the data sheet and see if sleeping deeply enough to run on batteries is going to be possible.

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ImaginaryAxis
 
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Re: Audio amp IC for battery power that doesn't pop when powered up

Post by ImaginaryAxis »

How much power do you need? What are your cost constraints?

The TPA701 is single-ended to Mono bridge-tied load and delivers 700mW at 5V. The "depop" circuitry implements capacitor scaling between the input caps and the bypass cap on the biasing block (probably just a voltage divider) to control the ramp rate - nothing elegant. Cheap, and reduces most pop noise.

The TPA6047 is a fully differential Class AB Stereo bridge-tied load and delivers 2W at 5V and also has a true ground (charge pump) headphone driver. Overkill, but the solution is an internal oscillator that does not allow the outputs to amplify until the internal bias nodes of the output stage ramp to Vdd/2. Expensive and eliminates pop noise.

Other class D solutions would be more expensive and would use a soft-mute and gain ramp during enable/disable to eliminate pop noise. Pop noise is a problem in Class D primarily due to the charging of the bootstrap capacitors on the switching node.

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zener
 
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Re: Audio amp IC for battery power that doesn't pop when powered up

Post by zener »

You could take a look at some parts from NJR such as NJM2076D. As for the popping, I don't know if that is typical, but you could put a fet or analog switch on the output and not turn it on until after the amp powers up.

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cstratton
 
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Re: Audio amp IC for battery power that doesn't pop when powered up

Post by cstratton »

L.A.W wrote:How much power do you need? What are your cost constraints?
That's something I obviously need to do more research on, but wanted to include some plausible candidates when I order the MCU eval boards.
The TPA701 is single-ended to Mono bridge-tied load and delivers 700mW at 5V. The "depop" circuitry implements capacitor scaling between the input caps and the bypass cap on the biasing block (probably just a voltage divider) to control the ramp rate - nothing elegant. Cheap, and reduces most pop noise.
Thanks - this looks like a good possibility, and it seems it will go into a sufficiently low power mode without needing an external switching circuit, which would be a big win.

I'll have to see if I can set up an experiment in the morning with something adjusted to comparable output power (which is going to be a bit less than 700mW on 3 cells anyway)
The TPA6047 is a fully differential Class AB Stereo bridge-tied load and delivers 2W at 5V and also has a true ground (charge pump) headphone driver. Overkill, but the solution is an internal oscillator that does not allow the outputs to amplify until the internal bias nodes of the output stage ramp to Vdd/2. Expensive and eliminates pop noise.
Cost isn't out of the question if I find I need that much power, but the package is not one I'd prefer to work with - ultimately I'll have to do a pcb, but hope to hand wire a few prototypes.

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ImaginaryAxis
 
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Re: Audio amp IC for battery power that doesn't pop when powered up

Post by ImaginaryAxis »

Zener wrote:You could take a look at some parts from NJR such as NJM2076D. As for the popping, I don't know if that is typical, but you could put a fet or analog switch on the output and not turn it on until after the amp powers up.
The switch introduces its own problems. It would have little effect on a single-ended design because of the di/dt of the capacitor into the voice coil of the speaker. A BTL design would nullify this, but the rds(on) of the switch is also a problem as it reduces power because most analog swiches had 3 to 10 ohms of on resistance. You also have to consider rds(on) flatness across the input signal range as this will modulate the on resistance of the pass gate introducing distortion and making the audio sound like BANNED. The only way to address this problem is to use 2 audio analog switches(>1 ohm rds(on) and .1 ohm for flatness on the output.

Best just to use an audio amp with depop.

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cstratton
 
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Re: Audio amp IC for battery power that doesn't pop when powered up

Post by cstratton »

L.A.W wrote:The TPA701 is single-ended to Mono bridge-tied load and delivers 700mW at 5V. The "depop" circuitry implements capacitor scaling between the input caps and the bypass cap on the biasing block (probably just a voltage divider) to control the ramp rate - nothing elegant. Cheap, and reduces most pop noise.
Just want to come back with an update - the project was long delayed, but I finally built it, and the TPA701D has been wonderful.

With a 36mm speaker it's conversational volume on 3v and pretty loud on 4.5v. And current draw in shutdown mode is infinitesimal.

I was only able to find the part in an 8-pin SOIC, but finally got around to making a breadboard adapter. I clamped the chip with one side of pins sticking up, tinned them, and soldered on inch long pieces of bare wire wrap wire, then did the other side. Then I cut up a piece of protoboard and soldered two rows of breakaway header pins to it spaced 7 rows apart, and suspended the chip in between, cutting the wires just outboard of the header pins. Later I made a home etched board using plain paper toner transfer - I tinned the copper pads before placing it and found soldering it a breeze.

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