WOPL 700B A/C hum ?

[mattoe]

Gentleman,

I just noticed that my 700 has a hum. I noticed this when working on speakers without any crossovers....
Connect the amp to the speakers , with no inputs to the amp and we get a low but obvious hum.
Connect The amp back to my system speakers ( paradigm signature s8"s ) and the hum is just about gone..
I suppose the crossovers take care of this ...
I have never noticed the hum because its always been running my paradigms ! till now...

All the ac wiring is relatively isolated to the top of the case and the dc and signal wiring along the bottom.
New 50 amp reg with .01 caps as snubbers... all good....
I did however remove the input potentiometers and connect directly to the board...
Do I have a floating input issue ??? I have heard some say to solder a 100K resistor from sig to ground at
the rca's ... also read to try a 220pf cap to ground ? Any of this sound familiar to anyone ?

Cheers !

Mattoe

 

[NavLinear]

Here's a section from the Phase Linear 700 700B service manual that may help. I've had a similar problem where tightening the transformer bolts corrected the issue.

 

[laatsch55]

Second what the Navo said. Also, is the wiring from the inputs to the board shielded?? What iteration of amp and control board do you have? No need for a resistor or cap.

 

[laatsch55]

Here's a section from the Phase Linear 700 700B service manual that may help. I've had a similar problem where tightening the transformer bolts corrected the issue.

What manual do you have Dennis, mine doesn't even mention hum>??

 

[mattoe]

Second what the Navo said. Also, is the wiring from the inputs to the board shielded?? What iteration of amp and control board do you have? No need for a resistor or cap.

Thanks for the reply,

The transformer bolts are tight, and yes... The input cabling is coaxial shielded... full WOPL rev "E" I believe....
When I connect the pre amp and bring up the volume the "hum " is negated.... The amp plays beautifully....
I never knew there was an issue until I used a speaker without any crossover ....

Mattoe

 

[mattoe]

Thanks for the reply,

The transformer bolts are tight, and yes... The input cabling is coaxial shielded... full WOPL rev "E" I believe....
When I connect the pre amp and bring up the volume the "hum " is negated.... The amp plays beautifully....
I never knew there was an issue until I used a speaker without any crossover ....

Mattoe

Any suggestions ?

 

[marcok]

Reverse the AC plug and check .
Short the inputs of PL using two shorted RCA plugs and check .
Replace the speakers with 2 other ones and check .
In this way you could find the issue and after the solution .
In other words first the diagnosis and after the therapy .
Ciao
Marco

 

[laatsch55]

Matt, are you running the White Oak back planes and Wattsabundant DC protect board?

 

[mattoe]

Matt, are you running the White Oak back planes and Wattsabundant DC protect board?

Yes Sir,

The "full monty"

Matt

 

[Northwinds]

Matt, are you using a power conditioner or plugged straight into the wall? Might be dirty power supply and you getting a ceiling fan hum or something similar. Also, I had a problem with an outlet that was part of a circuit with a dimmer rotary control making a hum which controlled living room ceiling fan and light. Once I used the power conditioner, hum gone

 

[WOPL Sniffer]

Ground loop??? Did you stick to Joe's Star-Ground scheme??? Maybe a pic of the build as it went along? Did you keep the power and signal wires at their respective sides of the chassis?

 

[WOPL Sniffer]

As Ron was saying, maybe trying a different outlet on a different circuit in the house? Did you use an IEC plug and ground it inside the amp? (I hope not).

 

[jbeckva]

Ground loop??? Did you stick to Joe's Star-Ground scheme??? Maybe a pic of the build as it went along? Did you keep the power and signal wires at their respective sides of the chassis?

I tell ya.. when I was doing a 700 (and you know who you are, person I built it for ), I missed the one single wire that should go from star ground at the caps to the chassis. But with my setup I could NOT hear the hum.. Unfortunately he could and we went back and forth for a bit.. even had a woofer shipped over to see if I could hear it then. Felt like an ass haha.. One of the first builds.. hey.. what can I say?

But anyway... basically what I learned from all that is that there should be only *one* connection to the chassis and direct from the star ground. On the inputs, both shields together at the RCA's and with the RCA's *isolated* from the chassis.. with a copper shield laying on top of the fiber swag washer installed on the inside Re-use a shield from a 400 .. works good.. or make your own.. or if you use different RCA jacks solder the two ground tabs together and run the combined shields L/R to the same point. Then back to each channels input on the Rev "E". I take the pots out of the circuit as well - maybe overkill but less wire, clutter, etc etc.

 

[Gepetto]

Exactly Jer. Like this...

 

[grapplesaw]

If I can add my five cense worth. The problem may lay more with that set of speakers. There may be inductance from the voice coil causing the hum when connect to that amp. Try putting a .1uf or .01uf cap across each channel’s Leeds. One per channel. Try it at not to high a power first.

 

[mattoe]

If I can add my five cense worth. The problem may lay more with that set of speakers. There may be inductance from the voice coil causing the hum when connect to that amp. Try putting a .1uf or .01uf cap across each channel’s Leeds. One per channel. Try it at not to high a power first.

Thanks for all the suggestions !

I'll try to touch on all these points...

My 700 build followed the White oaks instructions and yes I do have the single point ground scheme ... 2 prong plug not the IEC 3 ! The input rca's are insulated from the chassis with a copper strip and single point ground and directly to the board ,pots are eliminated.

The ac wiring is all along the top and dc and signal wiring along the bottom of the chassis

Monster power conditioner doesn't seem to make any difference.


I will try a cap across the speaker leads and post the result
Maybe flip off house breakers and move the amp to another house circuit just to see....

Matt

Thanks again for the input !

 

[WOPL Sniffer]

Insulated with a copper strip??? I hope you mean the one with the black insulators. Did you try the drinking straw trick on the transformer bolts? I never had to do it but some did.

 

[mattoe]

Insulated with a copper strip??? I hope you mean the one with the black insulators. Did you try the drinking straw trick on the transformer bolts? I never had to do it but some did.

Yes Sir,

the RCA's ground via a copper strip between and common to both ... Not directly to the chassis....
The drinking straw trick ??? That's new .... I can only assume insulating the bolts as they run the length of the
laminated stack ?

Matt

 

[WOPL Sniffer]

Yes Sir,

the RCA's ground via a copper strip between and common to both ... Not directly to the chassis....
The drinking straw trick ??? That's new .... I can only assume insulating the bolts as they run the length of the
laminated stack ?

Matt

Yeah, I think our guru's read that it can eliminate the eddy currents if you have any. Make sure you get the bolts nice and tight after. Are you sure it's the amp? And not another piece of equipment? I usually try just the amp, and a ipod with a set of speakers. The Ipod has enough juice to drive the amp. It does'n't sound the best but it helps track down noise.

 

[WOPL Sniffer]

I have had a preamp causing a hum.