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I am building a voltage regulator circuit for powering a pair of mini-speakers (about 3-5W) with built in amplifier and an old mp3-player/FM radio. The power source is an old mobile phone charger (switching PSU) with a no-load DC voltage of 7.3V, which is regulated to 4.2V by the LM317. I use the standard circuit exactly as recommended by the TI datasheet:

enter image description here

Besides the 2 resistors there are three capacitors, Cadj = 10uF, Co = 1uF electrolytic, Ci = 0.1uF ceramic.

The circuit works OK, noise levels are low. However, using an oscilloscope in AC coupling mode and testing the regulated voltage output by actually playing back music on the system, I see a significant variation in voltage that follows the audio signal’s transients. A kickdrum can drop the voltage by around 0.5V (i.e. about 12% of Vs) and I'm measuring peak-to-peak voltages of around 1V (24% of Vs). Adding a 1000uF electrolytic cap on the input side in parallel with Ci reduces peak-to-peak voltage swings drastically, to about 200 mV. Subjectively, the basses also sound fuller and deeper if I add this large capacitor.

My questions are the following:

  1. Is it correct to assume that an ideal (theoretical) PSU for audio applications should produce a constant voltage regardless of load variations (i.e. it is a voltage source)?

  2. In practice, what are acceptable levels of supply voltage variation due to transients in an audio application (in percentage of Vs or mV)?

  3. What is the correct approach to improve load transient response? Should I place a capacitor on the INPUT or OUTPUT of the regulator? Is there a rule of thumb/calculation for the required capacitance? Are electrolytic capacitors OK, or should I use polyester or tantalum caps (i.e. something with a lower ESR)?

UPDATE:

Regarding the input and output capacitors I found the following info on some hi-fi audio blogs and forums:

  • Large caps are recommended on the input for amplifier voltage regulators, i've seen LM317 schematics with around 2000uF and upwards. Cadj could also be somewhat larger for extra noise suppression (someone recommended 20uF or even 100uF).

  • The output capacitor should not be overly large, as this can cause stability issues with the LM317, however it could be higher than the 1uF in the datasheet (I've seen 100uF, someone stated that he uses 1000uF, however with a 0.33ohm series resistor for increasing ESR, see below).

  • The output and adjust caps should have a high ESR (i.e. they should be electrolytic caps, not polypropylene or tantalum ones), because low ESR caps introduce ringing, possibly in the audible range. Larger caps have lower ESR, so this consideration limits the size of the output cap. A possible solution to this problem is adding a small (0.2-0.3ohm) resistor in series with the caps to compensate for the lower ESR of larger caps.

  • If using higher Cadj and Cout caps, protection diodes D1 and D2 are strongly recommended to discharge the caps when powering off.

  • The issue described in my question (large transient voltage drops) was in part caused by transient voltage drops on the PSU, rather than the regulator circuit as pointed out by Peter Bennett. The regulator's input voltage fell a few hundred mV below the necessary input of the LM317 (Vout + 2.5V). These transient drops did not show up on the multimeter, but a scope revealed them:

transient voltage drops on a scope

Replacing the PSU with another unit that stays well above Vout + 3V still results in some transients on the regulated output, however the voltage drops are much smaller (60-90mV compared to 200-700mV). Replacing the 1000 uF input cap with 2x 2200 uF did reduce the transients further, but only by a fraction (to about 70 mV from 90 mV). Subjectively this 2% voltage fluctuation is not very noticable on this small amplifier so I consider this problem solved.

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  • \$\begingroup\$ A schematic would be much better than a non-link and page reference and bunch of words. There's a schematic button on the editor toolbar. My first thoughts are to check the peak current isn't exceeding the regulator's current limit causing it to shut down. \$\endgroup\$ – Transistor Jan 19 '16 at 23:53
  • \$\begingroup\$ I updated the post with a schematic. I am not sure how to measure transient peak currents, however I did measure the input voltage provided by the PSU, which is dependent on current. The PSU provides about 6.7V to the regulator when playing back music on full volume. It's nominal output is 5V at 350mA, so the current seems to be in the range of 2-300mA while the LM317 is capable of handling 1.5A as per the datasheet. The regulator does not shut down, but transient loads cause a 10-20% drop in voltage. \$\endgroup\$ – alchemist_anonymous Jan 20 '16 at 0:07
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    \$\begingroup\$ If the input voltage to the regulator drops below its [output voltage plus drop-out] level, the LM317 won't "shut down" - it will just reduce its output voltage to maintain the drop-out voltage. If you are seeing 6.7 volts with a normal DVM, I suspect that the LM317's output voltage will be dropping on audio peaks. Try looking at the LM317 input and output voltages at the same time, with your 'scope. \$\endgroup\$ – Peter Bennett Jan 20 '16 at 0:38
  • \$\begingroup\$ Yes, after some research on the topic I am suspecting more and more that this might be the issue. I will look at the input voltage with a scope tomorrow and also test the regulator using another PSU with a higher voltage. \$\endgroup\$ – alchemist_anonymous Jan 20 '16 at 2:16
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Is it correct to assume that an ideal (theoretical) PSU for audio applications should produce a constant voltage regardless of load variations (i.e. it is a voltage source)?

Yes. An ideal power supply for any application should be an ideal voltage source, which has a constant voltage.

In practice, what are acceptable levels of supply voltage variation due to transients in an audio application (in percentage of Vs or mV)?

This is dependent on your application. You have to evaluate your desired noise/distortion, the power supply rejection of the audio components you are using, and the way the circuit is constructed. 0.1% power supply variation translates to a -60dB noise floor, which might be sufficient.

What is the correct approach to reduce these V swings? Should I place a capacitor on the INPUT or OUTPUT of the regulator? Is there a rule of thumb/calculation for the required capacitance? Are electrolytic capacitors OK, or should I use polyester or tantalum caps (i.e. something with a lower ESR)?

Probably both. You should have both bulk capacitance on the output and low-ESR decoupling caps in close proximity to all active chips (op-amps, ADCs, DACs, etc). And some more capacitance on the input certainly wouldn't hurt.

Typically, you might use large electrolytics for bulk capacitance, and low-ESR ceramics for faster decoupling. Again, how much you need depends on the magnitude and characteristics of the wiggles on the supply rail. Also, carefully read the datasheet of the regulator and make sure you are within its comfortable operating region: the regulator has a response speed and current limits, as well as input ripple rejection specs.

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  • \$\begingroup\$ Thanks for your answer. What confuses me is the low capacitance values recommended in the datasheet, i.e. 1.0uF on the output and 0.1uF on the input. If I understand correctly these are suitable for filtering high frequency noise, but are insufficient to provide energy to smooth out large transients. \$\endgroup\$ – alchemist_anonymous Jan 20 '16 at 0:14
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    \$\begingroup\$ The capacitor values in the datasheet are typically the lowest recommended values, or nearly. However, the response time of the LM317 is well above the audio band, so you likely need more data to fully diagnose this issue. Put a scope on the input and output. Also, you may want to consider using a higher-performance regulator if you cannot get the LM317 to be stable enough for your purposes. \$\endgroup\$ – uint128_t Jan 20 '16 at 1:25

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