Dynaco Pas 3 Hiss from HV regulator - diyAudio
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Old 26th April 2008, 12:50 PM   #1
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Default Dynaco Pas 3 Hiss from HV regulator

Hi all,
I am upgrading a Pas 3 piece by piece, I built a LM317 regulated filament PS and it works fine no hum or hiss.

I built a LM317 + MOS HV power supply with a schematics from this site

Similar to others on this site.

The problem is quite strong Hiss on the mid-ranges and tweeters, with 0 volume , quite strong from listening distance.

I cannot see anything with a scope, just a 30 mV noise.

I made a few changes:

R5 = 240 ohm
R7 = 61k for V=320 Volts
R8 = 100
C5 = 22nF
R4 = 0
R1 = 0

What do you think ?
Should I try a P filter before C3 ? Which values ?

Rollo
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Old 26th April 2008, 01:51 PM   #2
SY is offline SY  United States
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I think your regulator may be oscillating. What's the value of the output cap? Are you using a gate-stopper resistor for the MOSFET? Is it positioned right at the gate?
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Old 27th April 2008, 07:45 AM   #3
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>>I think your regulator may be oscillating. What's the value of the output cap? Are you using a gate-stopper resistor for the MOSFET? Is it positioned right at the gate?<<

Yes oscillating, but how can I see it on a scope (100MHZ analog storage TEk) ?

there is a 470ohm gate stopper at 12 mm 1/2" from gate of IRF820.

the big cap is 330uF 450V 105 after 2 Hi Volt diodes from original trafo (I use only HiV section of trafo, I added a new toroid trafo for filament),
at the output I put a filter 100ohm + 223 (=22nanoF ?) between B+ and GND.

If I check with original PS which has a ? filter after Diodes of 20uF+10Kohm+20uF+10Kohm+30uF the Hiss is bearable, I wanted to cancel it with new PS.
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Old 27th April 2008, 11:09 AM   #4
Giaime is offline Giaime  Italy
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Hey, that's my Maida circuit. You could also ask the author for help...

However, check that output "filter" you made, it seems suspect to me. How have you come to those values (100ohm + 22nF)? That should only be a suitable capacitor (say from 100nF to 1uF) with a small (1ohm-10ohm) resistor in series to avoid oscillations in the LM317 because of too low phase margin.

What did you connect after the regulator? If you put big, low esr capacitors from B+ to ground, they can made the LM317 oscillate.

Check your layout also, for too long wires, wires running parallel, grounding, etc etc...

Have you tried bypassing the regulator (using a power resistor to imitate it's voltage drop)? Maybe that's the audio circuit that's oscillating...

I made a new page for the Maida circuit, check it out here.
http://giaime.altervista.org/maida.html
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Old 27th April 2008, 11:27 AM   #5
SY is offline SY  United States
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Rollo, go up to 10-20uF for the output cap (the 4R7 resistor is fine) and see what happens.
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Old 27th April 2008, 01:49 PM   #6
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Giaime
>>However, check that output "filter" you made, it seems suspect to me. How have you come to those values (100ohm + 22nF)?<<

From 1990 AN2-6 Linear Technology Applications (Jim Williams) see picture.
In fact
R8 = 10 ohm as I put it in the circuit.

>>If you put big, low esr capacitors from B+ to ground, they can made the LM317 oscillate<<

No capacitors on B+ 320V,
I put a 40 V Zener + bleeder resistor 390k + 6.8uF for the 280V (RIAA Board) which is not connected at the moment.

<<Have you tried bypassing the regulator (using a power resistor to imitate it's voltage drop)? Maybe that's the audio circuit that's oscillating...<<

Not yet but if the audio circuit is working fine with the standard Dynaco PS, should'nt that exclude the audio circuit ?

SY
>>go up to 10-20uF for the output cap (the 4R7 resistor is fine) and see what happens<

I tried 10 uF with 10 ohms and:

The hiss is quite less, now it is mainly in the midrange speaker.

Regards
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Old 27th April 2008, 02:37 PM   #7
SY is offline SY  United States
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Try 20u and 4-5R. Then go larger on the reference bypass cap, too.

Have you looked at Errol Dietz's work on noise versus bypassing (it's on National Semiconductor's website)? The Maida is not particularly quiet unless you let caps do some work.
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Old 28th April 2008, 07:34 AM   #8
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SY

>> Have you looked at Errol Dietz's work on noise versus bypassing (it's on National Semiconductor's website)? <<

I can't find it, Erroll Dietz is vice President but no articles found.

Any link ?

Regards
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Old 28th April 2008, 01:14 PM   #9
SY is offline SY  United States
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Hmmm, they seem to have taken that paper off the site. Sorry. If you have (or can get) a copy of Bob Pease's superb book on analog troubleshooting, the paper is published as an appendix.

Bottom line: big bypass and output caps are a good thing. So is running the current high.
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Old 28th April 2008, 02:05 PM   #10
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It's the ADJUST PIN -- You MUST bypass the adjust pin. I use 1uF 400V WIMA, but polyester will do fine.

SY -- Pease's book is must read as you so correctly mentioned a couple years ago.

I ran this test a few years back with the LR8N3 regulator -- in comparison to the LastPAS regulator board -- the test setup was a HP 3581 wave analyzer set to 3 Hz bandwidth:

Click the image to open in full size.
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