No more 60 Hz hum in a PL2000 preamp.

[George S.]

Any good ideas on how to reduce the 60 Hz hum and it's harmonics in a PL2000. Thinking of trying snap on ferrite chokes or isolating the transformer from the chassis. Very little of the issue is from the computer or sound card setup running REW that generated the graph. Using the preamp in the system, I can't hear any hum at all. This is the modded unit with Panasonic films, CD silver micas, and Browndogs. Power cord is wired 2 wire, flipping it has no effect.

 

[George S.]

I'm pushing a 113 mV RMS, 1000 Hz sine wave into the unit on the above graph.

 

[George S.]

Here's a 31 step, stepped sine wave graph. The peak at 20K Hz is a artifact of the stepping and doesn't factor in the results.

 

[Gepetto]

Have you snubbed the bridge rectifier and considered inserting ~10 ohms into each AC secondary leg where it connects to the bridge?

You have every harmonic of 60Hz represented in your spectrum analysis. 60, 120, 180, 240, 300, 360, 420, 480, 560, 600, 660 is quite muted, 720 and so on...

 

[George S.]

Thanks Joe, I'll take a look at it and see if I understand what your saying.

 

[Gepetto]

Thanks Joe, I'll take a look at it and see if I understand what your saying.

 

[George S.]

Yup, just got home from a short road trip so was thinking about this. Paralleling those diodes is what I was thinking from your suggestion. I already have the two resistors installed to drop the rail voltage, though I can't recall the values.
Got some chores to do first to keep the wife happy. Clean the garage, sharpen blades on the zero turn, get grass cut.
Later this evening I'll install the caps and post results. Pretty sure my analyzer setup is good enough for home use. Tried ARTA, Audiotester, and REW software with similar results. Found a desktop computer and internal card is too noisy. Laptop on battery power and USB sound card is way to go. Thanks!

 

[mlucitt]

You likely have a 60 Hz Fuse. They oscillate at 60Hz and every harmonic. I'd change it out for a Buss fuse, they sound better.
Interesting that you cannot hear that 60Hz hum in the system. No hum with no load on the transformer?
Did you change the grounding on the rear of the circuit board with the jumper between the solder tracks and isolate the 3rd board mounting screw?
Is it possible that the transformer is loose?
Convenience jack wiring too close to circuit board?
Please let us PL2000 owners know how you resolved this issue.

 

[George S.]

Mark, everything looks good. This is the unit I've been modding on and off for the past year. With the unit in the system, no media, and volume cranked, there is no hum.
I'm new to this testing and have little grasp of Decimal units and such, but think perhaps it's low enough level it wouldn't reveal itself as hum. I don't know.
The 2nd unit I picked up sometime back and is stock as a "control" unit also graphs similar. Going to go in now and look for the thread that states the value of the caps I need, think .1 uF are used on the amps rectifier, need to be certain.

 

[George S.]

.01 uF looks correct.

 

[George S.]

Adding the 4 .01 uF snubbers dropped the noise floor appx 4 dBFS. Made a noticeable improvement in the upper frequencies. I used 4 63V Wima polypropelene I had on hand. Not sure how to proceed from here other than try some chokes or experiment with floating the transformer above ground.

 

[George S.]

Going to try to find some info on snubbers, maybe a different value is needed here.

 

[George S.]

Good article here. Now I kind of understand Joe's 10 ohm snubbering resistor. Looks like I need to install the 10 ohm resistors where the voltage dropping resistors are now. The voltage dropping resistors to save the opamps will have to be further down the rail. Looks like it's best to reinstall the original opamps to save the Browndogs, tame the 60 hz, then adjust the rail voltage for the Browndogs. I'll sleep on it, read it again tomorrow and see if I really understand it.https://diyaudioheaven.wordpress.com/tutorials/power-supplies/snubbering/

 

[Gepetto]

Adding the 4 .01 uF snubbers dropped the noise floor appx 4 dBFS. Made a noticeable improvement in the upper frequencies. I used 4 63V Wima polypropelene I had on hand. Not sure how to proceed from here other than try some chokes or experiment with floating the transformer above ground. View attachment 52871 View attachment 52874 View attachment 52875

Your 60 Hz is down over 10dB

 

[George S.]

Your 60 Hz is down over 10dB

Yup, well worth while. Been reading about snubber R/C (resistor/capacitor) networks. Going to take a break and run a jumper from the IEC socket ground to a transformer mounting screw and retest. One thing I've read about is looking at the 60 Hz on the secondary with the scope. This is a deep, deep rabbit hole for me because I understand so little about it. But I got the test equipment so I can learn a little, so I'm pursuing this. Thanks Joe!

 

[George S.]

The IEC ground wire was a step backward. Taking it out. Going to set up the test equipment outside once it stops raining. Will run a single long cord to power the pre. It'll be interesting to test it away from all the electrical noise in the house.

 

[George S.]

I'm thinking this is as good as it gets. I seperated the primary bundle from the secondary and twisted the bundles fairly tight. Really no great change. I do have a electrically noisy environment here. The unmodded unit tests similiar for 60 Hz noise but has higher THD. So I'm out of ideas. Next weekend I'll take it all ouside to a electrically quiet place and retest. Until then I'll put it back into the system and enjoy it. The modded unit sounds so much better than the unmodded control unit.

 

[laatsch55]

Impressive stuff there George.,..

 

[George S.]

Just had a thought. Been using some older Nichicon Muse filter caps in that unregulated supply. Going to switch those out for some new Fine Gold. Probably have about 5 years use on those Muse. Listening to the preamp now, a old Morton Gould and Orchestra LP. Really is fantastic sounding. Cranked the volume before playing, no hum. I'm thinking that level of 60 Hz is just inherent in the design and at -80 dBFS is a non issue. Going to try to get it lower. I'm pretty happy with the THD at this point.

 

[laatsch55]

If you can hear no hum at cranked volume, George, Ya done good Lad!!