Low level 60Hz speaker hum

Magic Smoke · Nov 3, 2017

How y'all are?

As said. Depending on speaker efficiency, can be heard against driver or 4ft away. Amp otherwise sounds fine.

This amp has original drivers but using Joe's control board with new bulk caps for
early 4 fin model. Instant silence when plug pulled yet song plays on

Included scope shot shows a double bump as in 60 and maybe 120hz mixed on the either output. My old scope can't do much with such a low level signal.
Scope and Fluke measures left around 1mv to right 1.6 mv, right channel a bit higher. Fluke has trouble locking in Freq. but 60 Hz comes up the most.

Both channels BIAS sits @ 355 mv after warm up. X-former tight , tight, tight! Offset way below 1 mv but again right channel higher. Original 25 amp rectifier without .1mf - 500v caps.

Some guidence would be very mucho grateful. I would love to get her as quiet as my 80's Yama M-65, no?

Mike

Gepetto · Nov 3, 2017

Magic Smoke said:
How y'all are?

As said. Depending on speaker efficiency, can be heard against driver or 4ft away. Amp otherwise sounds fine.

This amp has original drivers but using Joe's control board with new bulk caps for
early 4 fin model. Instant silence when plug pulled yet song plays on

Included scope shot shows a double bump as in 60 and maybe 120hz mixed on the either output. My old scope can't do much with such a low level signal.
Scope and Fluke measures left around 1mv to right 1.6 mv, right channel a bit higher. Fluke has trouble locking in Freq. but 60 Hz comes up the most.

Both channels BIAS sits @ 355 mv after warm up. X-former tight , tight, tight! Offset way below 1 mv but again right channel higher. Original 25 amp rectifier without .1mf - 500v caps.

Some guidence would be very mucho grateful. I would love to get her as quiet as my 80's Yama M-65, no?

Mike

Some photos of the inside of your amp would be good Mike. Did you cut the bus wire between the speaker grounds and the copper plate that the input jacks tie to?

Lazarus Short · Nov 3, 2017

If your amp runs off a two-prong power plug and cord, try connecting the chassis to ground. It worked for my preamp.

Magic Smoke · Nov 4, 2017

This one?

Gepetto said:
Some photos of the inside of your amp would be good Mike. Did you cut the bus wire between the speaker grounds and the copper plate that the input jacks tie to?

Just making sure, you want me to cut this right?

I did notice that the ground plate brings the case to same potential as the copper bar on the bulk caps where I thought I read as the "Star" ground?

Gepetto · Nov 4, 2017

Magic Smoke said:
Just making sure, you want me to cut this right?View attachment 30728

I did notice that the ground plate brings the case to same potential as the copper bar on the bulk caps where I thought I read as the "Star" ground?

Yes, remove that bus wire and the white wire from the copper plate that runs to PAD 5R on the control board. Add a new wire from the copper bus bar between the 2 bulk caps to PAD 5L of the control board.

This eliminates a ground loop in the PL400 design.

That copper plate that the input jacks connect to must be floating from the chassis itself. There are insulators around the input jacks that achieve this separation. Make sure those are still in place.

Magic Smoke · Nov 4, 2017

buss wire gone

But no change. Both L5R and R5R have shield direct from jumpered L2 and R2 so looks like we just eliminated a second jumper?

Copper input plate good, all bushings in place. However the plate, star ground are still tied to chassis by xformer and bridge rectifier correct? I was looking at original schematics. Very little change with inputs shorted to chassis or just shorted together...

Oh snap, should I just cut both input shields at the copper plate?

Magic Smoke · Nov 4, 2017

Lazarus Short said:
If your amp runs off a two-prong power plug and cord, try connecting the chassis to ground. It worked for my preamp.

Chassis to ground? You ran a third wire to wall outlet ground on your preamp? Read several places this is bad for the PL-400?

marcok · Nov 5, 2017

Generally A/V gears must be disconnected from the ground .
There some exceptions : hi power amps and subwoofers . ( new regulations for safety )
In any case only one gear should be connected to ground .
Warning with the gears made for U.K . ( the UK regulations want the chassis connected to ground )
Many years ago I had a noise problem with my Nad 4150 .It took me 1 month to discover that ,for
a mistake, it was the UK version . Btw this matter is very complex .
Any comments ?

