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I have built a simple LM317 regulator power supply and would like to add a pass transistor to provide more current and lower output impedance of the circuit. Here are two alternatives, one I copied and adapted from another question here, using a PNP and NPN transistors, and another which is using only an NPN transistor (adapted from a source using LM338 and multiple 2N3055s in parallel).

The supply will be used for powering an Allo.com Katana DAC and a Raspberry Pi (two similar supplies) so the requirements are 5V at 1.2A (DAC) and 5v at 3A (Rpi with extra usb devices), with very low ripple (in microvolts) and very low output impedance. I have already measured the supply with and without Cadj, and the ripple is so low (50uV) that I decided not to add a capacitance multiplier. The problem is that the audible DAC sound dynamics were much lower than even a cheap smartphone charger, and it was pointed out to me that the regulator has a relatively high output impedance that I should mitigate by adding a pass transistor and/or much more output capacitance.

So my question is which of the alternatives should provide better performance, and does using only a NPN transistor negatively affect the ripple?

Circuit #1 - R values are copied and possibly should be revised

Circuit #2 - transistor voltage drop needs to be taken in account for output voltage

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    \$\begingroup\$ Why did you choose the LM317? This seems to be an extremely odd choice, seeing that you seem to need more current or voltage drop than the LM317 can provide itself. So, explain how you came to the conclusion that sticking to the LM317 was the right choice, instead of using a voltage regulator design that is actually meant for your use case. Also, you don't define anything about your requirements (voltage in, out, current, acceptable ripple), so how on earth are we supposed to assess whether your circuits fulfill the requirements? \$\endgroup\$ Aug 5, 2020 at 12:38
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    \$\begingroup\$ (and CADJ doesn't provide "ripple rejection" in general - it provides a modified control loop behaviour. Without thinking too much about this, a 100 µF capacitor in this position basically makes the regulator not react to any fast load variations at all – leading to basically no regulation, which is the oppositive of what you'd normally use a voltage regulator for. So, really, state your requirements before choosing an architecture and parts.) \$\endgroup\$ Aug 5, 2020 at 12:41
  • \$\begingroup\$ I’m voting to close this question because questions like this are best self-served by using one of the many free simulation tools available. \$\endgroup\$
    – Andy aka
    Aug 5, 2020 at 12:56
  • \$\begingroup\$ @MarcusMüller thanks for pointing out the lack of clarification and requirements, I edited the question. And for the choice of LM317 - I want to explore it's possibilities since I'm just learning electronics. I can use the LM338 for higher current, but that doesn't solve the output impedance issue. \$\endgroup\$
    – nahero
    Aug 5, 2020 at 14:06
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    \$\begingroup\$ You have a high end DAC coupled with the opportunity to learn electronics. I think a rethink is in order. \$\endgroup\$ Aug 5, 2020 at 19:00

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Looking at the board... apparently a switching converter is used to create +/- 15V for the discrete opamps on the opamp board, from the +5V power supply. Can you spot the two tiny "472" inductors?

enter image description here

Also there is a CLC filter on +5V input, which makes the output impedance of your LM317 power supply irrelevant. No matter how low the power supply impedance is, it'll be in series with the filter anyway.

On the DAC board we have lots of LDOs, presumably turning the incoming +5V into 3.3V and the other voltage ES9038q2m requires, I think it's 1V2 but I'm not sure I remember correctly.

enter image description here

Note, from the amount and type of decoupling caps visible, it looks like these guys don't own a network analyzer.

It is unnecessary to make a microvolt noise power supply to feed a LDO since the LDO will have a good amount of PSRR and will add its own noise anyway.

What is important is to have low HF noise because LDOs usually have low HF PSRR. ALso low common mode noise is important if you use an AC-DC switching supply. Since you use a linear power supply, these conditions should be satisfied without problems.

NEVER TRUST AUDIOPHILE REGULATOR DESIGNS unless they come with full specs, output impedance graphs, and specs about PSRR, noise etc, and especially stability. I've tested a number of these "audiophile regulators" and... some are pretty good, some are unstable, there was even one that managed to pick up some AM radio.

Now...

enter image description here

All regulators are closed loop feedback systems. They output a certain amount of current depending on how far the output voltage is from the desired value. You can represent that with an error amp with voltage gain A, followed by a transconductance G. This is explicit in most LDO datasheets which show an internal schematic with an error amp and a PFET as the transconductance device.

If the output voltage changes by dv, then the output current will change by AGdv, and the output impedance of the regulator is 1/(A*G).

An important point to consider is that the impedance Zo, which is load impedance in parallel with the output caps, is part of the feedback loop gain. So, the loop gain is LG=AGZo.

