TL431-related (circa) problem - TNT Indiscreto power supply - diyAudio
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Old 17th February 2013, 12:40 PM   #1
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Default TL431-related (circa) problem - TNT Indiscreto power supply

Dear guys,

I've just built a nice phono preamp, designed by Giorgio Pozzoli for TNT-Audio. Here's the English page, with description and schematics: inDiscreet Phono Pre-amplifier [English]

I did the first listening test and I was being surprised by its good sound, but all of a sudden a prominent hum appeared. First thing I did was to check the power supply, which must provide a +27V and a -9V rail.

Click the image to open in full size.

(note that U1 and U2 are TL431)

I read a good -9.1V, and a horrible +33V. I checked the reference voltage across the R12 resistor, which should have been 2,5V (reference to the TL431), and it was 0 V. On the other rail, there was the correct value of 2,5V, and in fact that rail, as already stated, was doing right.

I thought the TL431 in the faulty rail was probably defective, replaced it and followed the power-up process with a multimeter. I noticed that, despite the fact that it should have taken about 40 minutes to reach the nominal voltage, in less than circa 10 minutes I was already at 27V. The reference voltage across the resistor ramped proportionally, reaching about 2,5V at 27V rail. At past 27V rail, the reference dropped from 2,5-2,6V to zero (!). A few seconds later we were at 28.5V and going, still the reference across R12 dead to zero, and I quit the test.

I tried a couple TL431's with the same result.

I'd really appreciate some help in identifying a culprit, here...

Thank you in advance, have a nice Sunday!

Giacom
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Old 17th February 2013, 05:40 PM   #2
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Wow! the (R7, C8) timeconstant is 470 seconds; that's 7.8 minutes.

I guess I would temporarily disconnect the rest of the preamp from the power supply and install another brand new, known good TL431. If this one burns out then you know the fault is somewhere on the power supply board. If this one does not burn out then you begin to suspect that the fault is not on the power supply board.
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Old 17th February 2013, 06:34 PM   #3
Ketje is offline Ketje  Belgium
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R9 = 470 ohm ,did you use 47 ohm ?
Mona
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Old 17th February 2013, 06:46 PM   #4
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Quote:
Originally Posted by transistormarkj View Post
Wow! the (R7, C8) timeconstant is 470 seconds; that's 7.8 minutes.

I guess I would temporarily disconnect the rest of the preamp from the power supply and install another brand new, known good TL431. If this one burns out then you know the fault is somewhere on the power supply board. If this one does not burn out then you begin to suspect that the fault is not on the power supply board.
Thank you, I guess it was poorly described in the project page.

Well, that's a test I will surely do, but is there anything "past" that point that could fry the TL431? I didn't think so, therefore I didn't try disconnecting the preamp in the first place.

Click the image to open in full size.

Now that you pointed that out, I will try anyway.

Quote:
Originally Posted by Ketje View Post
R9 = 470 ohm ,did you use 47 ohm ?
Mona
I checked that already, it's the correct 470 ohm value. (hard to make a mistake there, since the resistors are PRP with clearly printed values)

Thank you both,

Giacomo
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Old 17th February 2013, 06:52 PM   #5
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The 431 can oscillate, after all it is a high gain amplifier inside.
Try 10nF ceramic cap across R11 / R13.
Solder them as close to the 431 pins as possible.
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Last edited by payloadde; 17th February 2013 at 06:57 PM. Reason: afterthought
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Old 17th February 2013, 07:30 PM   #6
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You are absolutely right about the pinning? If yes, did you load the regulator? Without load the TL431 should dissipate 400 mW or more = hot. What is your voltage after the fet?
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Old 17th February 2013, 07:50 PM   #7
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Quote:
Originally Posted by payloadde View Post
The 431 can oscillate, after all it is a high gain amplifier inside.
Try 10nF ceramic cap across R11 / R13.
Solder them as close to the 431 pins as possible.
I'll try this little mod asap (tomorrow).

It bugs me though, because many builders have done this phono stage without reporting any necessary modifications.

Thank you,

Giacomo
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Old 17th February 2013, 08:01 PM   #8
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Quote:
Originally Posted by peranders View Post
You are absolutely right about the pinning? If yes, did you load the regulator? Without load the TL431 should dissipate 400 mW or more = hot. What is your voltage after the fet?
Hi, yes the pinning is easy, since the PDF included within the project page has the PCB layouts and the components drawn "from above". Very easy to just copy and follow; anyway, since I had this issue, I already checked the pinout of both the TL431 and the IRF630, and they're all correct.

Also, I initially didn't know that the value had to be so precise (that should be the point of a regulated supply, now that you mention it... ), so when I tested it without load, and read about 32V, I thought it was due to my slightly high domestic AC, and went on connecting the boards as if everything was good. So, all my latest results are with the phono boards attached, under actual load conditions. Now that you make me think of it, I can therefore already answer that the phono section is OK (since I was measuring high right from the beginning, before connecting them), plus it also sounds very good until the hum emerges. It makes me guess that everything is just fine on that side of the preamp.

Should I measure the voltage past the IRF630 with or without load? And, with or without a (broken) TL431?

Thanks,

Giacomo

Last edited by giacomo_pagani; 17th February 2013 at 08:04 PM. Reason: highlighting a useful part
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Old 17th February 2013, 08:16 PM   #9
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A correct working 431 should have 2.50 volts across REF and ANODE. If you have zero volts you have some problem. You should also study the datasheet for the right capacitance value. How hot did the 431 get?
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Old 17th February 2013, 08:24 PM   #10
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Quote:
Originally Posted by peranders View Post
A correct working 431 should have 2.50 volts across REF and ANODE. If you have zero volts you have some problem. You should also study the datasheet for the right capacitance value. How hot did the 431 get?
Yep, I tried to see what was happening with the reference voltage already: put (several) new TL431's and powered up, with a voltmeter hooked up across the 1k resistor. The voltage would ramp slowly to 2.5V, and then suddenly drop to 0V. Meanwhile, the rail voltage would ramp slowly, without stopping at 27V; 27.x is the value when the reference voltage drops to zero, while it keeps ramping to 32, 33 or so.

I touched the TL431 immediately after the reference drop, and it doesn't feel hot.

Giacomo
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