Adjustable regulators vs. fixed regulators

[CurtZHP]

When building a power supply with a fixed DC output, is there any advantage to using adjustable regulators, even if the output voltage can be had with a fixed regulator, like 15VDC?

 

[JohnRoberts]

no

[edit- but check the data sheets for the regulators /edit]

JR

 

[Bo Deadly]

Some regulators are better than others. My understanding is that an LM317 is quieter than a 78L15. But either is considered to be quiet enough for a 15VDC audio circuit such that using something better will have no effect on overall noise because the noise floor of the circuit being powered is higher than the noise contributed by the regulator. If there is a specific reason why you need a part with some kind of an advantage over another, explain what that is. Linear Technology is known for making extra low noise ldo regulators but do you really need an extra low noise ldo regulator? For a conventional 15VDC audio circuit, almost certainly not.

 

[Monte McGuire]

With many adjustable regulators, you can add a capacitor to reduce the output noise as well as reduce the gain of the error amp (and thus decrease the output impedance of the regulator). If you do that, you also have to add a couple of diodes to protect the regulator while this capacitor discharges, but the whole process isn't that complicated. It all depends how much performance you need - some circuits do not care about noise or impedance, and some will benefit from improvements.

 

[Newmarket]

From memory LM3x7 is 'quieter' than 7*15.
I put inverted commas around 'quieter' because I recall looking at the frequency characteristics of the noise.
IIRC the LM317 (implemented with ADJ cap) had a 'smoother' spectrum - more like pink/white noise.
Of course, it may be that the parts are different now esp given the various sources over the years.
btw - I can't honestly say I experienced any difference in the audio output signal.

 

[Tubetec]

There might be an advantage to an adjustable regulator , suppose you want to supply a valve heater in either a microphone or preamp with remote power supply (to keep induced noise down) now the adjustable comes in handy and allows you to compensate for the loss in the cable. Ive used a common mode inductor and capacitors at the output of a regulator to gain a small improvement in the noise . Although its not something thats done often undervolting valve heaters is another place adjustable regs come in handy .

 

[JohnRoberts]

Missing from this discussion is what is the PS for? If you don't need an odd rail voltage the adjustable voltage feature seems like extra complexity with little benefit. Prudent design for audio circuits is to use topologies that are not overly sensitive to PS quality.

For TMI the old 78xx/79xx series of fixed VR (I remember when they were new) use internal process technology similar to op amps of their time, like the slow, noisy, limited gain bandwidth ua741 op amps. For those of you who never heard of a 741 op amp, you are lucky. One consequence of old technology inside the common 3 terminal regulators exhibit rising output impedance with increasing frequency. It is good practice to use capacitors in parallel to supplant rail impedance at higher frequency. I did a deep dive into this back in the 80s and discovered that using a 1,000uF electrolytic capacitor in parallel with 3 terminal outputs neatly flattened out the source impedance for a couple octaves above audio.

Newer adjustable regulators are designed with newer IC processes so just like newer op amps are higher performance than old 741, these newer regulators behave better too. Of course fixed 3 terminal regulators using these newer processes would also deliver better specs but the market is not demanding that and happy to purchase the old technology, lower cost parts.

There's more but thats enough to get you started.

JR

 

[CurtZHP]

Thanks for the info, guys. I'm powering your basic run-of-the-mill, op amp based audio circuit. Nothing out of the ordinary. I just happened to have a supply mostly put together using adjustable regulators and was wondering if I should go to the trouble of reworking it to use 7X15 regulators. I seem to remember reading somewhere (I wish I could remember where...) that the adjustable regulators were preferred, and as has been suggested here, that may have been true once upon a time, but not necessarily anymore.

Sounds like I'm not gaining or losing either way, so if it ain't broke....