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Old 22nd May 2009, 11:33 PM   #1
JesseG is offline JesseG  Canada
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Default Very strange gremlins in my phono PS

Hi Guys

I have been using my Torsten design phono stage for some months now. It sounds mostly fine, but has a hum in it that is finally getting on my nerves. So, it is back on the bench and I am trying to find out what is causing it.

First - test the tubes both OK.

Next, look at the output (with shorted input) - nasty ripple about +/- 2.5mV - not good for a phono stage.

OK, maybe DC heater - easy to complete, no change.

Time for the big guns (it's time I learned how to troubleshoot instead of just scrapping a disapointing project)

So, I hooked up the 'scope. What I found is this grunge on the B+. Added a BIG! cap - no change. Added a variety of smaller caps - no change

Next, I removed the tubes to look at the unloaded PS. Guess what? Same noise.

The PS is quite simple. 230-0-230 tx, CRCRC filter.

So, fellows, here is the question(s)

could this be generated by the 6X4 rectifier tube?

is it possible that this noise is coming form any of the passive devices?

how can I track this down and squash it like the smarmy little bug that it is?

BTW: for the trace shown below the scope was set at 50uS / 50mV.

The schematic for this is here:
Thorsten's simple phono stage
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File Type: jpg noise-sm2.jpg (4.9 KB, 211 views)
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Old 23rd May 2009, 02:25 AM   #2
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Hi,

Looks like a CFL bulb.

Try moving your phono and pre to a different part of the room and try a different outlet.

Cheers!
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Old 23rd May 2009, 02:34 AM   #3
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Looks like oscillations. Probably a ground somewhere misswired?
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Old 23rd May 2009, 05:16 PM   #4
JesseG is offline JesseG  Canada
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Quote:
Looks like a CFL bulb.
That's an idea Greg. There are none close to my workbench, but I do have a halogen nereby. I will try moving this.


Quote:
Looks like oscillations. Probably a ground somewhere misswired?
I have star-grounded this rig with determination. Still, There must be a way to prove this. Any ideas?
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Old 23rd May 2009, 07:55 PM   #5
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Quote:
Originally posted by JesseG


I have star-grounded this rig with determination. Still, There must be a way to prove this. Any ideas?
Any photos?
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Old 25th May 2009, 04:26 PM   #6
JesseG is offline JesseG  Canada
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Hi guys:

I think I found it. Turns out that the little bursts of energy laying on the trace are from the SMPS that I used for the DC heater supply. I have been fond of these 'cause they are small and very easy to implement.

This is the first time I have had a reason to look at the output of a SMPS in detail. It seems that they are not such a good choice after all. Probably also explains how a better wall-wart can improve the sound of some devices.

I have gone back to AC heater, carefully routed, and the hum has dropped (but not gone completely). This seems to indicate that the hum is heater induced.

I am planning a solid-state DC heater supply and we'll see if that kills it.
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