700B and speaker out DC protection?

[BMWR75]

You should have no noise, my WOPL is dead silent??? Even when I crank the preamp to the max, it's dead silent. Might want to ask Joe what would cause any background noise

I can't hear any background noise. Should have been more clear in what I wrote earlier.

 

[BMWR75]

All this amp needs now is a new set of PS filter caps. Well, need may be too strong a word, the amp's owner wants them just for piece of mind.

 

[Gepetto]

Please allow me to recap how this amp was fixed and answer all the questions posed in the prior posts.

First, the amp is working fine now!! BIG THANKS TO GEPETTO!!!

This amp was working fine before I installed the Watts Abundant DC Protection board (and is now working fine again after fixing the grounding scheme problem). The DC offset and bias were as shown below before the DCP board installation:

Left = 378 mV bias and -14.0 mV offset
Right = 379 mV bias and -14.8 mV offset​

I had previously installed the White Oak PCB14_20 board in this amp.

Followed Watts Abundants instructions on how to wire up the new DC Protection board. The instructions say connect three white ground wires to J6 (Pin 6) on the DCP board. When I did this, it caused me to get a bad dim bulb test. The dim bulb would burn bright, go about 1/2 dim and then cycle back to bright, then dim and just keep cycling (the DCP board relays were NOT clicking). As required in dim bulb testing, nothing was connected to the amp inputs or speaker outputs.

Followed Gepetto's instructions to only have one ground wire going into J6 on the DCP board. The J6 connection is the new white wire that is installed on the copper plate between the power supply filter capacitor posts. Next moved the white wire on 5R connection to the White Oak PCB14_20 to the 5L position on the same board (just to shorten the wire in keeping with good grounding protocol, 5L and 5R are jumpered together on the back of the board). Then removed the new white wire that was running from the L/R inputs on the amp to the DCP board (just snipped it off where it was soldered on the bare silver wire up near the inputs). The changes in this paragraph solved the dim bulb test failure problem.

Now when I put the amp on the dim bulb tester.....it burns bright initially, then dims down to nothing.....then the DCP board relays click. Watts Abundant's instructions say the DCP board relays won't click on the dim bulb, but they do for me.

Once again this amp is sounding great. On my test bench speakers it also has low background noise using pretty high sensitivity speakers.

Just like MAGIC Scott

With anything analog, grounding must be taken very seriously. There is a separation of ground rails within the White Oak board, you can see this in the schematic (2 different ground symbols used). These 2 separate ground rails are joined by R2L/R which is a 2.7 ohm resistor. This is done to keep the amp very low noise. When you join those 3 wires at the DCP, you defeat this designed in function and additionally introduce a trash signal which is the rectifier return path for the small power sense circuit on the DCP into the low level ground processing for the amp.

Glad you are all well again. When time permits, you should take some time to revamp the front end switch as in the photos I sent you. This is not urgent but will better improve the performance if you use the amp in the NORMAL mode especially. PL wired it so it would work but they did not wire for lowest noise and best signal fidelity.

 

[Gepetto]

All this amp needs now is a new set of PS filter caps. Well, need may be too strong a word, the amp's owner wants them just for piece of mind.

In the words of Columbo (for us that remember that TV show ) "One More Thing"

Keep those 2 new AC wires that go to the DCP board well away from anything that matters within the amp. in a general note, you will find that careful routing of the AC wiring within the PL700B will get you another 6dB of noise reduction. PL again did it so it would work but they did not spend any time fine tuning to achieve the best possible results. An amp with a black hole for background noise is something to be prized and if you fully optimize the AC routing in the PL700B, this can be achieved. I think Lee and others will attest.

 

[laatsch55]

You know you have a quiet amp when you go to bed and leave it on all night because there is no noise....and after a very loud crescendo that cuts off to nothing, the effect is even better when there is nothing to hear during that quiet period..

 

[BMWR75]

In the words of Columbo (for us that remember that TV show ) "One More Thing"

Keep those 2 new AC wires that go to the DCP board well away from anything that matters within the amp. in a general note, you will find that careful routing of the AC wiring within the PL700B will get you another 6dB of noise reduction. PL again did it so it would work but they did not spend any time fine tuning to achieve the best possible results. An amp with a black hole for background noise is something to be prized and if you fully optimize the AC routing in the PL700B, this can be achieved. I think Lee and others will attest.

I routed the two new green AC wires and one new white ground wire from the rectifier to the DC protect board as a twisted together 3-some lying on the bottom of the amp about 1.5 inches towards the front of the WO PCB and away from the other wire bundles.

Is this how you would have done it?

I'll take a look at the front end switch later this week.

 

[laatsch55]

Exactly. Nowadays though I have some SHIELDED, twisted, # 20 awg that I wire the DC protect AC feed with, then run it on the bottom and back up to the DC PCCB.