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5th element 12th August 2003 01:58 PM

Lm317/337 tracking preregs

Im in the process of adding regulators to my preamp and have built the tracking prereg circuit as illustrated in the LM317 data sheet

Now Im using apprx 18v input lines and the max voltage I can get on the output is only 8.54 volts. I was hoping to get about 15v, and im assuming that its impossible to get the full input on the output. Is the circuit running right?? and if so what can I change to increase the max output, if thats possible. Im all new to regulators so dont know much about them yet.

Cheers Matt

sam9 12th August 2003 03:16 PM

If you had said you were getting only 14V, but wanted 15V, I would have suggested the only route to be a bigger transformer :) But since what you are getting is only `8-1/2V, I suspect you have miswired the circuit. I can only suggest you re-check

8-1/2V just happens to be 1/2 of 17V which is a plausible voltage to achieve so keep an eye out for the possibility that you have inadvertently built in a 2:1 voltage divider somewhere. Most of my mistakes seem really simple and dumb - once resolved.

Bricolo 12th August 2003 04:10 PM

please tell us if this circuit works good, when it will work correctly.

I never built it, but it looks interesting

JeLL-O 12th August 2003 04:59 PM

THe circuit for the tracking pre-reg shown in the LM317 datasheet maintains a difference of 5 volts between the output of the first reg and the second reg. (720/240 +1 )*1.25=5 volts (ignoring Iadj)

Each LM317 needs a Voltage Out-In differential of at least 3 volts. This means that with an 18v input, the max you can hope for is 12v.

To get 15v, you need to start with approximately 25v, drop 5 volts in each reg, to end up with 15.

hugonot 12th August 2003 05:18 PM

Output Voltage
Minimum Input to Output Voltage is 3V for LM317, LM337. If you want you may use potentiometers in Adj paths, the you could regulate an output voltage.
LM317 and 337 have a cuurent limit. When current is too high then output voltage going low. Check output without preamp circuit. Those regulators have a thermal limit, so remember to use a heat sinks.

5th element 12th August 2003 06:12 PM

OK well I have it sorted, nothing really special needed. Loading the supply brought the supply rails up a few volts to 11.2. Then the variable resistor that adjusts the voltage when at max res1k gives the aforementioned 11.2volts. I added an extra 1k to this making min res 1k max 2k. Now I get 14.6 out of this which I assume is max for the loss of the regs.

I cant comment on the sound as of yet as im changing the case of my amp too which should be completed today or early tomorrow.
Pics to follow aswell. Ill keep you informed on the sound when I can


5th element 12th August 2003 06:13 PM

Thought I might add this quickly, I had previously listened to the pre at 17.5 rails unregulated, so it will be interesting to see how it sounds at 3v less but regulated.

ALW 12th August 2003 07:44 PM

You should be able to run the tracking pre-reg at 2.5V (i.e. both resistors of equal value) - the dropout of most 317's is nowhere near 3V at low currents, and certainly 5V is overkill.

Decouple BOTH adj pins too, for best performance / sound.


5th element 14th August 2003 12:41 PM

Ok well all is up and running. I had it going a few days ago but wanted to listen to evaluate the sound before posting.

The amp section I'd listened too before anyway and I knew it sounded good but not the preamp when regulated. The regs are all sorted and everything works fine.

All I can say is that it sounds better with new case and regs. I cant say if the amp sounds better with new case or the preamp sounds better with regs cause I changed both at the same time. But everything sounds more clearer, not clarity directly as clarity was never and issue before but more textured like everything is layered more clearly. Highs are more smooth, again wasnt an issue before really but they are, and stage depth (i find this really hard to actually perceive regardless of system) seems to be more realistic.

I will post pics when I get the digital cam upa nd running

Panelhead 14th August 2003 02:50 PM

LM337 Tracking Reg
It looks interesting. How did you impliment the LM337 tracking reg? The positive isa shown in the LM317 datasheet. How exactly did you build its compliment?


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