DC-DC Converter to power audio ?

[TechnoGilles]

Hi all,

I'm currently considering using an isolated DC-DC converter to obtain the power rails for an audio circuit (±10V or ±12V or ±15V, not sure yet), from a regulated quiet 5VDC or from an unregulated "raw" 6VDC.

I'm looking for parts from muRata (which were kindly suggested to me in this forum).

I'm wondering if the outputs of such converters are directly suited for audio application or if I need special filtering/regulation to smooth them our before powering the audio circuit.

Any thoughts ?

Thanks,
TG

 

[crutschow]

A properly designed and filtered DC-DC converter should be fine for powering an audio amp as long as it's switching frequency is above the audio range (20kHz).

 

[ronsimpson]

Depending on how good you want your amplifier: You might want to think about the sampling rate of digital audio. If you audio has 44.1khz in it and the power supply has 54.1khz noise then there will be a small amount of 10khz noise on the output of the amplifier.

 

[crutschow]

Depending on how good you want your amplifier: You might want to think about the sampling rate of digital audio. If you audio has 44.1khz in it and the power supply has 54.1khz noise then there will be a small amount of 10khz noise on the output of the amplifier.

That's only if the power supply switching frequency noise gets into the digital audio source and somehow non-linearly mixes with the digital audio before it is converted to analog.

 

[moffy]

With a switching supply it is best to pass it through a linear say LDO regulator first. The CMRR of amps drops off with frequency, so the higher frequncy switching of the switching supply the more likely it will get into your audio chain. Non linear mixing happens because of all the non linear components such as diodes, bjts etc. It only takes small amounts of an extraneous signal to be audible, or degrade your SNR.

 

[ronsimpson]

If you hope for 90db of noise, and the amplifier has almost no supply rejection at 50khz, then the supply must be very very quite. It is not the 50khz that you hear but the 50khz-44khz=6khz.

 

[Mr RB]

I would ask what is the power output you need? Generally +/-12v would be used to get a decent power output, but that might not be possible if the entire supply is derived from a 5v regulated supply.

Why not try using an LM386 on the 6v rail you have, and a lowish ohms speaker like 8 ohms or 4 ohms? That should be loud enough for all small applications.

 

[TechnoGilles]

Very interesting all that ! I'm learning !!

So here is the figure : my DC-DC converter is said to be "low-noise". It shows a 50mV p-p ripple at a switching frequency around 200kHz (min is 100kHz and max is 300kHz).

At 200kHz, the INA217 (which I use for my pre-amp) shows a power-supply rejection of -70dB.

Does that mean that the 50mV p-p ripple will appear in my audio signal as a 16uV p-p "noise" ?

If so, and considering that my ADC is setup for 1 Vrms (2.83 Vp-p), I should not worry, right ?

Thanks !!

 

[crutschow]

Very interesting all that ! I'm learning !!

So here is the figure : my DC-DC converter is said to be "low-noise". It shows a 50mV p-p ripple at a switching frequency around 200kHz (min is 100kHz and max is 300kHz).

At 200kHz, the INA217 (which I use for my pre-amp) shows a power-supply rejection of -70dB.

Does that mean that the 50mV p-p ripple will appear in my audio signal as a 16uV p-p "noise" ?

If so, and considering that my ADC is setup for 1 Vrms (2.83 Vp-p), I should not worry, right ?

Thanks !!

Yes, you would see 16µV p-p of noise at the amp output from the power supply noise on the amps power lines. That's likely not a problem. But you could reduce the noise further by placing small resistors in series with the power to the amp and add decoupling caps from the amp power pins to ground.

 

[ronsimpson]

A resistor reduces the power supply's regulation. Use a coil (inductor). It will have near 0 ohms in the audio range and maybe 10 ohms or more at 200khz. A LC filter will reduce the 200khz noise greatly.

 

[dougy83]

A resistor reduces the power supply's regulation. Use a coil (inductor).

Or maybe a mix of both: a low-Q inductor.

See: https://www.google.com.au/search?q=low-q+inductor+supply+filter

 

[crutschow]

A resistor reduces the power supply's regulation. Use a coil (inductor). It will have near 0 ohms in the audio range and maybe 10 ohms or more at 200khz. A LC filter will reduce the 200khz noise greatly.

Selecting a resistor value that only drops a volt or less at the amp supply current normally has no significant effect on the amp's operation since the amp has a high rejection of power supply variations. You can use a coil but that's generally more expensive and difficult to buy, and you have to be concerned about the resonant frequency of the LC combination, which can actually increase the noise at that frequency. I would only use an inductor if the resistor-capacitor combo wasn't sufficient.

 

[TechnoGilles]

I usually use the 10µF elec and 0.1µF ceramic combination as bypass capacitors. Would that be enough ?

If I was to add a resistor in series with the power line, how would I choose the value ?

Same question if I was using the LC option : how would I choose the values ?

Thanks