Shielded input cables, grounding scheme.

[Michael F]

So I put the newly WOPL`d 700 series II into the main system today and was quite pleasantly surprised at what I heard. This thing sounds real nice, gratifyingly detailed and smooth presentation with seemingly endless power and a killer bottom end, that aspect was kind of expected.
I do however have a minor gripe, I noticed a very low level buzz emanating from the speakers. By low level I mean you need to be right up to the speakers to hear it. Mind you, my speakers are relatively inefficient at 86db/1 watt/1 meter, the noise would certainly be more apparent with more sensitive speakers.
It`s not being induced upstream nor is there a ground loop with associated equipment, it`s there even with nothing plugged into the inputs and regardless the volume levels. That doesn't bode well with my OCD

I`m led to believe the issue may lie in the unshielded internal signal wires and/or a less than ideal grounding scheme or maybe the filter caps are on their way out.
I didnt have this problem with my 700B. I did rework the grounding as was recommended here but I cant recall if the inputs wires were shielded or not, I believe they may have been.
I still need to get inside the amp to replace the Zobel network resistors and tweak the meters some more. I`ll try removing the input wires from the control board to rule out wire induced noise.
What would you suggest I do as far as the grounding scheme?

Perhaps member Mark Comer may chime in with his experiences regarding shielded cabling, I know he was contemplating installing them in one of his amps at one point.

This is what she looks like, I love the metering too, very hypnotic

 

[MarkWComer]

Perhaps member Mark Comer may chime in with his experiences regarding shielded cabling, I know he was contemplating installing them in one of his amps at one point.

That scheme Phase Linear pulled with the two conductor shielded connection from the jacks to the sensitivity controls and then back to an unshielded twisted pair to the driver circuit didn't make any sense to me. If there's anywhere you'd want shielding, it would be in the inputs!

I tried a few things with my (still uncompleted) 700/II, taking just a standard patch cord and cutting the plugs off to provide a shielded path, but found that to be unwieldy. I bought some shielded cable from NavLinear to accomplish a fully shielded path. This is something that I didn't do with my 400/II (yet), but I haven't had any hum or buzz from the amp.

There was one issue concerning a bus wire from the jacks to the speaker output grounds that needed to be removed. This also went to the body of the "Direct Coupled / Normal" switch, remove all of those- maybe you have an internal ground loop problem that has nothing to do with ground looping between connected components.

Yeah... OCD... On the other hand, I may just be an insistent little prick...

Send a PM to me with your mailing address, I'll send some of that shielded cable to you. I have plenty.

 

[Michael F]

Hi Mark, I just dug up a photo of my 700B and the signal is indeed shielded from input connector to the control board. I think that is where I`ll begin, even if it doesn't solve my issue, its good practice and certainly cant hurt.
While in there, I`m thinking of running individual wires from the filter cap buss bar to the input and output connectors, and to terminals 5L and 5R on the control board.
As for the shielded cable, thank you kindly for the offer to send some over to me but I have some very nice Mogami and Canare cable on hand that will do the job.

 

[wattsabundant]

search this forum for 700 hum and you'll find all kinds of stuff. Joe does a fine explanation of transformer issues.

 

[Gepetto]

Hi Mark, I just dug up a photo of my 700B and the signal is indeed shielded from input connector to the control board. I think that is where I`ll begin, even if it doesn't solve my issue, its good practice and certainly cant hurt.
While in there, I`m thinking of running individual wires from the filter cap buss bar to the input and output connectors, and to terminals 5L and 5R on the control board.
As for the shielded cable, thank you kindly for the offer to send some over to me but I have some very nice Mogami and Canare cable on hand that will do the job.

The input connector ground comes from the shield connection of the input wiring. Running another ground to the input connections is quite likely to create a ground loop for you. Based on the photos you posted, each output binding post has a separate wire running from it to the bus bar connection.

 

[Michael F]

Apologies for resurrecting an old thread but I finally re-visited this issue and with Joe`s help I was able to resolved it. The amp was doing LFE duty all of this time so the hum wasnt really an issue until I decided one day to run it full range on some JBL L80T`s that I just restored.
The JBLs are quite a bit more efficient than the last speakers I tried the amp on so the hum was that much more audible.
On Joe`s suggestion, I removed the ground buss that ran from the copper plate over to the speaker terminals as well as the grey wire that ran from that buss to behind the right side back plane.
While in there I replaced the original unshielded wires coming off the pots into the driver board with some shielded Canare wire.
I really wanted to run that shielded wire from the board to the pots but unfortunately I didnt have enough of it on hand so I just terminated them as close to the original wire where the shielding ended.
That resulted in a 90% improvement. The last 10% sounded like ripple voltage and sure enough was taken care of when I installed a couple of Joe`s new filter caps.
Thanks to all for your help