modi 3 -> amp: pop when powering off amp

[ultrachrome]

I just replaced my preamp with a nanoDigi 2x8 so now my Modi 3 is connected directly to a vintage BrianGT LM3875 amp.

When I turn off the amp, I now get a brief pop/buzz sound out of my speakers. I've used this amp with several preamps and have never had this issue.

Yesterday, I swapped the Modi 3 with a Khadas Tone Board and the problem has gone away.

Other than the power off noise, the Modi 3 is normally quiet and well behaved.

Any ideas on why this occurring and how I can prevent it?

 

[ABSNimes]

Hi,
I'm just a noob, but pics of your built will help
From what I read & understand :
- NanoDigi connected to Modi 3 gives pop/buzz on power off.
- Khadas Tone Board connected to Modi 3 is pop/buzz free on power off.
Same RCA connectors I suppose ?
I suspect Gnd loop using NanoDigi... what about Gnd RCA onboard connexions ?
Khadas is USB fed. What about NanoDigi ?

Regards

 

[ultrachrome]

nanoDigi -> DAC -> Amp.

The issue is when the amp is powered off while nanoDigi and DAC remain on.

If DAC is Modi 3, turning off amp creates a pop from the speakers.
If DAC is Khadas Tone Board, turning off amp creates no noise.

Prior to nanoDigi, setup was Modi 3 -> preamp -> amp. No noise from amp when turning it off.

 

[ABSNimes]

Ok, so Modi 3 creates unwanted pop off noise.
Post pics please, it will really help
What about RCA connectors ? Different wiring from Modi 3 vs TB ?
Is there any cap in analog output on Modi 3 ?
Have a look at Khadas analog output to see if there are any caps there too, and values.
RCA Gnd trouble using both nanoDigi& Modi 3.
I'll give a try there at first.

Regards

 

[tomchr]

Lemme see if I can wrap my head around this:

All three components (source, DAC, amp) are on.
You turn the power off to the amp but leave the other two components on.

If the DAC is a Modi 3, you get a pop in the speakers. If the DAC is a different DAC, you get no pop.

Yeah. That's weird. I would measure the DC voltage at the output of the Modi 3. That could be causing a pop.

Tom

 

[ABSNimes]

Sorry, do you use same RCA cables from DACs (Modi & Khadas) to Amp ?

 

[ultrachrome]

Lemme see if I can wrap my head around this:

All three components (source, DAC, amp) are on.
You turn the power off to the amp but leave the other two components on.

If the DAC is a Modi 3, you get a pop in the speakers. If the DAC is a different DAC, you get no pop.

Yeah. That's weird. I would measure the DC voltage at the output of the Modi 3. That could be causing a pop.

Tom

Exactly.

I measured .003 VDC in one channel and .005 in the other.

 

[ultrachrome]

I emailed Schiit and they replied: "That's because the modi is technically outputting full voltage into the amp before you turn it off. The modi can't really be turned off, so all the excess current gets shunted. preamps generally have dc protect circuits on their front ends so this doesn't happen."

I don't quite follow.

To further confuse, I took just the DAC (not the nanoDigi) to the basement to try with another BrianGT LM3875 and a couple other amps. No pop at power off.

It will be some days before I can try different amps with the nanoDigi setup.