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[Stephen Swaffer]

Just speculation-could it be two signals combining or "heterodyning"?  Maybe a 40 kHz switching power supply mixing with a 48 kHz clock for a digital sample?

Otherwise, I'd be suspicious of a VFD. A lot of HVAC is going that way and a lot of them have an adjustable carrier freq.  Seems like 8 kHz would be in their range. Who knows who or why spmeone might set it at 8 kHz?

[Jason Glass]

Hi Mike,

I experienced a similar situation when a drummer on tour had his drum machine with a 1/4" balanced TRS output connected via TS cable to a DI with XLR from the DI to the splitter. Striking the DI and replacing it with a proper TRS to XLR balanced adapter solved the problem.

Sent from my mobile phone. Please excuse the inevitable spelling and grammatical errors.

[Jonathan Johnson]

The 8k whine has showed up on all of the monitor channels at least once, usually one monitor channel at a time.

Sounds like it could be the lead diva vocalist.

[John Escallier]

Also speculating:  is it possible that something is using an 8Khz PWM scheme to dim it's indicator led's?

John

[Mike Sokol]

Also speculating:  is it possible that something is using an 8Khz PWM scheme to dim it's indicator led's?

John

Here's a screen shot of the noise a few months ago when I isolated the noise as coming from the electric keyboard. Note there's a good bit of low freq hum and related harmonics in the spectrum. But that 8 kHz spike on the right is what's bothering them. The crazy thing is that the system is apparently making that same noise now, but randomly and without the electric keyboard or its power supply anywhere in the room. Something's rotten in the State of Denmark, but I can't find it until the new noise shows up while I'm in the room to troubleshoot.

[Mike Sokol]

This is a completely different problem, but with the same sort of failure mode. I just bought a $15 (with shipping) TosLink SPDIF to Analog converter for my Apple TV interface. It sounded fine for a few hours, but then I noticed a 450Hz tone in the output which kept getting louder until it was approaching -10 dBv on the output. Ugh... i shut it off and tried it again the next day. Sure enough it was perfectly quiet for the first hour or so, then started making the same 450 Hz tone once it warmed up. Since it used a little 5 volt DC power supply wall-wart, I grabbed another supply from the rack. Now it's quiet and works perfectly. Don't know if there's something similar causing my 8K tone at the church, but this is certainly something we should all watch out for. Stupid Power Supplies...

[David Buckley]

450Hz is far too low a frequency for a SMPS; that's a faulty unit.

Given these PSUs come off the production line by the billion, perhaps there are a lot of still-working-but-not-working-right PSUs out there.  It may not matter to recharge a phone, but the result of too low a switch frequency is more ripple on the DC output, and that may be important in audio electronics.

[Mike Sokol]

450Hz is far too low a frequency for a SMPS; that's a faulty unit.

Given these PSUs come off the production line by the billion, perhaps there are a lot of still-working-but-not-working-right PSUs out there.  It may not matter to recharge a phone, but the result of too low a switch frequency is more ripple on the DC output, and that may be important in audio electronics.

That's my point exactly. If a piece of audio gear is making too much noise, perhaps a power supply swap is needed. Whoda Thunk?

[Corey Scogin]

That's my point exactly. If a piece of audio gear is making too much noise, perhaps a power supply swap is needed. Whoda Thunk?

Did you check to see if the power supply was generating 450Hz noise directly?
I just wonder if it is that or sagging DC voltage causing the D/A converter to freak out.

[Mike Sokol]

Did you check to see if the power supply was generating 450Hz noise directly?
I just wonder if it is that or sagging DC voltage causing the D/A converter to freak out.

Not yet. I just swapped in a new supply and found that the audio was noise free. I'll do a look-see at the supply in a few days. It's definitely time/heat related since there's no noise when the supply is cold.

[ProSoundWeb Community]