Magic Smoke · Nov 5, 2017

Still no change, so...

Found this thread: http://forums.phxaudiotape.com/show...leshooting-an-humming-WOPL-need-help!/page13? Guy has a couple of issues so not sure if hum was 100% gone.

I went over post from Joe with the comments on the ideal ground scheme which I had other than the buss wire. I'm going order the snubber caps for bridge, then the bridge if no change. Anyone find leakage from the original delco's having like cause?

As before hum or buzz very faint, ear must be near woofer in quiet room..not sure this a safe procedure if something were to go .. :angel8:

I measure .84 mv at ether 120 or 60hz on left channel and .62 mv at 60hz on right terminals no speakers attached, inputs shorted. Fluke has hard time locking into frequency, so may be a mix from previous scope shot.

Anyone else other than complete rebuild?

Magic Smoke · Nov 8, 2017

Tried Bridge snubber caps...

Still hums, lol. Also tried the two .47 caps shown in the improved ground drawing. Next up, have 40 amp bridge in the mail.

Since noise in both channels, inputs shorted, what about reroute of the 117 to the thermal switch? Am I humming up the wrong tree?

Thx

Netfly · Nov 8, 2017

Just a thought - Could be a grounding issue. Test with a wire from a chassis screw to a negative input (shield).

Magic Smoke · Nov 9, 2017

Netfly said:
Just a thought - Could be a grounding issue. Test with a wire from a chassis screw to a negative input (shield).

Tried that, also that would be the removed buss wire from ground plate that runs back to star ground through both speaker grounds, no? Both input grounds go to 2L and 2R from the input ground plane which was removed, cleaned with 4 insulators in great shape. That plate is isolated from chassis.

I'm going to pull the AC wires from the thermal switch today. Did you remove yours, any hum from speaker outs?

Mike

Netfly · Nov 9, 2017

Magic Smoke said:
.. also that would be the removed buss wire from ground plate that runs back to star ground through both speaker grounds, no?

Mike

Ground yes, but if you assume all ground paths suspect, then injecting a solid chassis ground may find it. How much of that AC noise shows on the DC rails?

Magic Smoke · Nov 9, 2017

Netfly said:
Ground yes, but if you assume all ground paths suspect, then injecting a solid chassis ground may find it. How much of that AC noise shows on the DC rails?

Hey Netfly, using meter I rang out input grounds all the way to both ground chassis points from star buss bar. That is to and from board. Using clip leads direct to different board ground points had no effect. Both 15 vdc, bias, offset good as posted. Meter nulled before use.

BTW, cut AC wires to thermal switch, no effect. As before, low level hum same both channels. Wired switch back to circuit, 16ga teflon wire.

L Channel bulk 18-19 mv/ac Speaker out .7 mv

R Channel bulk 18-19 mv/ac Speaker out .5 mv

Think I missing specific ground points? Appreciate you looking!

laatsch55 · Nov 9, 2017

Have you tightened the 4 transformer bolts?

Netfly · Nov 9, 2017

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 Sounds like you checked a lot of grounds. I'm no expert on this amp, I'll refer you to Gepetto or one of our other experts as to what goes on regarding this main board, but I would look at pin 6 of the LF 351/356 op amp for noise and how much AC noise is on the 90v DC rails. My thought would be to try to isolate if its coming through the signal path or power supply.

Magic Smoke · Nov 9, 2017

laatsch55 said:
Have you tightened the 4 transformer bolts?

I tightened the crap out of it last week. So far THAT has made the biggest difference so far. I have new 40 amp bridge in the mail, maybe Tuesday if work leaves me alone...

Mike

laatsch55 · Nov 9, 2017

Ok Mike, just checkin....

Netfly · Nov 9, 2017

Magic Smoke;217722.. Instant silence when plug pulled yet song plays on :p ... Mike[/QUOTE said:
Sounds like your on the right track changing the bridge.

Magic Smoke · Nov 10, 2017

Netfly said:
Sounds like your on the right track changing the bridge.

I love this amp 'cause easy to work on..if it's not the bridge I can just live with the noise floor "as is" or move along and learn more.
I can only thank laatsch55 and Joe that have given their time and "grins" :scratch:

mike

Low level 60Hz speaker hum

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