Another look at the LM317 and LM337 regulators

[jbau]

Howdy folks, new member here. A recent injury has me sidelined from tennis this season, so I'm reviving some of my old audio hobbies for a while. Hopefully I'll bring some some interesting material to the table in the coming months. This intro will be a little long-winded to give some context.

Over the years, in preparation for transferring to digital some of the 70's/80's analog master tapes from my musician/REP days, I've been modding my tape decks, A/D and D/A converters with good results. But it's time to look a little deeper. At any given time, I have a decent selection of useful measurement tools at my disposal.

All of my machines still have the standard LM78xx and 79xx regulators (yuk) in them, so I decided to look into replacing them with LM317 and 337 types. First to get the treament will be the Sony 670 DAT machine (with many mods) that I use for D/A.

It's obvious from the 317 data sheet that the programming resistors, adjust bypass cap, and output cap have a huge impact on performance - both noise and output impedance. The output cap needs to be big to deal with the rising impedance with frequency, no issue there. But the optimum value range for the others has some tradeoffs. For ±15V output, the manual suggests values of 120/1k3 || 10uF. Steve Sandler suggested something like 240/2k7 || 82nF for phase margin and stability, with good measurements to back it up. And in an old article, Jung used 1k/11k || 100uF for lowest noise. Three very different approaches, optimizing different parameters. Obviously they can't all be "right."

So I set up to measure the noise of these program resistor/adjust capacitor combos, using an Amber 3501a with 30kHz lowpass filter engaged. (All have 1000uF || 1k2 ohms to ground on the output.)

240/2k7 || 82nF (82uV)
120/1k3 || 10uF (40uV)
1k/11k || 100uF (30uV)

Given the huge noise difference, I rejected the first one and decided to try the two others in the D/A: first 120/1k3 || 10uF, and then 910/9k1 || 100uF. Listening impressions compared to the 78xx/79xx:

On the first set: cleaner bass and mids, more aggressive highs. Some improvement but nothing I could live with.
On the second set: Even tighter/cleaner bass and mids, but a very aggressive "glare", like a broad boost the 2k-3k region. Good for radio but definitely nothing I could live with.

Obviously, broadband noise doesn't tell the whole story. Back to the data sheets. First observation is how my listening impressions resembled the output impedance curves. I don't have anything right now that will measure impedance with DC present. But the correlation of noise and output impedance that the manual suggests is very interesting. If true, then measuring the noise spectra should give clues to the output impedance too. So I set up to see the noise spectra of these combos. An FFT analyser is the right tool for this job, but unfortunately the only one I have right now doesn't have a sensitive enough input range to be useful, so I had to make do with the trusty HP 3580A, which has a great front end.

Dinner time - to be continued...

[mjf]

hello.
i think a bigger adjust cap (100uf or so for 10 uf) gives lower noise.
experiment: can you paralell a 100nf cap to the 100uf elco?
greetings...........

[SY]

Have you seen Earl Dietz's 1989 paper in EDN? He did a pretty comprehensive look at the noise performance of 317s. One commonly neglected variable is current draw- the noise is a strong function of current, and sometimes it's worth drawing more out of the regulator than you need...

[Gopher]

SY

I've searched the internet high and low for this paper with no success. Do you have a link or can you post a hard copy scan here please?

Thanks

[SY]

It was republished as an appendix to Bob Pease's Analog Troubleshooting book. I can't post it because of copyright issues, but you ought to own that book anyway.

If you want to try further searching, I am embarrassed to admit that I spelled his first name wrong- it's Erroll. I think.

[jbau]

mjf: Greetings, and thanks. There are proper 100nF bypasses, separate load sense wires, etc. but I've not mentioned these details so this doesn't become a novel.

Sy and Gopher: I too have searched for both of those but didn't find them.

Hold on until the next installment is posted and you'll see where this is going. My main interest is in the output impedance, and am using the noise/impedance correlation for clues into it, at least until I can get a rig up to measure impedance with DC present.

More later. Gotta keep the day job.

[marce]

The article is reproduced in the following publication.
Troubleshooting Analog Circuits” by Bob Pease

Whoops I should read post fully before engaging brain

Cheater Simpson as also done a few articles regarding this device, that may be helpful. One link below.
http://www.national.com/appinfo/power/files/f10.pdf

[jbau]

Now we get to look at some noise spectra. (Until I get confidence in how to post photos, I'm going to post this in chunks.) The following pics are all 0-5kHz sweeps, 10dB/division, -70dbV full scale, 30Hz filter BW with smoothing, and a painfully slow sweep rate. I didn't show higher than that because these voltage set components didn't change the noise spectra that much above 3kHz. With a smaller output cap they might.

The first pic shows the the extremes. Top trace is 120/1k3 with no cap. Bottom trace is 950/11k with 100uF cap. The top trace is the high trace all across, the two intersect right about 2.5kHz. The lower program current gives 2-3dB less noise at high frequencies. The real benefit of the adjust cap is below 1kHz, with a 15 to 25dB reduction in noise in the midband.

[jbau]

The second pic is the 950/11k || 100uF all by itself, so you can see the broad rise centered on 1.8kHz. This is Jung's configuration, which gives the lowest noise. It's also the one I described above as having an "aggressive glare" in the 2k region. I was a little bit off, my tone range identification skills are deteriorating a bit with age, I guess...

[jbau]

Now, as stated above, based on data sheet info, I am assuming that the noise curves are also giving us insight into the output impedance and interactions with the load. I'm not an EE so I can't explain the inner workings of the regulator. I'm more of a systems integration type and I look for problems such as exactly what I'm seeing here. And clearly, optimizing this thing for lowest noise creates impedance nonlinearities in a region that is very audible. I'm guessing the IC's error amp has run out of gain bandwidth and maybe becomes unstable until the large output cap finally takes over above 3kHz.

So I tried various combinations to find the ones that give the most linear noise spectrum below 3kHz and doesn't excite the resonance.

The third pic is the same 950/11k resistors with a .47uF cap. It gives up a little bit of noise below 1.2kHz but there is no sign of any resonance.