@matt related to this, what would be the best way to take a noise measurement directly from the output of the virtual ground circuit off the lm4562 before it goes into the opa1612 12db amplifier circuit. These are mounted in separate aluminum boxes connected with bnc cables.
@matt On your suggestion I did order a SilentSwitcher. I powered it with a 9v battery. It does produce +/-15v and ground.
But when I connect it to the notch and opa1612 I’ve been using successfully, I get the qa402 Atten going on and off repeatedly. I have to switch the qa402 to 18db attenuation to get it to stop. I measured the ac input to the qa402 as 25.6mvac. There is also 75mvdc at the qa402 input.
Why is that triggering the Atten on the qa402?
Everything in this setup is battery powered.
Have you connected the silent switcher ground to the BNC shell? What is likely happening is there’s a large DC offset between the SS ground and the wherever the BNC ground has floated, and when you connect the two, the offset is impressed through the input caps fo the QA402 and it triggers and overvoltage.
The other thing to try is connect everything with the atten engaged, then power on the SS, at that point, it will take a second or two for the DC points to be established, and then you can disengage the attenuator.
If the SS is powered but not connected to anything, what DC do you measure from the SS ground to the BNC shell on the QA402?
Thx @matt I tried measuring voltage between the powered up but disconnected silentswitcher ground to the bnc housing on qa402. 0vdc
I then tried your suggestion of hooking everything up but leaving the silentswitcher unpowered with atten on. I then powered the silentswitcher and then went to 0 db attenuation.
Same result with relay clicking. Turning on 18db atten again, I measured voltage between bnc on qa402 which is 0vdc because I use the silentswitcher ground as my ground for the notch/12db amplifier circuit.
However if I let the whole thing sit at 18db attenuation and take thd etc measurement I get this.
Then after a few minutes if I set it to 0db, I no longer get the Atten relay clicking but here is the screen.
I still measure 27mvac going to the qa402 but nothing happening on the screen.
I am confused.
Here is another pic of the last screen. No user weighting but Gen is still on and measured output from the opa1612 to qa402 input is the same 27mvac. 0db no clicking( have to wait about 2 minutes with 18db atten until it will allow me to go to 0db.
There is a 1khz signal coming out of the qa402 but you can’t see any evidence of it here.
@matt so following along the lines of dc being an issue, I added a 100nf cap to ground after the 50 ohm series resistor on the output if the opa1612. This seems to have solved it though I don’t know why. I was only measuring 75mvdc going into the qa402.
My question to you would be 1) why was that a problem
2) what might be a better value for that cap on the output
Hi @moto, are you using your laptop USB to power the silent switcher? Is the laptop plugged in or running from battery?
Are you sure your system has an earth ground someplace? Or is the entire setup floating?
@matt I was using a 9v battery to power the silentswitcher.
The system is floating except for the dac which is 110 3 wire cord into the chassis.
Is the SS ground tied to the system ground aka dac ground?
The ss ground is used as signal ground in the notch/opa1612 , i.e your AGND so that is connected to the bnc shell and thus rca outer shell on the dac.
Hi @Moto. I have read all the posts in this tread and found them interesting But everything hangs in the balance after this last post. I would like to know how the issue turned out. Was it solved? who was the “culprit”? Thank you
I had forgotten most of this but I believe I just left the cap in there. I dont use it anymore as I purchased a Cosmos apu to perform the function.
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Thank you for your response Moto. Sorry I don’t know who the “culprit” is! 