To have a stable feedback loop we need LG to have less than 180° phase shift as it reaches unity gain.

This is harder to do if LG has a high value at DC, because it will have to fall down all the way to unity gain without having 180° phase shift there.

And... adding transistors, as you did, increases loop gain while also adding more poles, therefore more phase shift. This means it makes the whole system less stable. These regulators "boosted" by a transistor are known to be finicky and prone to oscillations for this reason. This is most likely your issue.

Honestly, since the board already has DC-DC converters on it, I don't see any reason to use a linear power supply. You could simply use one of these Meanwell PCB mount bricks or a high quality wall wart. You can use the medical version for lower leakage and common mode noise.

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    \$\begingroup\$ lol, love the "one even managed to pick up some AM radio"; I can definitely see how that happens, and wouldn't it be so damning for these audiophile designs, it would be hilarious. Oh wait. It's hilarious. \$\endgroup\$ Feb 15, 2021 at 10:16
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    \$\begingroup\$ Yeah. My favorites are the super-ultra-low-µV-noise regulator used to power the input DC-DC on a Raspberry Pi, and the dude who put OSCONs everywhere and managed to make a LM317 oscillate. Many such cases. Not limited to gimmick tweaks though, lots of big manufacturers also sell unbelievable garbage. \$\endgroup\$
    – bobflux
    Feb 15, 2021 at 11:12
  • \$\begingroup\$ well, where there's people willing to believe the little gold contact fairies will make their 1960s-obsolete supply architecture deliver fantastic sound, there's a demand for people selling hand-tuned feedback-free headphone amplifiers with electrolytics the size of espresso mugs and screw contacts that you can reforge into a decent wedding ring... \$\endgroup\$ Feb 15, 2021 at 11:32
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So, yes, the LM317 is really not a good regulator.

You can certainly extend its capabilities with external pass transistors, but you're suddenly building a control system for a voltage of your own – the LM317 in itself has been designed to be stable given some capacitance and load variation, your own system needs to be designed so that the higher current gain doesn't lead to instability.

As you can see, that makes the whole thing rather complex (11 or > 15 components!).

Using another nearly 50 years old circuit, the LM338, doesn't make this any better. (And I heavily doubt your 50 µV measurement, unless you do have an oscilloscope.)

Pick a less ancient voltage regulator, if you need better regulation. Your use case suggest you'll want one supply for the digital part (the rPi + digital side of the DAC) and one separate one for the analog part (analog side of the DAC + amplification).


The design of your katana DAC thing doesn't seem to allow for that.

So, a super low-noise low-impedance power supply doesn't matter at all. You've got the noise generated by your rapidly switching digital logic in your analog supplies, and no matter how low the impedance of your voltage source gets, you won't get that out of your system. Go for a well-designed high-frequency switch-mode power supply design; 1 MHz switching speed is so far away from your audio frequency, that unless your system starts running on a synchronous clock to that, you'll not have anything in the audible / amplified frequency range. Adding low-noise linear regulation, again, doesn't help when the source of the noise is sharing the same supply.

Regarding your DAC board: They probably designed a voltage regulator stage in there, too, because no logic actually runs at 5V these days. However, they also decided to build opamps out of discrete parts, claiming these are better. Um. That's what us engineers call audiophile hogwash. A well-designed, factory-trimmed opamp IC will inherently beat discrete opamp designs by far, simply because of thermal coupling of the transistors on the die. So, probably things aren't going to get any better than using a normal not-the-cheapest-you-could-get USB power supply, and be done with it.

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  • \$\begingroup\$ Thank you for the break down, and I agree with most of what you say. Audio is finicky, and best discussed on a different forum. That being said, I didn't get an answer to my questions - how does adding pass transistor in the two variants affect impedance and ripple? Granted, I should build both and then test what I can myself, but an experienced opinion or a point in the right direction is much appreciated. \$\endgroup\$
    – nahero
    Aug 6, 2020 at 10:11
  • \$\begingroup\$ no, audio is not "finicky"; audiophile device suppliers are just borderline scammers. Engineers don't believe in technical magic. \$\endgroup\$ Aug 6, 2020 at 10:28
  • \$\begingroup\$ I did answer that: you're building a control loop of your own, now you have to measure and simulate. We can't tell you. \$\endgroup\$ Aug 6, 2020 at 10:28
  • \$\begingroup\$ Ok, thanks. Will return with some measurements once I build this. \$\endgroup\$
    – nahero
    Aug 6, 2020 at 11:03
  • \$\begingroup\$ Don't build it. You're very stubbornly trying to use the wrong tool for the job. Throw out your LM317, get a good voltage regulator that actually does the job you want. \$\endgroup\$ Aug 6, 2020 at 11:06

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